Key Single Line Diagram(1)

DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM

TABLE OF CONTENTS 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

INTRODUCTION.................................................................................... 2 BJECTIVE AND APPLICABILITY............................................................... 2 DEFINITION OF TERMS .......................................................................... 3 FUNCTIONAL DESCRIPTION OF SYSTEM / EQUIPMENT ............................. 6 DESIGN BASIS / CRITERIA ..................................................................... 8 QUALITY ASSURANCE AND TESTING ................................................... 13 APPLICABLE CODES AND STANDARDS ................................................. 13 INTERFACE REQUIREMENTS ................................................................ 13 CHECK LISTS ..................................................................................... 13 ATTACHMENTS.................................................................................. 13

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM 1.0 INTRODUCTION Key Single Line Diagram is intended to show the overall electrical power distribution arrangement for a plant. Considering the applicability in case of a power plant, one can have the following information from a Key Single Line diagram: o

Generation & Power Evacuation Voltages

o

Auxiliary Power Supply Voltages

o

Power Evacuation arrangement – i.e., Bus Configuration and No. of feeders in the EHV Switchyard

o

Auxiliary Power Distribution Arrangement – i.e., the feeding arrangement of Unit and Station Transformers, Major Buses at MV and LV levels and their feeding arrangements.

The Key SLD, however, does not show the details of all the loads connected at MV or LV buses, which forms the part of the specification of MV and LV Switchgears, MCCs and Distribution Boards In short, Key Single Line Diagram forms the basis of detail engineering and specifications of individual equipments like Power and Distribution Transformers, MV and LV Switchgears, EHV Switchyard etc. Keeping in view the context of the projects handled by us, Key Single Line Diagram as applicable for Power Plant has only been discussed in the current document. 2.0 BJECTIVE AND APPLICABILITY The purpose and objective of this document is to establish consistent guidelines for the following activities related to Key Single Line Diagram: To give a general idea about the contents of a Key Single Line Diagram •

General criteria for preparation of a Key Single Line Diagram

•

Basic Engineering related to preparation of a Key Single Line Diagram

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM 3.0 DEFINITION OF TERMS Key Single Line Diagram An electrical Single Line Diagram which shows the overall electrical power distribution arrangement starting from power receiving / evacuation at EHV level to the load distribution at LT Level upto the main Switchgears / Distribution Boards (without showing the load details). Generation Voltage The nominal voltage at the output of Generator Power Evacuation Voltage Nominal Voltage of the EHV Substation where the generator output is connected through the step-up Generator Transformer Generator Transformer The step-up transformer whose primary (LV) side is connected to the Generator output terminals and the secondary (HV) Terminals are connected to the EHV Substation Bus for evacuation of the generated power Unit Transformer The Power Transformer whose HV Side is connected to the output terminals of Generator and LV side to the so-called “Medium Voltage (Generally 11 kV) Unit Switchgear” in order to provide power supply to the Unit Auxiliary Loads. The standard Secondary side voltage level of Unit and Station Transformer in India is either 11 kV or 6.6 kV. However, for convenience of description, in current document we will describe the secondary voltage level of Unit orStation Transformer as 11 kV. Station Transformer Station transformer is the Power Transformer whose HV Side is connected to the EHV Bus of the evacuation substation and the LT Side to the so-called “Medium Voltage (Generally 11 kV) Station Switchgear” in order to provide power supply to the common station auxiliary loads. The standard Secondary side voltage level of Unit and Station Transformer in India is either 11 kV or 6.6 kV. However, for convenience of description, in current document we will describe the secondary voltage level of Unit or Station Transformer as 11 kV. Unit Auxiliary Transformer Document No.

