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IEEE Guide for the Design of Low Voltage V oltage Auxiliary Syste Systems ms for Electric Power Substations
IEEE Power and Energy Society
Sponsored by the Substations Committee
IEEE 3 Park Avenue New York, NY 10016-5997 USA
IEEE Std 1818™-2017
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IEEE Std 1818™-2017
IEEE Guide for the Design of LowVoltage Auxiliary Systems for Electric Power Substations Sponsor
Substations Committee of the
IEEE Power and Energy Society
Approved 28 September 2017
IEEE-SA Standards Board
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Abstract: Considered Considered in this guide are the components of both the ac and dc systems and the Abstract: provided guidelines and recommendations for designing the appropriate systems for the substation under consideration. This guide includes the low-voltage auxiliary systems from the source(s) to the distribution point(s). Reliability Reliability requirements and load characteristics are discussed and distribution methods are recommended. Keywords: ac system, auxiliary systems, battery, dc system, IEEE 1818, low voltage, station Keywords: power, station service
The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2017 by The I nstitute of Electrical and Electronics Engineers, Inc. All rights reserved. Published Published 15 December 2017. Printed in the United United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Oce, owned by The Institute of Electrical and Electronics Engineers, Incorporated. PDF:
ISBN 978-1-5044-4583-2
STD22934
Print:
ISBN 978-1-5044-4584-9
STDPD22934
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Participants At the time this IEEE guide was completed, the D9 Working Group had the following membership: Hanna Abdallah, Co-Chair Joseph Gravelle, Co-Chair Radoslav Barac, Secretary Gary Beane Steven Brown Donald Campbell Revinal Dela Rosa Brian Farmer Charles Haahr Jason Hawkins Zachary Homann Bruce Largent
Debra Longtin Reginaldo Maniego James Massura DJ Moreau Michael Nadeau Mike Noori Shashikant Patel Thomas Proios
James Purcell Christian Robles Oscar Santos Hamid Sharifnia Boris Shvartsberg Donald Wengerter Aaron Wilson Linda Zhao Adam Zook
The following members of the balloting committee voted on this guide. Balloters may have voted for approval, disapproval, or abstention. Hanna Abdallah William Ackerman
James Houston John Kay
Bansi Patel Shashikant Patel
Curtis Ashton Ian Backus Radoslav Barac Thomas Barnes G. Bartok W. J. (Bill) Bergman Bergm an Clarence Bradley Matthew Braet Derek Brown Kent Brown Gustavo Brunello Kevin Buhle William Bush William Byrd James Cain William Cantor
Yuri Khersonsky James Kinney Hermann Koch Boris Kogan Jim Kulchisky Saumen Kundu Mikhail Lagoda Chung-Yiu Lam Bruce Largent Thomas La Rose Albert Livshitz Jon Loeliger Debra Longtin Reginaldo Maniego James Massura Larry Meisner
Branimir Petosic Anthony Picagli Thomas Proios James Purcell Charles Rogers Thomas Rozek Ryandi Ryandi Steven Sano Bartien Sayogo Robert Seitz Nikunj Shah Devki Sharma Vinod Simha Jeremy Smith Jerry Smith Philip Spotts
Paul Cardinal Randy Clelland Timothy Conser Gary Donner Brian Farmer Paul Forquer Tirthatarun Tirthata run Ghosh Dastidar Joseph Gravelle Randall Groves Ajit Gwal Charles Haahr Werner Hoelzl Gary Homan
Daleep Mohla Carl Moller DJ Moreau Ryan Musgrove Michael Nadeau Dennis Neitzel Arthur Neubauer Michael Newman Joe Nims James O’Brien T. W. W. Olsen Ols en Lorraine Padden Chris Pagni
Ryan Stargel Wayne Stec Andrew Steen David Tepen Eric Thibodeau Michael Thompson Richard Tressler James Van De Ligt John Vergis John Wang Diane Watkins Donald Wengerter Kenneth White Jian Yu
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When the IEEE-SA Standards Board approved this guide on 28 September 2017, it had the following membership: Jean-Philippe Faure, Chair Gary Homan, Vice Chair John Kulick, Past Past Chair Konstantinos Karachalios, Secretary Chuck Adams Masayuki Ariyoshi Ted Burse Stephen Dukes Doug Edwards J. Travis Grith Michael Janezic
Thomas Koshy Joseph L. Koepnger* Kevin Lu Daleep Mohla Damir Novosel Ronald C. Petersen Annette D. Reilly
*Member Emeritus
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Robby Robson Dorothy Stanley Adrian Stephens Mehmet Ulema Phil Wennblom Howard Wolfman Yu Yua Yuan n
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Introduction This introduction is not part of IEEE Std 1818-2017, 1818-20 17, IEEE Guide for the Design of Low-Voltage Low-Voltage Auxiliary Systems for Electric Power Substations.
IEEE Guide 1818 was created by members of Working Group D9 and is under the sponsorship of the Substations Committee of the IEEE Power & Energy Society. Society. This guide provides guidance and information to substation engineers on factors to consider in the design of ac and dc auxiliary systems for application in electric substations. This guide references several existing standards and is not intended to replace existing documentation, but to provide guidance for the application of ac and dc systems specically in substation applications.
Acknowledgment The D9 Working Working Group would like to acknowledge Chuck Haahr for his ne work as technical editor.
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Contents 1. Overview Overview ................................................................................................................................................... 13 1.1 Scope Scope .................................................................................................................................................. 13 1.2 Purpose Purpose ............................................................................................................................................... 13 2. Normative reference referencess .............. ............................ ............................. ............................. ............................ ............................ ............................ ............................. ............................. .............. 13 3. Denitions Denitions ................................................................................................................................................. 13 4. Design of substati substation on ac auxiliary systems .............. ............................ ............................ ............................. ............................. ............................ .......................... ............ 16 4.1 Introduction Introduction ........................................................................................................................................ 16 4.2 Design criteria .............. ............................ ............................ ............................ ............................ ............................ ............................. ............................. ............................ ................... ..... 17 4.3 Station power source requiremen requirements ts ............. ........................... ............................. ............................. ............................ ............................ ............................ ................. ... 19 4.4 Load analysis ............. ........................... ............................ ............................. ............................. ............................ ............................ ............................ ............................ ...................... ........ 21 4.5 Conductor selection.............................. ........................................ ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ............ 23 4.6 Station Stat ion power transforme transformerr .............. ............................. ............................. ............................ ............................ ............................ ............................ ............................. ............... 29 4.7 Transfer switch switch ................................................................................................................................... 41 4.8 Bus layout and distribution distribution circuits circuits conguration .......... .................... ..................... ..................... .................... ..................... ..................... ................ ...... 44 4.9 AC distribution panelboards panelboards for electrical substations substations........... ..................... .................... ..................... ..................... ..................... .................... ......... 52 4.10 AC auxiliary auxiliary system protection protection .......... .................... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ............ 54 4.11 Equipment specications ......... .................... ..................... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ............ 55 4.12 Operation and maintenance maintenance considerations considerations ........... ..................... ..................... ..................... .................... ..................... ..................... ..................... ............. .. 56 5. Design of substation substation dc auxiliary auxiliary system................................................................................................... system................................................................................................... 58 5.1 Design criteria criteria .................................................................................................................................... 58 5.2 Typical equipment served by the dc system .............. ............................ ............................. ............................. ............................ ............................ ................. ... 60 5.3 One-line diagram .............. ............................ ............................ ............................. ............................. ............................ ............................ ............................ ............................. ............... 61 5.4 DC batteries........... ..................... .................... ..................... ..................... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ............ 62 5.5 Battery chargers........................... ...................................... ..................... .................... ..................... ..................... ..................... ..................... .................... ..................... .................... ......... 66 5.6 DC panels ............. ........................... ............................. ............................. ............................ ............................ ............................ ............................. ............................. .......................... ............ 70 5.7 Load transfer methods methods ........................................................................................................................ 70 5.8 Design considerations considerations......................................................................................................................... ......................................................................................................................... 72 5.9 Maintenance provisions provisions ...................................................................................................................... 77 Annex A (informative) Bibliography .............................................................................................................. 78 Annex B (informative) Conductor selection examples ................................................................................... 81 Annex C (informative) Battery sizing example .............................................................................................. 85
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List of Figures Figure 1—Block diagram for typical substation ac auxiliary power ............................................................... ............................................................... 17 Figure 2—Possible SSVT locations locations ............................................................................................................... 20 Figure 3—Conductor selection process ow chart .......... ..................... ..................... ..................... ..................... .................... ..................... ..................... ................ ...... 26 Figure 4—Single-phase-to-ground connection .......... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ............ 35 Figure 5—Single-phase transformer with phase-to-phase connections connections..................... ............................... ..................... ..................... ................ ...... 36 Figure 6—Delta-delta connection connection .................................................................................................................. 37 Figure 7—Delta-wye connection connection ................................................................................................................... 38 Figure 8—Grounded wye–grounded wye transformer connection connection ................................................................ 39 Figure 9—Two leg open delta from grounded wye wye ........................................................................................ 40 Figure 10—Two leg open delta from delta delta ..................................................................................................... 41 Figure 11—Simplest panelboards ......... .................... ..................... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ............ 45 Figure 12—Variation of simplest panelboard .......... .................... .................... ..................... ..................... ..................... ..................... .................... ..................... ............... .... 45 Figure 13—Sub-panelboa 13—Sub-panelboard rd .......... .................... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ..................... .................... ......... 46 Figure 14—Reliable and exible panelboard system system ..................................................................................... 47 Figure 15—Panelboards with backup generator ............................................................................................. ............................................................................................. 47 Figure 16—Expanded radial system system .............................................................................................................. 48 Figure 17—Primary selective system system ............................................................................................................. 49 Figure 18—Secondary selective system system ......................................................................................................... 50 Figure 19—Secondary selective system with backup generator ..................................................................... 51 Figure 20—Secondary selective system with backup generator and additional redundancy redundancy .......................... .............. ............ 51 Figure 21—Simplied dc system block diagram diagram ............................................................................................ ............................................................................................ 58 Figure 22—Possible load case .......... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... ..................... ..................... ................ ...... 63 Figure 23—Battery with breaker disconnect and charger at dc panels panels ........................................................... 68 Figure 24—Battery with fuse disconnect and charger at dc panels panels ................................................................. 68 Figure 25—Battery with no disconnect disconnect .......................................................................................................... 69 Figure 26—Simplied parallel/transfer parallel/transfer with two disconnects disconnects ....................................................................... 71 Figure 27—Simplied parallel/transfer parallel/transfer with one disconnect disconnect ......................................................................... 71 Figure 28—DC transfer scheme scheme ..................................................................................................................... 72 Figure 29—Rack designs designs ............................................................................................................................... 75
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Figure C.1—Substation one-line diagram diagram ...................................................................................................... 85 Figure C.2—Duty cycle tripping tripping .................................................................................................................... 87 Figure C.3—Completing the battery cell–sizing worksheet worksheet ........................................................................... 91
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List of Tables Table 1—Generic substation ac load study .......... ..................... ..................... .................... ..................... ..................... ..................... ..................... .................... .................. ........ 24 Table 2—Typical 2—Typical kVA ratings for distributi distribution on transformers transfo rmers ........................................................................... 31 Table 3—Distribution transformer short-circuit withstand capability ........... ..................... ..................... ..................... .................... .................. ........ 33 Table 4—Transforme 4—Transformerr BIL ratings (IEEE Std C57.12.20 [B23] [B23])) ........... ..................... ..................... ..................... .................... ..................... ............... .... 34 Table 5—Typical voltage ratings for ac panelboards ........... ..................... ..................... ..................... .................... ..................... ..................... ..................... ............. .. 52 Table 6—Working clearances for electrical panels panels ......................................................................................... 57 Table C.1—DC load table table .............................................................................................................................. 88
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IEEE Guide for the Design of LowVoltage Auxiliary Systems for Electric Power Substations 1. Overview 1.1 Scope This guide will consider the components of both the ac and dc systems and provide guidelines and recommendations for designing the appropriate systems for the substation under consideration. This guide covers the low-voltage auxiliary systems from the source(s) to the distribution point(s). Reliability requirements and load characteristics are discussed, and distribution methods are recommended.
1.2 Purpose The low-voltage ac and dc auxiliary systems comprise very important parts of the substation equipment. The design of the ac and dc auxiliary systems facilitates the safe and reliable operation of the substation. This guide considers various factors that aect the design of the ac and dc auxiliary systems such as reliability reliability,, load requirements, system congurations, personnel safety, safety, and protection of auxiliary systems equipment.
2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, used, so each referenced document is cited in text and its relationship relationship to this this document is explained). For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments or corrigenda) applies. IEEE Std 485™, IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications. Applications.1,2 IEEE Std 525™, IEEE Guide for the Design and Installation of Cable Systems in Substations. 3. Defnitions
For the purposes of this document, the following terms and denitions apply. apply. The IEEE The IEEE Standards Dictionary 3 Online should Online should be consulted for terms not dened in this clause. 1
The IEEE standards oravailable products referred in this clause are trademarks The Institute of Electrical Ele ctronics Engineers, Inc. IEEE publications are av ailable from ThetoInstitute of Electrical and Elec of tronics Engineers, 445 Hoes and Lane, Piscataway, NJ 08854, USA (http:// http://standards standards.ieee .ieee.org .org). ). 3 IEEE Standards Standards Dictionary Online subscription Online subscription is available at: http:// http://www www.ieee .ieee.org/ .org/ portal/innovate/ portal/innovate/ products/standard/ products/standard/ standards_dictionary.html standards_dictionary .html.. 2
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IEEE Std 1818-2017 IEEE Guide for the Design of Low-Voltage Auxiliary Systems for Electric Power Substations
authority having jurisdiction (AHJ): The organization, oce, or individual that has the responsibility and authority for approving equipment, installations, or procedures. available short-circuit current: (at a given point in a circuit) The maximum current that the power system can deliver through a given circuit to any negligible-impedance short circuit applied at the given point, or at any other point that will cause the highest current to ow through the given point. basic impulse insulation level (BIL): A reference reference impulse insulation strength expressed in terms of the crest
value of the withstand voltage of a standard full impulse voltage wave. battery duty cycle: The sequence of loads a battery is expected to supply for specied time periods. cell size: The rated capacity of a cell, or the number of positive plates in a cell. equalizing charge: A charge, charge, at a level higher than the normal oat voltage, applied for a limited period of time, to correct inequalities of voltage, specic gravity, or state of charge that may have developed between the cells during service. extra-high voltage (EHV): A maximum system syste m voltage that is greater gre ater than 242 kV but less than th an 1000 kV. kV. ferroresonance: (A) A phenomenon usually characterized by overvoltages and very irregular wave shapes and associated with the excitation of one or more saturable inductors through capacitance in series with the inductor. (B) An electrical resonant condition associated with the saturation of a ferromagnetic device, such as a transformer, through capacitance. Ferroresonance can arise when (1) due to dissimilar phase switching, the capacitance normally in shunt with the ferromagnetic device becomes energized in series with the device, (2) a weak source is isolated with a lightly loaded feeder containing power-factor-correction capacitors. For example, if the resulting voltage buildup produces saturation of the feeder transformers, there will be an interchange of energy between the system capacitance and the nonlinear magnetizing reactance of the transformers. oat charge: A constant-voltage applied to a battery to maintain it in a fully charged condition, while minimizing degradation or water consumption. oat service: Operation of a dc system in which the battery spends the majority of the time on oat charge with infrequent discharge. Syn: Syn: standby standby service. fully rated system: Every protective device is rated to at least the available fault current at the service point. high voltage: A class of nominal system voltages equal to or greater than 100 000 V and equal to or less than 242 000 0 00 V. V. low voltage: Voltage Voltage levels lev els that are less l ess than or equal e qual to 1 kV. medium voltage: A class of nominal nomina l system voltages voltag es greater than 1000 10 00 V and less than 100 000 V. V. molded-case circuit breaker (MCCB): A circuit circuit breaker that is assembled as an integral unit in a supporting and enclosing housing of insulating material. nominal battery voltage: The value assigned to a battery of a given voltage class for the purpose of convenient designation. The operating voltage of the system may vary above or below this value. nominal system voltage: The ac system voltage by which the system is designated and to which certain
operating characteristics of the system are related.
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IEEE Std 1818-2017 IEEE Guide for the Design of Low-Voltage Auxiliary Systems for Electric Power Substations
pad-mounted transformer: An outdoor transformer utilized as part of an underground distribution system, with enclosed compartment(s) for primary-voltage and secondary-voltage cables entering from below, and mounted on a foundation pad. panelboard: A single panel or group of panel units designed for assembly in the form of a single panel, including buses and automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front. (Adapted from the NEC.) See also: also: switchboard. period: An interval of time in the battery duty cycle during which the current (or power) is assumed to be constant for purposes of cell-sizing calculations. rated capacity (lead-acid): The capacity assigned to a cell by its manufacturer for a given discharge rate, at a specied electrolyte temperature and specic gravity, to a given end-of-discharge voltage. remote terminal unit (RTU): A piece piece of equipment located at a distance from a master station to facilitate monitoring and control the state of outlying power equipment and to communicate the information back to the master station or host. separately derived system: A wiring system whose power is derived from a generator, transformer, or converter windings and has no direct electrical connection, including a solidly connected grounded circuit conductor,, to supply conductors originating in another system. conductor series rated system: Each protective device needs to only be rated for the available fault current at its terminals. station service voltage transformer (SSVT): A transformer transformer that supplies power from a station high-voltage bus to the station station auxiliaries auxiliaries and also also to the unit auxiliaries during unit startup startup and shutdown, or when the unit auxiliaries transformer is not available, or both. switchboard: (A) A large, large, single-panel, frame, or assembly of panels on which are mounted, on the face, back, or both, switches, overcurrent and other protective devices, buses, and usually instruments. Switchboards are generally accessible from the rear as well as from the front, and are not intended to be installed in cabinets. (Adapted from the NEC.) (B) A metal-enclosed metal-enclosed panel or assembly of panels that may contain molded case, insulated case, or power circuit breakers, bolted pressure contact or fusible switches, protective devices, and instruments. These devices may be mounted on the face or the back of the assembly. Switchboards Switchboards are generally accessible from the rear as well as from the front; however, they can be front accessible only. switchgear: (A) A general term covering switching and interrupting devices and their combination with associated control, instrumentation, metering, protective, and regulating devices and covering assemblies of these devices with associated interconnections, accessories, and supporting structures used primarily in connection with the generation, transmission, distribution, and conversion of electrical power. (B) An assembly of equipment used to switch and control electrical power. tertiary winding: An additional winding in a transformer that can be connected to a synchronous condenser, a reactor, an auxiliary circuit, etc. For transformers with wye-connected primary and secondary windings, it may also help (1) to stabilize voltages to the neutral, when delta connected (2) to reduce the magnitude of third harmonics when delta connected (3) to control the value of the zero-sequence impedance (4) to serve load. valve-regulated lead-acid (VRLA) cell: A lead-acid cell that is sealed with the exception of a valve that opens to the atmosphere when the internal pressure in the cell exceeds atmospheric pressure by a preselected
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amount. VRLA cells cells provide a means for recombination of internally generated hydrogen and oxygen to limit water consumption. vented battery: A battery battery in which the products of electrolysis and evaporation are allowed to escape to the atmosphere as they are generated. These batteries are also commonly referred to as fooded as fooded .
4. Design of substation ac auxiliary systems 4.1 Introduction The objective of this section is to provide the required information for the substation engineer to design an ac auxiliary system as applicable for a substation. Figure 14 represents an ultimate station power conguration that can be applied to any substation depending on substation size, reliability, reliability, and load requirements. One ac source is designated as the normal or preferred source, and the second and third (if available or necessary) sources are designated as the backup source(s). A loss of the normal source may require transferring the load to a backup source. In substations with multiple sources, the sources are typically connected through a transferring scheme. One or more ac panels are used to serve the substation load as required.
4
Notes in text, tables, and gures gures of a standard are given for information information only and do not contain requirements needed to implement this standard.
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NOTE—The a bov bovee let letters ters r efer efer ence ence dier dier ent ent subsect subsections ions coveri covering ng specic specic compo componen nents ts of ac auxiliary power schemes as follows: A: see 4.2 4.2,, 4.3 4.3,, and 4.4 4.4;; B: see 4.5 4.5;; C: see 4.6 4.6;; D: see 4.7 4.7;; E: see 4.8 4.8;; F: see 4.9 4.9..
Figure 1—Block diagram for typical substation ac auxiliary power
In the rst step of the design process, the design engineer should review: a)
The design criteria for the station service.
b)
The number of station service sources available. The source type could be single-phase or three-phase.
c)
The load required to be served.
