2015 - IEEE C57.32 - IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices.pdf

IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

IEEE Power and Energy Society

Sponsored by the Transformers Committee

IEEE 3 Park Avenue New York, NY 10016-5997 USA

IEEE Std C57.32™-2015 (Revision of IEEE Std 32™-1972)

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IEEE Std C57.32™-2015 (Revision of IEEE Std 32™-1972)

IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices Sponsor

Transformers Committee of the

IEEE Power and Energy Society Approved 5 December 2015

IEEE-SA Standards Board

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Abstract: This standard applies to devices used for the purpose of controlling the ground current or the potentials to ground of an alternating current system. These devices are: grounding transformers, ground-fault neutralizers, resistors, reactors, or combinations of these. Keywords: arc-suppression reactors, ground fault neutralizers, grounding transformers, IEEE C57.32™, neutral grounding devices, reactors, resistors Acknowledgements: The author thanks the International Electrotechnical Commission (IEC) for permission to reproduce information from its International Publication IEC 60076-6 ed.1.0 (2007). All such extracts are copyright of IEC, Geneva, Switzerland. All rights reserved. Further information on the IEC is available from www.iec.ch. IEC has no responsibility for the placement and context in which the extracts and contents are reproduced by the author, nor is IEC in any way responsible for the other content or accuracy therein. •

The Institute of Electrical and Electronics Engineers, Inc. 3 Park Avenue, New York, NY 10016-5997, USA Copyright © 2016 by The Institute of Electrical and Electronics Engineers, Inc. All rights reserved. Published 15 April 2016. Printed in the United States of America. IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by The Institute of Electrical and Electronics Engineers, Incorporated. PDF: Print:

ISBN 978-1-5044-0588-1 ISBN 978-1-5044-0589-8

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Participants At the time this IEEE standard was completed, the Performance Characteristics - Neutral Grounding Devices Working Group had the following membership: Sheldon P. Kennedy, Chair Thomas R. Melle, Vice Chair Fred Elliott, Secretary Donald Ayers Peter Balma J. Arturo Del Rio Richard Dudley David Harris

Dan Kobida Sergio Panetta Klaus Papp Klaus Pointner Ulf Radbrandt Dinesh Sankarakurup

Steven Schappell Devki Sharma Michael Sharp Sanjib Som Vijay Tendulkar

The following members of the individual balloting committee voted on this standard. Balloters may have voted for approval, disapproval, or abstention. William Ackerman Saleman Alibhay Donald Ayers Roy Ayers Peter Balma Thomas Barnes Paul Barnhart G. Bartok Wallace Binder Thomas Blackburn William Bloethe Gustavo Brunello Carl Bush William Bush William Byrd Paul Cardinal Michael Champagne Yunxiang Chen Robert Christman Stephen Conrad John Crouse Glenn Davis Mamadou Diong Gary Donner Fred Elliott Jorge Fernandez Daher Sergio Flores Marcel Fortin Dale Fredrickson Fredric Friend Frank Gerleve David Gilmer Jalal Gohari James Graham Randall Groves Ajit Gwal Said Hachichi J. Harlow Lee Herron

Werner Hoelzl Robert Hoerauf Jill Holmes Richard Jackson Song Jin John John Laszlo Kadar Sheldon Kennedy Yuri Khersonsky James Kinney Boris Kogan Jim Kulchisky Saumen Kundu Chung-Yiu Lam Benjamin Lanz Thomas Lundquist Reginaldo Maniego Richard Marek Lee Matthews Omar Mazzoni William McBride Charles Mc Shane Tom Melle Daleep Mohla Jerry Murphy Ryan Musgrove K. R. M. Nair Martin Navarro Kris K. Neild Arthur Neubauer Michael Newman Joe Nims T. W. Olsen Sergio Panetta Klaus Papp Bansi Patel Dhiru Patel Branimir Petosic Donald Platts

Klaus Pointner Alvaro Portillo Tom Prevost Ulf Radbrandt Reynaldo Ramos Robert Resuali Jean-Christophe Riboud Michael Roberts Charles Rogers Thomas Rozek Daniel Sabin Dinesh Sankarakurup Steven Sano Bartien Sayogo Devki Sharma Michael Sharp Hyeong Sim Jeremy Smith Jerry Smith Gary Smullin Wayne Stec Kyle Stechschulte K. Stump Paul Sullivan Michael Swearingen Ed TeNyenhuis David Tepen Michael Thompson James Van De Ligt John Vergis Sukhdev Walia David Walker John Wang Daniel Ward S. Frank Waterer Yingli Wen Kenneth White James Wilson Jian Yu

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When the IEEE-SA Standards Board approved this standard on 5 December 2015, it had the following membership: John Kulick, Chair Jon Walter Rosdahl, Vice Chair Richard H. Hulett, Past Chair Konstantinos Karachalios, Secretary Masayuki Ariyoshi Ted Burse Stephen Dukes Jean-Philippe Faure J. Travis Griffith Gary Hoffman Michael Janezic

Joseph L. Koepfinger* David J. Law Hung Ling Andrew Myles T. W. Olsen Glenn Parsons Ronald C. Petersen Annette D. Reilly

Stephen J. Shellhammer Adrian P. Stephens Yatin Trivedi Phillip Winston Don Wright Yu Yuan Daidi Zhong

*Member Emeritus

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Introduction This introduction is not part of IEEE Std C57.32-2015, IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices.

Historically this standard developed from AIEE Standard 32, 1947. The standard was substantially revised to become IEEE Std 32™ in 1972 and was reaffirmed in 1978, 1984, and 1990. This standard is a revision of IEEE Std 32-1972, to which many changes have been made. The Standard 32 was originally developed by the Neutral Grounding Subcommittee of the Surge Protective Devices Committee of the IEEE Power Engineering Society. In 2003, the responsibility for upkeep and maintenance of this standard was transferred to the Performance Characteristics Subcommittee of the Transformers Committee of the IEEE Power and Energy Society. This version is being published under a new number, C57.32, and has been completely revised to bring it in line with the current technology. Each grounding device now has its own section with all requirements and test methods included. Old test methods that were developed in the 1930s and 1940s have lost their relevance over time. Conservative calculation methods were lost in history and did not correlate to modern practices. To maintain a link to the old methods for now, Annex A is a copy of the old test code and is provided solely for reference purposes. Note that capacitors have been removed from this standard. They had traditionally been used on occasion for front of wave surges. They are rarely used today. The use of capacitors has evolved into blocking devices in the neutral of transformers to block Geomagnetically Induced Currents (GIC) from Geomagnetic Disturbances (GMD). The IEEE Capacitor Subcommittee of the Transmission and Distribution Committee of PES has a task force that is handling this topic.

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Contents 1. Overview .................................................................................................................................................... 1 1.1 Scope ................................................................................................................................................... 1 1.2 General considerations......................................................................................................................... 1 2. Normative references.................................................................................................................................. 2 3. Definitions .................................................................................................................................................. 3 4. Grounding reactors ..................................................................................................................................... 4 4.1 General description .............................................................................................................................. 4 4.2 Safety ................................................................................................................................................... 4 4.3 Clearances: electrical, ventilation, and magnetic ................................................................................. 5 4.4 Mechanical considerations................................................................................................................... 5 4.5 Concrete foundation and mounting...................................................................................................... 6 4.6 Service conditions................................................................................................................................ 6 4.7 Basis for rating..................................................................................................................................... 6 4.8 Insulation levels ................................................................................................................................... 9 4.9 Limiting temperature rises ................................................................................................................. 12 4.10 Routine, design, and other tests for neutral grounding reactors ....................................................... 13 4.11 Dielectric tests ................................................................................................................................. 14 4.12 Temperature rise test........................................................................................................................ 14 4.13 Impedance and losses test ................................................................................................................ 15 4.14 DC resistance test ............................................................................................................................ 15 4.15 Seismic verification ......................................................................................................................... 15 4.16 Short-circuit test .............................................................................................................................. 15 4.17 Nameplates ...................................................................................................................................... 16 5. Ground-fault neutralizers (arc-suppression reactors)................................................................................ 16 5.1 General .............................................................................................................................................. 16 5.2 Service conditions.............................................................................................................................. 17 5.3 Design ................................................................................................................................................ 17 5.4 Bushings, insulators, and insulating liquids ....................................................................................... 17 5.5 Nameplates ........................................................................................................................................ 17 5.6 Tanks and enclosures ......................................................................................................................... 18 5.7 Ratings ............................................................................................................................................... 18 5.8 Temperature limitations ..................................................................................................................... 20 5.9 Insulation levels ................................................................................................................................. 21 5.10 Temperature rise .............................................................................................................................. 21 5.11 Routine, design, and other tests for ground-fault neutralizers ......................................................... 22 5.12 Dielectric tests ................................................................................................................................. 22 5.13 Measurement of current at rated voltage ......................................................................................... 23 5.14 Measurement of no-load voltage of the auxiliary and secondary windings ..................................... 24 5.15 Temperature rise test........................................................................................................................ 24 5.16 Measurement of loss ........................................................................................................................ 24 5.17 Measurement of linearity ................................................................................................................. 24 5.18 Measurement of acoustic sound level .............................................................................................. 25 5.19 Endurance tests of the inductance regulation mechanism................................................................ 25 6. Grounding transformers............................................................................................................................ 25 6.1 General description ............................................................................................................................ 25 6.2 Service conditions.............................................................................................................................. 25

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6.3 Unusual service conditions ................................................................................................................ 26 6.4 Basis for rating................................................................................................................................... 28 6.5 Insulation classes and dielectric withstand levels .............................................................................. 31 6.6 Temperature limitations ..................................................................................................................... 32 6.7 Tests................................................................................................................................................... 35 6.8 Construction ...................................................................................................................................... 39 7. Grounding resistors .................................................................................................................................. 40 7.1 Resistor element ................................................................................................................................ 40 7.2 Rated voltage ..................................................................................................................................... 40 7.3 Temperature coefficient of resistance ................................................................................................ 41 7.4 Mechanical considerations................................................................................................................. 41 7.5 Insulation levels ................................................................................................................................. 41 7.6 Routine, design, and other tests for neutral grounding resistors ........................................................ 41 7.7 Temperature rise test ......................................................................................................................... 42 7.8 Resistance test.................................................................................................................................... 43 7.9 Dielectric tests ................................................................................................................................... 43 8. Combination devices ................................................................................................................................ 46 8.1 Insulation levels ................................................................................................................................. 46 8.2 Routine, design and other tests for combination grounding devices.................................................. 46 8.3 Dielectric tests ................................................................................................................................... 46 Annex A (informative) Test code ................................................................................................................. 48 A.1 Resistance measurements ................................................................................................................. 48 A.2 Dielectric tests .................................................................................................................................. 50 A.3 Impedance and loss measurements ................................................................................................... 55 A.4 Temperature-rise tests ....................................................................................................................... 57 A.5 Temperature-rise calculations ........................................................................................................... 65 Annex B (informative) Bibliography............................................................................................................ 69

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IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices IMPORTANT NOTICE: IEEE Standards documents are not intended to ensure safety, security, health, or environmental protection, or ensure against interference with or from other devices or networks. Implementers of IEEE Standards documents are responsible for determining and complying with all appropriate safety, security, environmental, health, and interference protection practices and all applicable laws and regulations. This IEEE document is made available for use subject to important notices and legal disclaimers. These notices and disclaimers appear in all publications containing this document and may be found under the heading “Important Notice” or “Important Notices and Disclaimers Concerning IEEE Documents.” They can also be obtained on request from IEEE or viewed at http://standards.ieee.org/IPR/disclaimers.html.

1. Overview 1.1 Scope This standard applies to devices used for the purpose of controlling the ground current or the potentials to ground of an alternating current system. These devices are: grounding transformers, ground-fault neutralizers, resistors, reactors, or combinations of these devices.

1.2 General considerations The voltage, current, and insulation ratings for such devices shall consider their operation during normal and faulted system conditions. See also the IEEE C62.92™ series of guides [B29], [B30], [B31], [B32], and [B33] for additional information. 1,2,3

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IEEE publications are available from the Institute of Electrical and Electronics Engineers (http://standards.ieee.org/). The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc. 3 The numbers in brackets correspond to those of the bibliography in Annex B. 2

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

2. Normative references The following referenced documents are indispensable for the application of this document (i.e., they must be understood and used, so each referenced document is cited in text and its relationship to 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. Accredited Standards Committee C2-2012, National Electrical Safety Code® (NESC®). 4 ANSI/NEMA C29.8, Wet-Process Porcelain Insulators—Apparatus, Cap and Pin Type. 5 ANSI/NEMA C29.9, Wet Process Porcelain Insulators—Apparatus, Post Type. ANSI/NEMA C29.10, Wet Process Porcelain Insulators—Indoor Apparatus Type. ASTM D1816, Standard Test Method for Dielectric Breakdown Voltage of Insulating Liquids Using VDE Electrodes. 6 IEC 60076-5, Power Transformers—Part 5: Ability to withstand short-circuit, Edition 3.0, 2006-02. 7 IEC 60076-6, Power Transformers—Part 6: Reactors, Edition 1.0, 2007-12. IEEE Std 1™, IEEE Recommended Practice-General Principles for Temperature Limits in the Rating of Electrical Equipment and for the Evaluation of Electrical Insulation. IEEE Std 693™, IEEE Recommended Practices for Seismic Design of Substations. IEEE Std C37.30.1™, IEEE Standard Requirements for AC High-Voltage Air Switches Rated Above 1000 V. IEEE Std C57.12.00™, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std C57.12.90™, IEEE Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers. IEEE Std C57.12.91™, IEEE Standard Test Code for Dry-type Distribution and Power Transformers. IEEE Std C57.16™, IEEE Standard for Requirements, Terminology, and Test Code for Dry-Type Air-Core Series-Connected Reactors. IEEE Std C57.19.00™, IEEE Standard General Requirements and Test Procedures for Apparatus Bushings. IEEE Std C57.19.01™, IEEE Standard Performance Characteristics and Dimensions for Outdoor Apparatus Bushings. IEEE Std C57.91™, IEEE Guide for Loading Mineral Oil-Immersed Transformers and Step-Voltage Regulators.

4 The NESC is available from the Institute of Electrical and Electronics Engineers, Inc., 445 Hoes Lane, Piscataway, NJ 08854, USA (http://standards.ieee.org/). 5 ANSI/NEMA publications are available from the National Electrical Manufacturers Association (http://www.nema.org). 6 ASTM publications are available from the American Society for Testing and Materials (http://www.astm.org). 7 IEC publications are available from the International Electrotechnical Commission (http://www.iec.ch). IEC publications are also available in the United States from the American National Standards Institute (http://www.ansi.org).

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

IEEE Std C57.96™, IEEE Guide for Loading Dry-Type Distribution and Power Transformers. IEEE Std C57.106™, IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment. IEEE Std C57.110™, IEEE Recommended Practice for Establishing Liquid-Filled and Dry-Type Power and Distribution Transformer Capability When Supplying Nonsinusoidal Load Currents. IEEE Std C57.111™, IEEE Guide for Acceptance of Silicone Insulating Fluid and Its Maintenance in Transformers. IEEE Std C57.121™, IEEE Guide for Acceptance and Maintenance of Less Flammable Hydrocarbon Fluid in Transformers. IEEE Std C57.147™, IEEE Guide for Acceptance and Maintenance of Natural Ester Fluids in Transformers.

3. Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 8 acceptance test: A test to demonstrate the degree of compliance of a device with purchaser’s requirements. extended-time rating (of a grounding device): A rated time in which the period of time is greater than the time required for the temperature rise to become constant but is limited to a specified average number of days operation per year. ground current: The current flowing in the earth or in a grounding connection. ground end (of a neutral grounding device): The end or terminal that is grounded directly or through another device. ground-fault neutralizer: An electromagnetic device grounded through such values of reactance that, during a fault between one of the phases of the power system and earth, the rated-frequency current flowing in the grounding reactances and the rated-frequency capacitance current moving between the unfaulted phases and earth are substantially equal. Syn: arc-suppression reactor. grounded circuit: A circuit in which one conductor or point (usually the neutral conductor or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through a grounding device. grounding transformer: A transformer intended primarily to provide a neutral point for grounding purposes. Syn: neutral grounding transformer. insulating materials, thermal classification: For the purpose of establishing temperature limits, insulating materials should be classified as described in the latest revision of IEEE Std C57.12.80™. NOTE—For information on IEEE Std C57.12.80 [B26], please see Annex B.

iron-core: Having a magnetic core.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

losses (of a grounding device): The I2R loss in the windings, core loss (where applicable), dielectric loss, loss due to stray magnetic fluxes in windings and other metallic parts of the device, and, in cases involving parallel windings, losses due to circulating currents. The losses as here defined do not include any losses produced by the device in adjacent apparatus or materials not a part of the device. Losses will normally be considered at the rated continuous current, but in some cases may be required at other current ratings (if more than one rating is specified) or at no load, as for grounding transformers. neutral grounding wave trap: A neutral grounding device comprising a combination of inductance and capacitance designed to offer a very high impedance to a specified frequency or frequencies. The inductances used in neutral grounding wave traps should meet the same requirements as neutral grounding reactors. outdoor: Equipment with weatherproof construction suitable for operation in specified exposed environment. reactor (neutral grounding): A single-phase reactor for connection in the neutral for the purpose of limiting and neutralizing disturbances due to ground faults. resonant grounded: A resonant grounded system is one in which the capacitance current is tuned or neutralized by a neutral reactor or similar device. short-time rating (of a grounding device): A rated time of ten minutes or less.