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM Unit Auxiliary Transformer is a Distribution Transformer that takes power supply at HV Side from the MV Unit Switchgear and gives power supply to the incomer feeder of the 415 LT unit auxiliary loads. Station Auxiliary Transformer Station Auxiliary Transformer takes the power supply at HV Side from the MV Station Switchgear and gives power supply to the incomer feeder of the 415 V Station Auxiliary Switchgear, which feeds various LT common station auxiliary loads. Unit Switchgear MV Unit Switchgears take power supply at Incomer feeders from secondary side of Unit Transformer and feeds the HT Side of the Unit Auxiliary Transformers ( having secondary side voltage of 3.3 kV or 433 V) of various applications. The standard voltage level of Unit and Station Switchgear in India is either 11N kV or 6.6 kV. However, for convenience of description, in current document we will describe the voltage level of Unit or Station Switchgear as 11 kV. Station Switchgear MV (generally 11 or 6.6 kV) Station Switchgears take power supply at Incomer feeders from Unit Transformer and feeds the HT Side of the Station Auxiliary Transformers ( 11/0.433 kV) of various applications. The standard voltage level of Unit and Station Switchgear in India is either 11 kV or 6.6 kV. However, for convenience of description, in current document we will describe the voltage level of Unit or Station Switchgear as 11 kV. Unit Auxiliary Switchgear The LT Switchgears which are fed from the LT Side of the Unit Auxiliary Transformers and feed various LT Unit Auxiliary Loads. Station Auxiliary Switchgear The LT Switchgears which are fed from the LT Side of the Station Auxiliary Transformers and feed various LT Common Station Auxiliary Loads. Isolated Phase Bus Duct (IPBD) This type of Bus Duct is generally used to connect the output of the Generator to Generator Transformer and also for tapping to the Unit Transformer. Three phases are connected in the form of three runs of isolated bus ducts in three separate enclosures.

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM Phase Segregated Bus Duct (PSBD) This type of Bus Duct is generally used to in the incomer feeder for the Medium Voltage Switchgears. Here, three phases run in a common enclosure with insulated barrier between individual phases. Power Evacuation Substation This is the EHV Substation through which the generated power is evacuated. It contains a number of “Generator Transformer Bays” (each bay corresponding to each unit), which draws the generated power from the Generator Transformer HT Terminals and evacuates the power to the EHV grid through a number of “Line feeder Bays”. Centre-Break Disconnector This is an outdoor Disconnector at EHV Substation in which both contacts of each pole are movable and engage at a point substantially midway between their supports. Double-Break Disconnector This is an outdoor disconnector at EHV Substation which opens a circuit at two points. Earthing Switch Earthing Switch is a mechanical switching device for earthling parts of a circuit, capable of withstanding for a specified time currents under abnormal conditions such as those of short circuit, but not required carrying current under normal conditions of the circuit. Circuit Breaker This is a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of short circuit. Outdoor Circuit Breaker There are circuit breakers which are located at outdoor EHV substation and suitable for outdoor installation. They have generally SF6 or Vacuum as interrupting medium. Air Circuit Breaker (ACB)

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM These are circuit-breakers in which the contacts open and close in air at atmospheric pressure. They are used for carrying bulk current (800 A or more) at LT System and generally used at Incomer and Bus Coupler circuits of 415 V Unit and Station Auxiliary Switchgears. Moulded-Case Circuit-Breaker (MCCB) These are circuit-breakers having a supporting housing of moulded insulating material forming an integral part of the circuit-breaker. They are also used in LT Switchgear or DBs where current requirement is upto 630 A. Both Air circuit Breakers and Moulded-Case Circuit-Breakers are rated to interrupt the maximum LT fault current (generally rated at 50 kA) 4.0 FUNCTIONAL DESCRIPTION OF SYSTEM / EQUIPMENT As discussed earlier, Key Single Line Diagram gives the overall details of the plant electrical power distribution arrangement starting from EHV Substation upto LT Main Switchgears / Distribution Boards. Starting from the Generator Unit, the contents of a Key Single Line Diagram can be divided into two parts: a) Power Evacuation Arrangement and (upstream to the Generator) b) Auxiliary Power Distribution arrangement (downstream to the Generator). 4.1 Power Evacuation Arrangement The following details related to EHV and Generation voltage side are available in the Key Single Line Diagram I. No. of Units, Unit and Station Transformers with tentative ratings. II. Mode of connection of the Generator Units to Generator Transformers – whether Generator Circuit Breaker is used or not. III. Details of connection of Generator output to Generator Transformer and Unit Transformers ( generally by means of Isolated Phase Bus Duct) IV. Transformer vector groups and relative phasor displacement of EHV, Generator, MV and LV Voltages. V. Arrangement of EHV Substation, including: a. Switching Scheme (One and Half Breaker / 2 Main + 1 Transfer etc.)