4.2 Design criteria 4.2.1 Introduction In general, the design criteria of the ac auxiliary system are determined by the existing, proposed, and future substation loads, typically measured in kVA. kVA. Diversity of the total connected load needs to be considered as not all loads are concurrent. For example, the control enclosure cooling system should not run simultaneously with the heating system; redundant cooling systems are not concurrent; and spring charging motors for power circuit breakers may not run concurrently. concurrently.
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Also considered with the substation loads are the equipment voltage ratings and phase requirements (singleor three-phase) of the substation equipment to be installed. When sizing auxiliary transformers and other station power components, the designer should consider substation expansion and short-term loads, such as construction or maintenance loads. Timing of any proposed expansion may dictate initial installation or deferral of station power components. Some loads may be identied as critical, which requires ac service to be maintained continuously. continuously. Depending Depending upon such critical loads, the substation substation may require two or three ac station service sources with the ability to transfer loads between sources. Due to the importance of the station power to the operation and reliability of the substation, the following factors should be considered in order to determine the required station power congurations. This guide indicates various equipment ratings (voltage, ampacity, capacity, etc.) compliant with IEEE and NEMA standards. Equipment with other ratings conforming to standards published by other organizations is available. Design philosophies and practices presented in this guide should be adapted appropriately based on the equipment utilized in design and authority having jurisdiction.
4.2.2 System stability System stability considerations are important for the reliability requirements of the station power. If the loss of a substation results in a system disturbance to the electrical grid that could create a blackout condition in the area, the station service system should have an independent power source. The auxiliary power system requirements for redundant supply may also need to include the ability for the station to complete black start operations—meaning a local generation source is required to supply the station power system and battery chargers for the protection circuits in the event of a system collapse and subsequent repowering. See 4.3.5 4.3.5 and and 4.7.5.. 4.7.5
4.2.3 Customer service and loss of revenue Some substations serve critical loads such as hospitals, manufacturing complexes, government oces, schools, or serve large blocks of load where the substation reliability requirements are high. Some substations are connected to power plants that obtain at least a portion of their station service from the substation. Loss of the substation station service may result in tripping the plant and lead to a loss of revenue. These type of stations may need multiple station power sources. Other less critical substations may only have one station power source.
4.2.4 Equipment protection Substation equipment protection considerations should be given to all substations regardless of the size. High-voltage and extra-high-voltage substations contain expensive equipment such as transformers where the cooling system is important for operation and a backup source is generally required. Similarly, Similarly, protective relays or other electronic control equipment located in high-temperature areas may require a continuous cooling system and a second power source. Separately implemented control and protection schemes may be implemented to mitigate the likelihood of equipment damage. The control and protection schemes are outside the scope of this guide. For neutral grounding, there are several dierent grounding philosophies. The designer should ground station service transformer neutrals per utility practice or local jurisdictional requirements.
4.2.5 Design consideratio considerations ns The designer may consider the following list when designing an ac system for substations: a)
Location of ac equipment—indoor or outdoor
b) c)
Number of ac ac panels panels Essential loads
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d)
Non-essential loads
e)
Conductor types and sizes
f)
Voltage drop calculatio calculations ns
g)
Jurisdictional requirements such as the National Electrical Code® (NEC®) [B34]5
h)
Arc ash considerations
4.2.6 Selection of of auxiliary system voltage Several secondary voltage levels are available for ac auxiliary systems. When determining the secondary voltage, the designer may use a standard voltage level determined by the designer’s power system or use a voltage level based on the supplied equipment. Either way, the designer should consider the factors listed in 4.4 4.4.. Voltage Voltage ratings listed in this document are typical of North American American power systems, but are not all inclusive. Variations Variations to these voltages discussed in this document are common in other areas of the world.
4.3 Station power source requirements 4.3.1 Introduction Three ac sources are represented in Figure 1. 1. One source, typically the most reliable source, is designated as the primary, or normal, source. The second source is designated as the backup source and is used when the normal source is unavailable. The third source is used as a second backup and is utilized only when both the normal and secondary sources are unavailable. There are four sources that are commonly used as substation ac auxiliary power sources: a)
Power transformer tertiary
b)
Substation bus
c)
Distribution line
d)
Standby generators
Each source has advantages and disadvantages. Substation location, substation equipment, and bus congurations may dictate which source is normal. The selection of redundant sources is important so an outage would not remove both normal and alternate sources.
4.3.2 Power transformer transformer tertiary The tertiary of a power transformer in substations can provide a reliable source for station power applications. When the primary and secondary windings are connected wye, a third winding connected in delta is typically used for transformer stabilizing purposes. A tertiary winding presents a low impedance path for zero-sequence currents and harmonics, thereby reducing the zero-sequence impedance presented to the outside world, while avoiding the problem of tank heating. The tertiary winding typically has a volt-ampere rating between 20% to 30% of the volt-ampere rating of the primary winding. The tertiary winding typically has a medium-voltage rating up to 34.5 kV. kV. If there are plans to use the transformer tertiary for station auxiliary power purposes, the tertiary winding is brought out of the transformer through bushings. The volt-ampere rating of the tertiary winding typically exceeds the maximum volt-ampere requirement of a substation’ss ac auxiliary power load and is an adequate ac auxiliary power source. substation’
5
The numbers in brackets correspond to those of the bibliography in in Annex A. A.
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Consideration should be given to the available fault current at the tertiary bus. In the case that the fault current magnitude exceeds the interrupting rating of the protective equipment, such as fuses or circuit breakers, several options can be employed to mitigate the fault current. These options include installing current limiting fuses, resistors, or reactors; or increasing the transformer tertiary impedance.
4.3.3 Substation bus The substation bus is another available source for auxiliary station power. When distribution voltage is available, distribution transformers are typically utilized for station service. Transmission voltage buses can be used, but are not typically typically preferred preferred due to their relatively relatively high cost. cost. A station service service voltage transformer transformer (SSVT) is used to transform the transmission bus voltage to the ac auxiliary voltage. These transformers are available for voltages from 34.5 kV to 345 kV. One or more SSVTs might be required, depending on required station power load. See Figure 2 for 2 for possible connections.
Figure 2—Possible SSVT locations
The SSVT is located within the line or bus zone of protection. A fault on the SSVT may be cleared by the protective relay or by a high-side fuse. Depending on the size of the SSVT SSVT,, the the required required fuse ampacity may not be available available for certain voltage levels. The protection engineer should be consulted for the the nal nal location location when when determining the required SSVT protection. Low-side overcurrent protection of the secondary conductors used for auxiliary station service are typically applied as close to the secondary terminals as possible. Surge protection is typically needed on the the high-side connection of the SSVT SSVT.. If arresters protecting other equipment in the station are close enough to protect the SSVT, a dedicated arrester for the SSVT may not be required. Guidance on surge protection and separation eects can be found in IEEE Std C62.22™ [B27] [B27]..
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4.3.4 Distributio Distribution n feeders A common source for substation auxiliary station power is the use of nearby distribution feeder circuits. The feeder primary is typically connected to a step-down transformer located near the control enclosure. If the feeder is owned by another utility, utility, a revenue meter is installed. Since the feeder has more exposure to faults, it is typically used as a backup to the primary source.
4.3.5 Standby generators Generators may also be used as an ac auxiliary power source. In substations, generators are typically used as an emergency/backup power source. There are many disadvantages in using generators as a permanent ac auxiliary source. Choosing to use generators as a permanent ac auxiliary source requires additional design considerations. When using generators, designers should consider re-protection systems, fuel-storage systems, ventilation, and the climate. Generators may also be housed in a separate building structure, which requires the installation of a ventilation system. If the generators are located outdoors in the switchyard, there is a reduced need for re-protection installation, installation, fuel-storage systems, or building ventilation.
4.4 Load analysis 4.4.1 Introduction In order to design a reliable station service system, the ac loads for the system need to be identied and calculated. The designer should consider the ultimate plan for the substation in order to account for future loads anticipated at the station. After the ac loads have been identied, the demand and load factors for each load should be applied. The resultant ac loads are used to size the station service transformer(s) and determine associated conductor ratings. The use of demand and load factors allow for the economical selection of the transformer size without being overly conservative.
4.4.2 Load identicati identication on The following types of loads should be considered when identifying the overall station load: a)
Substation major equipment loads, including: 1)
Transformer Transfor mer cooling fans
2)
Transformer pumps
3)
Load tap changer motor drives
4)
Breaker ac charging motors
5)
Equipment heaters
6)
Yard loads—This loads—This load includes includes yard lighting lighting and receptacle receptacle loads for equipment testing. Also transformer oil-retention oil-retention pit pumps load should be included.
b)
Control enclosure enclosure loads: The control enclosure houses critical equipment equipment used for for the protection protection and operation of the substation. This equipment includes substation protective relays, metering, battery chargers, and control equipment. For optimum operation of this equipment, the cooling and heating of the control enclosure should be operated within a specied temperature range. Other loads may include control enclosure lights and receptacle loads.
c)
Maintenance equipment load: For large substations, maintenance crews may may elect to use a portable generator as a source for the maintenance equipment. If maintenance equipment is served from the station power transformer, transformer, the load of the maintenance equipment should be included.
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d)
Ancillary structures: The substation may have have additional additional structures structures for maintenance, maintenance, storage, or other purposes. Include any heating, ventilation, and air conditioning (HVAC), (HVAC), equipment, or structure auxiliary load if it is sourced from the substation ac system.
e)
Future build build out: In order to account for the ultimate load of the substation, substation, any future loads loads should be considered and included in calculating the maximum station load. This includes loads for additional power transformers, cooling and heating, heating, breakers, etc.
4.4.3 Equipment rating identication Once all planned and future loads have been identied, the ratings for each load need to be established. Ratings may be provided by the equipment manufacturer in amperes, watts, or kVA. kVA. In order to calculate the station load, a common basis needs to be used. Either current or kVA kVA are typically used because both are easy and straightforward. straightforwar d. If current is used, the total power should be calculated based on the respective voltage class of the equipment. Loads also should be segregated between single-phase and three-phase. For multiple-phase systems, loads need to be balanced between phases for optimal transformer loading. 4.4.4 Demand and load factors factors The demand factor is the ratio of the maximum coincident demand of a system, or part of a system, to the total load connected to the system, or part of the system, per IEEE Std 141™ [B3] [B3]:: demand factor =
maximum coincident system demand
(1)
total load connecttted ed to the syste system m Demand factors can also be established for a subset of similar equipment (such as receptacles) rather than only a single system-level demand factor factor.. The second consideration is the amount of time that a load runs based on a selected period of time, referred to as load factor . Load factor is the ratio of the average load over a designated period of time to the peak load occurring in that period per IEEE Std 141 [B3] [B3]:: average load load over a designated time period load factor = peak load occ ccuring uring in that period
(2)
The period for which the load factor is considered should be chosen based on the load capability of the transformer(s) that is used in the design. For best practice, loads operating for three hours or more should be considered asthe continuous load. load. High loads that operate for short periods of time also need special consideration in relation to entire system
4.4.5 Load calculations After equipment ratings have been established, the demand and load factors are selected. Selecting the demand and load factors often requires engineering judgment in terms of familiarity with substation operations and understanding how the loads such as receptacles, lighting, air conditioning, transformer cooling fans, etc., are applied. Applying Applying the demand and load factors provides a more realistic adjusted load (rather than simply summing up the nameplate ratings of the equipment) for sizing of the station service transformers. In the process of determining the ultimate loading, various conditions (generally the worst case scenario) under which loads may be operated should be considered, such as seasonal and/or time of day. The person responsible for the sizing of the transformer can perform the total adjusted load calculations. For information on the process of sizing transformers, see 4.6 4.6.. Be aware that the thermal time constants (overload capability) of transformers are determined by the manufacturers and are typically dierent for
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dry type and liquid-immersed units. Refer to IEEE Std C57.91™ [B24] [B24] and and IEEE Std C57.96™ [B25] [B25] for for additional information. A general example of a substation load analysis is shown shown Table 1. 1. The example only considers the case of loads running during the daytime in the summer summer.. It is not an exact or comprehensive analysis for substations, but intended to illustrate one practical approach to the process. Load analysis varies based on substation voltage class, capacity, capacity, climate, and any non-traditional loads. In the example, adjusted kVA load (for transformer sizing) is determined as follows: adj adjust usted ed kVA = quant quantity× ity× kVA per unit unit × dem demand and factor factor × lo load ad facto facto or or
(3)
Justication or reasoning for the demand and load factors should be documented, such as shown in the comments column of the example.
4.5 Conductor selection 4.5.1 Introduction Subclause 4.5 presents 4.5 presents general information useful in the selection of both line and load conductors. It describes various characteristics essential to conductor selection: conductor type, insulation type, insulation voltage rating, insulation temperature rating, conductor terminations, and conductor size. An essential document in understanding ac and dc cables used in substation design is IEEE Std 525™. A process ow chart has been developed to aid the designer/engineer in the conductor selection process. However,, any rules and restrictions set forth by the authority having jurisdiction (AHJ) in the area that work is However being performed supersede any documents documents referenced in this this section. section. This section covers the selection of both line and load conductors. There are six main characteristics to consider when selecting a conductor—conductor type, insulation type, cable insulation voltage rating, cable insulation temperature rating, the terminations being connected to (temperature rating, ampacity ampacity,, etc.), and conductor size. The engineer performing the conductor selection can use the process ow chart shown in Figure 3 and 3 and described in 4.5 4.5 for for guidance on conductor selection based on these characteristics. For specic examples on selecting conductors (control, instrument, power, and communication), refer to the annexes in the latest version of IEEE Std 525.
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e l / e l — o e c c ) p s r g c n t a i t e i r n a p t n C l v i c o n e s a r d A t e t c l e r c i V p s n e e u r b c i a 0 w a W k c k e - o r m 1 9 H R ( p
n i p ) o r l e i t t c a s a c r i e t n w p u o e c e k p r m o c 0 1 r ( C a
25 Copyright © 2017 IEEE. All rights reserved.
s l a t o t y r l A p V y A e r p k V m a u ø A r i o i l s 3 V M f 0 s x r e d k n a a u o w a a r t 2 t 4 n l p ø o 1 T
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Figure 3—Conductor selection process ow chart
4.5.2 Conductor type The rst step in conductor selection is to determine the type of conductor to be used. The conductor material and stranding type are the most important factors to consider (i.e., aluminum versus copper and stranded versus solid). There are advantages to using each type of conductor depending on the application. Consider characteristics such as their weight, conductivity, conductivity, and surrounding environmental conditions for the application of the conductors. Copper has historically been used for conductors of insulated cables due to its desirable electrical and mechanical properties. The need for mechanical exibility usually determines whether a solid or a stranded conductor is used, and the degree of exibility is a function of the total number of strands. A single insulated or bare wire is dened as a conductor, whereas an assembly of two or more insulated conductors, with or without an overall covering, is dened as a cable. All All of this information is typically available from the cable manufacturer. manufacturer. For additional information on conductor material and stranding, see IEEE Std 141 [B3] [B3] and IEEE Std 525.
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4.5.3 Cable insulation insulation voltage voltage rating Cable insulation voltage rating is selected based on the operating voltage, and the expected fault-clearing time. Further guidance on selecting the voltage rating of cables should be provided by the AHJ, or by specic product literature provided by the manufacturer manufacturer..
4.5.4 Cable insulation insulation type For theand insulation type should beTypically, selected toinformation meet the local suchjacketed as dry, conductors, wet, or both, chemical resistance. on environmental the applicationconditions— of dierent insulation types is available from the manufacturer. manufacturer. There may also be requirements by the AHJ. IEEE Std 141 [B3] and [B3] and IEEE Std 525 provides general guidance on cable insulation selection.
4.5.5 Cable temperature rating The temperature rating of the cable should be selected to withstand the ambient temperature of the environment in which it is installed, in addition to any self-heating that may occur. The designer selecting the conductor should note that the conductor installation may cross multiple environments, all of which should be considered. Typical conductor temperature ratings are 60 °C, 75 °C, 90 °C, and 105 °C.
4.5.6 Consideratio Consideration n for the characteristics of termination points and connected connected equipment The ampacity of a conductor with a given temperature rating may need to be reduced depending on the type of termination points to which the conductor is connected. The conductor should not be allowed to become hotter than the thermal rating of the interconnected equipment. Typical Typical equipment terminals are limited by the manufacturer to 75 °C. For an example involving cable selection based on termination ratings, see see Annex B. B.
4.5.7 Conducto Conductorr size calculations calculations 4.5.7.1 Introduction The following factors should be considered when selecting the conductor size: a)
Required ampacity (initial conductor size selection)
b)
Temperature, burial depth, and bundling bundling corrections corrections
c)
Voltage drop
d)
Short-circuit calculations
4.5.7.2 Required ampacity ampacity (initial conductor conductor size selection) selection) All conductors should be initially sized based on the ampacity of the load(s) they are supplying. The size of the conductor may be based on requirements provided by the AHJ, or by specic product literature provided by the manufacturer. manufacturer. For an example involving the initial conductor size selection, reference Annex B. B. Once the initial conductor type and size selection is made, verify the conductor has been sized to avoid overheating and excessive voltage drop. If the verications prove the conductor size to be inadequate, then the engineer should make an economically and practically sound decision to redesign the load-distribution scheme. The redesign decision could involve any of the following options: a)
Resize the conductor (reduces voltage drop)
b)
Reallocate loads to, or rebalance loads among, dierent circuits to adjust adjust load load distribution distribution
c) d)
Add additional additional circuits to accommodate accommodate new loads loads (may (may require require a larger larger ac panel) Decrease the distance of circuit run (reduces voltage drop)
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e)
Consider a higher voltage distribution system and local step-down transformers
4.5.7.3 Temperatur emperature, e, burial depth, and bundling corrections The ampacity rating of a cable may vary based on ambient temperature, burial depth, and proximity to other current-carrying conductors. The manufacturer can provide ampacity ratings based on a range of ambient temperatures. If the ambient temperature in the area of a particular cable installation is not within range of the ambient temperature specied by the manufacturer manufacturer,, then the ampacity should be adjusted. Guidance for this type of ampacity adjustment factor should be obtained from the cable manufacturer or the AHJ. For an example involving ampacity adjustment based on ambient temperature, see B.2 B.2..
4.5.7.4 Voltage drop vericat verications ions Losses by means of voltage drop across a conductor are directly proportional to the length and impedance of the conductor. Per Ohm’s Ohm’s law, the higher the current a conductor is carrying, or the higher the resistance of a conductor,, the greater the voltage drop. Voltage conductor Voltage drop can create under-voltage issues for substation equipment, leading to various malfunctions, depending on the type of equipment. Voltage drop calculation for a single-phase circuit: I × L ×[ R × pf 2× %V D =
X ×sin(arccos( pf )) ] ×100 V
+
(4)
Voltage drop dro p calculation calculatio n for a 3-Ø load: I × L × 3 ×[ R × pf %V D =
+
V
X ×sin(arccos( pf )) ] ×100
(5)
where V D
is the line-to-neutral voltage voltage drop of the conductor expressed in volts, for for a 1Ø conductor
or the line-to-line voltage voltage drop of the conductor expressed in volts for a 3Ø conductor V is the nominal voltage of the circuit circuit R is the alternating-current alternating-current resistance in ohms to neutral neutral per unit measurement X X is the alternating-current alternating-current reactance reactance in ohms to neutral per unit measurement measurement I is the load in amperes at 100% L is the length length of the conductor conductor in unit unit measurement being considered for the voltage voltage drop pf is the equivalent power factor being considered for the circuit. If this factor has been accounted for pf in the load study, then a value of 1.0 should be used in the voltage drop calculation After a conductor size is selected with an acceptable level of voltage drop, verify the terminal voltage delivered compared to the operating voltage range of the load. It is important that adequate voltage is delivered to critical loads (i.e., trip coils, battery chargers, etc.). It is common for the AHJ to provide standards that provide acceptable levels of voltage drop. For an example involving voltage drop calculations, see B.2 B.2..
4.5.7.5 Short-circu Short-circuit it calculations calculations Verify the conductor size can withstand the available short-circuit current at its termination point. Sizing of the conductor based on available fault current is a function of initial or continuous conductor operating temperature, the nal conductor temperature after a fault, the maximum possible fault-clearing time based on protective devices, and available fault current at the circuit’s termination point. The conductor nal temperature limits should be obtained from the cable manufacturer. manufacturer. Suggested methods and values for sizing a conductor based on short-circuit current can be found in IEEE Std 525 and IEEE Std 242™ [B4] [B4].. However,
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any rules and/or standards provided by the AHJ should be considered before any other methods of calculation are applied. For an example involving sizing a conductor based on short-circuit calculations, calculations, see Annex B. B.
4.6 Station power transformer 4.6.1 Introduction The objective of 4.6 4.6 is is to provide items for consideration to help the substation designer select the appropriate station service transformer for the substation. Subclause 4.6 discusses 4.6 discusses the required number of transformers, transformer power rating (kV (kVA), A), transformer connections, transformer short-circuit rating, and some other items to consider.