4. Grounding reactors 4.1 General description All parts of dry-type air-core neutral grounding reactors are “live,” unlike liquid-immersed reactors and transformers where the tank is grounded. The only external “live” parts of a liquid-immersed reactor or transformer are the bushings. Dry-type air-core neutral grounding reactors do not have an iron core. Therefore, the magnetic field is not constrained and will occupy the space around the dry-type air-core reactor. Although the magnetic field reduces in strength with increase in distance from the reactor, the presence of this field should be taken into consideration for the installation of dry-type air-core units. The extent to which care has to be taken is largely a function of the continuous kVA rating and is lower for low kVA units. Since neutral grounding reactors usually have small continuous current ratings, magnetic clearances are normally smaller than electrical clearances. In 4.2, 4.3, 4.4, and 4.5, applications related construction details and installation considerations will be discussed in order to provide guidance to the purchaser of dry-type air-core neutral grounding reactors.

4.2 Safety Since dry-type air-core neutral grounding reactors are not enclosed in a grounded tank, parts above the base support insulators must be treated as “live.” Therefore, in order to help meet personnel safety requirements, the reactor shall be installed with clearances established by National Electrical Safety Code® (NESC®) (Accredited Standards Committee C2-2012). 9 For transmission voltage class and distribution voltage class 9

Information on normative references can be found in Clause 2.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

equipment, standard methods of improving personnel clearance are the use of fencing and special support structures. In many cases, both methods are employed simultaneously. Distribution voltage class neutral grounding reactors are also installed in cells fabricated from standard building materials or vendor supplied enclosures.

4.3 Clearances: electrical, ventilation, and magnetic The clearance requirements are of three types: electrical, ventilation, and magnetic. As the normal dry-type air-core neutral grounding reactor has exposed live parts at essentially all points on its outer surface, provision must be made for electrical clearance from the reactor surface to nearby grounded surfaces and to other adjacent energized conductors. Standard electrical clearances to live parts are adequate. No special precautions over and above normal substation practices are required. Ventilation clearance requirements may vary somewhat depending on the reactor manufacturer. For instance, in the case of reactors employing vertical cooling ducts, provision must be made for the unimpeded entrance and exit of cooling air at the bottom and top of the cooling ducts, respectively. In some cases baffles are required for establishing sufficient cooling convection through the reactor ducts during conditions of consistent crosswinds. Generally, the ventilation clearance will be less than the magnetic clearance requirements and, as such, it will not prove to be a limiting factor when installing the unit. Irrespective of this fact, it is still important to recognize that there is a requirement for sufficient ventilation and the blockage of the cooling ducts with any foreign material, metallic or non-metallic, or interference with normal convective airflow must be avoided. Magnetic clearance requirements arise since dry-type air-core reactors have no iron core to capture the magnetic field. However, since continuous kVA ratings and hence continuous stray magnetic fields of neutral grounding reactors are relatively small, electrical clearances are usually greater than magnetic clearances. Nevertheless, during conditions when reactors carry thermal and mechanical peak currents, induced forces in large metallic objects or closed electrical loops near the reactors should be evaluated. Information regarding recommended magnetic clearances can usually be obtained from the equipment manufacturer and is often supplied on reactor outline drawings. Neutral grounding reactors may also be mounted directly on transformer tanks. In this case, possible elevated ambient temperatures and the magnetic clearance required to the transformer tank need to be taken into consideration in the reactor design. Therefore, it is important that the reactor specification includes details regarding the reactor mounting site on the transformer tank, tank dimensions, and expected ambient temperature range at the location where the reactor is to be installed.

4.4 Mechanical considerations Depending on the scope of supply, special attention should be given, by either the manufacturer or purchaser, to the interactive electromagnetic forces between the reactor and current carrying auxiliary parts (such as buswork) and between the reactor and bus or cable connections to the reactor. Support structure and bracing elements must be designed to resist the resultant loads. As standard practice, unless otherwise specified, peak wind loads and seismic loads or other specified loading are typically assumed to act on a non-coincidental basis. Reactors should be designed to facilitate lifting and should be provided with lifting eyes or similar devices to allow safe and rapid installation. Lifting and handling instructions should be clear and readily available.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

4.5 Concrete foundation and mounting Foundation and mounting requirements are a function of the rating (hence the mass of the reactor) and the dimensions of the supporting structure. To design the foundation, the design engineer will need the anchor bolt layout and an overall outline showing device dimensions and the centers of gravity. The foundation must consider wind, ice, and seismic loading along with the specific soil conditions. Most neutral grounding reactors can be mounted with no special precautions other than clearance considerations.

4.6 Service conditions Service conditions shall be in accordance with 4.1 of IEEE Std C57.16-2011.

4.7 Basis for rating 4.7.1 Conditions Ratings for neutral grounding reactors are based on standard operating conditions and shall include the following: a)

Service (indoor or outdoor)

b)

Current

c)

Voltage

d)

Frequency

e)

Time

f)

Basic impulse insulation level (BIL) and insulation class

g)

Circuit voltage of system

4.7.2 Rated current 4.7.2.1 Rated thermal current Unless otherwise specified, the basis for this rating shall be the thermal current. This will be the current through the neutral grounding reactor during a worst-case ground-fault condition. Implicit in the thermal current rating is an associated continuous current that, unless otherwise specified, shall bear the following relationship to the thermal current rating as shown in Table 1.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Table 1—Continuous duty in percent of thermal current rating for neutral grounding reactors Rated time

Continuous duty in percent of thermal current rating (%)

10 s

3

1 min

7

10 min

30

Extended time

30

Where there is a third harmonic component of current, it shall not exceed 15% of the rated continuous duty current (see IEEE Std C57.110). 4.7.2.2 Short-circuit current—asymmetrical Neutral grounding reactors shall be able to withstand, without mechanical failure, forces associated with the asymmetrical peak current. This peak current shall be determined from Equation (1):

I C = KIT

(1)

where Ic K IT

is the crest of the initial offset current, peak asymmetrical is the appropriate multiplier obtained from Table 2 or calculated with Equation (2) is the thermal current rating, rms symmetrical

 π r   − φ +    K = 1 + e  2  x sin φ  2  

(2)

where ϕ e x/r

is the arc tan (x/r), (radians) is the base of natural logarithm is the ratio of ohms reactance to ohms resistance in the system when the short-circuit occurs

When sequence impedances for the system are not specified, the ratio x/r shall be taken to be the ratio of ohms reactance to ohms resistance in the winding of the neutral grounding reactor through which the current flows. When the system to which the neutral grounding reactor is connected has an x/r ratio at that point which is greater than 10, the ohmic values of sequence impedances should be specified. When specified, the manufacturer shall combine them with the X and R of the neutral grounding reactor to determine the value of K to be used in calculating Ic. When neutral grounding reactors are located on or near generator or motor buses, sub-transient impedances shall be utilized in calculating system x/r ratios.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

K can be calculated using Equation (2); however, it is important to note that the expression for K is an approximation. The tabulated values for K given in Table 2 are calculated from this approximation and are accurate to within 0.7% of the values calculated by exact methods. Table 2—Values of K for Equation (2) for neutral grounding reactors r/x

x/r

K

0.001 0.002

1000.00 500.00

2.824 2.820

0.003

333.00

2.815

0.004

250.00

2.811

0.005

200.00

2.806

0.006

167.00

2.802

0.007

143.00

2.798

0.008

125.00

2.793

0.009

111.00

2.789

0.010

100.00

2.785

0.020

50.00

2.743

0.030

33.30

2.702

0.040

25.00

2.662

0.050

20.00

2.624

0.060

16.70

2.588

0.070

14.30

2.552

0.080

12.50

2.518

0.090

11.10

2.484

0.100

10.00

2.452

0.200

5.00

2.184

0.300

3.33

1.990

0.400

2.50

1.849

0.500

2.00

1.746

0.600

1.67

1.669

0.700

1.43

1.611

0.800

1.25

1.568

0.900

1.11

1.534

1.000

1.00

1.509

4.7.2.3 Maximum mechanical stress of neutral grounding reactors for short-circuit conditions For determination of maximum mechanical stresses, the initial thermal current shall be assumed to be offset. In a system with zero damping the maximum crest value of the short-circuit current is two times the crest value of the rms symmetrical thermal current. However, in reality the value is lower due to system damping effects. The first cycle asymmetrical peak current that the reactor is required to withstand shall be specified by the purchaser or based on knowledge of system damping, and can be determined as described in 4.7.2.2. If not 8

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

specified by the purchaser, the maximum asymmetrical crest value of short-circuit current should be considered to be 2.55 times the rms symmetrical current. The 2.55 value for the asymmetry factor K is based on a dc offset of 1.8 versus the theoretical maximum value of 2.0, i.e., K = 1.8 × 2 = 2.55 . 4.7.3 Rated voltage The rated voltage shall be taken as the product of the rated thermal current and the impedance of the neutral grounding reactor at rated frequency and at 30 °C. A neutral grounding reactor consisting of tapped sections or sections connected in series may have a rated voltage for each section determined from the impedance and the rated thermal current of the section as above. 4.7.4 Rated frequency The rated frequency shall be the fundamental power frequency. 4.7.5 Rated time Rated time for the duration of the rated thermal current shall be 10 seconds, 1 minute, 10 minutes, extended time, or another duration as specified. Extended-time operation shall not exceed an average of 90 days per year.

4.8 Insulation levels 4.8.1 Insulation classes All apparatus covered by this standard shall be assigned basic impulse insulation levels (BIL) and/or insulation class designations.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Table 3—Insulation classes for neutral grounding reactors Insulation class System insulation classa (kV) Class Column 1 1.2 2.5 5.0 8.7 15.0 23.0 34.5 46.0 69.0 115.0 138.0 161.0 230.0

Fault voltage criteriab kV Column 2 1.2 2.5 5.0 8.7 8.7 15.0 25.0 34.5 46.0 69.0 92.0 92.0 138.0

kV Column 3 1.2 2.5 5.0 8.7 8.7 8.7 8.7 15.0 15.0 15.0 15.0 15.0 15.0

NOTE 1—Once the insulation class value has been determined, the same value of rated system voltage can be identified in Table 4. From this table the appropriate dielectric test voltages can be chosen. 10 NOTE 2—For system voltages above 230 kV, fault voltage criteria have not been established for determining appropriate reduced insulation classes. a Where the insulation class for the terminal of a reactor is specified to be the system insulation class, the nominal system insulation level shall apply except that reduced BIL may be used where appropriate. b

When the fault voltage criterion applies, the maximum rms voltage that may exist between the terminal and ground, under fault conditions, is determined. If this fault voltage is less than or equal to the value in Column 2 but is greater than the value in Column 3 corresponding to the system insulation class in Column 1, the system insulation class at the terminal in question shall be the value in Column 2. If the fault voltage is less than or equal to the value in Column 3, the system insulation class at the terminal in question shall be the value in Column 3. If the fault voltage exceeds the value in Column 2 the system insulation class at the terminal in question shall be the value in Column 1.

The line end and ground end insulation levels shall be selected from Table 3 on the basis of fault voltage criteria, Column 2 and Column 3. Insulation levels for dry-type air core neutral grounding reactors are provided in Table 4. The nominal system voltage values in Table 4 are used as reference numbers and do not imply relation to operating voltages. For system voltages greater than 34.5 kV, the turn to turn test is not applicable and a full wave impulse test is to be performed as a routine test. In the case of neutral grounding reactors the BIL across the coil may be different than the BIL across the support insulators (to ground). The purchaser shall specify if the BIL across the coil or support insulators is to be at a reduced level. Such a decision should be based on factors such as knowledge of the system characteristics and protection practices. Reduced BIL levels shall be selected from the standard values in Table 4.

10

Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement this standard.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Table 4—Insulation test levels for neutral grounding reactors Column 1 Rated system voltage (kV)a 1.2 2.5 5.0 8.7 15 25 34.5 46 69 115 138

161

230 345 500 735 765

Column 2 BIL and full wave testb,g (kV peak) 30 45 45 60 60 75 75 95 95 110 150 200 200 250 250 350 350 450 550 450 550 650 550 650 750 825 750 825 900 1050 1175 1425 1550 1675 1950 2050

Column 3 Chopped – wave test c (kV peak) 33 50 50 66 66 83 83 105 105 120 165 220 220 275 275 385 385 495 605 495 605 715 605 715 825 900 825 900 990 1155 1290 1570 1705 1845 2145 2255

Column 4 Time to chopping (µs) 1.0 1.5 1.5 1.5 1.5 1.5 1.5 1.8 1.8 2.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

Column 5 Switching impulse testd (kV peak) — — — — — — — — — — — — — — — — 291 375 450 375 460 540 460 540 620 685 620 685 745 870 975 1180 1290 1390 1550 1700

Column 6 Turn-to-turn test voltagee (kV peak) 25 38 38 51 51 64 64 81 81 94 128 170 — — — — — — — — — — — — — — — — — — — — — — — —

Column 7 Power frequency withstand testf (kV rms) 10 10 15 15 19 19 26 26 34 34 50 70 95 95 140 140 173 173 173 207 207 207 242 242 242 242 345 345 345 518 518 750 750 750 830 880

a A reduced insulation level may be applied across reactor terminals if the reactor is adequately protected by a surge arrester. This could be an economical engineering solution for some neutral grounding reactors. For such cases, the insulation level across the reactor shall be one of the above standardized levels and shall be at least 1.25 times the 8 × 20 µs, 10 kA protective level of the surge arresters connected between terminals. b c

Alternate BIL test values are provided for each value of rated system voltage to reflect current practices by various users.

Chopped-wave test level is defined as 1.1 × BIL test level; including the appropriate “round off.”

d

Switching impulse test is applicable to the support insulator only and are defined for nominal system voltages 230 kV and higher.

e

Turn-to-turn test levels are defined for BIL equal to or less than 200 kV. Turn-to-turn test levels are defined to be approximately 90% of the rated BIL across reactor terminals. For BIL voltages greater than 200 kV, the turn-to-turn test is not applicable and a full-wave impulse test is to be performed as a routine test. Although the WG considered extending the turn-to-turn test up to BIL equal to or less than 550 kV, it was decided not to include test values in Table 4 as fully commercialized test equipment was not readily available or fully proven. However, the option remains to employ a turn-to-turn test at higher BILs based on a crest voltage of approximately 85% of BIL. Performance of the turn-to-turn test in lieu of the impulse test would be based on the availability of test equipment and agreement between the manufacturer and the purchaser. f

Power frequency withstand test is applicable to the support insulator only.

g

For reactors installed indoors the purchaser may specify a reduced BIL.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

4.8.2 Impulse insulation levels The standard impulse insulation levels for neutral grounding reactors are given in Table 4, Column 2. These values are expressed in terms of the crest value of a 1.2 × 50 microsecond full wave.

4.9 Limiting temperature rises 4.9.1 Temperature limitations at rated continuous current The inherent continuous current capability for a given rated thermal current is given in 4.7.2.1, Table 1. Table 5 provides average and hottest-spot temperature rises for various insulation systems for the designed continuous current rating of a neutral grounding reactor. Table 5—Limits of temperature rise at continuous rated current for neutral grounding reactors Item

Insulation materials, thermal classificationb, c (°C)

Average winding temperature rise by resistancea, d, e (°C)

1

105

55

85

130

80

110

155

100

135

180

115

160

220

140

200

a

Hottest-point winding temperature risea, d (°C)

2

Metallic parts in contact with or adjacent to the insulation shall not attain a temperature in excess of that allowed for the hottest spot of the windings adjacent to that insulation.

3

Metallic parts other than those covered in Item 2 shall not attain excessive temperature rises.

Temperature rise is rise above ambient in degrees Celsius.

b

A reactor with a specified temperature rise shall have an insulation temperature index that has been proven by experience or testing. c The insulation temperature index in Table 5 is supplied as a reference and is based on the preferred temperature index for insulation materials as defined in IEEE Std 1-2000. It should be noted that the preferred temperature index is the lowest value in a number range (of temperatures) into which insulation materials can be placed. This results in an element of conservatism in the thermal performance of an insulation system and thus constitutes part of the experience factors in assigning temperature rise limits. d The above average temperature rises and hot spot temperature rises are maximum upper limits. Hottest-spot winding temperature rise limits are based on the accepted experience that neutral grounding reactors are subject to significant variation in duty. The reference ambient temperature is considered to be a 20 °C annual average ambient temperature in conjunction with the capability to operate in a 40 °C maximum ambient when the 24 h average is 30 °C. Specified temperature limits may be lower due to such service considerations as high average ambient temperature conditions, prolonged exposure to high ambient temperatures, indoor vs. outdoor service, high continuous duty (limited “load cycling”), and loading profile (specified overloads). e

The average winding temperature rise limits are based on the attempt to achieve an approximate 40% differential between hot spot rise and average rise. However, it should not be construed that the difference between maximum hot spot rise and average rise is a hot spot allowance. There are so many design variables involved that it is not possible to arrive at a meaningful single value. Nevertheless, it should be stressed that the intent is that neither the average winding rise nor the hottest spot winding rise should be exceeded. Since it is generally hot spot temperature rise that determines the life of a reactor, the manufacturer and purchaser may agree that the average temperature rise can be safely exceeded for some reactor designs provided the hot spot rise limit is respected and verified.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

4.9.2 Temperature limitations under short-circuit conditions The temperature, as calculated by methods given in IEEE Std C57.16-2011, 11.7, of the conductor material in the windings of reactors under the short-circuit conditions specified in 4.7.2 shall not exceed the limiting values given in Table 6. Table 6—Limiting temperature for rated short-time current for neutral grounding reactors Insulation temperature index (°C)

Actual limiting temperature (°C)

105 130 155 180 220

205 285 315 350 400

The limiting temperatures in Table 6 are upper bounds based on the known capabilities of available materials. The actual limit used on design must be modified based on the winding insulation and encapsulation materials utilized.