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM b. No. of Bays / Diameters in the substation, identifying the Generator Transformer Bays and Line Bays. c. Location of the earth switches at feeders and bus. 4.2 Auxiliary Power Distribution System

Following details are available in the Key Single Line Diagram regarding the Auxiliary Power Distribution System: i.

Tentative details of the Unit and Station Transformers (No. of winding, vector group, cooling, type of tap changer etc).

ii.

Feeding arrangement of MV Unit and Station Switchgears

iii.

Feeding Arrangement of LT Unit and Station auxiliary Switchgears.

iv.

Tentative main LT switchgear / MCC buses meant for supplying various unit and station auxiliary services downstream to the Station Auxiliary Switchgears, or in some cases of Station Services, MV or LT Switchgear fed through dedicated transformers connected to 11 kV Station Switchgear .

power to Unit and Auxiliary auxiliary

Some examples of such dedicated switchgears are: - Unit auxiliary services: Turbine and Boiler Auxiliary MCC, ESP MCC, Cooling Tower Switchgear, Unit Emergency Switchgear etc. - Station Auxiliary Services: Coal Handling and Ash Handling Switchgear Generally 3.3 kV, fed directly from 11 kV Station switchgear through dedicated 11/3.3 kV Transformer), Fuel Oil Plant Switchgear, HVAC MCC, IA/PA Compressor MCC, DM Water Plant Switchgear etc. 4.2.1 MV and LV Unit and Station Auxiliary System Down below the main MV Unit or Station Switchgear (generally 11 kV) the auxiliary power is generally distributed at two separate voltage levels- namely, a lower Medium Voltage level ( generally 3.3 kV) and LT level. The purpose of insertion of a second MV level is as follows: - To provide power supply to the HT Motors (rated upto 2400 kW). The commonest motors in this category are ID/ PA Fans for Boiler, Condensate Extraction Pumps for Turbine, Raw Water Pump, and Cooling Water Pump etc. - To provide dedicated power supply to the station auxiliary services like Ash handling system, Coal handling System, Cooling water System which requires bulk power more than2000 kW. Document No.

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM Rest of the unit and station auxiliary loads are provided power supply at LT level. These are fed either from the LT Unit or Station Auxiliary Switchgear or directly from 11 kV Unit or Station Switchgear through dedicated 11/0.433 Kv transformer. 4.2.2 DG and Emergency Switchgear

Power supply to the safe shut down loads in case of plant shut down is provided from the Emergency Switchgear. Supply to the loads like Lub Oil Pump, Jacking Oil Pump, Seal Oil Pump, Barring Gear, Emergency Lighting, UPS System, AC input to Plant DC System etc are required to continue even after outage of the total plant to facilitate a safe shut down. To ensure this, the Emergency Switchgear has a dual feeding arrangement. One is from the 415 V Unit Auxiliary Switchgear and another from 415 V Diesel Generator (DG) Sets Feeding arrangement of 415 V Emergency Switchgear along with tentative rating of DG sets are to be indicated in Key SLD. 4.2.3 DC Distribution System DC power distribution arrangement with AC Incomer feeder, Battery Charger and battery and the main DC Distribution Board are to be shown in the key single line diagram. 5.0 DESIGN BASIS / CRITERIA 5.1 General

In line with the discussion in earlier section, the various steps of preparation of a Key Single Line Diagram will be as follows: 5.2 Power Evacuation Circuit For preparation of the EHV part of the Key Single Line Diagram, the following parameters are to be finalized:

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• No. of units and rating. This will determine the no. of Generator transformer feeders in the substation.