4.6.2 Transformer types The following transformer types are used in the substation: a)
Pole- or structure-mounted transformer: The primary primary is connected overhead overhead to the bus and the secondary can be brought to the main panel via conduit or trench. This transformer type is simplest when the load is single phase and less than 100 kV kVA A and the required secondary voltage is 120/240 V or 240/480 V. However, three-phase installations are common as well.
b)
Pad-mounted transformer: To limit the voltage drop and reduce the length of the secondary conductors, the transformers are typically located near the control enclosure. The location should not interfere with vehicle movement within the substation yard, and should be located near the cable entrance for easy access to the control enclosure load center. The primary cables are connected to the bus/ transformer tertiary and brought underground to the transformer. The secondary cables are connected to the ac system as required. This transformer type is typically used when medium voltage is available, the connected load is predominantly three-phase, and the total load is greater than 100 kVA. kVA.
c)
Station service voltage transformer transformer (SSVT): This transformer transformer type combines combines the the characteristics characteristics of a voltage transformer with convenient power capability. Used in the substation application if no lowor medium-voltage bus is available, or no nearby distribution feeder exists, or the cost of installing the feeder is high. One to three transformers can be installed depending the required kVA rating. The primary is is normally connected from from phase to ground. Typical secondary secondary ratings available 120/240 120/240 V, 277/480 V, 240/480 V, V, and 600 60 0 V (ac).
4.6.3 Number of of transformers transformers required The number of station power transformers required for a substation can be determined based on the design criterion discussed in 4.2.5 4.2.5.. One transformer may be acceptable for a low-load substation. For substations with high load or high reliability requirements, two or more station power transformers may be required. An important factor that can aect the number of station power transformers is the available sources for station power.. power Many utilities and power producers have developed standards and guidelines that help determine the number of station power transformers that are required for a particular substation. These guidelines are based on the utility system conditions and reliability requirements.
4.6.4 Single-ph Single-phase ase or three-phase transformer transformer requirements requirements The amount of station load determines whether single-phase or three-phase transformers are required. In general, single-phase transformers have been used for distribution substations when the load is single-phase and it has a low current rating. Three-phase transformers have been used for high-voltage and extra-highvoltage substations when the load is high and some station load requires three-phase voltage input. Using a single-phase transformer to serve large station load may result in a high level of secondary current. This could
29
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result in equipment with higher current rating as well as larger conductors due to excessive voltage drop. Other loads, such as maintenance and construction equipment, may dictate if three-phase transformers are required for station power.
4.6.5 Station service service transformer transformer rating 4.6.5.1 Introduction Station transformer ratings specied by the kVA rating, transformer voltages,service the short-circuit rating, and are the basic impulse insulation level (BIL) rating. primary and secondary
4.6.5.2 Transformer kVA rating The capacity of a transformer is determined by the amount of current it can carry continuously at rated voltage without exceeding the design temperature. Transformer Transformer ratings are given in kilovolt-amperes (kVA) (kVA) since the capacity is limited by the load current. Calculations of transformer KVA rating The kVA rating of the transformer should be selected to account for the expected load which the transformer is required to serve including anticipated future load. See 4.4 4.4 for detailed information regarding load classication and calculations. For a more general approach, the following methods can be used to determine the transformer kVA kVA rating. The 20% design margin used in this guide is conceptual. The designer may use a design margin as appropriate for the application and per the owner’s operating practice. Small substation with light load requir requirements ements For small substation with light ligh t load requirements, requiremen ts, the kVA kVA rating of the single-phase single-p hase transformer transfor mer is determined by calculating calculating the ultimate ultimate connected load and adding adding a margin of 20%: 20%: tr tran ansf sfor orme merr kV kVA A ra rati ting ng = 1.2 ×( ul ulti tima mate te co conn nnec ecte ted d lo load ad)
(6)
Medium to large large substations For large substations with high load requirements, the loads may be dierentiated as follows: a)
Continuous loads: Loads Loads that continue to to operate for three three hours or more are considered considered as continuous loads. In substations the following loads can be considered continuous:
b)
1)
Control building HV HVAC AC and lighting
2)
Transformer Transfor mer fans and/or pumps
3)
Battery chargers
4)
Equipment heaters
5)
Yard lighting
6)
Illuminated signs and miscellaneous inverters and receptacle loads
Non-continuous loads: Loads that are momentary are considered non-continuous loads. In substations the following loads can be considered non-continuous. For larger substations, the designer may want to consider not adding design margin to the non-continuous load. 1)
Breaker’s ac motor spring chargers chargers running current: Since Since this load type is momentary momentary and the possibility of more than one breaker charging motor starting at the same time is remote, it is
30
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suggested that the load of only two motors loads are added to the transformer kVA rating calculations. c)
Maintenance and construction construction loads: loads: Depending Depending on duration of these loads, loads could could fall under either continuous or non-continuous. 1)
Maintenance loads including transformer and breaker processing equipment.
2)
Construction loads including construction trailers and equipment.
Once the recommended transformer kVA rating is calculated, Table 2 can 2 can be used to select the appropriate transformer size for the application. Normally, Normally, the next transformer rating greater than the calculated value is selected. Sizes other than listed in Table 2 may 2 may be available from manufacturers. A list of preferred continuous kVA kV A ratings can be found fou nd in IEEE Std C57.12.00™ C57 .12.00™ [B21] [B21]..
Table 2—Typical kVA ratings for distribution dist ribution transformers transfor mers Overhead type Single-phase
Pad-mounted type
Three-phase
Single-phase
Three-phase
10
30
15
45
15
45
25
75
25
75
37.5
112.5
37.5
112.5
50
150
50 75
150 225
75 100
225 300
100
300
167
500
167
500
250
750
250
750
333
1000
4.6.5.3 Transformer voltage rating The primary and secondary voltage of the transformer should be specied. The following factors aect both the primary and secondary voltage: a)
Available source
b)
Transformer Transfor mer type: type: single-phase single-phase or three-phase three-phase
c)
Transformer Transfor mer connection
d)
Load voltage requirements
e)
Transformer impedance
For a single-phase transformer, the primary voltage can be specied phase-to-phase or phase-to-ground. For a three-phase transformer with a delta-connected primary or for a three-wire system, a phase-phase voltage is specied. For a four-wire source, or for a transformer with wye-connected winding(s), both phase-phase and phase-ground voltage are specied. specied. The following are typical substation secondary voltages (the list is not all inclusive): a)
For single-phase systems: 1)
240/120 V, single-ph single-phase, ase, three-wire
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This is basically a “residential” service, but applicable to small- to medium-sized substations. Panelboards that combine both power and lighting requirements can be used with this system, thus reducing the required number of panelboards. b)
For three-phase systems: 1)
480/240 V, three-pha three-phase, se, four-wire, mid-tap, delta When using this voltage conguration, larger equipment loads such as three-phase transformer fans need be specied at 480 V. V Other systemand loads can be specied at either 480 V or and 240oil V, pumps single-ph single-phase. ase.toThis type of system is. ungrounded may require a ground-detection system. If one phase of the system becomes grounded, an alarm is initiated to indicate a ground. If a second phase becomes grounded, then a phase-to-phase fault condition exists and a trip is initiated. This system has a high leg and requires that panelboards are labeled to identify that such a condition exists. This system is very uncommon.
2)
480/277 V, grounded wye connect connected, ed, three-ph three-phase, ase, four-wire When using this voltage conguration, larger equipment loads such as three-phase transformer fans and oil pumps need to be specied at 480 V. V. One advantage of this system is that luminaires can be equipped with 277 V ballasts allowing for a reduction in voltage drop for larger runs over the use of the more common 120 V lights.
3)
208/120 V, grounded wye-conne wye-connected, cted, three-ph three-phase, ase, four-wire In this system, either 208 V single-phase and three-phase or 120 V single-phase equipment can be used. Panelboards that combine both power and lighting requirements can be used, thus reducing the required number of panelboards. The savings realized by using fewer panelboards may be oset by the higher cost that is introduced by voltage drop issues in comparison to a 480 V system, as the typical solution to reducing voltage drop is to increase conductor size. In this system, an additional transformer is not needed to provide 120 V feeds to the receptacles.
4)
240 V, three-pha three-phase, se, three-wire, delta When using this voltage conguration, three-phase or single-phase transformer fans and oil pumps are specied specied at 240 V. This type of system is ungrounded ungrounded and and requires requires a ground-detection system. If one phase of the system becomes grounded, an alarm is initiated to indicate a ground. If a second phase becomes grounded, then a phase-to-phase fault condition exists and a trip is initiated.
5)
240/120 V, three-pha three-phase, se, four-wire, mid-tap, closed delta This voltage conguration is the most common for small to mid-sized substations. With this system, one phase of the auxiliary transformer is center tapped to obtain 120 V. V. Panelboards that combine both power and lighting requirements can be used, thus reducing the required number of panelboards. This system system has has a high leg and requires requires that that panelboards panelboards are labeled to identify identify that that such a condition exists.
6)
240/120 V, three-pha three-phase, se, four-wire, mid-tap, open delta This is essentially the same as the closed delta system with the exception that with only two transformers transforme rs the kVA kVA rating is only 58% of the kVA capacity of when wh en three transformers tran sformers are used. u sed. This system is more economical for a medium-sized installation or when used for a temporary installation. This system uses single-phase transformers and the third transformer can be added in the future to increase inc rease the overall ove rall kVA kVA capacity. This type of system sys tem is commonly used us ed when there is a small three-phase load and a large single-phase 120/240 V load. This system has a high leg and requires that panelboards are labeled to identify that such a condition exists.
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4.6.5.4 Transformer short-circ short-circuit uit rating The short-circuit ratings for distribution transformers are set by IEEE Std C57.12.00 [B21] [B21].. The maximum magnitude required for units with secondary voltages rated less than 600 V is given in the table below:
Table 3—Distribution transformer short-circuit withstand capability Single-phase KVA
Three-phase KVA
Rating (times normal)
5 to 25
15 to 75
40
37.5 to 100
112.5 to 300
35
167 to 500
500
25
750 to 2500
1/ZT
Two winding distribution transformers with secondary voltages rated above 600 V are required to withstand short-circuits limited only by the transformer’ transformer’ss impedance. The duration of the short-circuit current is determined by the following formulas: For transformer transforme r rated 500 kVA kVA or below (Category I): t
1250 =
I
t
=
I
2
(7)
(8)
where t t is the duration in seconds I is the symmetrical symmetrical short-circuit short-circuit current current in multiples multiples of normal normal base current current (per unit) unit) For transformer transf ormer rated rate d 501 kVA to 5000 kV kVA A (Category II): t
=
2s
t
=
I
(9)
(10)
where t t is the duration in seconds I is the symmetrical symmetrical short-circuit short-circuit current current in multiples multiples of normal normal base current current (per unit) unit) Transformers Transf ormers are required to withstand faults corresponding to the above criteria. Determination of short-circuit short-circuit capacity: The innite bus short-circuit capability of a transformer can be calculated as: I SC
I S
=
Z t
(11)
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where I S
is the rated secondary current at base kV kVA A
Z t
is the transformer impedance in percent
For example, a 50 kVA single-phase transformer with a 120/240 V (ac) secondary and a 3.5% impedance will have a short-circuit capability of: 50 000 240
0.0 35 35
=
59 95 5 2.4 A
(12)
Equipment connected to this transformer shall have the ability to withstand this current for the duration indicated above, and be capable of interrupting that current if the equipment is a protective device such as a circuit breaker or fuse.
4.6.5.5 Transformer impedance The station service transformer impedance should be considered when evaluating the ac system equipment rating. The ac equipment should withstand the maximum fault current and the circuit breakers should be capable of interrupting the fault. The transformer impedance has a direct eect on system fault current. The impedance determines the maximum short-circuit current. The impedance can be specied as low as 2% for small distribution transformers, andfor as high as 20%percentage for large power transformers. Impedance values outside this range are generally specied special applications.
4.6.5.6 Transformer BIL rating The BIL rating of overhead distribution distribut ion transformers transform ers 500 kVA kVA and smaller is its ability to withstand overvoltage overvolt age conditions resulting due to fault conditions, lightning surges, or any over-voltage due to switching surges. Table 4 meets 4 meets IEEE Std C57.12.20™ C57.12.20™ [B23] [B23] and and can be used to specify the BIL rating of the transformer.
Table 4—Transformer BIL ratings (IEEE Std C57.12.20 C57.12. 20 [B23] [B23])) Voltage range volts
Insulation class kV
BIL
480 to 600
1.2
30
2160 to 2400
5.0
60
4160 to 4800 7200 to 12 470
8.7 15
75 95
13 200 to 14 400
18
125
19 920 to 22 900
25
150
34 400
34.5
200
4.6.6 Transformer connections 4.6.6.1 Introduction Single-phase or three-phase transformers may be installed for station power applications.
4.6.6.2 Single-ph Single-phase ase transformer application Single-phase distribution transformers are manufactured one or two bushings. single primary-bushing transformers can be used only on groundedwith wye systems. For primary this connection, theThe H1 bushing
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is connected to an available phase. If a two-bushing transformer is used, the H1 is connected the same, and the H2 bushing is connected to ground as shown in Figure 4. 4.
Figure 4—Single-phase-to-ground connection
When a primary delta system is available, a phase-to-phase voltage is applied between the two bushings H1 and H2 as shown in Figure 5. 5.
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Figure 5—Single-phase transformer with phase-to-phase connections
The secondary voltage can also be 480/240 V if required.
4.6.6.3 Three-phase transformer connections connections Three-phase transformer connection can be achieved by using two or three single-phase transformers and connected as required. When a three-phase transformer is required, a pad-mounted three-phase transformer is normally used for the station power applications. A pad-mounted pad-mounted three-phase transformer is applicable to below-grade connection from both the primary and the secondary’ secondary’ss sides. The following are some examples of transformer connections that have been used for substation station service applications: Delta-delta connection The delta-delta connection shown in Figure 6 6 is suitable for both ungrounded and eectively grounded sources. Phase-to-phase voltage is applied to H1, H2, and H3 terminals of the transformer. For substation applications when the required voltage is 240 or 480, a three-wire connection is used. When the required voltage is 240/120 V or 480/240 V, a four-wire service can be used. The delta-delta four-wire service is accomplished by grounding the midtap of one of the transformer windings. However, However, if single-phase load is to be connected, connected, the three-phase three-phase capability of the transformer is derated. derated.
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Figure 6—Delta-delta connection
The advantages of the delta-delta connection are as follows: a)
System voltages are more stable in relation to unbalanced load.
b)
When three single-phase transformers transformers are used to form the phase bank, if one transformer fails, the remaining two transformers can be used at 58% of the total kVA rating. Single-phase loads are lost if the B phase tap is lost.
c)
The deltaremains delta connection provides a closed path path circulation of the third harmonic harmonic component component of current. The ux sinusoidal which results infor sinusoidal voltages.
The disadvantages of the delta-delta connection include the absence of a neutral terminal on either side. Another drawback is that the electrical insulation is stressed to the line voltage. Therefore, a delta connection requires increased insulation to accommodate the higher voltage across the line-line compared to the wyeconnection with line-neutral voltage for the same power. The delta connection is susceptible to ferroresonance. Delta-wye connection The delta-wye connection shown in Figure 7 is 7 is suitable for both ungrounded and eectively grounded sources. The transformer primary is connected delta, and therefore phase-to-phase voltages are connected to H1, H2, and H3 transformer terminals. The secondary is suitable for three-wire service or, if neutral is grounded, four-wire grounded service. In substation applications four-wire service is normally used. Typical Typical substation secondary voltages for this transformer connection are 480/277 V or 208/120 V. V.
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When the neutral is grounded the transformer acts as ground source for the secondary system. Fundamental and harmonic frequency zero-sequence currents in the secondary lines supplied by the transformer do not ow in the primary lines. Instead, these zero-sequence currents circulate in the closed delta primary windings. When supplied from an eectively grounded primary system, a ground relay for primary system does not see load unbalances and ground faults in the secondary system.
Figure 7—Delta-wye connection
When used in 25 kV and 34.5 kV three-phase four-wire primary systems, ferroresonance can occur when energizing or de-energizing the bank using single-pole switches located at the primary terminals. With smaller kVA transformers in the bank, the probability of ferroresonance is higher. Wye-wye connection The wye-wye connection shown in Figure 8 is 8 is best applied at the four-wire primary and secondary where both the primary and secondary neutrals are grounded. The high-voltage terminals H1, H2, and H3 are connected to the three-phases, and the H0 neutral is connected to ground. In a grounded wye-wye 240/120 V or 480/240 V cannot be supplied, only 208/120 V or 480/277 V can be supplied by this connection.
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Figure 8—Grounded wye–grounded wye transformer connection
The following operating conditions should be considered when this transformer connection is selected: a)
Excessive tank heating heating can result depending depending on the transformer transformer construction. For three-legged three-legged core core construction, excessive tank heating is probable. For ve-legged transformers, tank heating is possible if the load unbalance is high. Tank heating can be limited if the transformer bank is made from three single-phase transformers.
b)
Zero-sequence currents and harmonics transfer to the primary primary.. The secondary can act as high impedance ground source.
c)
A ferroresonance condition is unlikely if the transformer transformer bank is made from three three single-phase transformers, but is possible for a four- or ve-legged constructed transformer. transformer.
d)
Coordination between the source source ground protective device and the secondary secondary ground protective device is required because the secondary current can pass to the primary. primary.
Two leg open delta–open delta connection from grounded-wye primary The open delta–open delta connection as shown in Figure 9 shows 9 shows connection to a grounded wye-connected source such as a distribution bus. Phase-to-ground voltage is applied across the two transformer primary windings. This connection provides a 240/120 V secondary. The A-C A-C and A-B voltages are 240 V where A-n and C-n are 120 V. V. The B phase in this connection connect ion is the high leg 208 V B-n.
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Figure 9—Two leg open delta from grounded wye
Two leg open delta–open delta connection from a delta primary The open delta–open delta connection as shown in Figure 10 10 shows connection to a delta-connected source such as a three-phase transformer tertiary winding. Phase-to-phase Phase-to-phase voltage is applied across the two transformer primary windings. This connection provides a 240/120 V secondary. The A-C and A-B voltages are 240 V where A-n and C-n are ar e 120 V. V. The B phase in this th is connection connect ion is a high leg.
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Figure 10—Two leg open delta from delta
One benet for the above two applications is when there is a large amount of single-phase loads 240/120 V and a small amount of three-phase 240 V load. This can economically provide that with a larger single-phase transformer connected to C-phase primary (with secondary grounded neutral tap) and a smaller transformer connected to A-phase. Typical Typical substation ac station auxiliary loads tend to be single-phase with a small amount of three-phase loads (typically cooling pumps or larger three-phase battery chargers). Another benet is that the open delta connection avoids the ferroresonance issues of the closed delta transformer connections. The connection is good for substations with a lot of single-phase load and a small amount of three-phase loads. This is inecient for applications with substantial amounts of three-phase loads as you only get 58% of the capacity with only two transformers instead of three equally sized transformers.
4.7 Transfer switch 4.7.1 General The need for an auxiliary power system transfer switch is related to the criticality of the substation. If only one station service power source is available, a transfer switch is not required. If there are no critical ac system requirements, the dc battery system may be sucient to operate the critical dc systems until the ac station service power is restored. Most substations are provided with two sources of station service ac power. The two sources of station service power are generally designated as the normal source and the the alternate alternate (or backup backup or secondary) source.