4.10 Routine, design, and other tests for neutral grounding reactors Routine, design, and other tests for neutral grounding reactors are listed in Table 7. General test information is provided in 4.10 to 4.16. Further details regarding reactor testing may be obtained in IEEE Std C57.16-2011. Table 7—Routine, design, and other tests for neutral grounding reactors Test classification

Tests:

Routine

Resistance measurement: The dc resistance measurement shall be made on the full winding.

X

Impedance measurement: The impedance measurement shall be made on the full winding.

X

Total loss measurement: Total losses to be measured on all units.

X

Temperature rise test: This test is performed on one unit out of a number of units of the same design.

Other

X

Power frequency withstand test: The power frequency withstand test shall be made only on support insulators when specified. Turn-to-turn test: This test is performed for nominal system voltages of 34.5 kV and below.

Design

X X

Lightning impulse test: —

Nominal system voltage greater than 34.5 kV.

—

X

Nominal system voltage at or below 34.5 kV only when specified.

X

Chopped wave impulse test: The chopped wave impulse test shall be made only when specified.

X

Seismic verification test: Test performed only when specified.

X

Short circuit test: Test performed only when specified.

X

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

4.11 Dielectric tests Dielectric tests apply a higher than rated voltage for a specified time to determine the adequacy of insulating materials and spacing with respect to the specified insulation levels. 4.11.1 Turn-to-turn overvoltage test The turn-to-turn test for dry-type air-core neutral grounding reactors shall be made by applying between the terminals of each winding a series of high frequency, exponentially decaying sinusoidal voltages. The test sequence shall consist of a reduced sinusoidal voltage discharge followed by 7200 sinusoidal voltage discharges with a first peak voltage at least equal to the values specified in Column 6 of Table 4. The duration of the test is not to exceed 60 s. Further details regarding the turn to turn overvoltage test are provided in 11.3.5 of IEEE Std C57.16-2011. 4.11.2 Impulse tests For dry-type air-core neutral grounding reactors a lightning impulse test shall be made on each terminal of the reactor by applying a reduced wave and three full waves, all of positive polarity with crest voltages as specified in Table 4, Column 2. A chopped wave impulse test shall be made on each terminal of the reactor, when specified, by applying one reduced full wave, one full wave, two chopped waves, followed by two full waves (preferably within ten minutes after the last chopped wave), with crest voltage as shown in Table 4, Column 3 and time to chopping as shown in Table 4, Column 4. A reduced BIL may be specified to be applied at the reactor ground terminal. Further details regarding the impulse test are provided in 11.3.6 of IEEE Std C57.16-2011. 4.11.3 Power frequency withstand voltage test For dry-type air-core neutral grounding reactors, a power frequency withstand test shall be made, when specified, on the reactor’s supporting structure, including insulators. Test values are those indicated in Column 7 of Table 4. This test shall be made at the test voltage corresponding to the insulation class assigned to the ground end. Further details regarding the power frequency withstand voltage test (applied-voltage test) are provided in 11.3.3 of IEEE Std C57.16-2011.

4.12 Temperature rise test This test shall be made by passing continuous current through the reactor until the temperature rise becomes constant, and then determining the hottest-spot temperature rise, where practicable, together with the average winding temperature rise. The temperature rise tests shall be made in a room that is essentially free from drafts. When the available test power does not permit making the test at rated current, then the manufacturer shall demonstrate to the purchaser that reduced current testing produces sufficiently accurate results when

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

extrapolated to rated current level. The manufacturer shall notify the purchaser of reduced current level testing during the proposal stage. For higher current rated units, knowledge of in-service terminal temperature rise may be of importance. In this case, consideration should be given to performing the temperature rise test with the actual connectors to be used in service, especially if they differ from those recommended by the manufacturer. If the manufacturer does not supply the connectors, the purchaser should supply them. It is important to make every attempt to ensure conditions for such a test truly reflect in-service operating conditions or erroneous results may be obtained. Test conditions shall be agreed to by manufacturer and purchaser. The hottest-spot temperature rise for dry-type series reactors shall be determined by thermocouple or fiber optic sensor. A thermometer may be utilized where the use of thermocouples pose a hazard due to high voltage. Further details regarding temperature rise testing are provided in 11.5 of IEEE Std C57.16-2011.

4.13 Impedance and losses test Since dry-type air-core reactors have no iron core, the impedance and losses may be measured at any value of current and the losses corrected to rated current and reference temperature. If a wattmeter is used to determine losses then impedance can be calculated by measuring simultaneous current and voltage and dividing the voltage by the current. Bridge methods allow the simultaneous measurement of reactance (inductance) and effective resistance, from which impedance and losses can be calculated. Further details regarding impedance and loss measurements are provided in 11.4 of IEEE Std C57.16-2011.

4.14 DC resistance test DC resistance measurements are required for the calculation of conductor loss and for calculation of the winding temperature. Various methods of measurement are available but bridge methods are generally preferred due to increased accuracy. Further details regarding dc resistance measurements are provided in 11.2 of IEEE Std C57.16-2011.

4.15 Seismic verification When specified, a seismic performance verification shall be carried out using analytical methods, by testing under simulated seismic conditions, or by combined test and analysis such as described in IEEE Std 6932005. Although not directly applicable to electrical substation equipment, building codes are also at times referenced regarding the seismic performance of reactors.

4.16 Short-circuit test The test for the mechanical strength capability of neutral grounding reactor shall be made at a specified test current for a duration of not less than 10 alternating-current cycles at rated frequency with the first maximum crest value of the completely offset short-circuit not less than the maximum crest value specified by the purchaser. 15

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Full details regarding short-circuit testing and calculation methods is provided in 11.6 and 11.7 of IEEE Std C57.16-2011.

4.17 Nameplates A durable metal nameplate shall be affixed to each neutral grounding reactor. Unless otherwise specified, it shall be made of corrosion resistant material. It shall, as a minimum, include the following information: a) b) c) d) e) f)

g) h) i) j) k) l) m) n) o)

Name of manufacturer Serial number Name of device Type designation Impedance Rated currents 1) Short-time neutral current 2) Mechanical peak (crest of initial offset) current 3) Continuous neutral current (where applicable) Rated frequency Rated time Rated voltage BIL Indoor or outdoor service Mass Year of manufacture Number and year of this standard Maximum altitude (meters or feet above sea level) NOTE—This is the maximum altitude the device can operate at without derating or changes to any of its nameplate characteristics.

p) q)

System (or insulation) voltage rating Type of cooling

5. Ground-fault neutralizers (arc-suppression reactors) 11 5.1 General In this standard, the terms ground fault neutralizers and arc-suppression reactors are used interchangeably in describing these devices. Ground-fault neutralizers are single-phase reactors used to compensate for the capacitive current that flows during line-to-ground faults in a power system. They are connected between the neutral of a power transformer or a grounding transformer and ground in a three-phase power system. A system employing ground-fault neutralizers is also known as resonant grounding system. Such system can continue operation with a ground fault. Together with suitable electronic devices earth fault detection is possible. Additional information to be used in conjunction with this clause can be found in IEC 60076-6.

11

Clause 5 with permission from IEC. IEC 60076-6 ed 1.0 “Copyright © 2007 IEC Geneva, Switzerland. www.iec.ch.”

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

5.2 Service conditions Service conditions shall be in accordance with IEEE Std C57.12.00™-2010.

5.3 Design Ground-fault neutralizers are usually liquid-immersed natural cooled (ONAN), for indoor or outdoor installation. NOTE—The ground-fault neutralizer and the associated grounding transformer may be incorporated in a common tank.

Ground-fault neutralizers usually have adjustable inductance, either in steps or continuously, over a specified range to permit tuning with the network capacitance. A ground-fault neutralizer shall be a linear reactor, see 5.7.9. Ground-fault neutralizers may be provided with an auxiliary winding for measuring purposes and/or a secondary winding for connection of a loading resistor.

5.4 Bushings, insulators, and insulating liquids Bushings shall comply with IEEE Std C57.19.00 and IEEE Std C57.19.01 when specified. The dielectric strength of the insulating liquid when shipped shall meet the requirements for new condition as described in the appropriate IEEE standard or guide: 

IEEE Std C57.106 for mineral oil



IEEE Std C57.111 for silicone insulating liquid



IEEE Std C57.121 for less flammable hydrocarbon fluid



IEEE Std C57.147 for natural ester insulating fluid

5.5 Nameplates A durable metal nameplate shall be affixed to each neutral grounding device. Unless otherwise specified, it shall be made of corrosion resistant material. It shall, as a minimum, include the following information: a)

Name of manufacturer

b)

Serial number

c)

Type of ground-fault neutralizer

d)

Name of device

e)

Type designation (if any)

f)

g)

Rated current 1)

Continuous neutral current (where applicable)

2)

Short-time neutral current

Rated frequency

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h)

Rated time

i)

System (or insulation) voltage rating

j)

BIL/insulation levels

k)

Indoor or outdoor service

l)

Mass

m)

Mass of insulating liquid (where applicable)

n)

Untanking mass (for liquid-immersed ground-fault neutralizers)

o)

Transportation mass

p)

Year of manufacture

q)

Number and year of this standard

r)

Maximum altitude (meters or feet above sea level) — only applicable if above 1000 m (3300 ft) NOTE—This is the maximum altitude the device can operate at without de-rating or change to any of its nameplate characteristics.

s)

Type of cooling

t)

Maximum continuous voltage (if specified)

u)

Type of regulation (continuous or in finite steps)

v)

Thermal class of insulation (for dry-type only)

w)

Temperature rise of top liquid and average winding for rated current and duration (top liquid rise for liquid-immersed ground-fault neutralizers only)

x)

Type of insulating liquid, if not mineral oil (where applicable)

y)

Connection diagram regarding tappings and instrument transformers (where applicable)

z)

Type of tap changer (where applicable)

aa) A table or graph indicating the adjustment range in amperes or as a ratio (for ground,fault neutralizers with adjustable inductance)

5.6 Tanks and enclosures Tanks where applicable shall be designed to withstand without permanent deformation a pressure 25% greater than the maximum operating pressures resulting from the system of insulation preservation or ventilation used. Bolted or welded main cover construction shall be considered as alternate standards. Pigment paint shall be used when surfaces are painted. Corrosion protection system shall be stated by the supplier if not specified.

5.7 Ratings 5.7.1 Rated voltage The rated voltage shall be specified to be at least equal to the highest voltage which can occur between the neutral of the power transformer, or grounding transformer, and ground during a ground fault. NOTE—Usually, the rated voltage is specified to be equal to the line-to-neutral voltage of the power system.

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5.7.2 Maximum continuous voltage The maximum continuous voltage shall be specified by the purchaser to be not less than the voltage occurring at the neutral due to the voltage unbalance of the power system under normal operating conditions unless this value is less than 10% of the rated voltage. If the maximum continuous voltage is not specified it shall be taken as 10% of the rated voltage. The ground-fault neutralizer shall be designed to withstand the maximum continuous voltage. 5.7.3 Rated thermal current The rated thermal current shall be specified to be not less than the highest value of current under line-toground fault conditions. The ground-fault neutralizer shall be designed to carry this current for the rated current duration or continuously, if specified. The current of the main winding at minimum inductance and rated voltage shall be within ± 5% and current at other settings shall be ± 10% of specified values. 5.7.4 Rated thermal current duration The rated thermal current duration shall be specified by the purchaser to be not less than the expected maximum duration of a ground-fault unless the rated current duration is continuous. If successive faults can occur within a short period of time, the time intervals between applications and the number of applications shall be specified by the purchaser. The specified duration of rated thermal current shall be selected accordingly. Commonly used durations are 10 s, 30 min, 2 h, and continuous. For ground-fault neutralizers, a continuous duration would generally be specified for durations of more than 2 h. 5.7.5 Rated continuous current Implicit in the thermal current rating is an associated continuous current which, unless otherwise specified, shall bear the following relationship to the thermal current rating. The rated continuous current shall be taken into account as a pre-load to the rated thermal current in the thermal design of the ground-fault neutralizer. Table 8 shows continuous duty in percent of thermal current rating for ground-fault neutralizers. Table 8—Continuous duty in percent of thermal current rating for ground-fault neutralizers Rated time 10 s 1 min 10 min Extended time

Continuous duty in percent of thermal current rating (%) 3 7 30 30

Where there is a third harmonic component of current, it shall not exceed 15% of the rated continuous duty current (see IEEE Std C57.110).

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5.7.6 Adjustment range The current corresponding to rated voltage at rated frequency may be adjusted in one of the following ways: a) b) c)

By adding additional sections of the main winding in finite steps with an off-load or on-load tapchanger. Typically an adjustment range of not more than 2.5:1 is recommended. By reducing the air gap of the magnetic circuit by mechanical means. Typically an adjustment range of 10:1 is specified. By switching of single coils from a set of coils intended for parallel connection.

5.7.7 Auxiliary winding If the purchaser requires an auxiliary winding the current and voltage and the tolerances on these values shall be specified. No-load voltage of the auxiliary windings with rated voltage applied to the main winding, over the whole adjustment range, shall be ± 10% of specified values. NOTE—A typical rating for an auxiliary winding would be 100 V, 1 A.

5.7.8 Secondary winding If the purchaser requires a secondary winding the current and voltage shall be specified. No-load voltage of the secondary windings with rated voltage applied to the main winding, over the whole adjustment range, shall be ± 10% of specified values. NOTE—A typical rating for a secondary winding would be 500 V, 100 A.

5.7.9 Linearity of the ground fault neutralizer The ground-fault neutralizer shall be a linear reactor within a tolerance of ± 5% up to 1.1 times rated voltage, unless otherwise specified.

5.8 Temperature limitations 5.8.1 Limits Top liquid temperature rises determined under standard operating conditions shall not exceed the limits established in Table 9. Table 9—Limits of top liquid temperature rise for ground-fault neutralizers Rating duration Steady-state Extended-time Short-time

Ground-fault neutralizers 75 ºC 75 ºC 90 ºC

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Parts, other than current-carrying parts, in contact with or adjacent to insulation or insulating liquid shall not attain temperature rises in excess of those allowed for current-carrying parts. Other parts shall not attain temperatures that will be injurious to structures, or personnel, or produce excessive smoke or toxic fumes.

5.9 Insulation levels Insulation levels shall be in accordance with Table 4, Table 5, and Table 6 of IEEE Std C57.12.00. For nominal system voltages 69 kV and below, the insulation levels for Class I power transformers will apply. For nominal system voltages above 69 kV up to 138 kV, the insulation levels for Class II power transformers will apply. Refer to Table 10 for examples. Table 10—Example insulation levels (taken from IEEE Std C57.12.00-2010) Nominal system voltage (see NOTE 1)

Lightning impulse voltage test

Full wave level (see NOTE 2) kV rms Column 1

kV crest Column 2

25

150

115

450

Chopped wave level

Minimum time to voltage chopping kV crest µs Column 3 Column 4 Example 1: 165 3.0 Example 2 (see NOTE 4): 495 3.0

Lowfrequency overvoltage test

Applied voltage test (see NOTE 3)

kV rms Column 5

kV rms Column 6

29

50

120

173

NOTE 1—The nominal system voltage values given in Column 1 are used merely as reference numbers and do not necessarily imply a relation to operating voltages. NOTE 2—The minimum insulation levels for ground-fault neutralizer for the line end and ground ends should be the highest value selected from Table 17 on the basis of fault voltage criteria, Column 3 and Column 4, or of the insulation level equal to that of transformer neutrals in the power system. Reference is made to Table 17 (grounding transformer) as it is the upstream device and decisive for the insulation coordination. For the terminal of the groundfault neutralizer connected to ground, a lower insulation level may be specified (non-uniform insulation). A nonuniform insulation is usually specified for nominal system voltages of 46 kV and above, In case of a non-uniform insulation, the insulation level for the grounded end should be less or equal the values specified for a system voltage of 34.5 kV. Lightning impulse voltage test (Column 2 and Column 3) are only applicable for the line end of the ground-fault neutralizer. NOTE 3—The applied voltage test should be made at the test voltage given in Column 6. If the insulation class of the terminal connected to ground is lower than that of the line end (non-uniform insulation) a lower test voltage may be selected, subject to agreement between manufacturer and purchaser. NOTE 4—Usually ground fault neutralizers are not used at nominal voltages above 138 kV.

