Electrical Protection

Electrical Faults

� Phase to phase� Phase to ground� Phase to phase to phase� Phase to phase to phase to ground

Purpose of Protection System

� Minimize damage� Leave unaffected equipment in-service� Maintain equipment operating limits� Maintain electrical system stability

Requirements of a Protection System

� Speed� Reliability� Security� Sensitivity

Some terms

� Over-current� Overload� Inverse time

Protection Zones

Simple Zone

Double Protection

"A" Bus Protection

"B" Bus Protection

CircuitBreaker

X

Bus

CircuitBreaker

Y

CT2

CT3

CT3

CT1

Bus Zone

CT4

CT2

CT4

CT1

ProtectionZones

xBusH2

Y

Bus Zone H2

Bus H1

Bus Zone H1

Bus Zone H3

BusH3

TransformerT1

ZoneT1

ZoneG1G1

TransformerSST1

ZoneSST1

SST1LV Bus

Bus Zone L1Bus L1

Breaker Failure

� Minimizes the amount of equipment removed from service in event of a failure

� Failure Determination� Not started opening in a certain time� Not open in a certain time� Current not broken in a certain time

Duplicate Protection Schemes

A Protection B Protection

Breaker X Breaker Y Device ZAction Q

Initiate InitiateTrip TripClose Close

PT

CT2

CT1 Electrical bus Electrical

System

ProtectionLogic

ProtectionOutput

ElectricalDevices

SystemInput

Bus Protection

Bus Protection

� Over-current� Differential � Back-up� Under voltage

Over-current Relay

I 1P

I 1S

CT 2000/5 A

Differential Protection

I 1P

I 1S

CT1 2000/5 A

I 2P

I 2S

CT2 2000/5 ABusH1

X Y

I = I1S = I2S

I = I1S = I2S

I = I1P = I2P

I 2SI 1S

Fault Conditions

I 1P

I 1S

CT1 2000/5 A

I 2P

I 2S

CT2 2000/5 ABusH1

X Y

I 1SI 2S

Fault

I 1P

CT 2000/5 A

I 2P

CT 2000/5 ABusH1

X Y

I 1S I 2S

I 1S

I 3S CT 2000/5 A

I 3P

T1

I 2S

I3S

Bus Protection Scheme

X

YLoadL2

LoadL1

T1

S Bus

Bus Differential Backup

Zone L1

Zone L2

Back-up relay

X

YLoad

L2

LoadL1

T1

S Bus

Over CurrentBackup Relay Differential

Relay

Bus Under Voltage Protection

Armature shown in the non-energized (trip) position

NormallyOpenContacts

To PotentialTransformer

Bus Protection

� Over-current� Differential � Back-up� Under voltage

Transformer Protection

Transformer Protection

� Instantaneous� Differential� Gas� Thermal Overload� Ground

Transformer Characteristics

� High magnetizing inrush currents� Ratio mismatch with CTs aggravated by

tap-changers� Phase shifts� Transformers are affected by over-fluxing� Affected by over-temperature

Transformer Zone

T1

CT2

CT3

CT4

CT1

T1 Zone

CT3

CT1

CT4

CT2

"A" Transformer Protection

"B" Transformer Protection

Differential

I 1P

I 1S

CT1

I 2P

I 2S

CT2

I = I1S = I2S

T1

Operate Coil

Restraint Coil

Restraint Coil

I 1S I 2S

Gas Relay

Winding Temperature

Ground Fault Protection

CT1

T1

GroundBackup

DifferentialRelay

CT2

TransformerStar Point

GroundCT

IN

IN

CT3IF

Transformer Protection

� Instantaneous� Differential� Gas� Thermal Overload� Ground

Motor Protection

Service Factor

� Continuous allowable overload� Many motors some with a power rating and a

service factor� A 10 HP motor with a service factor of 1.15

has a maximum continuous output of 11.5 HP

Motor Protection Summary

� Instantaneous Over-current� Stall� Thermal Overload� Phase Unbalance� Ground

Inverse Time Relay

Overload

Ground Fault Protection

Single Phase to Ground Protection

Stalls

Thermal Overload & Phase Unbalance

Diagram of the Unit

Rotation as Ele me nts He at/Co ol

Phase Unbalance Contac tsLoad Indicating Po inte r

Adjus table Ove rload Contac t

Ove rloadSettingPo inte r

Shaft

He ater(e le c trical

c onnec tio ns no ts ho wn)

Ac tuating Bime tal

Ambie nt Co mpensating Bime tal

He at Shie ld

Motor Protection Summary

� Instantaneous Over-current� Stall� Thermal Overload� Phase Unbalance� Ground

Generator Protection

Classes of TG trips

� Class A� Trip generator breaker, field breaker and

turbine� Electrical trips before the output breakers

� Class B� Trip generator output but leave it supplying

station service� Electrical faults in the switchyard

Classes of TG trips

� Class C� Over excitation high V/Hz� Only used when generator is isolated from grid

� Class D� Trip turbine � Trip Generator after motoring is detected� Mechanical type turbine trips high condenser pressure

Generator Protection

� Over-current� Overload� Differential � Split phase differential � Ground� Rotor ground� Phase Unbalance� Low field� Under frequency� Out of Step� Reverse power

R

Prote ctionZo ne s

W B

Diffe re ntialRelay

Ne utral Co nne c tionto Gro und

a) He althy Phas e b) Faulted Phas e c ) Ope n Circ uit in the Phas e

Ge ne rator Terminals

Differential

G1

Operate Coil

Restraint Coil

Restraint Coil

I 1S I 2S

I 2P

I 2S

CT2

I 1S

CT1I 1P

GeneratorStar Point

Is = I1S = I2S

IP = I1P = I2P

To MainTransformer

Ground Fault Protection

T1

G1

GT1

Stator GroundRelay

IF

Rotor Ground Fault

Other Protections

� Phase Unbalance� Loss of field� Under frequency� Out of Step

Reverse Power

G

Re ve rsePowe rRe aly

V.T.

Generator Protection

� Over-current� Overload� Differential � Split phase differential � Ground� Rotor ground� Phase Unbalance� Low field� Under frequency� Out of Step� Reverse power

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