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To simplify the operation of the transfer between sources, a “break before make” operation is suggested. “Break before make” operations keep sources from operating in parallel. In the case of manual operation of the transfer switch, it may be desirable to disable or lock out one source while the other source is being used. In either case, sucient training should be provided so operators do not parallel sources. Most substation ac loads do not require continuous service to function as designed. The station service should be reviewed for sensitive loads that may require continuous ac service to function. Since the auxiliary power sources can be supplied at dierent voltages than the utilization voltage in the substation, the transfer switch or scheme can be applied at either the primary or secondary voltage. The higher voltage application ap plication results r esults in lower lo wer current rated r ated equipment. equip ment. 13.8 kV k V, 12.47 kV, 4.16 kV, kV, 480 V, V, and 240/120 240/ 120 V are common auxiliary power voltages and the transfer switch/scheme can be applied at any of these voltages. The auxiliary power source can be either three-phase or single-phase, depending on the station service requirements. Transfer Transfer switches typically can be purchased with two, three, or four poles. A four pole switch has the ability to switch the neutral and is necessary on a system that has separately derived neutrals. Using a transfer scheme at medium-voltage levels requires auxiliary voltage transformers and either programming protective relays or incorporating incorporating a programmable programmable controller for transfer and return return to normal functions. Smaller rated transfer switches can be wall mounted. Floor-moun Floor-mounted ted switches are common. Transfer Transfer switches can be purchased for indoor or outdoor mounting. The transfer switch may be as simple as two input sources with switching devices and one output to the load. The transfer system may be as elaborate as a unit switchgear consisting of two input switching devices, two transformers, two main circuit breakers, one tie circuit breaker, and multiple branch circuit breakers. Another consideration when designing the transfer system is the reliability of the transfer switch. It may be prudent to make provisions to bypass the switch in the event of the switch’ switch’ss failure, maintenance, or replacement. This may be accomplished by having a third source routed to the substation ac load center that is left normally open and locked out until it is needed. It may be more cost eective to route another set of conductors from either or both the normal and alternate source to the substation ac load center. Similar Similar to the transfer operation, training and procedures should be provided to the operator so that the operator is less likely to parallel sources during a bypass operation. When installing an auto-transfer switch to an ac system, an important consideration is to include switching of the neutral conductor in addition to the phase conductors. This is especially important on applications with one source provided from a separately derived ac system outside the substation with a dierent ground connection. This is important for both automatic and manual transfer switches. The switching of the neutral conductor provides isolation of a normal source from an alternate source which is important with separately derived sources. Another important consideration when installing a transfer switch is to specify a switch with break before make operation. This allows for a transfer of normal and alternate sources that may be out of phase or connected to dierent phases on single-phase systems. For example, a normal three-phase source connected to a distribution bus may be 30 degrees out of phase with an alternate source connected to the tertiary windings of a power transformer. A break before make operation allows a transfer between these two sources that are out of phase. It is important important to maintain maintain proper phase rotation rotation between dierent power sources.
4.7.2 Manual transfer switch For less critical substations, a manual transfer switch provides the capability of transferring from the primary to the alternate source. The manual transfer switch is a much simpler and lower cost switch than an automatic transfer switch. However, the use of the manual transfer switch requires station alarms to alert operations personnel of the the loss loss of of the the normal normal ac source and dispatching personnel to the substation to operate operate the manual transfer switch.
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Due to the loss of the normal ac source, many station devices will lose power. The battery charger cannot supply the charging source for the dc battery system. In this situation, the dc battery system is the source for station critical systems, such as system protection functions and control and breaker tripping, until operation personnel responds and manually operates the ac transfer switch. Other systems, such as control enclosure cooling/heating, may also be unavailable until operations responds. The designer should consider the eects of temperature on control enclosure components and response time when considering a manual transfer switch. If the substation has only one source of ac power, a manual transfer switch may still be desirable as a connection point for a temporary ac alternate alternate source, such as as a portable generator generator.. The manual transfer switch can consist of two manually operated switching devices (usually such as circuit breakers) capable of interrupting the load current of the transfer transfer switch switch or a manually operated switch switch similar similar to a disconnect switch that has on–o–on capability to select between the two sources. The two switching devices are typically mechanically interlocked so both ac sources are not connected in parallel. Fault current interruption capability is not required in the transfer switch, but a withstand rating should be specied. Indication of source status (hot or dead) is not typically provided. Some type of alarm is necessary to detect the loss of the primary (and perhaps secondary) ac source.
4.7.3 Automatic transfer switch switch Critical substations, or substations with critical ac loads, may require an automatic transfer switch between the normal and alternate sources. The transfer should occur only after a time delay to avoid inadvertent transfer and only when the alternate source is available. Automatic return to the normal source should occur only after the normal source has been restored for a specied time to conrm it is not an unstable source. The low-voltage (< 1 kV) automatic transfer switch consists of essentially a form C power relay capable of interrupting the load current of the transfer switch. Higher voltage transfer switches can be composed of two electrically operated switching devices (usually circuit breakers). The two switching devices can be electrically and/or mechanically interlocked to keep the ac sources from being connected in parallel. Fault current interruption capability is not required in the transfer switch, but a withstand rating should be specied. Detection and indication of source status (hot or dead) is required. Time delays and control sequencing is necessary to reduce the chance of transferring to a de-energized or unstable source. Indicating lights and relays are usually provided. Alarm indication of transfer should be provided. Close and latch capability should also be considered considered in equipment equipment rating.
4.7.4 Alternate methods Transfer Transf er switches can be cascaded to allow multiple sources to provide power to the station service system. Depending on the criticality of the substation, two ac sources (bus derived and distribution derived) can be normal primary and alternate, and that resultant connection can be further supported from an onsite generator to support essential ac loads such as battery chargers, control enclosure HVAC, HVAC, and communication systems as required. Another alternate would be for both sources to be designated as normal sources. The ac load can be divided between the two sources with the transfer switch system consisting of the two normally closed circuit breakers and a normally open transfer circuit breaker. breaker.
4.7.5 Alternate sources In some instances, depending on the criticality of the station, the transfer switch alternate source may be a local backup generator. generator . The transfer switch controller typically providesschedule. both the ac tnumber ransfer function ability to exercise the generator following a pre-determined maintenance A transfer of alarmsand arethe available
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(engine temperature, fail to start, engine running, oil pressure, etc.) and provisions need to be made to transmit these to the operations center. center.
4.8 Bus layout and distributi distribution on circuits conguration 4.8.1 Introduction The designer responsible for designing the bus layout and distribution circuit conguration of an auxiliary ac system should take the following parameters into consideration, at a minimum: a)
Essential versus non-essential load
b)
Load ampacity ampacity and overcurrent overcurrent protection requirements (see 4.4 4.4 and and 4.5 4.5))
c)
Voltage drop (see (see 4.5 4.5))
d)
Construction and maintainabilit maintainability y
e)
Cost
4.8.2 Essential load These loads are related to equipment operation and are necessary to the proper function of the substation. a)
Power transformer loads (cooling systems, fans, oil pumps, load tap changers, etc.)
b)
DC battery chargers
c)
Power circuit breaker loads (control, compressors, charging motors, etc.)
d)
Power equipment heating circuits
e)
Protective relaying, supervisory supervisory,, alarm, communications, and control equipment
f)
AC/DC converters for uninterruptable power supplies
g)
Control enclosur enclosuree HV HVAC AC systems
h)
Fire alarm and re suppression circuits
i)
Security lighting
4.8.3 Non-essential load These loads are not essential for functioning and reliability of the substation. a)
Outdoor lighting not essential for station security
b)
Outdoor receptacles
c)
Indoor lighting and receptacles
d)
Maintenance loads (SF6 gas carts, oil rening, receptacles, etc.)
e)
Construction loads (construction trailer trailer,, welding, drills, etc.)
4.8.4 Simple radial system In this system, a single normal service and station power transformer supply all auxiliary ac load. There is no duplication of equipment. System cost is the lowest of all the circuit arrangements.
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The simplest version of this system is shown in Figure 11. 11. It has panelboards supplied directly from station power transformer transformer.. Secondary Secondary breakers breakers may not be required on on the transformer as shown in Figure 11, 11, Figure Figure 12,, and Figure 13. 12 13. One of the panelboards (“A”) is used to connect a feed to another panelboard (“B”).
Figure 11—Simplest panelboards
A variation of this system is shown in Figure 12 where 12 where a power block is used to split a power supply coming from transformer breaker into cables feeding both panelboards “A” and “B.”
Figure 12—Variation of simplest panelboard
Another version of a simple radial system is shown in Figure 13, 13, where a main panelboard is connected directly to a station power transformer. Breakers are used to supply sub-panelboards.
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Figure 13—Sub-panelboard
The main deciency of the systems shown in Figure 11, 11, Figure 12, 12, and Figure 13 is 13 is that the panelboards do not have independent feeds from the main system and are connected to a station power transformer breaker through a single common cable susceptible to failure. In the case of the cable fault or a failure of one of the panelboards ahead of the internal main breaker, the whole whole auxiliary auxiliary ac power power system system becomes becomes de-energized. de-energized. To make a simple radial system more reliable and exible, the auxiliary bus with feeder breakers (switchboard or switchgear), shown in Figure 14 may 14 may be used. In this system, the auxiliary bus is connected directly to the transformer breaker through a bus, or cable run, and panelboards are connected to the bus via feeder breakers and separate individual cables. A failure of any panelboard or a cable feeding it should result in a tripping of the corresponding feeder breaker, leaving the rest of the ac system intact.
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Figure 14—Reliable and exible panelboard system
Further improvement of redundancy of a simple radial system may be achieved through installation of a backup generator, generator, which starts starts upon loss of the station station power transformer’ transformer’ss feed to the auxiliary bus, tripping tripping the transformer breaker and closing the generator breaker as shown in Figure 15. 15.
Figure 15—Panelboards with backup generator
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The main advantages of a simple radial system are low cost and operational simplicity. However, it has less reliability compared to more robust systems. A loss of the normal supply, supply, main cable, or station power transformer results in the interruption of auxiliary ac service for the entire substation. Another Another drawback of a simple radial system is the necessity to de-energize it to perform routine maintenance of its main elements (transformer, transformer breaker, auxiliary bus, etc.).
4.8.5 Expanded radial systems If a simple radial ac system is applied to a larger substation, its expanded version with two station power transformers may be used. See Figure 16. 16. The advantages and disadvantages of expanded radial systems are the same as those described for the simple ones. However, by having two transformers, a better redundancy of power supply is achieved. The panelboards can be fed through automatic or manual transfer switches, which can also provide added exibility in the continuity of power supply to the load if one of the transformers or buses is out of service.
Figure 16—Expanded radial system
4.8.6 Primary selective system Protection against loss of a primary power supply can be gained through the use of a primary selective system shown in Figure 17. 17. Each station power transformer is connected to two separate primary feeders through switching equipment to provide normal and alternate sources of power supply. Upon failure of the normal source, the transformer is switched to the alternate source. Switching can be either manual or automatic. Each panelboard can be fed through an automatic or manual transfer switch, which provides the continuity of power supply to the the load if one of the transformers or buses is out of service. service.
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Figure 17—Primary selective system
4.8.7 Secondary selective systems If a pair of station power transformers is connected through a secondary tie circuit breaker or automatic transfer switch, the end result is a secondary selective system shown in Figure 18. 18. If any of the primary feeders or transformers fails, power supply from the remaining source is maintained through the corresponding transformer’ss secondary breaker and a tie breaker. Tie breaker may be normally open. If this is the case, after transformer’ failure of one of the sources and opening of aected transformer’s secondary breaker, breaker, a tie breaker should be closed either manually or automatically to provide a power supply for the bus section normally connected to the failed source. When a power supply from this source is restored, a manual opening of the tie breaker and closing of the returning to service transformer’s breaker are recommended. Each panelboard can be fed through an automatic or manual transfer switch, which provides the continuity of power supply to the the load if one of the transformers or buses is out of service. service.
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NOTE—EO is electrically electrically operated, NO is normally open, and NC is normally closed. closed.
Figure 18—Secondary selective system
4.8.8 Secondary selective system with with backup generator generator If the level of redundancy provided by a secondary selective system shown in Figure 18 is 18 is not sucient, a backup generator with with a circuit breaker breaker may be added to it it as shown in Figure 19. 19. Normally, the generator’s breaker is open, and for a loss of a single primary feeder or transformer, transformer, this scheme works exactly like the one shown in Figure 18. 18. But upon the loss of both transformer feeds (both transformer secondary breakers are open) the backup generator starts automatically and its breaker closes, restoring power to both buses. Manual closing of the transformer breaker is recommended upon restoration of any primary feed after stopping the backup generator generator.. Each panelboard can be fed through an automatic or manual transfer switch, which can allow the continuity of power to the the load if one of the the transformers transformers or buses buses is out of of service. service.
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Figure 19—Secondary selective system with backup generator
If even more redundancy is needed, Figure 19 may 19 may be developed into a system with two tie breakers and possibly three transformers and a backup generator as shown in Figure 20. 20. For applications with processcritical equipment, additional provisions may be required for smooth transition during restoration of power. The operational logic for this scheme is consistent with the one described for the schemes shown in Figure 17 17 and Figure 18. 18.
Figure 20—Secondary selective system with backup generator and addition additional al redundancy
The size of cable feeding any load or panelboard is required to be selected in accordance with requirements of the AHJ or any applicable code, and to be protected by an upstream breaker or protective device.
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4.9 AC distributi distribution on panelboards for electrical substations 4.9.1 Introduction AC distribution panelboards are utilized for termination of service and feeder cable circuits and for origination of feeder and branch cable circuits for distribution of auxiliary ac electrical power to loads in electrical substations. An ac distribution panelboard is an assembly of bus bars, switching overcurrent protection devices, and connections housed in an enclosure with purpose to control and distribute auxiliary ac power to substations loads. AC distribution panelboards have a main bus bar for each phase, main lug only (MLO), or a main device such as a switch, fuses or molded-case circuit breaker (MCCB), and neutral and/or ground buses, if appropriate. Depending on voltage rating, ac distribution panelboards can be specied with a switch and/or overcurrent devices, such as plug or cartridge fuses or MCCBs, to serve as branch circuit devices. Most ac distribution panelboards utilized in modern industrial applications, such as electrical power substations, use MCCBs for main, feeder, and branch circuit overcurrent devices.
4.9.2 AC distribution panelboard application Bus bars in ac distribution panelboards are current density rated and meet temperature rise limitations established in UL 67 [B38] [B38] (UL (UL Standards are typical in the United States—other jurisdictions may have similar standards boards). Standard bus bar current densities are 750 amperes per square inch for aluminum bus bars and 1000 amperes per square inch for copper bus bars. Some ac distribution panelboard manufacturers oer reduced current densities of 600 amperes per square inch for aluminum bus bars and 800 amperes per square inch for copper bus bars. AC distribution panelboards used in electrical substation applications should be designed with consideration for the size of the conductors being terminated within the panelboard. The specied panelboards should accommodate the bending radius of conductors routed within them and should have adequate gutter spacing. The terminals of molded-case circuit breakers and other protective devices should be suitable for the wire size of the circuits on which the protective devices are applied. To be conservative, the designer designing the panelboard should account for the possibility of increases increases in wire sizing. The designer should also consider the space necessary for the electricians to perform terminations in the available space within the panelboard. For any application of ac panelboards, all panelboard manufacturers’ catalog and technical data should be considered carefully.
4.9.3 AC distribution panelboard ratings Panelboards can be single-phase or three-phase as required for the application. Typical voltage ratings for ac panelboards for dierent ac systems are given in Table 5. 5.
Table 5—Typical 5—Typical voltage ratings ra tings for ac panelboards Voltage ratings of ac panelboards Number of phases
Number of wires
AC voltage ratings—volts
1
2
120, 240, 277
1
3
120/240
a
3
208Y/120, 480Y/277
3
Table continues
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Table 5—Typical 5—Typical voltage ratings ra tings for ac panelboards (continued) Voltage ratings of ac panelboards
a
3
3
120, 240, 480, 600
3
Four-wire with neutral
208Y/120, 400Y/230, 480Y/277, 600Y/347
3
Four-wire delta with neutral connected at midpoint of one phase
240/120
Derived from three-phase, four-wire system
Although more ratings are available, typical nominal continuous rms current ratings of ac distribution panelboard main buses, main terminal lugs, main fuse and holder, and MCCB utilized in applications in electrical substations range between 100 A and 800 A. The maximum main current rating in ac panelboards is usually less than 1600 A. The current rating of an ac distribution panelboard should not be less than the feeder and branch circuit capacity required for the load. Typical nominal continuous rms current ratings of feeder and branch circuits range between 20 A and 400 A. The maximum feeder and branch circuit current rating in ac distribution panelboards is usually 1200 A. Unless marked to indicate otherwise, the provisions for cable terminations provided in ac distribution panelboards are based on the use of 60 °C temperature rise for wire sizes 14 AWG to 1 AWG AWG and 75 °C temperaturee rise for wire sizes 1/0 AWG and larger. temperatur Unless rated for 100 percent continuous load at its rated current, the total load on any overcurrent device utilized in an ac distribution panelboard should not exceed 80 percent of its nominal current rating.
4.9.4 AC distribution panelboard short-circ short-circuit uit rating The rms symmetrical and asymmetrical short-circuit current at an ac distribution panelboard location should be determined in accordance with methods provided in IEEE Std 141™ [B3] [B3],, unless otherwise directed by the AHJ. The rated rms symmetrical and asymmetrical interrupting current of an ac distribution panelboard should exceed the available short-circuit current at the location in the electrical system. Consideration should be given to possible future increases in available short-circuit current. Most ac distribution panelboards panelboards are selected to have a fully integrated short-circuit interrupting interrupting rating where the ac distribution panelboard and all overcurrent devices enclosed in the ac distribution panelboard have a short-circuit current rating greater than the available short-circuit current at the location in the electrical system, but series ratings may be utilized. Selectivity between overcurrent devices should be considered, if possible.
4.9.5 AC distribution panelboard standards AC distribution panelboards are typically designed and manufactured in accordance with NEMA PB1 [B31] [B31] and UL 67 [B38] [B38] or or similar standards, and are usually supplied in suitable cabinets or enclosures which are manufactured in accordance with standards such as NEMA 250 [B29] [B29] or or UL 50 [B37] [B37] and and designed to be mounted in or on a wall or other support structure and accessible only from the front. In general, ac distribution panelboards should be specied and applied in accordance with national or local standards, including all provisions for grounding. grounding. However However,, ultimate ultimate guidance for design, design, manufacturing, manufacturing, and installation/appli installation/application cation of ac distribution panelboards should come from the AHJ. Usual service conditions for ac distribution panelboards are ambient temperature of −5° C to 40 °C for ac distribution panelboards utilizing molded-case circuit breakers, and −30 °C to 40 °C for ac distribution panelboards utilizing utilizing enclosed switches. switches. Usual altitude altitude is not greater than 2000 m (6600 ft). ft). AC distribution panelboards for outdoor application can have a greater rated ambient temperature range and should be provided with with enclosures enclosures with a suitable suitable weatherproof weatherproof rating. For suggested suggested applications applications of enclosures based based on location, see NEMA 250 [B29] [B29]..
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AC distribution panelboards tested and certied to meet requirements of International Building Code (IBC) [B28] Zone [B28] Zone 3 or Zone 4 or other similar standards should be considered in seismically active areas. Ultimately, Ultimately, any guidance for panelboard design based on seismic conditions should originate from the AHJ. An ac distribution panelboard utilized for service equipment to provide main control and means of cuto of the supply conductors near the point of entrance of supply conductors of a building, structure, or other area or premises should meet all requirements requirements for service equipment required required in UL 67 [B38] [B38] and and UL 869A [B41] [B41],, unless otherwise dictated by the AHJ. Guidance for the specication of MCCBs is given in NEMA AB-1 [B30] [B30] and and UL 489 [B40] [B40].. Guidance for specifying fusible switches is given in UL 98 98 [B39] [B39]..
4.10 AC auxiliary system protection 4.10.1 Introduction Several studies may be performed for auxiliary system protection. These studies include a short-circuit current study for selection of equipment and cable sizing, a coordination study to evaluate and select the equipment rating and protective device rating of auxiliary systems, and an arc ash study study..
4.10.2 Panelboard or or switchboard protection protection The upstream feeder device safety fuses,and MCCBs, etc.) of the panelboard or switchboard should be protective sized to protect the(fused panelboard or switch, switchboard the feeder cable(s). Panelboard and switchboard may have a main incoming protective device. For a panelboard or switchboard with a main incoming protective device (breaker or fuses, breaker), the main incoming protective device should be sized to protect the panel bus bars. There is no limit to the number of circuits (fuses, MCCBs) in the panelboard or switchboard. At one time, this was a 42 circuit limit, and that limit may still be in in force force in certain jurisdictions.
4.10.3 Panelboard or switchboard switchboard circuit Panelboard and switchboard circuit protection (sizing) should be determined based on the terminal and load ampacity.. Typically ampacity Typically this is based on 100% of the non-continuous and 125% of the continuous load current with some design margin of the circuit load. Typical design margin is 10% to 20%.
4.10.4 Circuit breaker selection selection In order to properly protect the equipment and coordinate the fault clearing, circuit breakers should be properly selected. There are three important aspects to proper selection of circuit breakers. They are the rated maximum voltage, rated continuous current, and the short-circuit current rating. The voltage rating of the circuit breaker should be not less than the maximum operating voltage of the ac system. Typical low-voltage ac circuit breaker voltage ratings are 120, 120/240, 208Y/120, 240, 277, 347, 480Y/277, 480, 600Y/347, and 600 volts. The short-circuit current rating is the maximum short-circuit current that a circuit breaker can successfully interrupt. The circuit breakers for an ac system should have a current interrupting rating equal to or higher than the actual ac system maximum fault current. Typical Typical low-voltage ac circuit breaker current interrupting ratings are 7.5 kA, 10 kA, 14 kA, 18 kA, 20 kA, 22 kA, 25 kA, 35 kA, 42 kA, 50 kA, 65 kA, 85 kA, 100 kA, 125 kA, 150 kA, and 200 kA.