5.10 Temperature rise The temperature rises under the maximum continuous voltage shall be taken as the initial values for calculating the temperature rises due to rated current. The average temperature rise of the windings and the temperature rise of the top liquid at rated current shall not exceed the following values when tested according to 5.15: a)

80 °C for the windings and 75 °C for the insulating liquid, where the rated current duration is continuous

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

b)

100 °C for the windings and 90 °C for the insulating liquid, where the rated current duration is 2 h or less

The values of temperature rise take into account the fact that power system ground faults occur infrequently and have limited duration. Where short-time loading of a secondary winding of up to 10 s is specified, the temperature of the secondary winding shall not exceed the values prescribed for transformer windings under short time conditions in Table 19 of this standard. The temperature rise of the top liquid shall not exceed 90 °C.

5.11 Routine, design, and other tests for ground-fault neutralizers Table 11 shows routine, design, and other tests for ground-fault neutralizers. Table 11—Routine, design, and other tests for ground-fault neutralizers Test classification

Tests

Routine

Resistance measurement: The dc resistance measurement shall be made on the full winding. Measurement of current (5.13) at rated voltage: Measurement of no-load voltage of the auxiliary and secondary windings, where appropriate (5.14). Applied voltage test of the auxiliary and secondary windings and of the control and measuring wiring, where appropriate (10.6 of IEEE Std C57.12.90-2010). Operation tests of tap-changer or core air gap mechanism and of associated control and measuring equipment, where appropriate. Impedance measurement: The impedance measurement shall be made on the full winding. Total loss measurement (5.16): Temperature rise test (5.15): This test is performed on one unit out of a number of units of the same design. Lightning impulse test (5.12.1): — Nominal system voltage greater than 34.5 kV. — Nominal system voltage at or below 34.5 kV only when specified. Applied voltage test (5.12.2): Chopped wave impulse test: The chopped wave impulse test shall be made only when specified. Low-frequency withstand test (5.12.3): Acoustic sound level (5.18). Endurance tests of the inductance regulation mechanism (5.19). Seismic verification (4.15): Test performed only when specified. Short circuit test (per IEC 60076-5): Test performed only when specified. Measurement of linearity (5.17).

Design

Other

X X X X X X X X X X X X X X X X X X

5.12 Dielectric tests Dielectric test withstand levels shall be those found in 5.9 for applicable nominal system voltage. Ground-fault neutralizers with adjustable inductance shall be set for minimum current during these tests.

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5.12.1 Impulse tests Ground-fault neutralizers shall be designed to withstand impulse tests. The impulse tests are required for ground fault neutralizers rated 34.5 kV and above, unless specified as a special test. The lightning impulse test is applied on the terminal for connection to the neutral of the power or grounding transformer. The test is made in accordance with 10.3 of IEEE Std C57.12.90-2010. If the ground-fault neutralizer has a tapped winding, the lightning impulse test shall be performed with the ground-fault neutralizer on maximum tapping and repeated on minimum tap. NOTE—In 10.3.1.1. of IEEE Std C57.12.90-2010, a time to crest of greater than 2.5 µs for the impulse voltage test is allowed for windings of large impulse capacitance.

5.12.2 Applied voltage tests Applied-voltage tests are required. They shall be made at the test voltage corresponding to the insulation class assigned to the ground end if it is lower than the insulation class assigned to the line end. The external ground connection is removed for this test. If the ground end bushing is not capable of withstanding this test voltage, it is to be disconnected, and, if necessary, special insulation is to be provided for test purposes, see 10.6 of IEEE Std C57.12.90-2010. 5.12.3 Low-frequency withstand tests Low-frequency withstand tests are required. The ground end shall be connected to ground for this test. When taps are provided, the test shall be made on the tap which produces the highest voltage per turn in the winding and also on the connections having the greatest number of turns, but the voltage to ground at the line end shall not exceed the specified test voltage, see 10.7 and 10.8 of IEEE Std C57.12.90-2010. If the low frequency withstand test is impracticable, the test may be replaced by a lightning impulse test, subject to agreement between purchaser and manufacturer at the time of the order.

5.13 Measurement of current at rated voltage The measurement of current (routine test) shall be made over the whole range of adjustment. For groundfault neutralizers with finite steps the measurement shall be made at each step. For ground-fault neutralizers with continuously adjustable inductance, the current shall be measured at a minimum of five settings evenly distributed over the range. The measurement should preferably be made at rated voltage and rated frequency (type test). If this is impracticable, the test voltage chosen shall be as high as possible and should be agreed between manufacturer and purchaser at the time of order. In the case of routine testing of several identical units, this test may be performed at low voltage to be agreed between manufacturer and purchaser.

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5.14 Measurement of no-load voltage of the auxiliary and secondary windings The measurement of no-load voltages of any of the auxiliary and secondary windings shall be made over the whole adjustment range, at rated voltage on the main winding. If this is impracticable, the test voltage chosen shall be as high as possible.

5.15 Temperature rise test The test shall be made in accordance with Clause 11 of IEEE Std C57.12.90-2010. The terminals of any auxiliary and secondary windings shall be open during the test. If the rated current duration is not continuous, and the maximum continuous voltage is more than 30% of the rated voltage, the temperature rise test shall start with an application at maximum continuous voltage until the steady-state temperature is achieved. In all cases following the application of the rated current for the rated current duration, the winding temperature shall be determined using the resistance method and the top liquid temperature shall be measured by thermometers. If the maximum continuous voltage is less than or equal to 30% of the rated voltage and the application at maximum continuous voltage is not carried out, the initial temperature shall be determined by calculation and shall be added to the temperature rise measured by the temperature rise test.

5.16 Measurement of loss Ground-fault neutralizers with adjustable inductance shall be set for rated current during this test. The loss shall be measured at rated voltage and rated frequency. If, at rated voltage, the current measured is different from the rated current, the measured loss shall be corrected to rated current by multiplying the measured loss by the square of the ratio of rated current to measured current. Loss measurement shall be performed at factory ambient temperature and corrected to the reference temperature. The total loss is composed of ohmic loss, iron loss and additional loss. The ohmic loss portion is taken to be equal to I²R, with R being the measured dc resistance, and I being the rated current. Iron loss and additional loss cannot be separated by measurement. The sum of iron loss and additional loss is therefore the difference between the total loss and the ohmic loss. The ohmic loss is corrected to reference temperature according to 5.2 of IEEE Std C57.12.90-2010. A correction of iron loss and additional loss to reference temperature is normally not practical. Therefore, iron loss and additional loss shall be deemed independent of temperature; this assumption gives a higher loss figure at the reference temperature than actually exists. The total loss at reference temperature then is the sum of ohmic loss corrected to reference temperature and the measured iron loss and additional loss.

5.17 Measurement of linearity For ground-fault neutralizers with adjustable inductance this measurement shall be made at both maximum and minimum current settings.

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The measurement shall be made by applying a voltage in steps of approximately 10% at rated frequency up to 1.1 times the rated voltage. The linearity is determined by plotting a graph of the rms value of the voltage versus the rms value of current. Any point on this curve shall not deviate by more than ± 5% from a straight line drawn from zero through the point determined at rated voltage.

5.18 Measurement of acoustic sound level The measurement shall be made in general accordance with IEEE Std C57.12.90-2010 at rated voltage and rated current. Further measurements at other currents in case of ground-fault neutralizers with adjustable inductance shall be agreed between manufacturer and purchaser.

5.19 Endurance tests of the inductance regulation mechanism Where a ground-fault neutralizer has a mechanism for adjusting the inductance, the purchaser may require, in agreement with the manufacturer, additional endurance tests or verification procedures to demonstrate the integrity and satisfactory performance of the mechanism. The test may consist of a number of regulation operations of the ground-fault neutralizer reflecting the number of operations anticipated during the life time of the unit. A typical endurance test may consist of 1000 regulation operations over the full adjustment range. The ambient temperature during testing, for example, −20 °C, 20 °C, or 40 °C, should also be agreed between manufacturer and purchaser. NOTE—The mechanism may consist of, for example, a motor drive, switches, etc.

6. Grounding transformers 6.1 General description A grounding transformer is a transformer intended primarily to provide a neutral point for grounding purposes on three-phase ungrounded systems to provide a return path for fault currents and to support a faulted phase above ground. The most common connections used are either Wye-Delta or Zig-Zag (Interconnected Star). A grounding transformer is generally rated to withstand a specified short circuit current for a specific period of time without exceeding specific temperature limits. Grounding transformers are sometimes referred to as neutral grounding transformers.

6.2 Service conditions 6.2.1 Usual temperature and altitude service conditions Transformers conforming to this standard shall be suitable for operation at their ratings under the following usual service conditions listed in 6.2.2 and 6.2.3.

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6.2.2 Temperature The temperature of the cooling air (ambient temperature) does not exceed 40 °C and the average temperature of the cooling air for any 24-hour period does not exceed 30 °C. Minimum operating temperatures below −20 °C are considered unusual operating conditions. It is recommended that the average temperature of the cooling air be calculated by averaging 24 consecutive hourly readings. When the outdoor air is the cooling medium, the average of the maximum and minimum daily temperatures may be used. The value that is obtained in this manner is usually slightly higher than the true daily average but not by more than ½ °C. 6.2.3 Altitude The altitude does not exceed 1000 m (3300 ft).

6.3 Unusual service conditions Conditions other than those discussed in 6.2 are considered unusual service conditions and, when prevalent, should be brought to the attention of those responsible for the design and application of the equipment. Examples of some of these conditions are described in 6.3.1 through 6.3.4. 6.3.1 Operation at altitudes in excess of 1000 m (3300 ft) Standard transformers may be applied in locations having an altitude in excess of 1000 m (3300 ft), but the dielectric strength of air insulated parts and the current-carrying capacity will be affected. 6.3.2 Insulation The dielectric strength of air-insulated parts of a given insulation class at or above 1000 m (3300 ft) should be multiplied by the proper correction factor, as given in Table 12, to obtain the dielectric strength at the required altitude.

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Table 12—Dielectric strength correction factors for grounding transformers at altitudes greater than 1000 m (3300 ft) Altitude (m)

Altitude (ft)

Altitude correction factor for dielectric strength

1000 1200 1500 1800 2100 2400 2700 3000 3600 4200 4500a

3300 4000 5000 6000 7000 8000 9000 10000 12000 14000 15000a

1.00 0.98 0.95 0.92 0.89 0.86 0.83 0.80 0.75 0.70 0.67

a

An altitude of 4500 m (15000 ft) is considered a maximum for standard transformers.

6.3.3 Operation at rated current Grounding transformers with standard temperature rise may be used at rated current at altitudes greater than 1000 m (3300 ft) provided the average temperature of the cooling air does not exceed the values in Table 13 for the respective altitudes. Under these conditions, standard temperature limits will not be exceeded. Table 13—Maximum allowable average ambient temperature, in °C of cooling air for grounding transformers carrying rated currenta Method of cooling apparatus

1000 m (3300 ft)

2000 m (6600 ft)

3000 m (9900 ft)

4000 m (13200 ft)

Liquid-immersed self-cooled

30

28

25

23

105

30

27

24

21

130

30

26

22

18

155

30

24

18

12

180

30

23

16

9

220

30

22

15

7

Dry-type self-cooled insulation index (°C)

aa

For recommended calculation of average temperature, see 6.2.2.

6.3.4 Other unusual service conditions Other unusual service conditions are as follows: a)

Exposure to damaging fumes, radiation, or vapors; excessive abrasive or magnetic dust; explosive mixtures of dust, vapors, or gases; steam, excessive moisture, dripping water, fog or spray; salt or acid; etc.

b)

Abnormal vibration, shock, or tilting from earthquakes or other causes. 27

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c)

Ambient temperatures outside of normal range.

d)

Unusual transportation or storage conditions.

e)

Unusual space limitations.

f)

Unusual maintenance problems.

g)

Abnormal operating duty, frequency of operation, poor waveform, excessive unbalance voltage, special insulation requirements, lack of normal lightning arrester protection, unusual magnetic shielding problems, harmonics in excess of 15% or other than those expressed in IEEE Std C57.110.

h)

System having a ratio of reactance to resistance X/R greater than 10, see 6.4.2.2.

6.4 Basis for rating 6.4.1 Conditions Ratings for transformers are based on standard operating conditions and shall include the following: a) b) c) d) e) f) g) h) i)

Rated thermal current Rated continuous current Voltage Frequency Basic impulse insulation level (BIL) and insulation class Circuit voltage of system Service (indoor or outdoor) Time (duration) Impedance

6.4.2 Rated current 6.4.2.1 Rated thermal current Unless otherwise specified, the basis for this rating shall be the thermal current. This will be the current through the grounding transformer during a worst-case ground-fault condition. Implicit in the thermal current rating is an associated continuous current that, unless otherwise specified, shall bear the following relationship to the thermal current rating (see Table 14). Table 14—Continuous duty in percent of thermal current rating for grounding transformers Rated time

Continuous duty in percent of thermal current rating (%)

10 s

3

1 min

7

10 min

30

Extended time

30

Where there is a third harmonic component of current, it shall not exceed 15% of the rated continuous duty current (see IEEE Std C57.110). 28

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6.4.2.2 Short-circuit current—asymmetrical Grounding transformers shall be able to withstand, without mechanical failure, forces associated with the asymmetrical peak current. This peak current shall be determined from Equation (3):

I c = KI T

(3)

where Ic K IT

is the crest of the initial offset current, peak asymmetrical is the appropriate multiplier obtained from Table 15 or as calculated in Equation (4) is the thermal current rating, rms symmetrical

 π r   − φ +    2 x  K = 1 + e sin φ  2  

(4)

where ϕ e x/r

is the arc tan (x/r), (radians) is the base of natural logarithm is the ratio of ohms reactance to ohms resistance in the system when the short-circuit occurs

When sequence impedances for the system are not specified, the ratio x/r shall be taken to be the ratio of ohms reactance to ohms resistance in the winding of the neutral grounding device through which the current flows. When the system to which the neutral device is connected has an x/r ratio at that point which is greater than 10, the ohmic values of sequence impedances should be specified. When specified, the manufacturer shall combine them with the X and R of the device to determine the value of K to be used in calculating Ic. When grounding transformers are located on or near generator or motor buses, subtransient impedances shall be utilized in calculating system x/r ratios. K can be calculated using Equation (4); however, it is important to note that the expression for K is an approximation. The tabulated values for K given in Table 15 are calculated from this approximation and are accurate to within 0.7% of the values calculated by exact methods.

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Table 15—Values of K for Equation (4) for grounding transformers r/x

x/r

K

0.001

1000.00

2.824

0.002

500.00

2.820

0.003

333.00

2.815

0.004

250.00

2.811

0.005

200.00

2.806

0.006

167.00

2.802

0.007

143.00

2.798

0.008

125.00

2.793

0.009

111.00

2.789

0.010

100.00

2.785

0.020

50.00

2.743

0.030

33.30

2.702

0.040

25.00

2.662

0.050

20.00

2.624

0.060

16.70

2.588

0.070

14.30

2.552

0.080

12.50

2.518

0.090

11.10

2.484

0.100

10.00

2.452

0.200

5.00

2.184

0.300

3.33

1.990

0.400

2.50

1.849

0.500

2.00

1.746

0.600

1.67

1.669

0.700

1.43

1.611

0.800

1.25

1.568

0.900

1.11

1.534

1.000

1.00

1.509

6.4.2.3 Rated continuous neutral current The rated continuous neutral current is the current that the transformer neutral can carry in steady-state conditions. This current results from continuous unbalance between the power system phases. The inherent capability for a given thermal current capability is given in Table 14 in 6.4.2.1. Desired continuous current requirements different from this must be specified in the transformer specification. The continuous neutral current in amperes should be listed on the nameplate. 6.4.3 Rated voltage The rated voltage of grounding transformers shall be taken as the rated phase-to-phase voltage of the system to which it will be connected. 30

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6.4.4 Rated frequency The rated frequency shall be the fundamental frequency of the system. 6.4.5 Rated time Rated time shall be 10 s, 1 min, 10 min, and extended time. Extended-time operation shall not exceed an average of 90 days per year. 6.4.6 Impedance For grounding transformers zero sequence impedance shall be rated in ohms per phase. NOTE—This is a physical value not calculated on kVA base ratings. The reason for this is that there is possible confusion between continuous and fault kVA ratings and their associated percent impedance values.