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• Basic philosophy of power evacuation – i.e., decision regarding the choice of following two alternatives:

Use of common Generator and Station Transformer with Generator Circuit Breaker.

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM -

Separate Generator and Station Transformer connected to EHV Substation and Circuit Breaker placed after Generator Transformer at EHV side.

In case the second alternative is chosen, there will be a “Station Transformer bay” corresponding to each unit. •

No. of feeders to be used for connecting the substation to the grid. These feeders will be used both for evacuation of the generated power to the grid as well as drawing the start-up power from the grid. For example if it is decided that two nos. Double circuit lines are to be used for evacuation of power that means there will be four nos. “Line Feeder.

•

In each bay, Breakers and isolators will be located based on the requirement of the corresponding switching scheme (i.e., one-and-half breaker scheme, two-main-one-transfer bus scheme etc.)

•

Earth switches are to be located in the bus and feeders keeping in view the maintenance requirement. For example, in case of Generator transformer Bays, the earth switch is to be located ‘before’ ( i.e., at the power station side) the isolator and for the line feeders, the earth switch will be located ‘after’ ( i.e., further end from the substation) the isolator. The purpose is to earth the equipment connected in each line, like LA, CVT, wave trap when the respective bay is switched off

•

Bus coupler shall be placed at suitable location on the buses.

5.3 Auxiliary Power Distribution Arrangement The following issues are to be addressed for preparation of Single Line Diagram for the auxiliary power distribution. 5.3.1 Connection from Generator to Transformers Connection of Generator output terminals to the Generator Transformer and also the tap-off connection to Unit Transformer are generally done by means of Isolated Phase Bus Ducts. This is shown in the key SLD with proper symbol 5.3.2 Transformer sizing criteria

a) Unit Transformer Primary side of Unit transformers are rated for generator output voltage ( say 22 kV) and the secondary side is rated for the Medium Voltage for Unit Auxiliary Power Supply ( generally, 11 kV). Hence a typical voltage rating of a Unit Transformer is 22/ 11 kV.

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM For unit size upto 210 MW, one (1) no. Unit Transformer is considered per unit. For Unit size higher than 210 MW, generally two (2) numbers of unit transformers are considered for each unit. Each unit auxiliary transformer is to be sized based on the 50% unit loads. 5% margin on the aforesaid loads will be considered. No contingency factor will be considered for unit loads in case actual unit loads from the BTG supplier is available. Otherwise 85% contingency factor will be considered. Final transformer capacity will be adequate for starting largest motor with other loads in running condition. Generally ONAN/ONAF type cooling and On Load tap Changer (OLTC) is considered. Although exact range of tapping is subject to detail system study, a typical figure of ±10% @ 1.25% can be considered at initial stage as a tentative data. b) Station Transformer Purpose of Station Transformers are not only to provide power supply to auxiliary loads, but also to provide the start-up and shutdown power corresponding unit and also to provide power supply to the unit auxiliary partly) in case of failure of Unit transformer. Hence, the sizing criteria for transformer is little bit complicated compared to unit transformer.

station to the loads ( station

For one single unit, Station transformer supplies the Station auxiliary Power requirement for one unit only. However, in case of more than one unit, although one number station transformer for each unit is considered, but the sizing of the transformer depends on division of common station auxiliary loads. Two alternative divisions of station auxiliary loads may be possible: - ‘Unitized’ station auxiliary loads. That means, like Unit auxiliary Loads, the station auxiliary loads are also independent (and obviously identical) for each unit, with no common station auxiliary load. - Station auxiliary load for a pair of unit is considered as a module and two station transformers are considered to provide power supply to station auxiliary loads of two units. In view of more common application and more complicated sizing criteria, the sizing criteria corresponding to the second option is discussed. However, once this criterion is understood, it can be suitably modified to size station transformer in first case as well. Rating of station transformer shall be decided to meet worst of the following loading conditions and also shall meet the requirement of interrupting capacity of 11kV switchgear. Condition -1 Document No.