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The circuit breaker current rating should not be less than 125% of the calculated maximum load. 100% rated breakers are available available if required, required, or or may provide benet to accommodate accommodate preferences with frame size. It may may be appropriate appropriate to include include a 10% design design margin. margin. In some cases, thermal trip units or electronic trip units should be selected based on equipment protection requirements or the arc ash energy limitation requirements. The trip unit setting should be clearly identied in the circuit breaker order and design document. For more information on the selection and application of molded-case breakers, see IEEE Std 1458™ [B17] [B17]..
4.10.5 Selection of circuit fuses Appropriate fuse selection is important for the protection and fault-clearing coordination coordination of the ac auxiliary power system. system. The important important ratings ratings to consider when when properly selecting fuses for ac auxiliary auxiliary power system protection are voltage voltage rating rating and current current rating. rating. The ac voltage rating of the fuse should not be less than the operating voltage of the ac auxiliary power system. Typical ac fuse voltage voltag e ratings are a re 125 V, V, 250 V, V, and 600 V; 300 V and 480 V ratings are also available. availa ble. The ac current rating of a fuse is the maximum ac continuous current that can ow through a fuse without interrupting. When the rating is exceeded, the fuse blows, opening the circuit. The maximum ac continuous current required to supply an ac load should be considered when selecting the ac fuse rating. Typical ac continuous current fuse ratings range from 1 A to 600 A.
4.11 Equipment specications 4.11.1 Introduction Documents for specifying equipment include the necessary information for manufactures or suppliers to prepare and submit a rm rm proposal to furnish the the requested equipment. The equipment specication specication usually comprises both commercial and technical requirements. The commercial requirements are typically a set of terms and conditions that address how how,, when, and to whom the proposals are to be returned. Other information included may be legal considerations, such as taxes or liabilities. Commercial Commercial requirements are not discussed in further detail. The technical requirements include the description of the necessary performance requirements for the equipment. The information in the description should include, as needed, the operational criteria of the equipment related to its design, construction, testing, and shipment. Subjects that need to be addressed when specifying auxillary power equipment include voltage/current levels, service conditions, code requirements/res requirements/restrictions, trictions, delivery dates, delivery/transportation to site, and temporary storage of equipment. Designers should be aware that the standard equipment oered by suppliers may not meet the robust requirements needed for some substations. For instance, the size and layout of the substation may warrant larger cables be used between equipment. These larger cable sizes require larger cable bending space and termination sizes, and hence bigger enclosure sizes. Numerous standards have been written to specify specify requirements requirements of equipment to be used in ac auxiliary auxiliary power systems. These standards cover transformers, surge arresters, transfer switches, panelboards, medium- and low-voltage fuses, medium- and low-voltage circuit breakers, etc. Some of these standards are:
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IEEE Std C57.12.00, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers [B21] IEEE Std C62.22, IEEE Guide for the Application of Metal-Oxide Arresters for Alternating-Current Systems [B27] NEMA PB 2, Deadfront Deadfront Distribution Switchboards UL 489 (NEMA AB 1), Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures [B30] Enclosures UL 891, Switchboards Switchboards [B42] UL 991, Tests for Safety-Related Controls Employing Solid-State Devices [B43] UL 1008, Transfer Switch Equipment [B44]
4.11.2 NEMA standard for indoor/outdoor operation NEMA (National Electrical Manufacturers Association) creates ratings for equipment based on expected performance. NEMA does not require require independent testing to verify verify that the the manufacturer is compliant to the standard. Compliance to the standard is up to the manufacturer. NEMA 250 [B29] [B29] describes describes types of enclosures for electrical equipment up to 1000 V maximum. NEMA publishes descriptions of their enclosure types for both non-hazardous and hazardous locations. They also dene which enclosure types may be used for indoor/outdoor use and which enclosure types may be used for indoor use only. The designer should choose the type of enclosure specic to environmental, atmospheric, and site conditions. For example, a NEMA Type Type 1 enclosure provides a minimum degree of protection for indoor use in a nonhazardous location, while a NEMA Type Type 3R enclosure provides a minimum degree of protection for outdoor use in a non-hazardous location. The degree of protection oered by these types of enclosures may be sucient for a particular substation environment.
4.12 Operation and maintenance considerations 4.12.1 Operational and and maintenance provisions provisions There are several features that should be considered to enhance the operation and maintenance of the ac station service system. a)
Provide disconnect switches that can be visibly veried and used for electrical electrical clearance clearance points points on the high side of the station service transformer, transformer, and between the transformer and the service panel.
b)
Provide transfer switches to allow transferring load to an alternate source when the normal source (primary bank, bus, station service bank, or line) needs to be cleared. Transfer switches are typically break before make and need a mechanical mechanical interlocking means to avoid avoid paralleling paralleling the sources. sources.
c)
Indoor and and outdoor panels need need to have adequate adequate working working space. Recommended Recommended depth, width, and head room clear distances are shown in in Table 6 and 6 and accompanying notes.
d)
Panels should be dead-front design, and outdoor panels should be lockable.
e)
Clearly mark phases at the transformer bank and in the distribution panels to facilitate future trouble trouble shooting.
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Table 6—Working clearances for electrical panels
Minimum clearance for depth Nominal voltage to ground
No live or grounded parts opposite (see Note 1)
Grounded parts opposite (see Note 2)
Unguarded live parts opposite
0 V to 150 V
900 mm (3 ft)
900 mm (3 ft)
900 mm (3 ft)
151 V to 600 V
900 mm (3 ft)
1070 mm (3.5 ft)
1200 mm (4 ft)
NOTE 1—Also 1—Also applicable applicable if if exposed live parts parts on both sides are eectively eectively guarded by wood or other insulating insulating materials. Insulated wire or insulated bus bars operating at 300 V or less are not considered live parts. NOTE 2— Concrete, Concrete, brick, and wall tiles are considered as grounded. NOTE 3—The 3—The width of the working space in front of the equipment should be the width width of the equipment equipment or 30 inches, whichever is greater. greater. The headroom should be the greater of 1.98 meters (6.5 ft) or the height of the equipment. NOTE 4—This 4—This information information is based on NFP NFPA A 70, Article 110-26, and and NESC [B1] [B1],, Section 125.A.
4.12.2 Low-mainten Low-maintenance ance design For optimal performance and to reduce maintenance, consider the following features for outdoor distribution panels: a)
Utilize rain-tight construction: Minimum NEMA 3R or equivalent. Include a drip shield to reduce the likelihood of water entering the panel.
b)
Steel cabinets cabinets should have a minimum of 4 mils paint by wet process or powder-coat powder-coat type. type. Epoxy or two coat epoxy/polyester are common for good durability. Aluminum, or in highly corrosive areas stainless steel, cabinets may also be used.
c)
Do not use piano-type piano-type (continuous) hinges on doors. doors. Use a multi-point multi-point latching system.
d)
Cover all vents with small mesh mesh screen to to reduce insect or rodent infestation. infestation. Secure Secure in place. place. Use material such as brass that does not corrode over time.
4.12.3 Standby backup ac system The purpose of the standby ac system would be to provide continued ac power to essential systems for a set period of time after all sources to the auxiliary power system are unavailable. The essential systems may be dened as the dc power systems that provide the power required for relaying, control, telemetry telemetry,, and communications, and any ac power needed for breaker operation. Factors that may determine the need for a standby backup ac system are the criticality of the substation, the battery life life for the essential systems, and and the reliability reliability of of the ac sources for the auxiliary auxiliary system. If there is a possibility that an event can occur occur where the minimum time period to provide provide dc power power is exceeded, a standby standby backup ac system system may may be considered. The standby backup ac system should be a stand-alone unit that provides power without the support of the overall electric power system. An automatic start for the system may be desirable, considering that telemetry and communications functions may be disabled. Control of the generator would be through an auto-transfer switch. Manual control may also be available. Isolation of the sources to the auxiliary power system is necessary before connecting the standby backup ac system. The designer should avoid ac system paralleling. The standby backup ac system used in substation normally consists of a generator. The fuel source for the standby generator should be selected based on regional conditions, such as temperature and availability of fuels. The generator is normally used in the substation for one of the following reasons:
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a)
Used as backup to the normal normal source when only only one source is available, and the substation requires two redundant ac sources.
b)
Used as the third source when two two sources sources are are available, available, and the the substation substation requires three ac sources. sources.
c)
Under emergency emergency condition when all all the normal ac sources sources are not available, available, the the backup generator is used to restore the system.
When the generator is used as aby backup more normal fer ac system(s), themanual stationtransfer, load can berequired. transferred to the generator automatically the usetoofone an or automatic-transfer automatic-trans switch or by transfer , as
5. Design of substation dc auxiliary system 5.1 Design criteria 5.1.1 Introduction Prior to the start of the dc system design, the designer should consider several factors that are crucial to successful implementation. Typically Typically in substation applications, the primary purpose of dc auxiliary systems is to provide a reliable power source for the power system protection. DC systems provide power to operate protective relays, monitoring equipment, and control circuits that operate power circuit breakers or other fault-isolating fault-isolatin g equipment. The dc systems are designed to provide power for these protection systems during outages and when the power systems are intact. Several key factors are listed below. Figure 21 is 21 is a simplied dc block diagram.
Figure 21—Simplied dc system block diagram
5.1.2 Reliability The reliability requirements of the power system are typically dened by the system protection design. For example, the design requirements for transmission equipment is likely dierent than the requirements for distribution equipment. These designs determine the robustness requirements for the systems. System reliability standards should be reviewed to determine if back-up equipment or automatic switching is required in the event of one piece of equipment failing.
5.1.3 Redundancy The redundancy requirements of dc systems aredesigned typicallywith related to the power system protection requirements. For example, a transmission substation may be redundant components of the protection system. Redundant components may include ac voltage sources, protective relays, breaker trip coils, and duplicate
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components of the dc systems. Failure of one or more components of a non-redundant dc system may prevent operation of the protection and control systems, which could lead to system outages or stability problems. Providing redundant dc system components, such as batteries, chargers, and panelboards, may improve the integrity of the power system in the event of power system faults, dc system maintenance, or other utility operation. A distribution system may not have similar design requirements or concerns related to system stability. A designer could consider that the criticality of a certain distribution substation, such as the nature or location of the load or customers served by that substation, may justify the addition of redundancy in the dc system. Consideration for system back-up methods for failed equipment such as mobile substations or eld ties to an alternate source, could provide a more economical or acceptable solution to dc system redundancy redundancy.. There may be regulatory requirements requiring redundancy. redundancy.
5.1.4 Environment The environment that dc systems are exposed to impact the reliability of battery performance including the capacity and life of the battery battery.. Key environmental components include: temperature, vibration, cleanliness, and ventilation. Some applications may be susceptible to seismic considerations.
5.1.5 Design consideratio considerations ns The dc system design should be based on capacity and performance. Applicable criteria should be reviewed to conrm a reliable and cost-eective system has been selected for the life of the installation. Some factors to consider: a)
Load on on the dc system system when the the maximum maximum output of the battery charger is exceeded. exceeded.
b)
Demand on the battery when the the output output of the charger is interrupted. interrupted.
c)
Demand during the duty cycle.
d)
Battery re-charging time.
e)
DC system redundancy requirements.
f)
The battery battery standby standby duration (e.g., 2 h, 4 h, 8 h, 12 h), when auxiliary auxiliary ac power is lost.
g)
Battery life—What life—What is the projected minimum minimum life life of the the dc system? system? Are battery battery life cycle costs factored
h)
into cost of operation? Battery type
i)
Cost/reliability—What was the cost and quality Cost/reliability—What quality of the battery battery initially initially selected? Does operational operational history align with published life/costs?
j)
Available fault current of the dc system. system.
k)
Arc ash hazards hazards—Reference —Reference NFP NFPA A 70E [B36] [B36]..
l)
Operating temperatures— temperatures—Is Is the battery to to be subjected to temperature temperature extremes? extremes? When air conditioning is lost, what is the expected minimum or maximum temperature the battery can be expected to reach? What are the expected times to reach these temperatures?
m)
Maintenance intervals—T intervals—The he overall reliability of the battery depends on proper maintenance. maintenance.
n)
Location—Is the battery located where where required maintenance can can be completed? completed? Is the battery battery properly ventilated? Is any any associated associated equipment equipment susceptible to damage damage from from electrolyte? electrolyte?
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o)
Vibration/shock—Is the battery Vibration/shock—Is battery located located near rotating equipment? Lead-acid batteries easily shed their their active materials from the surface of the plates, aecting battery life.
p)
Weight/size—P eight/size—Physical hysical size and and weight weight can can play a signicant signicant role in determining determining the type type of battery to be selected. Is there enough enough room for the battery and rack in the proposed location? location? Can the location of the battery accept the oor loading? Can the battery cells be replaced with all adjacent equipment installed, or are lifting measures required (e.g., a multi-cell jar can easily weigh over 50 kg)? Is adequate space allocated to get either a permanent or portable lifting device installed? Parallel strings
q)
could be considered to reduce weight and size. Design process—Does process—Does the design process account for verication verication of the dc system loads loads for all additions or changes?
r)
Changed state state loads—Does loads—Does the design design need to account for loads loads that may change change state? state? Examples Examples are breaker spring charging motors that run on dc on loss of ac, or a supervisory control and data acquisition (SCADA) (SCADA) computer monitor that is fed from an inverter source that fails to dc on loss of its normal ac service.
s)
Is emergency lighting required? If so, can an alternate alternate source be provided?
t)
Does the dc system have alternatives in the substation emergency power system?
u)
Safety components in the dc design include mitigating mitigating arc ash, ash, electric electric shock, shock, and short circuits. circuits.
The design considerations need to accommodate both the owner’s requirements and those of any regulatory agency, AHJ, or quasi-regulatory agency. agency, agency. Other considerations may include those of any insurer or transmission operator (e.g., black start plans). For example, a black start, or system restoration plan, may require more than one attempt to close a transmission path and re-establish a secure source of the station ac service. During these attempts, breaker spring motors may have to charge on the station battery, which may be overlooked in an existing load case and may need to be accounted for in a new design.
5.2 Typical equipment served by the dc system The dc system in a substation serves many critical and non-critical functions and equipment. Some typical equipment served may include: a)
Circuit breakers
b)
Circuit switchers
c)
Motor operators
d)
Protective relay systems
e)
SCADA
f)
Fire protection/detect protection/detection ion
g)
Emergency lighting
h)
Security systems
While most of the equipment is required to be operational at all times, some may be dened as non-critical and may be segregated to reduce loads in the event where the battery of the dc system is required to carry substation loads without the battery charger available. Consideration should be given to limit the amount of non-critical loads connected to the battery to provide reliability to the system protection and to limit the size of the battery. The equipment may require dc voltages at dierent values such as 125 V (dc) for circuit breaker controls and 12 V (dc) or 24 V (dc) for a radio communication system. The designer needs to determine the best method
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to supply the various voltages. It is not recommended to tap a larger voltage battery for lower voltages (i.e., 24 V tap on a 125 V [dc] battery). If alternate voltages are required to be supplied from a single battery, battery, dc– dc converters are typically utilized for smaller non-critical loads at a lower voltage or a second dc system dedicated to the communications equipment could be installed. It is not recommended to install many dc–dc converters to provide dierent voltages. Vendors Vendors should be consulted to determine if alternate power supplies can be used.
5.3 One-line diagram 5.3.1 Introduction To start the design process, it is recommended that the designer create a one-line diagram showing the battery (or batteries), charger (or chargers), dc panels, and all connected loads. Consideration should also be given for future load growth. A review of the overall substation one-line may aid in determining future possible additions. The one-line diagram is very important. The following should be considered during the design of the one-line diagram: a)
System redundancy (number of chargers and battery banks)
b)
Fault currents and arcing arcing currents currents
c)
Distribution panel connections (number of panels and how they are connected)
d)
Cross tie breakers (if necessary)
e)
Isolation points (for isolating equipment for maintenance)
f)
Protection devices (e.g., battery bank disconnect disconnect switches switches and and fuses for downstream downstream protection of cables and panels)
g)
Commissioning and maintenance procedures (ability to transfer loads and isolate sources)
5.3.2 Number of battery battery systems The designer should evaluate the criticality of the substation facilities and owner’s preference or regulatory requirements. High-voltage and extra-high voltage (EHV) protective relay systems are normally designed with two independent systems. The systems are inclusive from the dc feeds to independent trip coils in the circuit breakers. Theindependent designer should revieworwhether separate battery systems and panelssystems are required, a single battery system with dc panels, one battery system and panel. Independent may provide better opportunities for maintenance or replacement in the event of equipment equipment failure or the need to upgrade in the future. The ability to tie redundant dc systems may also aid in maintenance activities. The number of battery systems may depend on the voltage level of the equipment. For example, if a communication system requires 48 V (dc) and the substation equipment is 125 V (dc), the designer needs to consider whether the communication equipment would be supplied by its own battery and charger as noted in 5.2,, or be supplied by a dc-dc converter. The decision should consider reliability 5.2 reliability and control enclosure space among other issues. The number of battery systems has a direct impact on the size of the control house as battery systems typically occupy wall space that can can dictate dictate building building size.
5.3.3 Load transfer The designer needs to account for any dc load transfer requirements. Load transfer could be automatic or manual andneed serves to backup system the event of a can charger failureby from another systemeor event. The to transfer andone the dc details of ain transfer scheme be dictated owner owner’s ’s preference preferenc orsimilar design
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criteria, criticality criticality of the substation, or other similar reasons. All equipment that could serve additional load upon transfer should be sized appropriately for that additional load. There may be regulatory requirements that require the ability to transfer the dc load to enhance reliability of the protection systems for the electric transmission system. In North America, America, the North American American Electric Reliability Corporation (NERC) (NERC) and regional transmission organizations have established requirements for dc system reliability.
5.4 DC batteries 5.4.1 Battery types Battery types and their characteristics are discussed extensively in several other IEEE guides (refer to Annex Annex A). Common types of batteries used in substation applications include: valve regulated lead acid (VRLA), vented lead acid (VLA) which are commonly referred to as fooded as fooded , and nickel-cadmium (NiCd). This may change with time due to continued development of new battery technologies. Vented Vented lead acid batteries are the most common battery types used in substation applications. The intent of this document is not to focus on lead acid batteries, and any references or examples that utilize lead-acid batteries is for convenience. The type of battery used should be based on reliability and economic criteria. Designers, through the use of various IEEE guides, manufacturer’ manufacturer’ss specications, and owner’s preference, should familiarize themselves with the impact of each type of battery on the design of the overall dc system. Considerations for selecting dierent battery types should include: battery load requirement, environmental environmental conditions (temperature range, moisture), battery life, design, duty cycle, capacity, and planned maintenance cycle. In most utility substation applications, the battery is not exposed to many deep cycles, so the ability to accommodate many cycles may not be as important compared to other factors, such as battery life and maintenance. Typically ypically,, the battery charger supports substation loads with the battery available to supply energy for shortduration activities, such as breaker trips and closes where the battery charger response time or capacity cannot support the transient. The battery is also available to supply critical long- and short-duration loads when there is loss of dc output from the battery charger.
5.4.2 Criteria for for battery rating rating 5.4.2.1 Introduction IEEE Std 485™, IEEE Std 1115™, and IEEE Std 1189™ are standards that should be referenced for determining the battery size needed (based on the type of batteries used) for the dc system of substations. These standards include requirements a designer should consider for obtaining the appropriate battery rating. However,, to aid the designer, some considerations are repeated here. In addition, this guide places emphasis However on substation specic application considerations.
5.4.2.2 Continuous loads First using the one-line or equivalent document, the designer should review all the continuous loads such as protective relays, relays, SCADA SCADA systems, emergency emergency lighting, lighting, indicating indicating lights, communication equipment (power (power line carrier, radio, telecom, microwave, ber optic), security systems, re protection, etc. Continuous loads can be obtained for new substations by reviewing vendor literature or calculations from previous designs. For upgrades at existing facilities, the data may need to be obtained by eld testing, or by examining the existing charger load, as vendor data may not be readily available. The eld-obtained continuous load measurements should be evaluated for end-of-load-cycle voltage and operating experience. When reviewing the literature, the continuous loads should be evaluated at the nal battery voltage (end of discharge or minimum cell voltage)
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selected (for example 105 V). For example, if a device has a load of 125 W, W, one may be tempted to have the load at 1 A for a 125 V (dc) system. However, However, at nal battery voltage of 105 V the load would be 1.19 A. Care should be taken to tabulate all known loads. The designer should also review the design for future loads and “phantom” loads that may be added by personnel other than the substation designer. For example, the control enclosure may be designed by another person who includes a re-protection system to meet local codes and may add dc emergency lighting. Substation designers should consider limiting loads connected to substation batteries used primarily for protection purposes to provide a longer-lasting longer-lasting source to the the protective protective system. Reduction of continuous continuous loads to help reduce the required battery size may be considered.