6.5 Insulation classes and dielectric withstand levels 6.5.1 Basic impulse insulation levels and insulation classes Grounding transformers covered by this standard shall be assigned BIL and/or insulation class designations. The line end and ground end insulation levels shall be selected from Table 17 on the basis shown in Table 16: Table 16—Basis for selecting line end and ground end insulation levels for grounding transformers Line end

Ground end

Disconnectable from ground

System insulation class Column 1 and Column 2

System insulation class Column 1 and Column 2

Permanently grounded

System insulation class Column 1 and Column 2

Fault voltage criteria Column 3 and Column 4

Basic impulse insulation levels and insulation class designations used with grounding transformers are given in Table 17.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Table 17—Insulation classes for grounding transformers, ground-fault neutralizers, and combination devices Insulation class System insulation classa (kV)

Fault voltage criteriab

Class Column 1

BIL Column 2

kV (Line end) Column 3

kV (Ground end) Column 4

1.2

45

1.2

1.2

2.5

60

2.5

2.5

5.0

75

5.0

5.0

8.7

95

8.7

8.7

15.0

110

8.7

8.7

23.0

150

15.0

8.7

34.5

200

25.0

8.7

46.0

250

34.5

15.0

69.0

350

46.0

15.0

115.0

550

69.0

15.0

138.0

650

92.0

15.0

161.0

750

92.0

15.0

196.0

800

115.0

15.0

230.0

1050

138.0

15.0

NOTE—The two ends of the winding of a grounding transformer may be assigned different insulation levels. a

Where the insulation class for the terminal of a neutral grounding device is specified to be the system insulation class, the nominal system insulation level shall apply except that reduced BIL may be used where appropriate.

b

When the fault voltage criterion applies, the maximum rms voltage that may exist between the terminal and ground, under fault conditions, is determined. If this fault voltage falls between the values in Column 3 and Column 4 corresponding to the system insulation class in Column 1, the system insulation class at the terminal in question shall be the value in Column 3, which equals or is next higher than the maximum fault voltage. If the fault voltage is less than the value in Column 4, the system insulation class at the terminal in question shall be the value in Column 4. If the fault voltage exceeds the value in Column 3, the system insulation class at the terminal in question shall be the value in Column 1.

6.5.2 Protective devices Suitable protective devices shall be provided whenever necessary to hold transient over voltages at the terminals of a transformer to values within the limits set by the selected insulation levels.

6.6 Temperature limitations 6.6.1 Limits Top liquid temperature rises determined under standard operating conditions shall not exceed the limits established in Table 18. Parts, other than current-carrying parts, in contact with or adjacent to insulation or liquid shall not attain temperature rises in excess of those allowed for current-carrying parts. Other parts shall not attain temperatures that will be injurious to structures, or personnel, or produce excessive smoke or toxic fumes. 32

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Table 18—Limits of top liquid temperature rise for grounding transformers Rating duration Steady-state Extended-time Short-time

Grounding transformers 60 °C 60 °C 75 °C

6.6.2 Steady-state temperature rises The steady-state average winding temperature rise shall be determined by test (see 6.7). The steady-state hot-spot winding temperature rise shall be determined when specified. Steady-state and rated-time temperature rises for current-carrying parts, determined under standard operating conditions, shall not exceed the limits established in Table 19. Table 19—Limiting temperature rises above 30 ºC ambient for current carrying parts of grounding transformersa 65 °C liquid immersed

Dry-type insulation temperature class 130 ºC

155 ºC

180 ºC

200 ºC

220 ºC

Temperature rise (°C) Steady-state continuous current ratings

Short time (≤ 10 min) thermal current calculation (see 6.6.4) Long time (> 10 min) thermal current calculation

Winding hottest spot rise Average winding rise

80

90

115

140

160

180

65

75

90

115

130

150

Copper

250

300

350

400

400

400

Aluminum

200

200

200

200

200

200

Aluminum alloy

250

250

250

250

250

250

Liquid filled

Refer to IEEE Std C57.91.

Dry type

Refer to IEEE Std C57.96.

NOTE—Other factors may limit temperature rises in a specific design. For example: 1) The reduction in the mechanical strength and increase in elongation of copper at temperatures above 300 °C and aluminum at temperatures above 350 °C. 2) Gas evolution from insulation and liquid adjacent to hot conductors. 3) Auto-ignition of insulation or liquid. a

The values in Table 19 are based on the thermal aging characteristics of the insulation. Grounding transformers built to these thermal limits will have normal insulation life.

6.6.3 Rated-time temperature rises 6.6.3.1 Ten-second and one-minute ratings The rated-time temperature rise of 10-second and 1-minute transformers is an average winding rise. 33

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

The rated-time temperature rise of 10-second and 1-minute transformers shall be taken as the sum of the steady-state rise determined as in 6.6.2 and the additional rise caused by the flow of rated thermal current for rated time, determined by calculation, using Equation (5). 6.6.3.2 Ten-minute ratings The rated-time temperature rise of 10-minute transformers is an average winding rise. The rated-time temperature rise of 10-minute transformers shall be taken as the winding rise above ambient resulting from the flow of rated thermal current for 10 min starting with an initial temperature rise equal to the steady-state rise of the transformer. The rated-time temperature rise of 10-minute transformers shall be determined by routine test. 6.6.3.3 Extended-time ratings The rated-time temperature rise of extended-time transformers is an average winding rise. The rated-time temperature rise of extended-time transformers shall be taken as the winding temperature rise above ambient resulting from the continued flow of rated thermal current. The rated-time temperature rise of extended-time transformers shall be determined by routine test. 6.6.4 Thermal short-time capability calculations for grounding transformers The final winding temperature, Tf, at the end of a short circuit of duration, t, shall be calculated as shown in Equation (7) through Equation (10), on the basis of all heat stored in the conductor material and its associated turn insulation. All temperatures are in degrees Celsius. Tf =

(T

k

+ Ts ) m (1 + E + 0.6 m ) + Ts

(5)

where m=

Ws t

(6)

C (Tk + Ts )

These equations are approximate formulas, and their use should be restricted to values of m = 0.6 or less. For values of m in excess of 0.6, the following more nearly exact formula should be used:

(

) − 1 + T

Tf = ( Tk + Ts )  e + E e − 1



2m

2m

(7)

s

where t is the time in seconds Tf is final winding temperature 34

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Tk is 234.5 for copper is 225 for EC grade aluminum (the appropriate values for other grades may be used) Ts is the starting temperature. It is equal to one of the following: a) A 30 ºC ambient temperature plus the average winding rise plus the manufacturer’s recommended hottest-spot allowance, b) A 30 ºC ambient temperature plus the limiting winding hottest-spot temperature rise specified for the appropriate type of transformer e E

is the base of natural logarithm, 2.718 is the per-unit eddy-current loss, based on resistance loss, Ws, at the starting temperature

 T + Tr  E = Er  k   Tk + Ts 

2

(8)

where Er is the per-unit eddy-current loss at the reference temperature Tr is the reference temperature, which is 20 ºC ambient temperature plus rated average winding rise Ws is the short-circuit resistance loss of the winding at the starting temperature (W/kg) of conductor material Ws =

where Wr N M C

Ai Ac

Wr N 2 Tk + Ts M Tk + Tr

(9)

is the resistance loss of winding at rated current and reference temperature (W) is the ratio of symmetric short-circuit current magnitude to normal rated current is the mass of winding conductor (kg) is the average thermal capacitance per kg of conductor material and its associated turn insulation (Ws)/oC. It shall be determined by iteration from either of the following empirical equations: [384 + 0.0496(Ts + Tf) + 110 Ai/Ac] for copper [893 + 0.220(Ts + Tf) + 360 Ai/Ac] for aluminum is the cross-sectional area of turn insulation in mm2 is the cross-sectional area of conductor in mm2 CAUTION

When applying the preceding formulas for calculation, care shall be taken to assure consistent application of units for mass, weights, and cross sectional areas. This will involve the variables Ws, M, C, Ai, and Ac. Mixing of units between SI system and the U.S. customary units (in, lb) will yield incorrect results.

6.7 Tests 6.7.1 Routine, design, and other tests for grounding transformers Specific tests for grounding transformers are shown in Table 20 and shall be conducted as detailed in IEEE Std C57.12.90-2010 for liquid-filled transformers or IEEE Std C57.12.91 for dry-type transformers.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

6.7.1.1 Routine tests A routine test is a test made on each and every unit of a specific design and is primarily a verification of quality. Routine tests shall be made on all grounding transformers as covered in the clause of this standard that is set aside for the grounding transformer being tested. 6.7.1.2 Design tests A design test (also referred to as a type test) is a test carried out on a single unit of a specific design and is primarily a verification of the ability to meet in-service application requirements. Design tests shall be made on all grounding transformers when specified. 6.7.1.3 Other tests A test designated as “other” is a test performed on one or all units of a specific design if requested by the purchaser and is usually requested to demonstrate conformance to special application requirements as opposed to the more general application requirements covered by design tests. When specified (as individual tests) “other” tests shall be performed. Table 20—Routine, design, and other tests for grounding transformers Test classification

Tests

Routine

Resistance measurement: The dc resistance measurement shall be made on the full winding.

X

Ratio tests on the rated voltage connection and on all tap connections.

X

Polarity and phase relation tests on the rated voltage connection.

X

Impedance measurement: The impedance measurement shall be made on the full winding.

X

Total loss measurement: No load and load losses to be measured at rated frequency and/or rated voltage or current.

X

Temperature rise test: This test is performed on one unit out of a number of units of the same design.

Design

Other

X

Applied voltage test.

X

Induced voltage tests.

X

Lightning impulse test: —

Nominal system voltage greater than 34.5 kV.

—

Nominal system voltage at or below 34.5 kV only when specified.

X X

X

Seismic verification test: Test performed only when specified.

X

Short circuit test: Test performed only when specified.

X

6.7.2 Test sequence The listing of tests shown for specific grounding transformers does not necessarily indicate the sequence in which the tests shall be made. All tests are defined and shall be made in accordance with 6.7.1.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

6.7.3 Dielectric tests Dielectric test withstand levels shall be those listed in Table 21, Table 22, and Table 23 for the applicable insulation class. Table 21—Dielectric test voltages for (liquid filled) Zig-Zag or Grounded Wye-Delta transformers Nominal system voltage (kV rms)

Applied potential (kV rms)a

Induced voltage (kV rms)b

Full wave (kV crest)

(Commonly used BIL)

Enhanced 7200 cycles

One hour

Minimum

Commonly used BIL

Alternate

Column 1

Column 2

Column 3

Column 4

Column 5

Column 6

Column 7

1.2

15

1.4

30

45

2.5

19

2.9

45

60

5

26

5.8

60

75

8.7

34

10

75

95

15

34

17

95

110

25

50

29

125

150

34.5

70

40

150

200

46

95

53

200

250

69

140

80

250

350

115

185

120

105

350

450

138

230

145

125

450

550

650

161

275

170

145

550

650

750, 825

230

325

240

210

650

750

825, 900

550

a

For neutrals with a reduced BIL level, applied potential test level shall be selected based on neutral BIL level.

b

For system voltage 69 kV and below, values are taken from Table 4 of IEEE Std C57.12.00-2010 (Class I Power Transformers). For voltages 115 kV and above, values are taken from Table 5 of IEEE Std C57.12.00-2010 (Class II Power Transformers).

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Table 22—Dielectric test voltage and time to test gap flashover (liquid-filled) Zig-Zag or Grounded Wye-Delta transformers Impulse test voltage Full Wave

Chopped wave

kV crest

kV crest

Minimum time to flashover (µs)

Column 1

Column 2

Column 3

30

33

1.0

45

50

1.5

60

66

1.5

75

83

1.5

95

105

1.8

110

120

2.0

125

138

2.3

150

165

3.0

200

220

3.0

250

275

3.0

350

385

3.0

450

495

3.0

550

605

3.0

650

715

3.0

750

825

3.0

825

910

3.0

900

990

3.0

NOTE—Values taken from Table 6 of IEEE Std C57.12.00-2010.

Table 23—Dielectric test voltage and time to test gap flashover (dry-type) grounding transformers Nominal system voltage

Impulse tests Chopped wave (commonly used BIL)

Full wave (kV crest)

Applied potential test

kV rms

Minimum

Commonly used BIL

Alternates

kV crest

Minimum time to flashover (µs)

kV rms

Column 1

Column 2

Column 3

Column 4

Column 5

Column 6

Column 7

1.2

10

20/30

10

1

4

2.5

20

30/45

20

1

10

5

30

45/60

30

1

12

8.7

45

60/95

45

1.25

19

15

60

95/110

60

1.5

34

25

95

110

125/150

110

1.8

50

34.5

125

150

200

150

2.25

70

NOTE 1—Values from Table 4 and Table 5 of IEEE Std C57.12.01™-2015 [B25]. NOTE 2—Specifying engineer need to coordinate insulation levels with system voltages and protection.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

6.8 Construction 6.8.1 Bushings, insulators, and liquid Bushings shall comply with IEEE Std C57.19.00 and IEEE Std C57.19.01. Insulator units shall comply with ANSI/NEMA C29.8, ANSI/NEMA C29.9, and ANSI/NEMA C29.10. The dielectric strength shall be tested per ASTM D1816 of new insulating liquid as received, with the acceptable limits as described in the following appropriate IEEE standard or guide: 

IEEE Std C57.106 for mineral oil



IEEE Std C57.111 for silicone fluid



IEEE Std C57.121 for less flammable hydrocarbon



IEEE Std C57.147 for natural ester fluid

6.8.2 Nameplates A durable metal nameplate shall be affixed to each grounding transformer. Unless otherwise specified, it shall be made of corrosion resistant material. It shall, as a minimum, include the following information: a)

Name of manufacturer

b)

Serial number

c)

Name of transformer

d)

Type designation (if any)

e)

Impedance in ohms per phase

f)

Number of phases

g)

Temperature coefficient of resistance and resistance at 25 °C

h)

Rated continuous neutral current

i)

Rated short time neutral current When a grounding transformer has more than one current-time rating, each rating shall appear on the nameplate

j)

Rated frequency

k)

Rated time

l)

Rated voltage

m)

BIL of “line”

n)

Indoor or outdoor service

o)

Mass

p)

Volume of liquid (where applicable)

q)

Year of manufacture

r)

Number and year of this standard

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

s)

Maximum altitude (meters or feet above sea level). Note this is the maximum altitude the transformer can operate at without derating or change to any of its nameplate characteristics

t)

System (or insulation) voltage rating

u)

Type of cooling

The nameplate shall be affixed to each transformer by the manufacturer. Unless otherwise specified, it shall be made of corrosion-resistant materials. On transformers with one or more taps not brought out of the case, a diagram shall be included. 6.8.3 Tanks and enclosures Tanks where applicable shall be designed to withstand without permanent deformation a pressure 25% greater than the maximum operating pressures resulting from the system of insulation preservation or ventilation used. For tanks, the maximum operating pressures (positive and negative) for which the tank is designed shall be indicated on the nameplate. Tanks shall be designed for vacuum filling (external pressure of one atmosphere, essentially full vacuum) in the field on all transformers with high-voltage insulation levels of 350 kV BIL and above. In addition, the following tank vacuum data shall be provided: Tank designed for ___ kPa (lbf/in2) vacuum filling. Bolted or welded main cover construction shall be considered as alternate standards. A pigmented paint shall be used when surfaces are painted.

7. Grounding resistors 7.1 Resistor element A resistor element is the conducting unit which functions to limit the current flow to a predetermined value. The element material shall possess a balanced combination of properties, uniformity of resistance, and mechanical stability over the intended operating temperature range, without any injurious effects to the elements and its associated insulation. 7.1.1 Conductor connections All conductor terminations shall be bolted, welded, or brazed. Low-melting alloys used to join connectors that would be adversely affected by the resistor operating temperatures shall not be used. All conductor terminations must be mechanically secure to provide continuous electrical continuity throughout the life of the device.

7.2 Rated voltage Since the active material used in resistors has an appreciable temperature coefficient, the resistance is materially changed during the time of operation causing the voltage to increase or the current to decrease. When the product of the fault current and resistance at 30 °C exceeds 80% of the line-to-neutral voltage of

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

the circuit, the resistor shall be rated for constant voltage and the rated voltage shall be taken equal to the line-to-neutral voltage.

7.3 Temperature coefficient of resistance The conductor element resistance changes to some extent with temperature. The change may be calculated from the temperature coefficient of resistivity.

α=

R2 − R1 R1 (θ 2 − θ1 )

(10)

R2 = R1 [1 + α (θ 2 − θ1 )]

(11)

R1 and R2 are resistances in ohms at temperatures θ1 and θ2 in degrees Celsius, respectively, and α is the temperature coefficient of resistance. The temperature coefficient of resistance, based on the temperature interval 30 °C to 790 °C shall not exceed 0.000263 per degree C. This is to help ensure that the final fault current has a value sufficiently high to allow protective circuitry to operate as intended. Where a special temperature coefficient is required, such data are to be brought to the attention of those responsible for the design of an unusual service condition. It is recommended that neutral grounding resistors rated less than continuous duty should be equipped with a resistor monitor to verify the tolerance and continuity of the resistance. The resistor shall be monitored continuously.

7.4 Mechanical considerations As standard practice, unless otherwise specified, peak wind loads and seismic loads or other specified loading are typically assumed to act on a non-coincidental basis. Resistors should be designed to facilitate lifting and should be provided with lifting eyes or similar devices to allow safe, rapid installation. Lifting and handling instructions should be clear and readily available.

7.5 Insulation levels The line end and ground end insulation levels shall be selected from Table 26 on the basis of fault voltage criteria, Column 4 and Column 5.

7.6 Routine, design, and other tests for neutral grounding resistors A routine test is a test made on each and every unit of a specific design and is primarily a verification of quality. Routine tests shall be made on all neutral grounding resistors in accordance with the requirements of Table 24.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

A design test (also referred to as a type test) is a test carried out on a single unit of a specific design and is primarily a verification of the ability to meet in-service application requirements. Design tests shall be made on all neutral grounding resistors in accordance with the requirements of Table 24. A test designated as “other” is a test performed on one or all units of a specific design if requested by the purchaser and is usually requested to demonstrate conformance to special application requirements as opposed to the more general application requirements covered by design tests. When specified (as individual tests) “other” tests, as listed in Table 24, shall be made on neutral grounding resistors. Table 24—Routine, design, and other tests for neutral grounding resistors Test classification

Tests

Routine

Resistance measurement: The dc resistance measurement shall be made on the full resistor.

