Station Protections

7/31/2019 Station Protections

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STATION PROTECTION

ENGINEERING

Knowledge Management System

ELECTRICAL


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STATION PROTECTIONS

By

NTPC


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Specifications for Bus bar protection

OPERATING TIME OF 15 MS AT 5 TIMES SET VALUE

INCLUDE CT SUPERVISION

INDIVIDUAL TRIP RELAYS(DUPLICATED) FOR EACH FEEDER

TRANSIENT FREE OPERATION

DC SUPPLY SUPERVISION

IN /OUT SWITCH FOR EACH ZONE

MODERN NUMEIRCAL BUSBAR PROTECTIONS AREAVAILABLE IN CENTRALISED AND DECENTRALISED FORM

IN DUPLICATED BUS BAR SCHEME, PREFERABLY ONESCHEME SHALL BE LOW IMPEDANCE BASED AND SECONDSHALL BE HIGH IMPEDANCE BASED


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Bus bar prot scheme design

R1

R2


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LOGI C OF MAI N AND TRANSFER BUS SYSTEM


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Bus section protection


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Designed stability level Shall correspond to swgr ratingeven if the available SC level is less

Settings

Vs = If(Rct+2 Rl)

Vk not less than 2 x Vs

Effective current settingIr = Is+ n, Ie

Is= relay circuit current setting

Ie= Magnetising curent

N= No of CTs in parallel

I pry = Ir X turns ratio

CT

Matching CT ratio to avoid spill current duringhealthy state

Less Rct/ less Ie/high CT ratio


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BUS BAR PROTN (contd)

Supvn of CT circuit Detected by using a 3 phase rectifier relay to

effect the summation of the bus wire voltages andshort the pilot wire from the affected phase

Routing of CT connection

Looped at the yard itself to ensure minimum loopresistance and thus a minimum setting voltageand a minimum Vk for a given stability limit


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LOW IMP BUS BAR

PROTECTION Use biased differential PRINCIPLE.

Stability for through fault achieved by restraining quantities proportional tothe feeder current.

Less stringent CT parameters

CT wires directly to the relay

More stability for spill current

CT mismatch (typ of the order of 1:5 ) can be accommodated. More suitable for numerical integrated protection systems

AS THE CTs CAN BE SHARED FOR MANY FUNCTIONS.

BUILT IN LBB FUNCTION IS ACCEPTED IN BUS BAR PROTN.


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Use of bias

Compensate for small ratio mismatch AC switching of feeder current through isolator

contacts are avoided and switching is done on thesec side of aux CTs forming part of therelay.Alternatively in static / numerical relays,thebus bar replica is made inside the relay through

isolator contacts and the logic is done by thesame.

Aux CT takes care of ratio mismatch of CTs

A current operated auxiliary relay is used todetect any unbalance sec current for supervisionof the CT ckts. Current setting of the supvn relay

must be less than that of main diff relay and morethan the lowest loaded feeder


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Setting criteria

Main diff relay is set above the highestloaded feeder.

Must be more than 30% of the min fault

level.


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Numerical bus bar protections

Centralized: all functions in a centre unit Decentralized : Peripheral units attached to

each bay and a central unit forrelay logic.

Have many zones of protection in the samerelay

Many added functions like LBB, Feeder back upprotection

Event logging Dist recording


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ISLANDING SCHEMES


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Islanding scheme

n DEPENDS ON TYPE OF GENERATING SYSTEM

n GRID CONNECTED GENERATING STATIONS

GENERATOR IS CONNECTED TO THE GRID THROUGHEHV TRANSMISSION LINES.

CAPTIVE GENERATING STATION

DEDICATED GENERATOR(S) SUPPLIES POWER TO APARTICULAR UTILITY/ESTABLISHMENT.

NOT CONNECTED TO THE GRID.


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TYPE OF ISLANDING SCHEMES.

FOR GRID CONNECTED GENERATORS

CRITERIA:

UNDER FREQUENCY AND/OR RATE OF

CHANGE OF FREQUENCY

ACTION:

STAGGERED TRIPPING OF GRID LINES ATPREDTERMINED LOGIC AT THE PRE DECIDEDSEQUENCE.

CONCERN: VERY DYNAMIC SYSTEM CONDITIONS


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ISLANDING SCHEME FOR GRIDCONNECTED GENERATORS.

KEY FEATURES INTEGRATED SCHEME FOR THE WHOLE PLANT

FREQUENCY BASED

TRIGGERING BASED ON THRESHOLD AND RATE OFCHANGE OF FREQUENCY

U/F RELAYS CONNECTED TO BUS CVT

BASED ON 2 OUT OF 3 LOGIC

INDIVIDUAL TRIP RELAYS FOR EACH FEEDER

STAGGERED TRIPPING SCHEME TO BE ENGINERED INCOORDINATION WITH RESPECTIVE GRID AGENCYDEPENDING ON THE SYSTEM CONDITIONS AND THEPOWER NUMBER OF THE CONNECTED GRID.

SUCCESS CHANCE MORE IF RADIAL LOAD IS AVAILABLE.


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CVT SUPPLY FOR U/FRELAYS CONNECTED TO THE SELECTED

BUS CVT SUPPLY

OR

DISTRIBUTED AMONG THE 2 BUS CVTS

{2 ON ONE BUS CVT (ON 2 CORES) & 1 ON OTHER BUS CVT}


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TYPICAL SWYD SLD

1

381F

2

3

281F

10

11

4

181F

6

5

7

8

12 13

14

15

481F581F 681F

19

9 16

R2 R2

18

17

20

21


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TYPICAL ISLANDING LOGIC

FOR SELECTED BUS CVT


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TYPICAL ISLANDING LOGICFOR NON SELECTED BUS CVT SUPPLY


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ISLANDING FOR CAPTIVE

GENERATING PLANTS. FEATURES

BASED ON LOAD GENERATION BALANCE.

SCADA BASED DYNAMIC LOGIC

ADAPTIVE TO SYSTEM CHANGES

PRIORITY GROUPS ARE SET FOR ISLANDING

TRIPPING SEQUENCE IS DECIDED BY THE SOFTWAREBASED ON THE PRIORITY AND THE DYNAMIC LOADGENERATION BALANCE.


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THANK YOU

FOR

YOUR TIME

Station Protections

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