5.4.2.3 Momentary loads Momentary loads are those such as breaker open or close that occur at various times through the duty cycle (see IEEE Std 485). Many substation momentary loads such as breaker operations, lockout relays, and communication system operations operate in time frames of several cycles (electrical cycles or Hz, not to be confused with duty or load cycles) and careful analysis using IEEE guides and the battery manufacturer may be required. For example, example, an EHV EHV system system may detect detect a fault fault in ¼ cycle, initiate communications communications for 1 cycle, cycle, operate protective devices in ½ cycle, and open the circuit breaker(s) in 2 cycles. The whole operation is over in less than 5 cycles from detection. Typical Typical sizing per IEEE Std 485 looks at loads of 1 minute as the shortest period. After all momentary momentary loads are considered and the initial battery battery size selected, it may be advisable advisable to contact the battery to verify the selected battery can respond to the expected loads and duration of the load. See Figure 22vendor 22. .
Figure 22—Possible load case
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If a discrete load sequence can be determined, the peak one-minute load can be determined more accurately than if the loads are summed. For example, if a substation bus trips on dierential via a lockout relay (LOR) that trips three breakers with logic that opens a motor-operated disconnect (MOD) after the breakers open, the peak current would be either the LOR current, the sum of the three breaker trip coil currents, or the motoroperated disconnect locked rotor current. The single max current (breaker trips or locked rotor of MOD) would be used as the peak one-minute load. This reduces the likelihood of an overly conservative battery size. It requires careful examination of the trip sequence to understand the peak momentary loads. Computer analysis programs may be used. As described in IEEE Std 485, all load cases should be analyzed to verify that the proper case is identied. A traditional traditional load case that may have been used over an eight-hour period, for example, may not be applicable in a situation where the substation may be required to cycle multiple loads, or an extended period in order to restore the system after a blackout. When sizing momentary loads for motor-operate motor-operated d disconnects, the locked rotor value should be used for the dc load of the motor operator to accommodate for misoperations of the motor-operated switch. Multiple protection events should be considered, and the highest current draw should be the worst case momentary load. Examination of the station’s ac single lines and dc protection schematics is required required to determine determine the protection protection events. Another important issue when determining the worst case momentary load is whether to consider a breakerfailure situation where a breaker-fail relay can operate a group of devices around a failed breaker to isolate the fault. When utilized, breaker-failure relaying is a form of the secondary power system protection that requires a second contingency to operate. If breaker-failure protection is used, a second contingency to operate the breaker fail may provide the worst case tripping scenario, and this contingency should be considered to properly size the battery. battery. In many cases, the breaker-fail operation operation may put a larger load on the battery, battery, and both loads may occur within a minute minute time frame because the breaker fail would occur in a matter matter of cycles. In a breaker-failure event, the highest fault current in the sequence of events in that one-minute duration should be used for the worst case. If the original trip included a motor-operated device, it would still be operating when breaker fail occurred, and thus should be included in both conditions before and after the breaker-fail operation to determine the worst case scenario. As mentioned above, restoration from “black-start,” or system restoration scenario, may need to be considered. During “black-start” or system restoration, several trip and close cycles may be required to restore the transmission system after a collapse. It would not be uncommon for two or three attempts to be made to get the system to restore and become stable. As part of the “black start,” all the station breakers may be opened prior to closing closing in a selected transmission path.
5.4.2.4 Duty cycle The duty cycle of a battery is dened in IEEE Std 485 as the loads a battery is expected to supply during specied time periods. The duration of the duty cycle and the specic loads on the battery during that time period determines determines the size of a battery based on IEEE IEEE Std 485 battery sizing. An important consideration for determining the length of the duty cycle is the response time required to restore the ac and dc auxiliary systems to normal operation. For example, a realistic sequence of events that would follow a battery charger failure may include the following: a)
Charger fails and initiates an alarm to SCADA
b)
Dispatcher notices alarm
c)
Dispatcher attempts to contact substation personnel
d)
Substation personnel drives to substation
e)
Substation personnel investigates alarm
f)
Substation personnel determines determines that charger charger has failed failed and noties dispatcher that a technician is needed to repair the charger
g)
Dispatcher attempts to contact technician
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h)
Technician drives to substation
i)
Technician attempts to repair charger
j)
Technician determines that the the charger charger cannot cannot be repaired and the substation supervisor is notied notied
k)
Substation supervisor locates spare charger
l)
Substation supervisor attempts to contact additional substation personnel
m)
Additional substation personnel personnel report to service center to pick up vehicle and charger
n)
Additional substation personnel drives to substation
o)
Charger is replaced
It is not dicult to imagine this process taking longer than the 8-hour duration typically used in substation designs. Under certain circumstances (particularly during major storms where there are multiple station outages) the acknowledgement of the initial alarm is likely delayed due to other priorities, thus increasing the battery duty cycle duration. The lack lack of availability of personnel to respond to an alarm may may also increase the duration during weekends or holidays. The battery may function properly supporting continuous load during an extended time to replace the charger, but may not fulll its design basis if called upon. Remote devices may be needed needed to clear a fault fault having having a greater impact. Another important impact is loss of ac to the control enclosure. Similar to the loss of the charger, the battery supports loads during this type of event. However, many control enclosures mayavailable. not have been designedcritical to limitstation temperature minimums or maximums without the heating or cooling systems The designer should review the battery capability during this type of event.
5.4.2.5 Battery voltage voltage and number number of cells The operating voltages of batteries are usually greater than their nominal voltage ratings. For example, on a 48-nominal-volt systems, operating voltages are typically over 50 V and operating voltages are typically over 130 V for 125-nominal-volt systems. The operating voltages vary depending on the chemistry and specic gravity of the battery electrolyte. The oat voltages (voltage in the nominal charged condition) for an individual cell vary from approximately 2.17 V per cell to 2.25 V per cell, depending on the type of battery and number of cells. In some cases, these batteries are equalize charged (continuation of the regular charge at a higher voltage). It is important to verify that the equalization charge voltage does not exceed the maximum system voltage of the dc system which is typically dictated by equipment ratings. In substation applications, the maximum dc system voltage is typically limited to 140 V. In this case, the maximum cell voltage depends on the number of cells in the battery. The designer should review with the owner if the required equalization voltage would exceed alarm limits or normal equipment ratings (typically 140 V for 125 V (dc) systems). In that case, the number of cells may need to be reduced or the equalization voltage reduced, increasing the recharge time. The minimum voltage for lead acid battery cells is typically 1.75 V per cell, which is normally considered fully discharged. Other battery types will have diering discharge values. The designer should verify that the nal battery voltage would support the equipment terminal voltage sucient for the equipment operation. Voltage drop calculations need to be included in this consideration. Make sure to check connected equipment ratings if there are any questions. The voltage of the battery is calculated by using the following formula: volt ltag agee of ( vo
th thee ce cell ll) × (nu (numb mber er of ce cell llss in se seri ries es)) = ba batt tter ery y sy syssttem em vol voltag tagee
(13)
The number of cells and the end voltage of a battery system can be calculated using the following formulas:
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number of cells =
en end d of di disc scha harg rgee =
maximum allowable battery voltage maximum cell vo oltage oltage required for charging minimum mini mum allowabl allowablee battery battery vol voltage tage numb number er of ce cell llls ls
×
→
105 V 60 cell cellss
140 V 2.33 V pc
=
60 cells
= 1.75 V
(14)
(15)
NOTE— Equation Equation (14) and (14) and Equation (15) include (15) include the equation and an example in the second half that results in, or uses, u ses, 60 cells.
5.5 Battery chargers 5.5.1 Introduction Battery chargers are discussed in detail in other IEEE guides. The battery charger is the dc power supply that is normally used to provide the continuous loads of the station, and as a means to maintain charge on the battery,, recharge after an event, and to battery to provide provide an equalizing charge to bring the battery back into specication when cell voltages are outside manufacturer’s tolerances. The charger ltering requirements requirements may dier per connected load. The ltering levels are typically adequate to accommodate the continuous load currents to the duty cycle, however they may not be adequate to accommodate the tripping transient current requirements on most substation applications. Thus, even with a battery eliminator type ltering, removing the battery from the dc circuit may also compromise system protection requirements. There are four types of battery chargers commonly available as described in IEEE Std 1375™ [B16] [B16]:: a)
Ferroresonant and controlled ferroresonant
b)
Phase control silicon controlled rectier (SCR)
c)
Magnetic amplier chargers
d)
High-frequency switched-mode power supply (SMPS)
Battery charger type depends largely on owner’s preference or design criteria.
5.5.2 Battery charger charger sizing sizing Battery charger sizing is based on the amount of energy required to recharge a battery that has been discharged per its full design duty cycle, the desired recharge time, the continuous dc load supported by the charger during the recharging period, and various factors. For a given battery duty cycle, the amount of amp-hours removed is known from the battery sizing calculation (either manual or via computer program). This amount of charge is what the battery charger needs to supply in order to recharge the battery in a certain amount of time. If the amp-hours removed is not known from an available design calculation, a conservative method is to use the 8-hour amp-hour rating of the battery. This method will typically lead to a larger-than-necessary larger-than-necessary charger, as the amp-hours removed from a battery during a full duty cycle is typically less than the amp-hour rating of the battery. For the recharge time, the designer should consider the owner’s preference or design criteria. Typical times of 8 hours to 24 hours are used. While a shorter recharge time may restore a fully discharged battery faster, this may cause other problems. A faster recharge may lead to plate damage of the battery due to overheating, or the charger being oversized for day-to-day operations. The designer needs to review the probability of a worst case event happening during recharge, and use that to help determine battery size. For large charger sizes, the designer may consider installing two chargers operating in parallel. Since, under normal operating conditions,
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the full capacity of the charger is not needed, it can allow for routine maintenance, or even a single charger failure to occur, without an effect on battery performance. The recharge factor accounts for additional energy needed to fully recharge the battery. During charging, the recharge efficiency of the battery should be considered, including losses (e.g., heat) experienced during recharging that are not included in the amp-hours removed. The recharge factor depends on the battery technology. The battery manufacturer’s specifications and literature should be consulted, but typical values include 1.1 for VLA, 1.15 for VRLA, 1.3 for vented Ni-Cd, and 1.4 for valve-regulated Ni-Cd. A design margin factor may be included at the discretion of the designer. While chargers do not age or lose capacity over time like batteries, it may be desirable to add an additional design margin to account for future station load growth, changes in the battery duty cycle, or other factors. If the battery duty cycle amphours removed is used in the charger sizing, then there is typically no additional design margin already included. If the conservative method of using the 8-hour amp-hour rating of the battery for the amp-hours removed in charger sizing, then a design margin from the battery sizing calculation may already be included. Note the charging rate should be limited to 20% of the 8-hour capacity per battery manufacturer recommendations. An altitude/temperature altitude/temperature correction factor may be needed based on the installation installation conditions or the charger. The charger manufacturer’s specifications and literature should be consulted to determine these factors. Sizing—The following following formula may be used to determine the required dc output of the battery charger.
I
éæ A ö ù = êçç ÷÷÷ e + I C ú ( d)(k ) êëèç t ø úû
(16)
where I A
t
e I C d k
5.5.3
is the calculated battery charger output, dc amperes is the ampere-hours to be replaced is the time in which the battery should be recharged is the recharge factor is the continuous dc load current is the design margin factor is the altitude correction factor
Battery charger connections
The designer should review the owner’s preference, or design criteria, regarding the method of connecting the battery charger to the dc system. All connection methods have benefits and drawbacks. The charger can be connected at various various points in the the system including: a)
Directly to the battery terminals
b)
Source side of battery disconnect switch, if one exists
c)
Load side of battery disconnect switch, if one exists
d)
DC panel main lugs
e)
DC panel branch circuit
f)
DC bus terminal erminal block
23,, Figure 24, 24, and Figure 25 25 demonstrate demonstrate three connection options. Figure 23
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Figure 23—Battery with breaker disconnect and charger at dc panels
Figure 24—Battery with fuse disconnect and charger at dc panels
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Figure 25—Battery with no disconnect
If the charger is connected directly to the battery or on the source side of the disconnect switch, it could be considered a reliable method of charging the battery battery,, since there are minimal points of failure in between the charger anda battery. battery since the charger also servesfor to replacement supply power(by to continuous under normal operation, fault on. However, the battery battery, , or removal of the battery opening theloads battery disconnect switch [Figure [Figure 23] 23] or disconnecting the main battery leads/cables [Figure [ Figure 25]), 25]), may disconnect the charger from the loads. The charger size does not typically accommodate the worst case tripping current requirements. Most chargers do not have capability to source more current than the rating or a tripping transient (even with the battery eliminator option on some chargers). Substation design and operation activities need to coordinate to prevent operating or sourcing continuous load with charger and without a battery. Even though this may be functional, it removes the capacity of the battery to accommodate the higher loads of worse case tripping scenarios, and thus provides a false sense of security by compromising the capability of the dc system to provide the required dc power for the system protection. If the charger is connected on the load side of the battery disconnect switch (Figure ( Figure 23), 23), or at the dc panel (Figure 24), 24), it maintains a connection to the continuous loads even in the event of a battery failure or replacement. However, However, if the charger gets disconnected from the battery due to an event at the dc panel, the battery loses its means means to to re-charge. re-charge.
5.5.4 Charger circuit protection protection Although the charger may be equipped with integral ac and dc circuit breakers or fuses, the designer may consider external protection as well. The ac feed breaker from the main ac source should be protected in accordance with applicable local codes. The dc output may need to be connected with another overcurrent device to coordinate with the overall dc system. Typical charger overcurrent overcurrent protection is conservatively sized at 140% of the charger current rating. The cables connecting the charger to the dc system need to be sized to accommodate the overcurrent protection ratings of the charger dc output and the overcurrent protection in the dc cabinet (if the charger is connected to a dc panel with overcurrent protection). This overcurrent device could be either a fuse or circuit breaker depending in owner preference, local codes, or coordination needs. Both the ac and dc external protection should be used to protect the external circuit and cabling. The current limiting characteristics characteristics of the selected charger should be reviewed in accordance with IEEE Std 1375 [B16] [B16]..
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5.6 DC panels 5.6.1 Introduction The dc panels are used to distribute power to various loads in a substation and can come in many varieties. Panels can come with overcurrent protection on the main feed or main lug only (where the main dc feed connects directly to the dc bus). Branch circuits can be protected by circuit breakers, fuses, fuses with knife blade isolation, or combinations of these, these, such as a circuit circuit breaker in the positive leg and knife switch isolation in the negative. The designer should review applicable local codes and owner’s preference as to what type should be used.
5.6.2 Critical and non-critical loads The designer should review if there is separation required by local codes, owner’s preference, or design criteria. This could be based on whether there is a need to separate loads as critical or non-critical. Critical loads are those that would be required to have dc power under unusual system conditions, such as loss of power to the the site, site, black black start start path, path, loss loss of the charger charger,, etc. etc.
5.6.3 Circuit size The designer should size the dc panel to accommodate the required number of circuits needed for existing load as well as planned load growth. Branch circuits should be sized in accordance with the NEC, local codes, or owner’s design criteria, as applicable. Branch circuits should coordinate with downstream devices, such as fuses or circuit breakers. The installed cable should be sized to exceed the required load. Circuit size should also account for any voltage drop. Voltage Voltage drop includes the eects of current through all interconnecting cable to and from the remote device. The cable should be sized so the device can operate at minimum battery voltage (i.e., 105 V [dc] on a 125 V [dc] battery) so that the minimum device voltage (90 V [dc] typical minimum pickup) is available at the remote device. It may be prudent to build some conservatism in the design calculation to allow for variations in eld conditions due to cable lengths, device tolerances, etc.
5.7 Load transfer methods 5.7.1 Introduction To provide for a more robust dc system, it may be determined that a load transfer or paralleling scheme is required. The designer should consider the additional load that will be applied in a paralleling scheme and is accounted for in calculations that size the battery battery,, charger, cables, etc. that are part of the dc system(s) that may accommodate the added load. The specic details and method of transfer should also be reviewed. When designing a load transfer between two dc systems, the fault currents and arcing currents should be considered. Panels and protection devices should be rated for the maximum fault current of the entire system. The designer should also consider if the two systems should be run in parallel or interlocked to not allow parallel operation. operation. Paralleling the battery battery banks will result in in increasing the available fault fault current. It is also also recommended that paralleled battery banks should be the same type and size to ensure equal load sharing. Additionally, if battery chargers are to be operated in parallel, the designer should verify that the selected chargers will operate when paralleled. Consideration of paralleling two batteries with dierent state of charge may cause unexpected current ows and excessive loading on the good battery battery..
5.7.2 DC paralleling—dc paralleling—dc transfer transfer scheme Manual transfer of dc load can be accomplished with disconnect switches or temporary cables. Manual load transfer should be accomplished in a safe manner using switching procedures, electrical isolation, physical locks, other methods. The equipment (cable, switch, etc.) that as actually thegrowth. load from one systemand to the other should be sized for the expected load to lugs, be transferred, well astransfers future load Figure Figure 26 and 26 and Figure 27 show 27 show two possible manual transfer schemes.
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Figure 26—Simplied parallel/transfer parallel/transfer with two disconnects
Figure 27—Simplied parallel/transfer with one disconnect
In the above examples, the manual disconnect switches are sized for the larger of the two connected panel loads. The cabling to the switches is sized for the total dc system loads. Means to provide isolation of the switches for maintenance should be considered. A simple paralleling/transfer system may be the use of a normally open breaker or fuse position in each of the main dc panels tying the two together when both are closed. Administrative Administrative control procedures should be established to implement the paralleling or transfer of the two systems. Both battery systems will be sized for the total station dc load and load prole. While more costly than battery systems designed for single segregated loads, it provides for maintenance of the battery(s) with no disruption in supply.
5.7.3 DC transfer DC transfer could be accomplished via transfer switches similar to those used on ac systems. Figure 28 28 illustrates one version of that method. This conguration creates a single point of failure, and a complete
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dc outage would be required to upgrade or replace a failed automatic transfer switch (ATS). This may be acceptable in non-critical applications since there is only one circuit to the dc panels. Other methods may include bypass switches to allow maintenance of the transfer switch while still powering the dc loads. Automatic transfer switches can be purchased with internal bypass switches and removable transfer switches for maintenance. As with the transfer/paralleling schemes, the batteries and equipment in the transfer scheme are to be sized for the total station dc load.
Figure 28—DC transfer scheme
5.8 Design consideration considerations s 5.8.1 Battery monitorin monitoring g 5.8.1.1 Introduction The battery and dc system has many options for monitoring. The battery charger itself itself may be equipped with monitoring functions such as loss of dc, low dc, battery grounds, and loss of charger ac. Some microprocessor based chargers have programmable exibility to provide many other forms of battery monitoring, such as battery temperature, impedance, and an on-line partial battery capacity test. Many microprocessor microprocessor-based -based relays have the option to monitor the dc source voltage to the relay and can provide additional alarm capability. capability. An auxiliary relay may be used to monitor systems where automatic monitoring may not be available. Through the use of communication links, continuous loads may be monitored from the charger directly to a SCADA remote terminal unit (RTU) (RTU) or other similar device. A dc dc shunt may be used to measure battery current directly and connect to a monitoring device. Please refer to IEEE Std 1491™ 1491™ [B18] [B18]..
5.8.1.2 Battery location 5.8.1.2.1 Fire consideratio considerations ns While the battery is not normally a direct re hazard, several conditions may present hazards. If the battery main terminals become shorted between the main terminals, and there is no protection (fuse or circuit breaker) as allowed by IEEE Std 1375 [B16] [B16] for for overcurrent, the short-circuited battery would become a re hazard. The availability of re-resistant jars may be specied to reduce re hazards. Thermal runaway conditions also present re hazards. Another common hazard is the generation of hydrogen gas produced by VLA, NiCad, and VRLA batteries during charging—especially charging—especially when an equalizing charge is applied. Removal of any potential hydrogen build-up should be considered by the designer. This build-up may be removed through normal building exhaust or leakage, direct exhaust of the battery area, or by inclusion of fresh air into the
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building ventilation system. IEEE Std 1635™ [B20] [B20] notes notes several other recommendations. The designer should be aware of any restrictions imposed by the AHJ in regards to battery ventilation. IEEE Std 979™ [B10] provides [B10] provides guidance for re protection in substation applications. IEEE Std 1375 [B16] [B16] provides provides some additional guidance as well on physical protection of batteries. Local codes or the owner’s preference should be reviewed reviewed as to to whether whether the the battery battery should should be housed in its its own room or enclosure. The battery charger also does not present any direct re hazard. However they generate heat as part of the ac–dc conversion and care should be taken to restrict ammable material from being located above the vent openings. Working clearance meeting the requirements of the NESC [B1] [B1] Tabl Tablee 125-1 (or local codes) should be used to provide safe access to the the equipment equipment for for workers workers and in the event of an emergency emergency..