X

Impedance measurement: The impedance measurement shall be made on the full resistor.

X

Temperature rise test: This test is performed on one unit out of a number of units of the same design. Applied voltage test: The applied voltage test shall be made only on support insulators when specified.

Design

Other

X X

Lightning impulse test: —

Nominal system voltage greater than 34.5 kV.

—

Nominal system voltage at or below 34.5 kV only when specified.

X X

Seismic verification test: Test performed only when specified.

X

7.7 Temperature rise test This test shall be made by passing rated current through the resistor until the temperature rise becomes constant, and then determining the hottest-spot temperature rise. The temperature rise tests shall be made in a room that is essentially free from drafts. The limits for the temperature rise shall be according to Table 25. Table 25—Limiting temperature rises above 30 ºC ambient for current carrying parts of neutral grounding resistors Temperature rise (°C) Steady State for continuous current ratings

Rated Time for thermal current ratings

Steady state (hot-spot)

385

Extended-time (average)

610

Ten-minute (average)

610

Less than 10 min (average)

760

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

7.8 Resistance test Overall resistance shall be measured to determine that the resistance is within the design value. Unless the application requires close resistance tolerance, the dc resistance shall not vary more than 10% from the guaranteed value.

7.9 Dielectric tests Dielectric test withstand levels shall be those listed in Table 26. Table 26—Insulation and dielectric requirements for neutral grounding resistors System insulation (kV)

Distributiona BIL (kV)

Powera BIL (kV)

Neutrale terminal (kV)

Groundd terminal (kV)

Dielectric testing

Column 1

Column 2

Column 3

Column 4

Column 5

Column 6

0.60

10

10

0.35

0

(see footnote b)

1.20

30

45

0.70

0

(see footnote c)

2.50

45

60

1.44

0

(see footnote c)

5.00

60

75

2.89

0

(see footnote c)

8.70

75

95

5.02

0

(see footnote c)

15.0

95

110

8.66

0

(see footnote c)

23.0

125

150

13.3

0

(see footnote c)

34.5

150

200

19.9

0

(see footnote c)

46.0

200

250

26.6

0

(see footnote c)

69.0

250

350

39.8

0

(see footnote c)

92.0

350

450

53.1

0

(see footnote c)

115.0

550

550

66.4

0

(see footnote c)

138.0

650

650

79.7

0

(see footnote c)

161.0

750

750

93.0

0

(see footnote c)

180.0

825

825

103.9

0

(see footnote c)

196.0

900

900

113.2

0

(see footnote c)

230.0

1050

1050

132.8

0

(see footnote c)

a

Distribution and power refer to the insulation levels of the transformer connected to the neutral grounding resistor. Dielectric testing shall be 2 times neutral terminal plus 1000 V. c Dielectric testing shall be 2.25 times neutral terminal plus 2000 V. d Ground terminal may be isolated for instrumentation and/or dielectric requirements. e The neutral terminal voltage shall be the voltage to establish the insulation levels in the absence of any phase voltages. b

7.9.1 Impulse tests Impulse tests are not required for resistors where the system nominal voltage is 34.5 kV or less. Resistors at nominal system voltages greater than 34.5 kV shall be impulse tested on the basis of system insulation class, Table 26, Column 2 or Column 3.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

7.9.2 Applied-voltage tests Applied-voltage tests are required. They shall be made by applying between the neutral terminal and its enclosure for the complete device, or between terminals of each unit and its own individual enclosure, the specified voltage from a suitable external source. When specifications do not require that such a resistor be completely assembled at the factory, it shall be permissible for the manufacturer to waive the applied voltage test of the complete device, substituting the applied-voltage test of each section, supplemented by insulation which will show that the complete resistor will meet the insulation requirements of service and would pass the applied-voltage test when assembled. In many cases resistors are made in sections insulated from each other and from ground by standard apparatus insulators whose insulation value has been established. Each section may consist of one or more resistor sections or unit assemblies of resistance material supported on a suitable framework. In such cases each resistor section or unit assembly shall receive an applied-voltage test. The voltage applied from the terminals of each section or assembly to its own enclosure shall be twice the rated voltage of the section of which the frame is a part plus 1000 V when rated 600 V or less, or 2.25 times the rated value plus 2000 V when rated over 600 V (see Note 2 and Note 3 of Table 26). 7.9.3 Low-frequency withstand tests Low-frequency withstand tests (except as may be incidental to any required temperature test) are not required for resistors. 7.9.4 Thermal capability calculation for neutral grounding resistors 7.9.4.1 Equations for temperature rise and current density, when current is constant The eddy-current loss may usually be ignored due to the high-resistance materials used in neutral resistors. The temperature rise when the current is held constant, and all heat is assumed to be stored in the active material, shall be computed by the following equations, where all quantities have been defined in Table 27.

θ=

1 a0

2   −1  0.104a0 r0tJ 0   − 1 + θ1  log  10   Cδ    

(12)

For design purposes, it is more convenient to insert the desired temperature rise and derive the current density which will produce the desired temperature rise. Thus,

J0 =

9.62Cδ log10 [1 + a0 (θ − θ1 )] a0 r0 t

(13)

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Table 27—Nomenclature for Equation (12), Equation (13), Equation (14), and Equation (15) Symbol

Identity

Metric

θ

Final temperature rise

°C

θ1

Initial temperature rise

°C

θo

Initial temperature

°C

ao

Temperature coefficient of resistance, change in resistance per degree, at initial temperature

1/°C

δ

Density of material

g/cm3

C

Effective integrated specific heat

Cal/g·°C

Jo

Initial current density

A/cm2

ro

Resistivity at initial temperature

Ω-cm

Time

s

t −1

x

NOTE 1—Log10 = 10

NOTE 2—For cast iron, over the range of temperature covered by this standard, C shall be taken as 0.130.

Equation (12) and Equation (13) apply only when the temperature coefficient of resistance a0 is substantially constant over the temperature range used, and must not be used for materials for which the coefficient varies 7.9.4.2 Equations for temperature rise and current density, when voltage is constant For some resistors (see 7.2) temperature rise is computed on the basis that constant voltage is maintained between the terminals, the current being allowed to decrease as the resistance increases with temperature. The temperature rise, with all heat stored in the active material and with constant voltage, shall be computed by the following equations where all quantities have been defined in Table 26.

θ=

1 a0

2   − 1 + 1 + 0.478 J 0 r0 a0 t  Cδ 

  +θ  1 

(14)

For design purposes, it is more convenient to insert the desired temperature rise and derive the current density that will produce the temperature rise with the voltage maintained. Thus,

J0 =

4.18Cδ r0 t

 (θ − θ1 )2  θ − θ1 + a0  2  

(15)

Equation (15) applies only when the temperature coefficient of resistance ao is substantially constant over the temperature range used, and must not be used for materials for which the coefficient varies greatly.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

8. Combination devices Combination devices are any combination of devices of Clause 4 through Clause 7, including distribution transformers, with resistance, or inductance, or combination of two or more of these elements connected in their low-voltage winding or the delta of such low-voltage windings.

8.1 Insulation levels Combination devices covered by this standard shall be assigned BIL and/or insulation class designations. The line end and ground end insulation levels shall be selected from Table 27 on the basis shown in Table 28. Table 28—Basis for selecting line end and ground end insulation levels for combination grounding devices Line end

Ground end

Disconnectable from ground

System insulation class Column 1 and Column 2

System insulation class Column 1 and Column 2

Permanently grounded

System insulation class Column 1 and Column 2

Fault voltage criteria Column 3 and Column 4

8.2 Routine, design and other tests for combination grounding devices Tests for combination devices shall be made on the fully assembled grounding device. Routine, design, and other tests shall be based on the test requirements for the individual devices comprising the combination device as described in Clause 4 to Clause 7, unless otherwise agreed to between the purchaser and manufacturer at the time of purchase.

8.3 Dielectric tests Dielectric test withstand levels shall be based on the test requirements for the individual devices comprising the combination device as described in Clause 4 to Clause 7, unless otherwise agreed to between the purchaser and manufacturer at the time of purchase. 8.3.1 Impulse tests Combination devices shall be designed to withstand impulse tests, but the impulse tests are required only when specified. Components not requiring impulse tests shall be disconnected or suitably protected. 8.3.2 Applied-voltage tests Applied-potential tests are required. They shall be made by applying between terminals and ground for the complete device, or between terminals of each unit and its own individual frame, the specified voltage from a suitable external source. When specifications do not require that such a device be completely assembled at the factory, it shall be permissible for the manufacturer to waive the applied-potential test of the complete device, substituting the applied potential test of each section, supplemented by insulation data which will show that the complete device will meet the insulation requirements of service and would pass the applied-potential test when assembled. 46

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8.3.3 Low-frequency withstand tests Low-frequency withstand tests shall be made as required for each component part but not on the whole.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Annex A (informative) Test code 12 NOTE—The following test code from IEEE Std 32-1972 (Reaffirmed 1990) [B24] was retained in this informative annex for historical purposes only. Tables referenced by this annex have been included for convenience.

The usual program of testing a neutral grounding device includes some or all of the following tests: a)

Resistance measurements

b)

Dielectric tests

c)

Impedance and loss measurements

d)

Temperature-rise tests

A.1 Resistance measurements A.1.1 Necessity for resistance measurements Resistance measurements are of fundamental importance for two purposes: a)

For the calculation of the conductor I2R loss.

b)

For the calculation of winding temperatures at the end of a temperature test.

A.1.2 Determination of cold temperature The cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance. The precautions listed in A.1.2.1 through A.1.2.3 shall be observed. A.1.2.1 General Cold resistance measurements shall not be taken on a device when it is located in drafts or when it is located in a room in which the temperature is fluctuating rapidly. A.1.2.2 Windings out of oil The temperature of the windings shall be recorded as the average of several thermocouples or thermometers inserted between the winding sections, with extreme care used to see that their junctions or bulbs are as nearly as possible in actual contact with the windings. It should not be assumed that the windings are at the same temperature as the surrounding air.

12

Original test code from ANSI/IEEE Std-32 1972 is provided as a historical reference only.

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A.1.2.3 Windings immersed in oil The temperature of the windings shall be assumed to be the same as the temperature of the oil, provided the device has been under oil with no current in its winding from 3 h to 8 h before the cold resistance is measured, depending upon the size of the device. A.1.3 Drop-of-potential method The drop-of-potential method is generally more convenient than the bridge method for measurements made in the field. In all cases, greater accuracy may be obtained by the use of potentiometers for the measurement of both current and voltage although the setup may be rather cumbersome.

Figure A.1—Connections for the drop-of-potential method of resistance measurement

Measurement is made with direct current, and simultaneous readings of current and voltage are taken using the connections of Figure A.1. The required resistance is calculated from the readings in accordance with Ohm’s Law. In order to minimize errors of observation, the measuring instruments insofar as possible shall have such ranges as will give reasonably large deflections. The voltmeter leads shall be connected as closely as possible to the terminals of the winding to be measured. This is to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of leads. To avoid dangerous induced voltages, a rheostat should be used to reduce the current to less than ½% of rated winding current before opening the circuit. Also, to protect the voltmeter from injury by off-scale deflections, it should be disconnected before switching the current on or off. If the drop of potential is less than 1 V, a potentiometer or milli-voltmeter shall be used. Readings shall not be taken until after the current and voltage have reached steady-state values. Readings shall be taken with not less than four values of a current when deflecting instruments are used. The average of the resistances calculated from these measurements shall be considered to be the resistance of the circuit.

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The current used shall not exceed 15% of the rated continuous current of the winding whose resistance is to be measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance. A.1.4 Bridge methods Bridge methods are generally preferred because of their accuracy and convenience, since they may be employed for the measurement of resistances up to 10 000 Ω. The rheostat should always be turned to minimum current before opening the circuit. Bridge methods are especially recommended for all measurements that are to be used in connection with temperature-rise determination.

A.2 Dielectric tests A.2.1 Test procedure Unless otherwise specified, dielectric tests shall be made in accordance with IEEE Std 4™-1968, Techniques for Dielectric Tests (ANSI C68.1-1968) [B23]. NOTE—Where a sphere gap is used to measure voltage, IEEE Std 4-1968 provides for reduction of the series resistance when the test frequency exceeds 1 kHz. The resistance should be in an inverse ratio to the frequency, which for current-limiting reactors means the sphere gap resistance must be short-circuited.

A.2.1.1 Factory dielectric tests The purpose of dielectric tests in the factory is to check the insulation and workmanship, and, when required, to demonstrate that the device has been designed to withstand the insulation tests required by the purchase specifications. Impulse tests, when required, shall precede the low-frequency tests. Dielectric tests should preferably be made at the temperature assumed under normal operation or at the temperature attained under the conditions of commercial test. A.2.1.2 Insulation resistance The insulation resistance of machinery is of doubtful significance as compared with the dielectric strength. It is subject to wide variation with temperature, humidity, and cleanliness of the parts. When the insulation resistance falls below prescribed values, it can, in most cases of good design and where no defect exists, be brought up to the required standard by cleaning and drying the machine. The insulation resistance, therefore, may afford a useful indication as to whether the machine is in suitable condition for application of the dielectric test. The insulation-resistance test shall be made with all circuits of equal voltage above ground connected together. Circuits or groups of circuits of different voltage above ground shall be tested separately.

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A.2.1.3 Periodic dielectric tests in the field It is recognized that dielectric tests impose a severe stress on the insulation, and if applied frequently will hasten breakdown or may cause breakdown, the stress imposed, of course, being the more severe the higher the value of the applied voltage. Hence, practice in this matter has varied widely among operating companies, and the advisability of periodic testing may be questionable. It is recommended that field tests of insulation should not be in excess of 75% of the factory test voltage; that for old apparatus rebuilt in the field, tests should not be in excess of 75% of the factory test voltage; and that periodic insulation tests in the field should not be in excess of 65% of the factory test voltage. These recommendations relate to dielectric tests applied between windings and ground and to tests between turns. Under some conditions, devices may be subjected to periodic insulation test using dc voltage. In such cases, the test dc voltage should not exceed the original factory test rms alternating voltage; for example, the factory test was 26 kV rms, then the routine test dc voltage should not exceed 26 kV. Periodic dc tests should not be applied to devices of higher than 34.5 kV rating. A.2.1.4 Tests on bushings When tests are required on bushings separately from the devices, the tests shall be made in accordance with IEEE Std 21™-1964, IEEE Standard Requirements and Test Code for Outdoor Apparatus Bushings [ANSI C76.1-1964 (Reaff 1970)]. A.2.2 Applied-potential tests The terminal ends and taps brought out of the case of the winding under test should all be joined together and to the line terminal of the testing transformer. The duration of these tests shall be 1 min and of a value given in Table 4, Table 5, and Table 6 of IEEE Std C57.12.00. All other terminals and parts (including magnetic shield and tank) should be connected to ground and to the other terminal of the testing transformer. The ground connections between the apparatus being tested and the testing transformer must be a substantial metallic circuit. All connections must make a good mechanical joint without forming sharp corners or points. Small bare wire may be used in connecting the respective taps and line terminal together, but care must be taken to keep the wire on the high-voltage side well away from the ground. The high-voltage lead from the testing transformer should preferably be at least 1/8 in (3 mm) in overall diameter. Neutral Capacitor Terminal-to-Case. The terminal-to-case applied-potential test shall be made by applying between terminal and case, with the case connected to ground, the specified alternating voltage. Neutral Resistor. Tests shall be made by applying between terminals and ground, for the complete device, or between terminals of each unit and its own frame, for individual frames, the specified alternating voltage. No appreciable resistance should be placed between the testing transformer and the device under test. It is permissible, however, to use reactive coils at or near the terminals of the testing transformer. 51

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A sphere gap set at a voltage 10% or more in excess of the specified test voltage shall be connected during the applied-potential test. For devices to be tested at 50 kV or less, it is permissible to depend on the ratio of the testing transformer to indicate the proper test voltage and also to omit the sphere gap, if the kVA equivalent parts are less than 100 for grounding transformers and ground-fault neutralizers or 500 for current-limiting reactors and other devices. A.2.3 Induced-potential tests A.2.3.1 Grounding transformers and ground-fault neutralizers As this test over-excites the device, the frequency of the applied potential should be high enough to prevent the exciting current of the device under test from exceeding about 30% of its rated-load current. Ordinarily, this requirement necessitates the use of a frequency of 120 Hz or more, when testing 60 Hz units. When frequencies higher than 120 Hz are used, the severity of the test is abnormally increased, and for this reason the duration of the test should be reduced in accordance with Table A.1. Table A.1—Duration of induced-potential tests related to frequency Frequency in Hz

Duration in seconds

120 and less

60

180

40

240

30

360

20

400

18

To avoid switching surges, the voltage should be started at one-quarter or less of the full value and be brought up gradually to full value, within a period of 15 s. After being held for the duration of time specified in the table above, it should be reduced gradually to onequarter of the maximum value within a period of 5 s, at which time the circuit may be opened. A.2.3.2 Dry-type current-limiting reactors Because of the low impedance of current-limiting reactors, inducing voltage between turns will have to be done with a high frequency, well above the power frequency range. This can be obtained by repeatedly charging a capacitor and discharging it into the reactor winding. The number of discharges should be sufficient to produce 7200 cycles of high frequency having crest values outlined in 7.2.3 of IEEE Std 32-1972 [B24] except that testing time shall not exceed 60 s. If the available equipment cannot induce sufficient voltage in the reactor coil directly, then turns of insulated cable may be placed around the mid-section of the reactor as a primary of a Tesla coil and the capacitor discharged into it. Where a sphere gap is used to check, the current-limiting resistance normally in series with the spheres should be short-circuited. Detection shall be by noise, smoke, or spark discharge in the windings.

