April 10, 2023 1

AC Supply System

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April 10, 2023 2

•Importance of Elect System for Auxiliary•Classification of Auxiliaries•Choice of Voltage for Aux• Aux. Power supply system•Emergency AC Supply

Presentation outline

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Auxiliary Power Supply System

The purpose of an Electrical system is to generate and supply electrical energy .

The system should be designed and managed to deliver power with Reliability and Economy.

As these two requirements are quite opposed a balance is required to have an optimum design of Auxiliary power supply system

The auxiliaries have Ratings range from a few watts to few Mega Watts & Located over a large area

Work under varied Operating Conditions

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Auxiliary Power Supply System

Failure of Auxiliaries can cause Outage of Power Plant

Auxiliaries need Reliable Power Supply

System designed to cater to this need

Loads segregated on the basis of

Rating (to decide Voltage)

Location (to decide the

Feeding Point) Change-over schemes & Alternate Power Supply

arrangement to ensure Reliability

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Elements of Aux. Power System

More than 95% of the auxiliaries are driven by Motors

Different Voltage levels are interconnected through

Transformers

Miscellaneous Elements

Incomers

Ties

Diesel Generators

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Auxiliary Power Supply

• Voltage Rating of Motors Decided based on kW Rating Rating ≤ 160kW – 415V (LV), Contactor > 160kW & ≤ 200kW – 415V (LV), Breaker > 200kW & ≤ 1500kW – 3.3kV (MV), Breaker > 1500kW – 11kV (MV), Breaker Frequent Start / Stop Operations (Coal Conveyor

Motors) – MV Vacuum Contactor

• List of Boards / Feeding Points arrived at based on Location of Load Centers / Systems

• Quantity & Rating of Transformers calculated based on Load & Operational Requirements

• Rating of Board Calculated based on the Operating Load & Transformer Rating

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MV Switchgears – System Parameters

Nominal System voltage 6.6kV 11kV 3.3kV

Highest System voltage 7.2 kV 12kV 3.6kV

Rated Frequency 50Hz 50Hz 50Hz

Number of phases/ poles Three Three Three

System Neutral EarthingEarthed through Resistance of 6.35Ω to Limit E/F Current to 300A

One minute power frequency withstand voltage

20kV 28kV 10kV

Impulse withstand voltage 60kV (peak) 75kV (peak) 40kV (peak)

Maximum System Fault Level 40kA (rms)

Short time rating of Eqpt. 40kA (rms) for 1 second

Dynamic withstand rating 100kA (peak)

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LV Switchgears – System Parameters

Nominal System voltage 415V, ±10%

Rated Frequency 50Hz

Number of phases Three

System Neutral Earthing Solidly Earthed

Maximum System Fault Level 45kA (rms)

Short time rating of Eqpt. 45kA (rms) for 1 second

Dynamic withstand rating 105kA (peak)

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Auxiliary Power Supply System• Sizing of Unit & Station transformer• Start up source short circuit power• Feeding offsite areas loads• Rating of the Switchgear• Loads• Power Supply arrangement• 6.6& 0.415kV System• 11 & 3.3 & 0.415kV System

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Auxiliary Power Supply INTRODUCTION

• Commonly used voltage level up to 200 MW unit size- 6.6kV, 415V

Initially same voltage level (6.6 kv, 415v) were adopted for 500 MW unit size as well.

With 500 MW unit, quantitative and qualitative change in auxiliaries took place.

However no significant change in SWGR available in market.

Need for optimizing and cost effective solution

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Aux. Supply system(500MW)

• Grid Voltage- Normally 400kV for most of the200, 500MW units in India except few deviations

• Any Intermediate Voltage Available-to provide reliable start-up power source and to meet initial commissioning activities as well as to supply bulk loads(CW system, make-up water etc.) away from the main plants.Generally used Intermediate voltage levels are 220kV, 132kV, 33kV

• Nature of Auxiliaries i.e. ratings, basic characteristics and location

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Auxiliary Power Supply-500MW

• Selection of voltage levels to be adopted for 500MW and above unit size.

• a) 6.6 KV and 415 V system• b) 11& 3.3 kV system

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HT/LT Supply System

G

UAT-A Dyn1 UAT-B

16.5 KV

6.6 KV Ynd11 GT

Ynyn0 Station TRF-1

UNIT SWGR--AUNIT SWGR--B

Stn. SWGR-SA SB

UST‘A’

ESP TRF-A

S/Serv.

Trf.-BUST-B ESP TRF-B

USS-A USS-B 415 V Unit EMG SWGR

X

From DG SWGR

XBlr MCCTurb MCC

BFPID Fan

Air Compr Trf

X X

X

Stn Serv Trf-A

DMP Serv Trf

1)SWYDServTrf-12)AshHandl T/F1600KVA

6.6 KV

B.C.

N.O.

TMCC

Fdr toClarifWater SWGR

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Transformer feeder Details

To PMCC

MCC/PCCMedium Voltage

Magnetic Contactor

SFUCB

To ACDB

Motor Feeder

Unit Service Transformer

To PMCC

Magnetic Contactor

SFU

To ACDB

Motor Feeder

Dyn11

C.B.

415 V

6.6 KV

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X X

X X

X XX X

DG-1 DG-3 DG-2

DG SWGR-1 DG SWGR-2

UNIT EMERSWGR-1 UNIT EMER

SWGR-2

UNIT EMERSWGR-3

UNIT EMERSWGR-4

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EMCC

(1) AOP-1 (2) Em. Lighting Trf.

(3) TVDC Reserve Supply (4) UPS Ch.

(5) Scanner Air Fan (6) BVDC Reserve Supply

(7) AC Seal Oil Pump(8) Air Heater Lub Oil Panel

(9) Jacking Oil Pump (10) Stator Water Pump-B

(11) Barring Gear (12) +26 v DC ATRS Ch. ‘B’

(13) -26V DC ATRS Ch.’B’ (14) 26V DC FSSS Ch. ‘B

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From Unit SWGR ‘A’ Normally

(1) BFP-1C (2) BFP-1A

(3) PA Fan-1A (4) ID Fan -1A

(5) UST-1A (6) ESP T/F -1A

(7) FD Fan -1A (8) Coal Mill- 1A, C, E

(9) CEP-1A,C (10) ACW Booster Pump-1A

(11) Bus V.T.

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USS ‘A’

(1) Unit Em. SWGR (2) Boiler MCC

(3) BVDC ‘A’ (4) Turb. MCC ‘A’

(5) 220V Dc Ch. ‘A’ (6) +24 DC ATRS Ch. ‘A’

(7) -24V DC ATRS Ch.’A’ (8) Turb ACDB ‘A’

(9) +24 V DDCMIS Ch. ‘B’ (10) Gen Control Panel

(11) Boiler ACDB ‘A’ a) Grav. Feeder A,B, C,D,E,F

b) S/B MCC Feeder-1

c) INC From USS ‘B’

(12) MOT Centrifuge

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USS ‘B’

(1) UPS Ch. (2) +24V FSSS Ch.

(3) TVDC (4) Boiler MCC ‘B’

(5) Turb MCC ‘B’ (6) Turb ACDS ‘B’

(7) Boiler ACDB ‘B’

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THANK YOU