5.8.1.2.2 Maintenance consideratio considerations ns As discussed in 5.8.1.2.1 5.8.1.2.1,, working space meeting the requirements of NESC [B1] [B1] Table 125-1, or other jurisdictional codes, should be maintained. In retrot designs of older stations, the designer should check clearances that may have been compromised over the life of the substation, or in replacing equipment that was installed prior to code applicability. applicability. Consideration should also be given to a method for removing battery cells in the future. Space for a permanent or temporary lifting device may be needed. Typical substation battery cells weigh 20 kg to 70 kg (44 lb to 154 lb). Lifting cells of that weight can be very dicult for maintenance from upper steps or tiers of a battery rack. An eyewash station (or equivalent device) should be available to support workers in the event of acid contact. Provisions should be made for storing the specic gravity tester and an acid-resistant cloak if required by the owner.. Consideration should be given to using a spill containment system around the battery to absorb acid in owner the event of a catastrophic cell failure. Refer to to 5.8.1.3 5.8.1.3 and and IEEE Std 1578™ [B19] [B19] for for further information.
5.8.1.2.3 Reliability consideratio considerations ns The designer should review owner’s preference or local codes for separation of multiple battery systems. Physical separation or barriers may be required for multiple systems to reduce the likelihood of a catastrophic event (e.g., re or short circuit) on one dc system propagating to other dc systems. This can include physical separation by air gap or installation of a barrier (a wall or locating batteries in separate rooms). As the battery system is crucial in allowing most substation equipment to successfully operate, care should be given to provide as much much protection protection to the the battery battery system system as reasonably reasonably possible. Reliability is also dependent on battery area temperature. Battery area temperature should be monitored and kept constant (refer to 5.8.1.2.5 5.8.1.2.5 and and 5.8.1.4 5.8.1.4). ). Owner’s operating practice for response to building high or low temperatures should be reviewed to determine eect on battery performance and reliability reliability.. Low or high temperatures outside the design of the battery load prole can impact reliability. reliability. Reliability of the dc system is also aected by the placement location of dc panels. Separation of dc panels may reduce the likelihood of a single panel re removing both dc systems, and should be considered. Cable routing should also be reviewed. Some utilities run dc cables from dierent systems in separate locations to enhance reliability.
5.8.1.2.4 Battery room door requirements requirements If the battery is placed in its own room due to owner’s preference or local codes, the battery room door should have a re rating at least equal to the re rating of the walls. The battery room door should also incorporate all necessary signage to inform workers of potential hazards of the area, such as acid containing, explosive mixtures, etc., as required by the AHJ. Interior signage should identify the exit doors. Depending on room design and local codes, the battery room door may also need to incorporate a blast louver to relieve pressure in the event of a hydrogen build-up and explosion. The battery room door should have a panic bar on the
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inside, and open outward into the control room or outside to allow safe egress of personnel in the event of an emergency.. Requirements for securing the door such as locks should be reviewed by the designer emergency designer..
5.8.1.2.5 Battery area area temperature temperature Battery temperature plays a key role in battery performance. Battery specications specications are generally published at 25 °C (77 °F) and temperatures that vary from this can aect performance. During the battery sizing calculation the designer should consider the minimum and maximum temperature that the battery area could reach. For example, in a cold weather climate in winter winter,, the battery area could easily reach 13 °C (55 °F) during a loss of ac to the substation, depending on building insulation levels during the needed response time. Conversely, Conversely, in a warm weather climate in summer, summer, the same loss of ac could drive the battery area over 40 °C (104 °F). Normal operating practices should also be reviewed to determine baseline conditions as part of the battery calculation. If the owner keeps the battery area cooler than the battery manufacturer’s recommended temperature, battery performance may may be below published data and the designer designer should account for the discrepancy discrepancy in the design calculation. Batteries Batteries that are installed outdoors, or in non-climate control enclosures, may be subject to large variations in temperature.
5.8.1.3 Acid spill spill containment containment The designer should review applicable local codes regarding acid containment. It is typical practice to install a spill-containment spill-containme nt system that contains the acid to an area immediately adjacent to the battery and neutralizes it for safe handling and disposal. Use of acid-resistant paint on the oors and walls of the battery area is recommended to reduce damage to the building in the event of a spill. If permanent spill containment is not installed, the designer should review local codes or owner’s preference to determine if on-site temporary acid-absorbent material or temporary containment is required. For example, in the United States, the NFP NFPA A1 [B33] requires [B33] requires spill containment for an individual vessel with more than 208 liters of electrolyte or multiple containers exceeding 3785 liters. Most substation batteries have electrolyte volumes below those limits. Refer to IEEE Std 1578 1578 [B19] [B19] for for further information. The designer should review the footprint required for a containment system. The designer should consider adequate worker access and remove tripping hazards that may be created by installation of a mechanical containment system.
5.8.1.4 Battery racks When selecting a battery rack, there are several things that should be considered, including temperature dierences, weight of the battery, available space, and maintenance requirements. Battery racks generally come types—step, tier,another or stepped tier as shown 29 step A rack is designed the battery levels in arethree “stepped” from one (usually oset by in theFigure depth 29. of .aAcell). tiered rack hasso the levels of batteries on top of each other. A stepped tier is a combination of the the two. two.
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Figure 29—Rack designs
For substation applications, steps and tiers are usually limited to two levels. Step racks generally have a larger footprint than an equivalent tiered rack, and cells can be easier to access. Tiered racks tend to save oor space due to their smaller footprint. Other considerations with larger batteries include height and weight. Battery weight can also be an issue for battery installation or removal, especially in tight space and/or with taller racks. Battery weight should be considered during structural design. The height variations between upper and lower levels of a battery rack are a concern. Height variations can cause cell temperature dierences within the same battery system. Since cell temperature can impact battery characteristics,, interconnecting cells at dierent temperatures can lead to an early failure of the battery system. characteristics As a general rule, temperature gradients in excess of 3 °C should be avoided. Battery racks should have an acid-resistant coating applied to the structural frame to preserve its integrity. integrity. It may also be advantageous to have a liner of polyethylene or similar material on the support rails to further protect the rails rails from from damage damage and provide provide electrical electrical isolation. The battery rack should be specied based on its correct seismic zone. A seismic rack has the same basic design as a non-seismic rack with additional bracing applied to hold the rack and cells in place.
5.8.2 Circuit consideratio considerations ns 5.8.2.1 Grounded and ungrounded systems Substation batteries used for operation and control of interrupting devices and protection system, SCADA, etc. are typically ungrounded with ground fault detection. Communication systems, such as those used by telecom companies, are typically a positively grounded 24 V (dc) or 48 V (dc) system. The designer should be aware of the dierence and not mix the two. Direct contact input to opposite systems should be avoided and use of interposing relays or devices should be used. Addition of unintentional grounds should be reviewed during the design and installation process.
5.8.2.2 Isolation of main dc cables As discussed further in 5.8.2.3.2 5.8.2.3.2,, the battery is the source of fault current for the dc system. The cables between the main battery terminals and the rst overcurrent protection device (breaker or fuse) are usually unprotected (unless using a mid-point fuse). Thus, designs should place the main battery overcurrent protection as close to the main terminals as possible to reduce this exposure. A short circuit to any portion of the battery main
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terminals can produce extreme heat and re hazard. Any damage to the cables from the battery can subject a worker to the full short-circuit capability of the battery battery.. The designer should review the owner’s preference to separate the positive and negative cables of the battery to reduce the possibility of a direct short circuit being applied to the battery. When separating cables, cables should be placed in non-magnetic conduits to reduce induced elds from causing other potential hazards. With multiple battery systems, the designer should not route main dc cables near one another to preserve independence and reliability reliability.. IEEE 1375 [B16] gives [B16] gives deviceStd from the battery battery. . Theyadditional include: guidance on the methods of protecting the main dc feed to the load a)
Battery fuse (in both positive and negative leads for ungrounded systems)
b)
Battery circuit breaker (including both positive positive and negative negative leads leads for for ungrounded ungrounded systems) systems)
c)
Battery disconnect switch (fused or non-fused) non-fused) that allows the battery battery to be disconnected disconnected from from the load circuits
d)
Mid-point battery fuse fuse which protects for internal internal and external faults and limits fault energy energy by up to half of the battery capacity for certain types of faults; cable only, no overcurrent provided
IEEE Std 1375 1375 [B16] [B16] gives gives a description of the advantages and disadvantages of each method.
5.8.2.3 Circuit protection and coordination 5.8.2.3.1 Coordinat Coordination ion of overcurrent overcurrent protection protection The designer needs to review the coordination between all devices in the dc circuit in accordance with the NEC [B34] [B34],, local codes, or owner’s owner ’s design criteria. Overcurrent Overcurrent protection devices should be sized such that an upstream device does not trip for a downstream operation. For example, if a dc panel circuit feeds both a relay panel fuse and a circuit breaker trip coil, the relay panel fuse should operate due to a protective relay power supply or circuit circuit failure and leave leave the circuit breaker trip coil operational.
5.8.2.3.2 Short-circu Short-circuit it levels levels Since the battery is the primary current source in case of short circuit, the battery data sheet or manufacturer should be consulted to determine available fault current. The interrupting devices in downstream circuits should be reviewed for their dc ratings. Many devices may appear to have sucient interrupting capability, capability, but do not have the appropriate asymmetrical interruption current (AIC). Without Without proper AIC, a breaker may not interrupt the current. It may closed injury or opentowithout the ability dissipate the energy. These conditions could result in damage to weld equipment, personnel, and/ortoother unintended operations. Similar conditions apply to fuses used for interrupting faults. The designer should consider protection of the main dc feed by use of circuit breakers or fuses. Subclause Subclause 5.8.2.2 and 5.8.2.2 and IEEE Std 1375 1375 [B16] [B16] give give more guidance on protection of the battery main feed.
5.8.2.3.3 Fuse and circuit breakers breakers The designer should consider local codes as well as owner’s preference or design criteria when selecting circuit breakers or fuses. fuses. Fuses may have a lower initial installed cost, but may may require require additional additional spare material to be stored on site to allow for replacement in the event of an operation. Fuses may also require a fuse monitor to be installed installed to detect and provide provide indication indication that they have have operated. operated. Circuit breakers may have have a higher initial installed cost, but they provide indication they have operated, and usually do not require replacement after they have operated.
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5.8.3 Equipment rating 5.8.3.1 Indoor and outdoor equipment ratings The dc equipment should be selected to be of the proper rating for their intended location. Outdoor rated equipment may be installed within indoor substation locations, but indoor rated equipment should not be installed outdoors. It may be advantageous to have some dc panels placed closer to the loads they support, such as circuit breakers in a large transmission substation. In this application, outdoor rated equipment may be required, such s uch as NEMA 3R or NEMA 4.
5.8.3.2 Equipmen Equipmentt current and voltage voltage ratings As discussed previously, the dc equipment should be rated for interruption of fault current. System conguration should be considered for determining ultimate fault current availability. If a main breaker is used, it should be able to interrupt the maximum short-circuit current available from the battery for the life of the battery. The designer should review interrupting capability during a battery replacement. Continuous current rating should match or exceed the current drawn by existing loads and allow for future growth. Voltage Voltage rating should match or exceed the maximum battery voltage (i.e., 250 V [dc] for a 125 V [dc] battery). Faultinterrupting current ratings at a dc level should be known. A large battery may be capable of currents over 10 kA. DC interrupting capability of the main fuse or circuit breaker should be reviewed. The interrupting rating of the distribution panel is based on the breaker(s) with the lowest fault current rating.
5.9 Maintenance provisions 5.9.1 Isolation switches The designer should review local codes and owner’s preference or design criteria regarding the need to provide isolation isolation switches for the battery and charger charger.. Main isolation switches switches can allow a temporary temporary battery battery to be installed during maintenance, upgrades, or replacement. Since it is usually not feasible to shut down an entire substation during a battery change out, providing an isolation switch where a temporary battery can be connected can be advantageous during upgrades or emergencies, such as battery failure. Similar logic can be applied to chargers, though in case of a charger failure or replacement, it is usually easier to connect a charger temporarily than a battery.
5.9.2 Equipment accessibility As discussed previously, access per NESC [B1] [B1] Table Table 125-1 or other local codes should be maintained. Table 125-1 provides minimum clearances, but owner’s preference and design criteria should also be reviewed. Battery cells/jars can be heavy enough that workers may not be able to lift without mechanical assistance. Access room may need to be maintained for mechanical lifting devices to install or remove battery cells/jars. Safe working clearances between the battery and other equipment should be maintained. Overhead lifting devices may need to be anchored to building supports to remove battery cells. Battery chargers may also require lifting devices. The designer should also consider the heat generated by chargers when evaluating equipment accessibility accessibility..
5.9.3 Back-up supplies The designer should review owner’s preference for any back-ups and/or spare parts. Based on the importance of the substation, there may be a need for back-up equipment (either charger or battery bank). As discussed previously,, if provisions are made during design, then back-up supplies previously supplies can easily be connected. If back-up supplies are required, the design should account for the time frame required to facilitate timely or permanent connection of any back-up supplies, including location of back-up or temporary designer needs to review if automatic actions arethe required to place any back-up suppliesconnections. in service. Also, the
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Annex A (informative)
Bibliography Bibliographical references are resources that provide additional or helpful material but do not need to be understood or used to implement this standard. Reference to these resources is made for informational use only. [B1] Accredited Standards Committee C-2, National Electrical Safety Code® (NESC®).6,7 [B2] Distribution Trans [B2] Distribution Transformer former Handbook, Handbook, First First Edition, Transformer Transformer connections, General Electric, October 1951. [B3] IEEE Std 141™, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants ( IEEE IEEE Red Book ™). ™). [B4] IEEE Std 242™, IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems ( IEEE Bu Book Book ™). ™). [B5] IEEE Std 446™, IEEE Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications ( IEEE IEEE Orange Book ™). ™). [B6] IEEE Std 450™, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications. Applications. [B7] IEEE Std 484™, IEEE Recommended Practice for Installation Design and Installation of Vented LeadAcid Batteries for Stationary Applications. Applications. [B8] IEEE Std 485™, IEEE Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications. [B9] IEEE Std 946™, IEEE Recommended Practice for the Design of DC Auxiliary Power Systems for Generating Systems. [B10] IEEE Std 979™, IEEE Guide for Substation Fire Protection. [B11] IEEE Std 1106™, IEEE Recommended Practice for Installation, Installa tion, Maintenance, Maintena nce, Testing, Testing, and Replacement Replace ment of Vented Nickel-Cadmium Batteries for Stationary Applications. [B12] IEEE Std 1115™, IEEE Recommended Practice for Sizing Nickel-Cadmium Batteries for Stationary Applications. [B13] IEEE Std 1187™, IEEE Recommended Practice for Installation Design and Installation of ValveRegulated Lead-Acid Batteries for Stationary Applications. [B14] IEEE Std 1188™, IEEE Recommended Practice for Maintenance, Testing, and Replacement of Valve Regulated Lead-Acid (VRLA) Batteries and Stationary Applications.
6
The IEEE standards or products referred to in in Annex A are A are trademarks owned by the Institute of Electrical and Elect ronics Engineers, Incorporated. 7 IEEE publications are available from the Institute of Electrical and Electronics Engineers (http:// (http://standards standards.ieee .ieee.org/ .org/). ).
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[B15] IEEE Std 1189™, IEEE Guide for Selection of Valve-Regulated Lead-Acid (VRLA) Batteries for StationaryApplications. [B16] IEEE Std 1375™, IEEE Guide for the Protection of Stationary Battery Systems. [B17] IEEE Std 1458™, IEEE Recommended Practice for the Selection, Field Testing, and Life Expectancy of Molded Case Circuit Breakers for Industrial Applications. [B18] IEEE Std 1491™, IEEE Guide for Selection and use of Battery Monitoring Equipment in Stationary Applications. [B19] IEEE Std 1578™, IEEE Recommended Practice for Stationary Battery Electrolyte Spill Containment and Management. [B20] IEEE Std 1635™, IEEE/ASHRAE Guide for the Ventilation and Thermal Management of Batteries for StationaryApplications. [B21] IEEE Std C57.12.00™, IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. [B22] IEEE Std C57.12.10™, IEEE Standard Requirements for Liquid-Immer Liquid-Immersed sed Power Transformers. [B23] IEEE Std C57.12.20™, IEEE Standard for Overhead-Type Distribution Transformers 500 kVA and Smaller: High Voltage, 34 500 V and Below; Low Lo w Voltage, Voltage, 7970/13 79 70/13 800Y 800 Y V and Below. [B24] IEEE Std C57.91™, IEEE Standard for Loading Mineral-Oil-Immersed Mineral-Oil-I mmersed Transformers and Step-Voltage Step-Voltage Regulators. [B25] IEEE Std C57.96™, IEEE Guide for Loading Dry-Type Distribution and Power Transformers. Transformers. [B26] IEEE Std C57.105™, IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems. [B27] IEEE Std C62.22™, IEEE Guide for the Application of Metal-Oxide Surge Arresters for AlternatingCurrent Systems. [B28] International Code Council (ICC), International Building Code (IBC).8 [B29] NEMA 250, Enclosures for Electrical Equipment (1000 Volts Maximum). [B30] NEMA AB-1, AB-1, Molded-Case Circuit Breakers, Molded Case Switches, and Circuit-Breaker Enclosures. [B31] NEMA PB 1, Panelboards. 9 [B32] NEMA PB 2, Deadfront Distribution Switchboards. [B33] NFPA 1, Uniform Fire Code®. 10 [B34] NFPA 70, National Electrical Elec trical Code® (NEC®). 11
8 9
The Uniform Building Code is available from the International Code Council (http:// ( http://iccsafe iccsafe.org .org). ). NEMA publications are available from the National Electrical Manufacturers Association (http:// (http://www www.nema .nema.org/ .org/). ). 10 NFPA NFP A publications are published by by the National Fire Protection Association (http:// (http://www www.nfpa .nfpa.org/ .org/). ). 11 National Electrical Code, Code, NEC, and NFPA 70 are registered trademarks of the National Fire Protection Association.
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[B35] NFPA 70, 2014 Edition, Editio n, National Electrical Elect rical Code® (NEC®). [B36] NFPA 70E, Standard for fo r Electrical Safety in the t he Workplace Workplace.. [B37] UL 50, Enclosures for Electrical Equipment, Non-Environment Non-Environmental al Considerations.12 [B38] UL 67, Standard for Panelboards. [B39] UL 98, Enclosed and Dead-Front Switches. [B40] UL 489, Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Bre Circuit-Breaker aker Enclosures. [B41] UL 869A, Reference Standard for Service Equipment. [B42] UL 891, Switchboards. [B43] UL 991, Standard for Tests for Safety-Related Controls Employing Solid-State Devices. [B44] UL 1008, Transfer Switch Equipment.
12
UL publications are available from Underwriters Laboratories (http:// http://www www.ul .ul.com/ .com/). ).
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Annex B (informative)
Conductor selection examples B.1 Example B-1 A designer is tasked with designing a load-distribution scheme for station service in an electrical substation. The designer completes a load study, and once nished, contacts an ac panel supplier to purchase their ac panel for the load-distribution scheme. The authority having jurisdiction (AHJ) has mandated that all station service installations follow the NEC 2014. While waiting for specication back from the panel vendor, the designer begins selecting the cables for the the scheme. scheme. One of the cables the designer is selecting will supply a 120 V, V, 200 A single-phase load, will be located outdoors, and will be assumed to be connected to 75 °C termination. Based on the given information, the designer selects a 600 V, 75 °C, 4/0, cross-linked polyethylene high heat-resistant water-resistant (XHHW) conductor, per NEC NEC 2014 2014 Tables Tables 310.104(A), 310.15(B)(16), and cable manufacturer recommendations. Upon receiving receiving specication back from the panel supplier, the designer discovers that the branch circuit breaker terminations on the panel are only rated for 60 °C. The designer should now nd a dierent solution for the 200 A load, load, as their cable should be derated to the 60 °C temperature rating: 195 A [see Table 310.15(B)(16)]. 310.15(B)(16)]. In addition to temperature rating of terminations, the availability of terminations at the connected equipment may be a limiting factor as well. For example, when sizing conductors it is determined that a single 250 kcmil conductor would meet the ampacity requirements. The equipment terminations are the tap-screw type, however, and only allow two connections with sizes ranging from 4 AWG to 4/0 AWG. The designer determines that two 1 AWG conductors would still meet the ampacity requirements of the circuit. So in this case, the availability of terminations ultimately governed the conductor size. The designer veries the conductor is adequately sized based on the available fault current at the circuit breaker termination. The AHJ has dictated that the short-circuit capability ratings be determined based on IEEE Std 242-2001 [B4] [B4].. The available fault current in this case is 20 kA. Based on the manufacturer specs and associated time-current trip curves, the circuit breaker feeding the load should trip within a maximum time of 1.5 cycles. The designer has selected 1 AWG XHHW copper conductors, with a continuous operating temperature of 60 °C, and a short-circuit temperature rating of 250 °C. The designer rst calculates the virtual available fault current based on IEEE Std 242-2001 [B4] [B4],, Figure 9-4: K T
=
1.2
virtual available fault current
=
20 kA
×
1.2
=
24 kA
Based on the virtual available fault current, and a fault-clearing time of 0.025 s, the designer determines that a single 2 AWG AWG copper conductor would be sucient, based on IEEE Std 242-2001 [B4] [B4],, Figure 9-2. Since two 1 AWG AWG conductors are being used per phase, there is no need to change the size of the conductor.