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A.2.3.3 Oil-immersed current-limiting reactors For oil-immersed reactors, each line terminal of each winding shall be induced-potential tested as outlined in 7.2.3 of IEEE Std 32-1972 [B24]. A.2.3.4 Neutral capacitors The terminal-to-terminal induced-potential test shall be made by applying between the terminals of each unit the specified alternating voltage. A.2.3.5 Neutral resistors No induced test required. A.2.4 Standard impulse tests The standard impulse test consists of applying, in the following order, one reduced full wave, two chopped waves, and one full wave. A.2.4.1 Reduced full-wave test For this test, the applied voltage wave shall have a crest value between 50% and 70% of the full wave in accordance with 3.2 of IEEE Std 32-1972 [B24]. Crest voltages near the lower limit are preferable. A.2.4.2 Chopped-wave test For this test, the applied voltage wave shall be chopped by a suitable air gap. It shall have a crest voltage and time to flashover in accordance with 3.2 of IEEE Std 32-1972 [B24]. To avoid recovery of insulation strength if failure has occurred during a previous impulse, the time interval between application of the last chopped wave and the final full wave should be minimized, and preferably should not exceed 5 min. A.2.4.3 Full-wave test For this test, the voltage wave shall have a crest value in accordance with 3.2 of IEEE Std 32-1972 [B24] and no flashover of the bushing or test gap shall occur. To avoid flashover of the bushing during adverse conditions of humidity and air density, the bushing flashover may be increased by appropriate means. In general, the tests shall be applied to each terminal one at a time. All impulses applied to a device shall be recorded by a cathode-ray oscillograph if their crest voltage exceeds 40% of the crest of the full wave in accordance with 3.2 of IEEE Std 32-1972 [B24]. When reports require oscillograms, those of the first reduced full wave, the last two chopped waves, and the last full wave of voltages shall represent a record of the successful application of the impulse test to the device. 53

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

A.2.4.4 Connections for impulse tests One terminal of the winding under test shall be grounded directly or through a small resistance if current measurements are to be made. Other terminals in the same winding not being tested or terminals in the windings of other phases may be protected by grounding or other appropriate means. In some cases the inductance of the winding is so low that the desired impulse voltage magnitude and the duration to the 50% point on the tail of the wave cannot be obtained with available test equipment. Such low-inductance windings may be tested by inserting a resistor of not more than 500 Ω in the grounded end of the winding. The secondaries of current transformers, either on bushings or permanently connected to the equipment being tested, shall be short-circuited and grounded. Any magnetic shielding or metallic housing shall be grounded for all impulse tests. A.2.4.5 Detection of failure Because of the nature of impulse test failures, one of the most important matters is the detection of failure. There are a number of indications of insulation failure. Some of these are as follows: a)

Noise; presence of smoke or bubbles; failure of the gap or bushing to flash over, although the oscillogram indicates a chopped wave.

b)

Any difference between the reduced full wave and the final full wave detected by superimposing the two oscillograms or any difference between the two chopped waves from each other or from the full wave up to the time of flashover similarly detected. Such deviations may, however, be caused by conditions in the impulse test circuit external to the device and should be fully investigated.

c)

Measurement of the current in the grounded end of the winding tested. The current is measured by means of a cathode-ray oscillograph connected to a suitable shunt inserted between the normally grounded end of the winding and the grounded tank. Any deviation of current-wave shape obtained during the reduced full wave and full-wave tests indicates changes in impedance arising from insulation breakdown within the device, or changes in the impulse circuit external to the device, and the cause should be investigated.

A.2.4.6 Wave to be used for impulse tests A nominal 1.2 × 50 µs wave shall be used for impulse tests. Either, but not both, positive or negative waves may be used. Waves of negative polarity for oil-immersed apparatus and of positive polarity for dry-type or compoundfilled apparatus are recommended and shall be used unless otherwise specified. If in testing oil-immersed apparatus the atmospheric conditions at the time of test are such that the bushings will not withstand the specified polarity wave, then a wave of the opposite polarity may be used. The time to crest on the front from virtual time zero to actual crest shall not exceed 2.5 µs, except for windings of large impulse capacitance (low-voltage high-apparent-power and some high-voltage highapparent-power windings). To demonstrate that the large capacitance of the winding causes the long front, the impulse generator series resistance may be reduced, which should cause imposed oscillations. Only the inherent generator and lead inductances should be in the circuit.

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For convenience in measurement, the time to crest may be considered as 1.67 times the actual time between points on the front of the wave at 30% and 90% of the crest value. The time on the tail to the point of half-crest voltage of the applied wave shall be 50 µs from the virtual time zero. The virtual time zero can be determined by locating points on the front of the wave at which the voltage is, respectively, 30% and 90% of the crest value and then drawing a straight line through these points. The inter-section of this line with the time axis (zero-voltage line) is the virtual time zero. If there are oscillations on the front of the wave, the 30% and 90% points should be determined from the average, smooth wave front sketched in through the oscillations. The magnitude of the oscillations preferably should not exceed 10% of the applied voltage. (With superimposed oscillations of high magnitude, evaluation of wave crest is difficult, while if generator characteristics are such as to give a completely smooth wave it may be difficult to detect failures of small portions of the winding insulation by means of the cathode-ray oscillograph. If the impulse generator is sufficiently flexible, a good compromise is the use of generator constants such that the device impedance largely determines the length of the tail of the applied wave.)

A.3 Impedance and loss measurements It is not practical to measure the resistive and reactive components of the impedance voltage separately, but after the total impedance loss and impedance voltage are measured, the components may be separated by calculation. Connections for impedance voltage and impedance loss tests are shown in Figure A.2.

Figure A.2—Connections for impedance voltage and impedance loss tests Resistance and reactance components of the impedance voltage are determined by the use of the following equations: Er =

P I

(A.1)

E x = E 2 − E r2

(A.2)

where E Er

= potential impressed on winding = component of E that is in phase with I 55

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Ex P I

= component of E that is in quadrature with I = power measured in impedance test of winding carrying current = current in winding on which voltage is impressed

The I2R component of the impedance loss increases with the temperature, the stray-loss component diminishes with the temperature, and, therefore, when it is desired to convert the impedance losses from one temperature to another, as for instance when calculating efficiency that calls for 75 °C losses, the two components of the impedance loss are converted separately. Thus,

Pr' = Pr

Ps' = Ps

T +θ ' T +θ

(A.3)

T +θ

(A.4)

T +θ '

where T T

= 234.5 for copper = 225 for aluminum ' ' Pr and Ps' are desired resistance and stray losses, respectively, at the specified temperature θ

Pr and Ps are measured resistance and stray losses at temperature. A.3.1 Wattmeter method Voltage at rated frequency is applied and adjusted to circulate rated current in the winding (see Figure A.2). With current and frequency adjusted to the rated values as nearly as possible, simultaneous readings should be taken on the ammeter, voltmeter, wattmeter, and frequency meter. Because of the extremely low power factor, correction must be made for phase angle and losses of meters and metering transformers. Test current and potential leads should leave the winding terminals in a radial plane for a distance not less than one coil diameter when measuring coils in air. In the case of reactors of the current-limiting type, when reactor windings are being measured outside of enclosures or shielded tanks, the measurement may be made at lower than rated current to minimize the effect of short-circuited loops of nearby magnetic materials that are not part of the reactor and would have a disproportionately larger current induced in them at rated current. The temperature of the winding shall be taken immediately before and after the impedance measurements in a manner similar to that described in A.1. The average shall be taken as the true temperature. The I2R loss of the winding is calculated from the resistance measurements (corrected to the temperature at which the impedance test was made) and the currents which were used in the impedance measurement. These I2R losses subtracted from the impedance losses give the stray losses of the winding. When reactor windings are enclosed in shielded housings or tanks, part or all of which are magnetic material, part of the stray loss must be considered with the winding I2R when correcting losses from measured temperature to other temperatures. Since this varies with proportions of design and type of shield, it will have to be approximated for each design but can be checked by measurement of loss at the start and finish of the temperature run. Per-unit values of the resistance, reactance, and impedance voltage are obtained by dividing E, Er, Ex in Equation (A.1) and Equation (A.2) by the rated voltage. Percentage values are obtained by multiplying perunit values by 100. 56

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Temperature correction shall be made as in Equation (A.3) and Equation (A.4). The stray-loss component of the impedance watts is obtained by subtracting from the latter the I2R losses of the reactor. A.3.2 Bridge method Bridges are frequently used and are generally more accurate than the wattmeter method.

A.4 Temperature-rise tests A.4.1 Loading for temperature-rise tests Neutral grounding devices shall be tested under loading conditions that will give losses approximating, as nearly as possible, those obtained at rated frequency with rated current in the device. Units having taps for more than one rating shall be tested on the connection providing the highest losses. For devices rated less than 10 min, the loading shall be adjusted to obtain a steady-state rise approximating that listed in Table A.2 (original Table 6 from IEEE Std 32-1972 inserted next for convenience) for the class of insulation involved. The data so obtained shall be used to determine the degree of compliance with the short-time temperature requirements by calculation using the method of A.5.1.

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Less than 10 minutes (average) section)

Steady state (hot spot section) Steady state (average section) Extended-time (average section) Ten-minute (average section)

Column 2

65 55 75 125 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 300 320 340

55 75 125 120 125 130 135 140 145 150 155 160 165 170 180 190 200 210 220 230 240 250 260

103 000 53 300 30 000 16 700 9 330 5 330 3 330 2 000 1 270 767 500 33.3 257 150 103 73 50 26 14 7.7

Column 4 55 °C Dry-type Temp. Time rise factor†† seconds °C

65

22 500 15 000 9 000 6 700 4 300 3 000 2 100 1 500 1 030 733 517 270 143 77 40 25 15 8.8 5.3 3.3

Column 3 55 °C Oil-immersed Temp. Time rise factor†† seconds °C

160 170 180 190 200 210 220 230 240 250 260 280 300 320 340 360 380 400 420 440

185

110

80

110

143 000 86 700 53 700 33 300 21 000 13 700 9 170 6 170 4 170 2 830 1 970 967 500 277 157 94 61 34 22 14

Column 5 80 °C Dry-type Temp. Time rise factor†† seconds °C

250 260 270 280 290 300 310 320 330 340 350 360 380 400 420 440 460 480 500 520

275

200

150

180

98 400 63 000 41 000 26 600 18 000 12 000 8 340 5 670 4 000 2 800 1 968 1 434 750 406 243 140 84 51 32.4 20.5

Column 6 150 °C Dry-type Temp. Time rise factor†† seconds °C

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58

NOTE—The values of both C and P should be taken at the temperature corresponding to θ1 and standard ambient conditions.

Rated Time for thermal current ratings (rated voltage for certain resistors)

Steady State for continuous current ratings

Column 1

760

610

610

385

510

460

385

385

Column 7 Resistors Temperature rise °C Stainless Cast steel** grid #

Table A.2—Limiting temperature rises above 30 °C ambient for current carrying partsNeutral devices†* (Originally Table 6 from IEEE Std 32-1972 (Reaff 1990) [B24])

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59

= Steady-state hot-spot temperature rise at continuous current rating either above top oil temperature for oil- immersed equipment or above the ambient air temperature for dry-type equipment. θ2 = Average winding rise over ambient for rated continuous current under standard operating conditions = Top oil rise over ambient for rated continuous current under standard operating conditions θ3 P = Specific power (watts per unit mass of conductor material) C = Specific thermal capacitance (joules per degree Celsius unit mass) of conductor material and its associated insulation, as calculated in Equation (A.10) and Equation (A.11) M = Multiplier from Table A.3 [original Table 7 from IEEE Std 32-1972 inserted below for convenience] ** The temperature rise limits for extended time, ten-minute or less rated resistors are hot-spot values.

where θ1

‡ The time factor of a device for use in determining the limit of temperature rise shall he calculated as follows: F = (Cθ1 Mseconds)/P = 1.182 θ2 — θ3 for 55 °C oil-immersed devices θ1 = 1.182 θ2 for 55 °C dry-type devices = 1.373 θ2 for 80 °C dry-type devices = 1.200 θ2 for 150 °C dry-type devices

† No limits have been established for capacitors.

* The values in this table are based on the thermal aging characteristics of the insulation. Devices built to these thermal limits will have normal insulation life. Other factors may limit temperature rises in specific designs. For example: 1) The reduction in the mechanical strength and increase in elongation of copper at temperatures above 300 °C and aluminum above 350 °C. 2) Gas evolution from insulation and oil adjacent to hot conductors. 3) Auto-ignition of insulation or oil.

(Originally Table 6 from IEEE Std 32-1972 (Reaff 1990) [B24])

Table A.2—limiting temperature rises above 30 °C ambient for current carrying partsNeutral devices†* (continued)

IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Table A.3—Time factor multiplier for use in equation in Table A.2 (Originally Table 7 from IEEE Std 32-1972 (Reaff 1990) [B24]) Oil-immersed 55 °C rise Steadystate average Multiplier winding M rise 55 1.0 54 0.13 53 0.074 52 0.053 51 0.043 50 0.037 48 0.031 46 0.026 44 0.024 40 0.022 35 0.021 0.020 < 30

Dry-type 80 °C rise Steadystate average Multiplier winding M rise 80 1.0 79 0.18 78 0.098 77 0.069 76 0.056 75 0.047 74 0.041 73 0.037 72 0.034 70 0.030 68 0.027 66 0.025 64 0.024 62 0.023 60 0.022 55 0.021 0.020 < 50

55 °C rise Steadystate average Multiplier winding M rise 55 1.0 54 0.15 53 0.088 52 0.066 51 0.052 50 0.045 49 0.040 48 0.036 46 0.031 44 0.027 42 0.025 40 0.024 35 0.022 30 0.021 0.020 < 25

150 °C rise Steadystate average Multiplier winding M rise 150 1.0 149 0.21 148 0.12 147 0.085 146 0.067 145 0.056 144 0.049 142 0.040 140 0.034 138 0.031 136 0.028 134 0.026 132 0.025 130 0.024 128 0.023 124 0.022 120 0.021 0.020 < 115

A.4.2 Determination of average measured winding temperature by the hot-resistance method The average measured temperature of the winding conductor may be determined by either of the following equations:

θ=

R (T + θ O ) − T RO

(A.5)

θ=

R − RO (T + θ O ) + θ O RO

(A.6)

where T T θ θO R RO

= 234.5 for copper = 225 for aluminum = temperature in degrees Celsius corresponding to hot resistance R = temperature in degrees Celsius corresponding to hot resistance RO = hot resistance (see A.4.5) = cold resistance determined in accordance with the rules in this standard

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

The induction time for the measuring current to become stable should be noted during the cold-resistance measurements, in order to help ensure that sufficient time elapses for the induction effect to disappear before hot-resistance readings are taken. When tests are made at an altitude not exceeding 1000 m (3300 ft) above sea level, no altitude correction shall be applied to the temperature rise. When a device which is tested at an altitude less than 1000 m (3300 ft) is to be operated at an altitude in excess of that altitude, it shall be assumed that the observed temperature rise will increase in accordance with the following relation: Increase in temperature rise

= observed rise ×

( A − 1000) (F )

= observed rise ×

(B − 3300) (F )

100

or

330

where A B F

is the altitude in meters is the altitude in feet is the empirical factor equal to 0.004 for oil-immersed self-cooled and 0.005 for dry-type selfcooled

The observed rise in the foregoing equation is as follows: 

Top oil rise, or average oil rise, over the ambient temperature for oil-immersed self-cooled devices.



Winding rise over the ambient temperature for dry-type self-cooled devices.



The winding rise for oil-immersed devices over ambient at altitude A is the observed winding rise over ambient plus the calculated increase in temperature rise.