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM • 50% station load of one pair of units with total four station transformers in service • Load of one UT (the UT being out of service) with the unit operating at full load. • Start-up load of one unit • Capacity shall be adequate for starting largest motor with other loads in running condition. Condition-2 •

Each station transformer should have sufficient capacity to meet following simultaneously loading.

•

00% station load of one pair of units with one station transformer out of service.

•

Start-up load of one unit

•

Capacity shall be adequate for starting largest motor with other loads in running condition. In order to avoid uneconomic and oversized design of transformer rating the following considerations have been made.

5.3.3

•

The probability of simultaneous outage of two transformers, i.e one (1) ST and one (1) UT is remote and is not considered in the design intent.

•

When One (1) ST is out of service, its bus shall be fed from the other healthy ST

MV Station and Unit Switchgear • The incomer of MV Unit and Station Switchgear are to be rated for the full load secondary current of unit and station transformer respectively. • In order to realize the redundancy in the capacity of the station transformer as discussed above, suitable tie feeder is to be provided between the station and unit switchgears. Following tie feeders are to be provided: - Between Station to each Unit Switchgear. - Between two station switchgear, in case of “pair-wise module” concept is adopted for station auxiliary loads ( as discussed in item 5.3.1 b) above).

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM • Outgoing Feeders Outgoing feeders of the Unit and Station Switchgears are to be finalized based on the available information about the Unit and Station Auxiliary Loads. Direct HT Motor loads are to be connected to the 11 kV Unit / Station Switchgear or 3.3 kV Unit / Station auxiliary Switchgear depending upon the ratings. As per convention, following standards regarding supply voltage to motors are followed: - Motors upto 160 kW : LT Supply - 160 – 2400 kW : 3.3 kV - Above 2400 kW : 11 kV The common HT Motors used in a power plant are Boiler Feed Pump ( in case motor driven pumps are used) , ID Fan, FD fan, PA Fan (for boiler), Condensate Extraction Pump for Turbine, Raw Water Pump, CW Pump ( station auxiliary) • A tentative list of other Unit Auxiliary Loads which are commonly connected to the 11 kV Unit Switchgear are given below: - 3.3 kV Unit Auxiliary Transformer - 415 V Unit Auxiliary Transformer - 415 V ESP transformer - 415 V Cooling Tower Transformer • A tentative list of common Unit Auxiliary Loads downstream to the 415 V Unit Auxiliary Switchgear are as follows: - 415 V Turbine MCC - 415 V Boiler MCC - 415 V Soot Blower MCC - 415 V Boiler Valve MCC • A tentative list of other Station Auxiliary Loads which are commonly connected to the 11 kV Station Switchgear are given below: - 3.3 kV Station Auxiliary Transformer - 3.3 kV Coal and ash handling Transformers Document No.

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DESIGN GUIDE FOR KEY SINGLE LINE DIAGRAM - 415 V Station Auxiliary Transformer - 415 V Water Treatment Plant transformer - 415 V A/C and Ventilation Transformer (Note: depending on loading, some of the above transformers may not be required and the corresponding MCCs may be directly fed from 415 V Station Auxiliary Switchgear) 6.0 QUALITY ASSURANCE AND TESTING This clause is not applicable for this Design Guide 7.0 APPLICABLE CODES AND STANDARDS This clause is not applicable for this Design Guide 8.0 INTERFACE REQUIREMENTS 8.1 Design Interface

From the discussion made so far, it is clear that in order to have necessary design input, close interface with the following authorities are required: - Client (For finalization of unit size, no. of unit, operational philosophy, operating voltages, power evacuation voltage, no. of lines, etc.). - Contractor / Bidder (for auxiliary load information) & Mechanical engineering 9.0 CHECK LISTS List of inputs required for preparation of Key Single Line Diagram ha already been discussed in item 5.2 & 5.3 10.0 ATTACHMENTS Following drawings / documents have been attached: - Attachment-I: Key Single Line Diagram for a typical 4 x 660 MW Thermal Power Station

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