B.2 Example B-2 A designer is tasked with designing a load-distribution scheme for station service in an electrical substation. The AHJ has mandated that all station service installations for the project follow the NEC 2014 [B35] [B35].. A majority of the scheme is designed to be three-phase, 208/120 V. V. The designer checks the design criteria, nds
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that the ambient temperature for the area is 29 °C, and as a result selects all of their cables based on 30 °C ambient temperature ratings. One of the branch circuits is designed to feed an SF6 gas cart for outdoor HV breaker servicing. The gas cart product manual states that it has a power demand of 60 A, a power factor of 0.9, and requires a 208 V, 3-Ø, fourwire supply. supply. The circuit run is 61 m (200 ft) long and routed through a 4 in polyvinyl chloride (PVC) conduit with two other 3-Ø, four-wire circuits. The supply conductor will be connected to copper termination points, rated at 75 °C. Gas carts are rarely operated for more than two hours. Any load that is not expected to run for three hours or more is considered a non-continuous load, per NEC Article Article 100. Based on this information and NEC 210.19(A)(1), 310.15(B)(16), the designer designer initially initially selects selects the the branch circuit conductor that that supplies supplies the SF6 gas cart to be 6 AWG, copper thermoplastic thermoplastic heat and water-resi water-resistant stant nylon-coated (THWN) rated at 75 °C. Once the initial selection has been made, the designer should account for conductor bundling eects and voltage drop. The calculation for bundling eects is performed rst, per NEC 310.15(B)(3)(a). There are two ways that this process can be executed; the rst one shown below is not recommended, but has been given for demonstration purposes. Note that that the value used used in the calculation calculation is 70%, since, since, out of the total of 12 conductors conductors in the PVC conduit, only 9 are current-carrying: 0.7 = 65 ×0.7 = 45.5 I ′ = I ×
A
(B.1)
where I I '' is the adjusted conductor conductor ampacity per NEC Table Table 310.15(B)(3)(a) 310.15(B)(3)(a) I is the NEC conductor conductor ampacity ampacity per NEC Table 310.15(B)(16) 310.15(B)(16) It is clear from this conductor bundling calculation that the 6 AWG AWG conductor does not have sucient ampacity to supply the load. At this point there are two ways to nd the appropriate conductor to supply the load per NEC 310.15(B)(3)(a): 310.15(B)(3)(a): 1) select a conductor of the same type, type, yet with higher ampacity ampacity rating, and calculate the adjusted ampacity of the conductor per NEC Table Table 310.15(B)(3)(a) until a sucient conductor is selected, or 2) apply the adjustment factor to the full load amperes, and then select a conductor of sucient ampacity from NEC Table 310.15(B)(16). The second option is the easiest route, as it only requires one calculation, instead of iterative calculations: I L ′ = I L ÷ 0.7 = 60 ÷ 0.7 = 85. 71 A
(B.2)
where I L
' is the adjusted load amperes for conductor bundling
I L
is the load amperes
Based on this calculation, the designer would select 3 AWG copper THWN. THWN. The designer also has the option of routing one or two of the sets of 3-Ø, four-wire circuits through another raceway, raceway, in order to decrease, or possibly eliminate, the correction factor for conductor conductor bundling. Next, the designer should consider consider the voltage drop of the conductor. conductor. The designer has designed designed the system to where the voltage drop of the feeders does not exceed 2% to the point of termination at the panel, and 5% overall. The voltage drop is calculated based on the initial information given, formula given in NEC 2011, Table 9, Note 2, as well as the values given in Table 9, as shown below.
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First, eective Z is calculated for a 3 AWG AWG copper wire running through PVC conduit. Note that if available, the impedance given by the manufacturer should be used in the calculation. NEC Table 9 provides values based on uncoated uncoated wires, wires, and could could be used for for estimation: estimation: Z e
=
R × pf
+
X L ×sin (arccos arc cos( pf ))
(B.3)
Z e = 0.25× 0.9 + .04 0 47 ×sin arccos 0.9
(
(
)) =
Ω 0.2253
kFT
(B.4)
Next, the voltage voltage drop drop is calculated based on the eective impedance: V D
=
I L × Z E × L×
3
(B.5)
V D = 60 × 0.22537 × 0.2×1.73 = 4.68 V
%Drop =
V D V L
=
4.68 V 208 V
(B.6)
100 = 2. 2 25 5%
(B.7)
×
where V D
is the voltage voltage drop expressed expressed in volts
V L
is the required load load voltage expressed in volts
I L
is the load amperes
Z E is
the eective impedance impedance of the conductor conductor
L is the total distance distance of wire in circuit run, run, given in kFT Note that the calculation above is essentially the same as the 3-Ø voltage drop calculation except it is simplied into two separate equations. In this case, the voltage drop is acceptable, as it is below the required 3% per NEC 210.19(A) (Note 4). Had it been excessive, the designer would have to select a conductor with a lower impedance cable (usually larger size), or nd an alternate route to decrease the distance of the circuit feeding this load. Further into the design of the auxiliary system, the designer nds out that the ambient temperature at the substation is not 29 ° C—it is actually 33 °C. The load amperes should be recalculated to account for the change in ambient temperature, per NEC Table Table 310.15(B)(2)(a): I L ′′ =
I L
0.94
=
85.71 0.94
= 91.18 A
(B.8)
where I L " is the adjusted load amperes for change in I L
ambient temperature
' is the load amperes adjusted for conductor bundling
In this case, the conductor size does not need to be adjusted, as it can satisfy the load amperes requirement per NEC Table 310.15(B)(16). 310.15(B)(16). The last check the designer performs is for the short-circuit rating of the selected conductor. The AHJ has dictated that the short-circuit capability ratings be determined based on IEEE Std 525. The available fault current in this case is 10 kA. Based on the manufacturer specs and associated time-current trip curves, the
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circuit breaker feeding the load should trip within a maximum time of 0.5 cycles. The designer has selected 3 AWG copper THWN conductor, with a continuous operating temperature of 75 °C, and a short-circuit temperature rating of 250 °C. Based on the information provided, the designer performs a short-circuit capability calculation, based on IEEE Std 525: A =
I
0.0297 t
log10
T 2 + 234
in circular mils
(B.9)
T 1 +234
where I is the short circuit circuit in in amperes = 10 000 A t t is the time of of short circuit in seconds = ½ cycle = 0.0083 sec T 1 is the maximum conductor operating operating temperature temperature = 75 °C T 2 is the maximum maximum conductor short-circuit temperature = 250 °C A =
10 000 0.0297 250 + 234 log10 0.00833 75 + 234
= 11
996.5 in circular mils ≈ 9AWG
(B.10)
The chosensize conductor oninthe calculated amount of area, is 9 AWG. AWG. This is smaller than selected conductor 3 AWG,size, so nobased change conductor size is necessary.
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Annex C (informative)
Battery sizing example Materials and information needed for calculation: a)
Metering and relaying drawing showing relaying and tripping sequences
b)
DC system schematic showing dc loads loads connected connected to to the battery
c)
Materials and information needed for a battery sizing calculation
d)
Battery discharge curves or tables
Figure C.1—Substation one-line diagram
Options for this example: a)
Single battery supplying dc power for the substation
b)
Two independent independent redundant batteries—l batteries—load oad is split between two batteries into primary and secondary secondary systems with no ties
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IEEE Std 1818-2017 IEEE Guide for the Design of Low-Voltage Auxiliary Systems for Electric Power Substations
c)
Two batteries with an automatic transfer switch
From a battery sizing standpoint, options A and C would require batteries with the same total A-hour requirements as they both would require a single battery to accommodate the substation dc load. Option B would likely require two smaller batteries, possibly dierent sizes depending on how the primary and secondary loads are split. Examples of steps to size a lead-acid battery based on IEEE Std 485: 1.
Determine the voltage voltage and number of cells—Sele cells—Select ct the number of battery cells cells to be used to support voltage level
2.
Battery sizing considerations— considerations—Determine Determine adjustment factors like growth, aging, design margin, and temperature correction
3.
Select the battery type and and determine determine the characteristics of the the cell— cell— Battery type type is the specic manufacture and style; characteristics of the cell include include amperes per positive plate and construction (lead calcium, lead selenium, etc.)
4.
Determine the time time span of the duty cycle—How cycle—How long long the system has has to run without without the battery under charge
5.
Construct the minute-by-minute minute-by-minute load prole prole (the duty duty cycle), cycle), which which is very site site specic—Determ specic—Determine ine the continuous loads ondc thesystem) dc system, and determine the momentary, momentary, worst case switching event (the maximum stress on the
6.
Calculate the required required positive plates of the battery battery for each period in the duty cycle utilizing the cellsizing worksheet—Figure worksheet—Figure 3 in IEEE Std 485
Step 1 This example considers a 125 V nominal system with maximum dc voltage = 140 V and utilize a 60 cell battery.. The example battery example utilizes a 1.75 1.75 V per cell cell end-of-life end-of-life cycle = 105 V for 60 60 cell battery battery.. Step 2 Design margin = 10% The design margin provides additional capacity to accommodate future substation additions or expansions without requiring an upgrade to the substation battery due to capacity. Temperature correction factor = 1.15 (68 °F) Because a battery’s performance is aected by temperature, the temperature correction factor is needed to adjust the required battery capacity for any environment above or below the standard battery temperature rating. Aging factor = 125% IEEE Std 450™ [B6] [B6] and and IEEE Std 1188™ [B14] [B14] recommend recommend that a battery be replaced when the actual capacity drops to 80% of its rated capacity. Based on end-of-life capacity, a 125% aging factor is typically used. Step 3 Select the battery type and cell characteristic characteristics. s.
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IEEE Std 1818-2017 IEEE Guide for the Design of Low-Voltage Auxiliary Systems for Electric Power Substations
This example looks at several types of ooded batteries, including lead selenium and lead calcium, and selects the correct size for each type. Step 4 Determine the time span of the duty cycle. This example assumes a 12-hour duty cycle with the event starting the duty cycle being a failure of the battery charger. Thus, when the battery supplies the complete dc power for 12 hours, then the worst case tripping would occur. The The denition of worst case tripping in this case is a fault and sequence of events that would lead to the highest tripping current during the last minute of the 12-hour duty cycle. Step 5 Construct the minute-by-minute load prole (the duty cycle). One of the most variable components of battery sizing is dening the duty cycle or the load(s) over a dened time period that the battery may be required to supply dc power. It is not the intent of this example to dene the duty cycle for every battery application, but to provide guidance and discussion on some of the issues that should be taken into consideration. Some utilities may have a standard duty cycle dened for simplicity, or to provide consistency in the battery sizing applications. Considerations should include: duration of duty cycle, worst case tripping current (applied at beginning and/or end of duty cycle), continuous loading, and random loads. The duty cycle with 16.5 A continuous load and 63 A worst case tripping, is shown in Figure C.2. C.2.
Figure C.2—Duty cycle tripping
Calculating continuous current: New substation—A substation—Add dd all loads connected connected to battery battery that are are on for the 12-hour 12-hour duration duration of of the duty cycle. cycle. Existing substation expansion—Record the charger output current and oat voltage under oat charging. Multiply the current by a correction factor (ratio of end-of-life voltage to oat voltage) in order to accommodate the current at the lower end-of-life voltage. Then, add all new loads connected to the battery being installed on the expansion project.
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In manufacturer documentation for relays or similar devices, power supply burden/load is typically listed at a range or a higher value than what is observed or calculated from battery charger readings. The maximum current draw listed is generally assuming a maximum, or more signicant, amount of data processing or contact operation than what occurs during normal operation. During a fault or switching event, the load drawn by a relay or other device is generally higher than what is measured during normal operation. In lieu of determining load for every device in an existing station, the designer may consider adding a multiplier or safety factor to the current measured on the battery charger in order to account for increased “continuous” loads during fault or switching events. Additional load calculation items for new or substation modications: From the dc panel schematic, add up the continuous loads to calculate the continuous current. The total watts is divided by the end-of-life voltage (105 V in this example) to get the total calculated continuous current. Note that Table C.1 is C.1 is an example only and the designer should verify the manufacturer’ manufacturer’ss published data.
Table C.1—DC load table Equipment
Continuous load
Quantity
Watts
Total
Dierential relay
525 kV bus primary
2
20
40
Dierential relay
525 kV bus secondary
2
20
40
Distance relay Distance relay
525 kV line primary 525 kV line secondary
2 2
35 30
70 60
Communication
Line ber pilot
2
10
20
Communication
Line carrier pilot
2
27
54
Communication
Satellite clock
1
15
15
Control
Process automation controller
6
10
60
Dierential relay
Transformer primary
2
35
70
Dierential relay
Transformer secondary
2
13
26
Dierential relay
34.5 kV bus primary
2
13
26
Voltage dierential relay
Capacitor bank primary
2
17.5
35
Over current relay
Capacitor bank secondary
2
15
30
Breaker fail relay
Breaker fail
0
17.5
0
Over current relay
Feeder relay
12
25
300
M650 meter
Transformer meter
2
20
40
M650 meter
Feeder meter
12
15
180
Lights
Lights
40
3
120
Communication
Router
1
25
25
Tel protection
Tel protection
1
25
25
SCADA
Remote terminal unit/ human machine interface
1
325
325
Fault recorder
1
15
15
Communication switch
Ethernet switch
2
30
60
Miscellaneous
Miscellaneous
1
100
100
Total watts
Total watts
1736
Tot otal al ad addi diti tion onal al am ampe perres
Tot otal al ad addi diti tion onal al am ampe perres
16.5
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IEEE Std 1818-2017 IEEE Guide for the Design of Low-Voltage Auxiliary Systems for Electric Power Substations
For continuous current calculations, use the total watts divided by the end-of-life voltage. Continuous current =
1736 W 105 V
=
16.5 A ,
Review scenarios to determine determine worst case tripping—T tripping—Two wo scenarios scenarios:: Case 1 = fault on 34.5 kV bus 1 Sequence
Fault scenario
Devices actuated
Load ty type
Amperes
Amperes
Less than 5 cycles
Greater than 5 cycles
New additional load
Continuous
16.5
16.5
Existing continuous load
Continuous
0.0
0.0
42
1st
34.5 kV bus 1 fault
Trip breakers FB1— FB6 (6 breakers) Trip Cap bank
Then
Breaker failure on breaker BT1
7
Trip breaker BT1
7
Trip bus tie breaker TB1
7
LOR (BT1)
5
Trip breakers 5B2 and 5B3 TC2 (ABC)
42
Total
79.5
63.5
Case 2 = fault on TR1 Sequence
Fault scenario
Devices actuated
Load type
Amperes
Duration
Ampere loading Greater
Less than 5 cycles
than 5 cycles
New additional load
Continuous
16.5
16.5
Exiist Ex stin ing g co cont ntiinu nuou ouss loa oad d
Con onttinu nuou ouss
0.0
0.0
42
7
1st
TR1 fault
Trip breakers 5B2 and 5B3 TC1 and 2 (ABC) Trip breaker BT-1 TC1
Then
Restore 34.5 kV bus 1 via breaker TB1
Close TB1
3.5
Total
Step 6
65.5
20.0
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Calculate the required positive plates of the battery for each period in the duty cycle utilizing the cell-sizing worksheet—Figure worksheet—F igure 3 in IEEE Std 485. Since dierent types of batteries, and similar batteries by dierent manufacturers, have dierent plate designs they also have dierent discharge curves. Thus it is important to calculate this step separately for each dierent type of battery or manufacturer. You can calculate the amperes per positive plate for a particular battery from the vendor battery discharge curve or table of discharge rates for specic time and divide it by the number of positive plates. If the manufacturer data provides the total number of plates, the number of positive plates can be calculated by the following: ( total plates
1)
−
RT T
positive plates
discharge rate =
(C.1)
=
2
positive plates plates
(C.2)
where RT T
is the amperes per positive plate at time T Typical battery discharge rate in amperes to 1.75 VPC at 25 °C (77 °F)
Minutes Total plates plat es
Positive plates
1
720
7
3
330
24.0
11
5
550
39.0
The discharge rate above shows the calculations of RT of RT for for Vendor Vendor B at 1 minute and 720 7 20 minute rates. rates . Continue this for other manufacturers to ll out table of amperes per positive plate ( RT RT ). ). Minutes
Vendor A
Vendor B
Vendor C C
RT
RT
RT
1
63
110
47
720
7.3
8
4.8
Complete the battery cell–sizing worksheet from the vendor’s discharge curves that can be found in their literature (see Figure C.3). C.3).
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Source: Figure 3 from IEEE Std 485.
Figure C.3—Completing C.3—Completing the battery cell–sizing worksheet
Multiply the uncorrected values by the temperature correction factor, design margin, and aging factors to calculate the number of positive plates and the total number of plates. Then, round the calculated number of positive plates up to the next next whole whole number. number. Match the number of plates to the battery manufacturer’s size to obtain the 8-hour AH rating of the battery of each type. Manufacturer Manufactu rer A = 456 AH Manufacturer B = 440 AH Manufacturer C = no match for 6 V block type battery Compare the cost of Manufacturer A and Manufacturer B battery sizes to select the most economical battery that is properly sized for this application. Considerations for battery sizing: Determining the worst worst case tripping tripping current Per IEEE Std 485 battery sizing guidelines, the time increment of the duty cycle in battery sizing should be in one minute increments. Thus when determining the worst case scenario, the designer should look at the sequence of events that would occur in the last minute and select the one that sums up to the highest value. For example if there is a fault on transformer T1, the sequence of operations would be: trip breakers 5B2, 5B3, and BT1. After BT1 is tripped, it would be likely that there would be an auto-restoration function to restore the 34.5 kV bus via the bus tie breaker breaker.. This would likely occur in the same minute that the fault occurred and tripped the breakers on the bus. However, all of the original tripping would have occurred prior to the reclosing function. Therefore, the designer should look at the tripping load and the resulting restoration load and select the higher value for the worse case tripping during the last minute. A more likely scenario would be for a breaker-failure breaker-fai lure condition condition with motor operators operators rather rather than breakers. The motor motor operators operators would likely still still be operating when the breaker-fail function tripped more devices. In order to fully understand the sequence of events, it is important to review a relay and metering diagram that shows what devices would trip for various faults on the system. It is also important to include auxiliary relays, such as lock out relays if they are used. For example, a bus dierential relay may operate a lock out relay to trip all the equipment on the bus. The load
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of the lockout should be included in the calculation. The metering and relay diagram would also show all the devices that would be tripped, reclosing schemes, and breaker-fai breaker-faill schemes. Device load should be taken from the manufacturer’s nameplate data, for example breaker trip coil ratings. For motor operators, the locked rotor value should be used in order to accommodate the worst case scenario of operating a switch that may be iced up, or that the blade may be corroded and stuck in the jaws due to lack of frequent operation. Battery charging sizing From 5.5.2 5.5.2,, the following formula is used to determine the required dc output of the battery charger.
A e + I d I = C t
( )(k )
(C.3)
where I A t t e I C
is the the calculated calculated battery charger output, dc amperes amperes is the A-hours to be replaced is the the time time in which the battery should be recharged recharged is the recharge factor is the continuous dc load current
d d is the design margin factor k k is the altitude correction factor (charger (charger manufacturer data) From the duty cycle shown above:
1 A = 16.5 A×12h + 79.5 A× hours = 199.325 Ah removed 60 I C
=
16.5 A
This example assumes the battery recharge time = 8 h The recharge eciency factor = 1.1 The design margin factor = 1.1 The estimate assumes the altitude is below 3300 ft and k = = 1.0 (verify manufacturer data)
199.325 ×1.1 + 16.5×1.1×1 = 48.29 8
= I =
From manufacturer available sizes, select the closed size that is equal or greater than this value. Suggested battery ba ttery charger size siz e = 50 A charger. From the battery charger manufacturer data, verify the dc output breaker size to coordinate the cable size between the charger charger and the dc system. system. For a 50 A battery charger charger,, the the dc breaker size is 70 A.
The cable should be sized to 125% should be sized to accommodate 87.5of A.overload device. The cable to connect the charger to the dc system
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From cable capacity tables select a 4 AWG copper conductor for this application.
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