transmission system protection

7/31/2019 Transmission System Protection

1/24

REYROLLECreated:

Transmission System Protection


2/24

REYROLLECreated:

Overall Power System

Network

GENERATORS

TRANSMISSION NETWORK

DISTRIBUTION

NETWORK

MG

BusbarsAuto-Transformers

Interconnected Feeders

BusbarsInterconnectors

Double Wound TransformersRadial Feeders

Ring MainsEmbedded Generation

Motors


3/24

REYROLLECreated:

Power System Design Criteria

Power system should be planned, constructed & operated such that there is

sufficient generation to meet predicted demand as economically as possible.

Must withstand most faults without repercussions to customers .

Designed to avoid loss of synchronism, voltage collapse, load shedding or

unacceptable frequency/voltage variations when faults occur and/or plant is

switched out of service.

All under varying conditions of production and consumption of power. Provision

must also be made for necessary maintenance to be carried out


4/24

REYROLLECreated:

General Protection Requirements

Fast, Accurate Fault Detection

Discrimination (Isolation of Minimum amount of plant)

Back-up (Redundancy)

Cost Effective


5/24

REYROLLECreated:

Protection Requirements at Transmission Voltages

High energy levels

High capital cost of plant

Impact of loss of equipment

Danger of loss of synchronism on Network

Very Fast

Fully Discriminative

Extremely Reliable

Cost Effective

For Transmission Systems:

Technical Considerations are more Important than Cost!

TRANSMISSION

SYSTEM

CONSIDERATIONS

TRANSMISSION

SYSTEM

REQUIREMENTS


6/24

REYROLLECreated:

Back-Up ProtectionDifferent Operating Principle

Slower (Often time delayed)

Circuit Breaker FailProtection

Usually 400/275kV only

Trip Upstream Breakers

Last Resort (Loss of Discrimination)

MAIN PROTECTION400/275kV Two Independent Protection Systems

Individual Trip Circuits (Separate DC Supply)

132kV - Single Protection System

If possible, different operating principles

If not, different Relay or Manufacturers

Transmission Protection Philosophy (UK

Practice)


7/24REYROLLECreated:

General Considerations when Specifying Protection Systems

Nature of protected plant

Fault conditions

Normal operating conditions

Integration with other protection systems

Signalling channels available

Instrument transformer position



CTs on busbar side of line Circuit Breakers

& near side of Bus Section Breaker

CTs on Line side of Line Circuit Breakers

& far side of Bus Section Breaker

Instrument Transformer

Placement

Overlapped Zones of Protection

provide cover for Circuit Breakers and connections.



Feeder Protection

2 Independent Feeder Protection systems using separate CTs

Preferably utilising different principles of operation

Different relay or hardware platform to avoid common mode failure

Any signalling or communications channels should be diversely

routed

Inherent back-up Protection Zones 2 & 3 of distance protection

Back-up IDMTL Overcurrent device at each feeder endWhere distance protection is not used and additional 2-phase IDMTL

IDMTL device is also applied

Main Protection

Back-up Protection



OHL

Feeder Protection-1

1ST Main (Current Diff)

2ND Main (Distance)*

Back-up O/C (IDMTL)*


2ND Main (Distance)

Back-up O/C (IDMTL)

Circuit Breaker Fail


Channel 1Channel 2

* Additionalback-up

protection is

provided by

Zones 2 & 3 of

the distance

protection



OHL

Feeder

Protection-2


2ND Main (Current Diff)

IDMTL Overcurrent


2ND Main (Current Diff)

IDMTL Overcurrent


2-phase Overcurrent

Intertrip

Channel 1

Intertrip

Channel 2

2-phase Overcurrent


* 2 back-up overcurrent

protection systems are

provided as back-up



Auto Transformer Protection

2 Independent Systems of overall differential protection

Both systems will use common set of CTs

These Systems will operate 2 separately energised trip circuits

2-Stage HV O/C Protection

Stage 1 - Faults beyond LV circuit breaker

Stage 2 - Time Delayed - Trips all circuit breaker capable of supplying

fault current

HV Three phase overcurrent from the same CTS

LV 3-phase overcurrent protection

LV E/F from residually connected IDML relay

Main Protection

Back-up Protection



SGT1

Auto Transformer Protection

2-Stage HV OC Guard

2-Stage HV OC

Transformer Overall 1

HV Overcurrent

Transformer Overall 2

LV Earth Fault

LV OC

To Trip

Circuit 1

To Trip

Circuit 2

Winding Temperature

VT Buchholtz Gas/Surge

Main Buchholtz Surge

HV Connections

LV Connections



2-Winding Transformer Protection

2 Independent Systems of overall protection (common set of CTs)

Restricted Earth Fault for HV and LV winding respectively (Earth Faults only)