Devices shall be completely assembled and if oil-immersed they shall be filled to the proper level. If the devices are equipped with thermal indicators, bushing-type current transformers, etc., such apparatus shall be assembled with the device. The temperature-rise test shall be made in a room that is essentially free from drafts. The temperature of the surrounding air (ambient temperature) shall be determined by at least three thermocouples, or thermometers, spaced uniformly around the devices under test. They should be located about one-half the height of the device, and at a distance of about 1 m to 2 m (3 ft to 6 ft ) from the device. They should be protected from drafts and abnormal heating. For dry-type reactors, they should be located one coil diameter from the coil at the level of the bottom of the coil.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

To reduce to a minimum the errors due to time lag between the temperature of the devices and the variations in the ambient temperature, the thermocouples, or thermometers, shall be placed in suitable containers that shall have such proportions as will require not less than 2 h for the indicated temperature within the container to change 6.3 °C if suddenly placed in air that has a temperature 10 °C higher, or lower, than the previous steady-state indicated temperature within the container. The temperature rise of metal parts (other than the winding conductor) in contact with, or adjacent to, insulation, and of other metal parts, shall be determined by thermocouple or by thermometer. Provision shall be made to measure the surface temperature of iron or alloy parts surrounding or adjacent to the outlet leads or terminals carrying large currents. Readings shall be taken at intervals or immediately after shutdown. The determination of the temperature rise of metal parts within the case, other than winding conductors, is a design test and shall be made when so specified unless a record of this test made on a duplicate, or essentially duplicate, unit can be furnished. This test will not be made unless definitely specified because provision for the proper placement of the thermocouples and leads must frequently be made during the design of the devices. Comparison with other devices having metal parts of similar design and arrangement, but not necessarily having the same rating, will in many cases be adequate. A thermocouple is the preferred method of measuring surface temperature. When used for this purpose on tanks or enclosures, the thermocouples should be soldered to a thin metal plate or foil approximately 25 mm (1 in) square. The plate is to be placed and held firmly and snugly against the surface. In either case, the thermocouple should be thoroughly insulated thermally from the surrounding medium. It is permissible to shorten the time required for the test by the use of initial over-loads, restricted cooling, or any other suitable method. The temperature rise of the winding shall be determined by the resistance method, or by thermometer when so specified. The ultimate temperature rise is considered to be reached when the temperature rise becomes constant; that is, when the temperature rise does not vary more than 2.5% during a period of three consecutive hours. A.4.3 Temperature-rise tests—oil-immersed devices The top oil temperature shall be measured by a thermocouple or alcohol thermometer immersed approximately 51 mm (2 in) below the top oil surface. The average temperature of the oil shall be determined when it is to be used. The average oil temperature is equal to the top oil temperature minus one-half the difference in temperature of the moving oil at the top and the bottom of the cooling means, as determined by suitable measurements. For devices with external cooling means, this temperature difference may be closely approximated by careful determination of the temperature on the external surfaces of the oil inlet and oil outlet of the cooling means by the use of thermocouples. The ambient temperature shall be taken as that of the surrounding air which should be not less than 10 °C or more than 40 °C. No corrections for variations of ambient temperature within this range shall be applied.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Temperature tests may be made with ambient temperature outside the range specified, if suitable and agreed upon correction factors are available. A.4.4 Temperature-rise tests—dry-type devices When the ambient air temperature is other than 30 °C, a correction shall be applied to the temperature rise of the winding by multiplying it by the correction factor K, which is given by the following ratio:

K=

T + 30 T +θ

(A.7)

where T T θ

= 234.5 for copper = 225 for aluminum = ambient air temperature in degrees Celsius

When the temperature-rise tests by thermometer are required, it is important that the coil thermometers be properly placed in the air ducts in such a manner as to indicate the winding temperature and yet not restrict the ventilation. This may be accomplished by means of grooved sticks of dry wood or some other kind of insulating material slightly larger than the thermometer bulbs. When the thermometers are used for measuring temperature of apparatus other than oil-immersed, the bulbs shall be covered by felt pads cemented to the equipment. When pads interfere with ventilation, as in ventilating ducts between coils, grooved wooden sticks may be used. Dimensions of felt pads for use with large apparatus should be approximately 40 mm × 50 mm × 3 mm (1.5 in × 2 in × 0.125 in). When the temperature rise has become constant, the test voltage and current should be removed and the blowers, if used, shut off. Immediately thereafter, the coil thermometers and any other temperatureindicating devices should be read continually in rotation until the temperature begins to fall. If any of the thermometer temperatures are higher than those observed during the run, the highest temperature should be recorded as the final thermometer temperature. The temperature rise of the device above the specified ambient temperature, when tested or calculated in accordance with the rating, shall not exceed values given in Table A.1 (originally Table 6, IEEE Std 321972 [B24]). For times of one minute or less, the temperature rise shall be determined in accordance with 4.3.1 of IEEE Std 32-1972 [B24]. For a time of 10 min and greater, 4.3.2 and 4.3.3 of IEEE Std 32-1972 [B24], the temperature rise is the limiting observable temperature rise using the methods of measurement as follows: 

Resistance method for grounding transformers, ground-fault neutralizers, and reactors.



Thermometer method and radiation thermometer method for resistors.

When the rated voltage of a resistor is equal to the line-to-neutral voltage of the circuit, the specified temperature limits are based upon the application of line-to-neutral voltage at rated frequency to the resistor for a time equal to the rated time, the current being allowed to decrease during the rated time. 63

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

NOTE—This assumes there will be sufficient additional impedance in the circuit to keep the current at the start of the test from exceeding the mechanical current rating when the product of the rated current and the resistance at the starting temperature is less than 83 1/3 % of the line-to-neutral voltage.

A.4.5 Correction back to shutdown A.4.5.1 Empirical method The empirical method utilizes correction factors that represent average results from usual commercial designs. This method is not to be used for forced-oil-cooled devices, nor for those designs that deviate considerably from usual commercial proportions. In such cases, the cooling curve method should be used (see A.4.5.2). Take one hot-resistance reading on each winding, record the time after shutdown, and determine the corresponding temperature rise. When the conductor loss of oil-immersed equipment does not exceed 15 W/kg (7 W/lb) for copper or 26 W/kg (12 W/lb) for aluminum, an arbitrary correction of 1 °C per minute may be used. The conductor loss in watts per pound in a winding shall be taken as the sum of the calculated I2R and eddy-current loss at a temperature equal to the rated temperature rise plus 20 °C divided by the weight of the active conductor in pounds. A.4.5.2 Cooling curve method Take a series of at least four, preferably more, readings on one phase of each winding, and record the time after shutdown for each reading. The readings should be time spaced to help ensure accurate extrapolation back to shutdown. The overall reading time should exceed 4 min and may extend considerably beyond. The first reading on each winding should be taken as quickly as possible after shutdown, but not before the measuring current has become stable, and must be taken within 4 min. Plot the resistance time data on suitable coordinate paper, and extrapolate the curve back to instant of shutdown. The resistance value so obtained shall be used to calculate the average winding temperature at instant of shutdown. The resistance time curve obtained on one phase may be used to determine the correction back to shutdown for the other phases provided the first reading on each of the other windings has been taken within 4 min after shutdown. If necessary the temperature test may be resumed so that the first readings on any windings may be completed within the required 4 min.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

A.5 Temperature-rise calculations A.5.1 Thermal short-time capability calculations for reactors, ground-fault neutralizers, and transformers used for grounding The increase in winding temperature θf during short-time conditions shall be estimated on the basis of all heat stored in the conductor material and its associated turn insulation. The thermal capability of the conductor material shall be taken as the average of the values at the starting and finishing temperatures. All temperatures are in degrees Celsius. The increase in winding temperature θf that will occur during a specified short time t shall be calculated by the following equation:

θ f = (T + θ s )[(1 + e )m + 0.6m 2 ]

(A.8)

where m a=

t T T θf θs

Ws Wr e

= at Ws Wx 1 = C av T + θ s C av (T + 75)

(A.9)

= time, in seconds = 234.5 for copper = 225 for aluminum = increase in winding temperature during time t (not to be greater than the difference between starting temperature θs and the limiting temperature during short-circuit conditions given in Table A.3 (originally Table 7, IEEE Std 32-1972 [B24]). = starting temperature—reference ambient temperature (30 °C) plus steady-state hottest-spot temperature rise above reference ambient temperature at continuous current rating, using 1) measured hottest-spot temperature rise, if tested 2) standard hottest-spot temperature rise, if not temperature tested = short-circuit resistance loss at starting temperature θs, watts per kilogram or pound of conductor material = short-circuit resistance loss at 75 °C, watts per kilogram or pound of conductor material = per-unit eddy-current loss, based on resistance loss at the starting temperature 2

e e75 Cav

 T + 75  = (e 75 )  T + θ s  = per-unit eddy-current loss, based on resistance loss at 75 °C = average specific thermal capacity in Joules per degree Celsius, per kilogram or pound of conductor material and its associated turn insulation over the range of increase in winding temperature

NOTE—Equation (A.8) is an approximate formula and its use should be restricted to values of m = 0.6 or less. The exact equation is θ f =

( T + θ )  s

e

2m

(

+e e

2m

)

− 1 − 1  where = ε 2.7183 = base of the natural logarithms



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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

The thermal capacity Cx at any temperature θx below 500 °C may be closely estimated from the following empirical equations:

Metric

C x = 384 + 0.099 θ x + 243

English C x = 174 + 0.045 θ x + 110

Metric

C x = 893 + 0.441θ x + 794

Ai per kilogram of copper Ac Ai per pound of copper Ac Ai per kilogram of aluminum Ac

A English C x = 405 + 0.200 θ x + 360 i per pound of aluminum Ac

(A.10)

(A.11)

where Ai Ac

= cross-sectional area of insulation = cross-sectional area of conductor Table A.4—Respective Metric System and English System nomenclature for Equation (A.12) and Equation (A.13)

aFor cast iron, over the range of temperature covered by this standard, C shall be taken as 0.130.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

A.5.2 Thermal capability calculation for neutral resistors A.5.2.1 Respective Metric System and English System equations for temperature rise and current density, when current is constant The eddy-current loss may usually be ignored due to the high-resistance materials used in neutral resistors. The temperature rise when the current is held constant, and all heat is assumed to be stored in the active material, shall be computed by the following equations, where all quantities have been defined in Table A.4.

Metric

θ=

2 1  −1  0.014a 0 r0 tJ 0 log 10  a0  Cδ  

1 English θ = a0

  a r tJ 2 log −110  0 0 O  2430Cδ  

   − 1 + θ   1  

   − 1 + θ   1  

(A.12)

For design purposes, it is more convenient to insert the desired temperature rise and derive the current density which will produce the desired temperature rise. Thus,

Metric English

J0 =

9.62Cδ log 10 [1 + a 0 (θ − θ 1 )] a 0 r0 t

J0 =

2430Cδ log 10 [1 + a 0 (θ − θ 1 )] a 0 r0 t

(A.13)

NOTE—Equation (A.12) and Equation (A.13) apply only when the temperature coefficient of resistance a0 is substantially constant over the temperature range used, and should not be used for materials for which the coefficient varies greatly.

A.5.2.2 Respective Metric System and English System equations for temperature rise and current density, when voltage is constant For some resistors (see 10.1.1 of IEEE Std 32-1972 [B24]) temperature rise is computed on the basis that constant voltage is maintained between the terminals, the current being allowed to decrease as the resistance increases with temperature. The temperature rise, with all heat stored in the active material and with constant voltage, shall be computed by the following equations where all quantities have been defined in Table A.3. 2    − 1 + 1 + 0.478 J 0 r0 a 0 t  + θ 1 Cδ     1.898 × 10 −3 J 2 0 r0 a 0 t 1  − + + English θ = 1 1 a 0  Cδ 

Metric

θ=

1 a0

  +θ  1 

(A.14)

For design purposes, it is more convenient to insert the desired temperature rise and derive the current density that will produce the temperature rise with the voltage maintained. Thus, 67

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(θ − θ1 )2  4.18Cδ  θ − θ 1 + a 0  2 r0 t  

Metric

J0 =

English

(θ − θ1 )2  105Cδ  J0 = θ − θ 1 + a 0  2 r0 t  

(A.15)

NOTE—Equation (A.15) applies only when the temperature coefficient of resistance a0 is substantially constant over the temperature range used and should not be used for materials for which the coefficient varies greatly.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

Annex B (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. Books [B1] Central Station Engineers, Electrical Transmission and Distribution Reference Book, Chapter 19, Westinghouse Electric Corporation, East Pittsburgh, Pennsylvania, 1964. [B2] Willheim, R., Waters, M. Neutral Grounding in High Voltage Transmission, Elsevier Publishing Company, New York, 1956. Transactions, Proceedings, Conferences, Reports, etc. [B3] AIEE Committee Report, “Guide for Application of Ground-Fault Neutralizers,” AIEE Transactions on Power Apparatus and Systems, vol. 72, pp. 183–190, April 1953. [B4] AIEE Committee Report, “Present Day Grounding Practices on Power Systems,” AIEE Transactions on Power Apparatus and Systems, vol. 66, pp. 1525–1548, 1947. [B5] Auer, S., Capra, R. L., “Limiting Fault Currents on the Primary Distribution System,” Transmission & Distribution, pp. 22, 24 & 48, April 1980. [B6] Clerfeuille, J., Juston, P., Clement, M. “Extinguishing Faults Without Disturbances,” Transmission & Distribution World, pp. 52–59, August 1997. [B7] Druml, G., Kugi, A., Parr, B., “Control of Peterson Coils,” International Symposium on Theoretical Electrical Engineering, August 2001. [B8] Dunki-Jacobs, J. “The Historical Development of Neutral Grounding Practices,” IEEE Industrial Applications Magazine, vol. 3, pp. 10–20, March/April 1997. [B9] Emms, M., “Neutral Impedances in Fault Analysis,” IEEE Transactions on Power Systems, pp. 274– 279, vol. 13, no. 2, May 1998. [B10] Fulczyk, M., Bertsch, J., “Ground-Fault Currents in Unit-Connected Generators with Different Elements Grounding Neutral,” IEEE Transactions on Energy Conversion, vol. 176, no 1, pp. 61–66, March 2002. [B11] Gross, E., Gulachenski, E., “Experience of the New England System with Generator Protection by Resonant Neutral Grounding,” AIEE Transactions on Power Apparatus and Systems, vol. PAS-92, no. 4, pp. 1186–1194, August 1973. [B12] Gulachenski, E. M. and Courville E. W., “New England Electric’s 39 Years of Experience with Resonant Neutral Grounding of Unit-Connected Generators,” IEEE Transactions on Power Delivery, vol. 6, pp. 1016–1024, July 1991.

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IEEE Std C57.32-2015 IEEE Standard for Requirements, Terminology, and Test Procedures for Neutral Grounding Devices

[B13] Kappenman, J., “Neutral Blocking Device Combats Currents Caused by Geomagnetic Storms,” Transmission & Distribution Magazine, May 1972. [B14] Khunkhun, K., Koepfinger, J., Haddad, M., “Resonant Grounding (Ground Fault Neutralizer) of a Unit Connected Generator,” IEEE Transactions on Power Apparatus and Systems, vol. 38, no. 6, pp. 1633– 1640, November/December 2002. [B15] Nelson, J., “System Grounding and Ground-Fault protection in the Petrochemical Industry: A need for a Better Understanding,” IEEE Transactions on Industrial. Applications, vol. IA-99, pp. 672–679, November/December 1973. [B16] Niggli, M., “Neutral Reactors Limit Catastrophic Failures,” Transmission & Distribution, pp. 50–52, August 1979. [B17] Owen, E., “The Historical Development of Neutral-Grounding Practices,” IEEE Industry Applications Magazine, pp. 10–20, 1997. [B18] Papp, K., “Arc suppression coils for neutral grounding of distribution systems,” Power Technology International, Sterling Publications, London, pp. 197–200, 1992. [B19] Powell, L., “Influence of Third Harmonic Circulating Currents in Selecting Neutral Grounding Devices,” IEEE Transactions on Industrial. Applications, vol. IA-99, pp. 672–679, November/December 1973. [B20] Tomlinson, H. “Ground-Fault Neutralizer Grounding of Unit Connected Generators,” AIEE Transactions on Power Apparatus and Systems, pt. III, vol. 72, pp. 953–966, October 1953. [B21] Webb, C., “Determining the Rating of a Generator Neutral Grounding Reactor,” Industrial Power Systems, General Electric Company, Schenectady, NY, December 1970. Standards [B22] IEEE Standards Dictionary Online subscription is available at: http://ieeexplore.ieee.org/xpls/dictionary.jsp. [B23] IEEE Std 4™, IEEE Standard Techniques for High-Voltage Testing. [B24] IEEE Std 32™-1972 (Reaff 1990), IEEE Standard Requirements, Terminology, and Test Procedures for Neutral Grounding Devices. [B25] IEEE Std C57.12.01™, IEEE Standard General Requirements for Dry-Type Distribution and Power Transformers, Including Those with Solid Cast and/or Resin Encapsulated Windings. [B26] IEEE Std C57.12.80™, IEEE Standard Terminology for Power and Distribution Transformers. [B27] IEEE Std C57.19.100™, IEEE Guide for Application of Power Apparatus Bushings. [B28] IEEE Std C57.98™, IEEE Guide for Transformer Impulse Tests. [B29] IEEE Std C62.92.1™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems—Part 1: Introduction. [B30] IEEE Std C62.92.2™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part II—Grounding of Synchronous Generator Systems.

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[B31] IEEE Std C62.92.3™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part III—Generator Auxiliary Systems. [B32] IEEE Std C62.92.4™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution. [B33] IEEE Std C62.92.5™, IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part V—Transmission Systems and Subtransmission Systems.

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I EEE s t andar ds . i eee. or g Phone:+17329810060 Fax:+17325621571 ©I EEE Authorized licensed use limited to: UNIVERSITY OF SUSSEX. Downloaded on May 12,2016 at 18:21:13 UTC from IEEE Xplore. Restrictions apply.