1 Overall biased differential Protection (Slower, but reacts for all faults )

2-Stage HV O/C Protection

Stage 1 - Faults beyond LV circuit breaker

Stage 2 - Time Delayed -

HV Three phase overcurrent from the same CTS

LV 2-stage LV unrestricted earth faultStage 1 - Faults beyond LV circuit breaker

Stage 2-Time Delayed - All circuit breakers supplying fault current

Main Protection

Back-up Protection



Transformer Protection

SGT1

2-Stage HV OC Guard

LV Unrestricted EF St 1

Biased Differential

HV Overcurrent

LV Unrestricted EF St 2

HV REF

2-Stage HV OC

Tapchanger Selector Buchholtz

Tapchanger Diverter Oil Surge

LV REF

Main Buchholtz Surge

Winding Temperature

To Trip

Circuit 1

To Trip

Circuit 2

CT/VT Gas Detection

Aux Buchholtz Surge

* HV & LV

connections

protection

may also be

included

(using CTs

mounted on

Transformer

HV & LV

Bushings)



Protection for Double Busbar Stations

2 Independent Fault Detection Systems (from separate CTs)

Each Fault Detection System consists of 2 relays and a CT supervision relay

1 Zone of protection for each section of Busbar

1 Check zone for entire substation

To Trip all breakers connected to a busbar section, BOTH an individual zoneand the check zone MUST operate (2 out of 2 Logic)

Afforded by IDMT Overcurrent protection on Bus Sections & Bus Couplerswhere appropriate

Further back-up protection provided by Zones 2 & 3 of any remote Feeder

distance protection

Main Protection

Back-up Protection



Double Busbar

Protection

106

104

186

184

Main1

Res.1

206

204

286

284

Main2

Res.2

166 169

106

104

186

184

206

204

286

284

166 169

RESERVE 1

MAIN 1

RESERVE 2

MAIN 2

Check

180 280205105

Any one of the Discriminating Zones AND the check

zone must operate to trip the breakers



Mesh Corner

Protection

Mesh Corner 1 Mesh Corner 4

Mesh Corner 2 Mesh Corner 3



Feeder End

Transformer End

Mesh Corner Overall 1

Mesh Corner Overall 2

CT Supervision

Detailed Mesh Corner



Circuit Breaker

Fail

BFC

CK

Prot.

CB 'A'

CB 'E'CB 'C'

CB 'F'

CB 'D'CB 'B'

Backtrip

Intertrip

+

-+

+

+BFC

CK

-

-

Back tripdiscriminating

bus wires

BTD

BTRTR

TD0.15s

CBTCT

T

m

r

m

r

I

I

BTRTR - Back trip receive trip relay

BTD - Back trip discriminating relay

TD - Time delay relay

T - Trip relay

CBTC - Circuit breaker trip coil



CB Fail Timing

Diagram

Circuitprotectionoperation

Triprelayoperated-CBtripinitiated

CBFtimerstarted-curren

tcheckrelaycontactsoperated

System Fault

CBtripoperationcompleted

CBFcurrentcheckrelaydropoff-CBFtim

erstopped

100

80604020 20

028

026

024

022

0160

180

140

120

40ms CBtrip op.

60ms discrim.

margin

Timer setting 105ms

CBFtimerope

rationcompleted-backtripinitiated

Backtripreceiverelays&ICTrelayoperated

Backtripreceivetriprela

yoperated

LocalCBsviabacktrip

Carrier

VHSPO

Remot

eCBs

viaint

ertrip

Total clearancetimes for 40ms CBs

Time (ms)



Circuit Breaker Fail Protection

Required in the event that the circuit breaker fails to trip following receipt of a trip

command

Designed to trip all Circuit breakers capable of supplying fault infeed if

Fault current is still present after a given time delayAND

Trip signal is still present after a given time delay



Protection Requirements at Transmission Voltages

High energy levelsHigh capital cost of plant

Impact of loss of equipment

Danger of loss of synchronism on Network

Very Fast

Fully Discriminative

Extremely Reliable

Cost Effective

For Transmission Systems:

Technical Considerations are more Important than Cost!

TRANSMISSION

SYSTEM

CONSIDERATIONS

TRANSMISSION

SYSTEM

REQUIREMENTS


24/24

REYROLLEC t d

Back-Up ProtectionDifferent Operating Principle

Slower (Often time delayed)

Circuit Breaker FailProtection

Usually 400/275kV only

Trip Upstream Breakers

Last Resort (Loss of Discrimination)

MAIN PROTECTION

400/275kV Two Independent Protection Systems

Individual Trip Circuits (Separate DC Supply)

132kV - Single Protection System

If possible, different operating principles

If not, different Relay or Manufacturers

Transmission Protection Philosophy (UK

Practice)

transmission system protection

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