Electrical Protection

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Electrical Protection

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Electrical Faults

•• PPhhaasse e tto o pphhaassee •• PPhhaasse e tto o ggrroouunndd

•• PPhhaasse te to po phhaasse te to o pphhaassee

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Purpose of Protection System

• Minimize damage

• Leave unaffected equipment in-service • Maintain equipment operating limits • Maintain electrical system stability

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Requirements of a Protection

System

• Speed • Reliability • Security • Sensitivity

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Some terms

• Over-current • Overload

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Double Protection

"A" Bus Protection "B" Bus Protection Circuit Breaker X Bus Circuit Breaker Y CT 2 CT 3 CT 3 CT 1 Bus Zone CT 4 CT 2 CT 4 CT 1

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Protection

Zones

x Bus H2 Y Bus Zone H2 Bus H1 Bus Zone H1 Bus Zone H3 Bus H3 Transformer T1 Zone T1 Zone G1 G1 Transformer SST1 Zone SST1 SST1

LV Bus Bus Zone L1 Bus L1

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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

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Duplicate Protection Schemes

A Protection B Protection

Breaker X Breaker Y Device Z Action Q

Initiate Trip Close Initiate Trip Close

PT CT

2 CT

1

Electrical bus Electrical

System Protection Logic Protection Output Electrical Devices System Input

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Bus Protection

• Over-current • Differential • Back-up

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Over-current Relay

I _1P

I _1S

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Differential Protection

I_1P I_1S CT1 2000/5 A I_2P I_2S CT2 2000/5 A Bus H1 X Y I = I_1S= I_2S I = I_1S= I_2S I = I_1P= I_2P I_2S I_1S

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Fault Conditions

I_1P I_1S CT1 2000/5 A I₂ I_2S CT2 2000/5 A Bus H1 X Y I_1S I_2S Fault

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I_1P CT 2000/5 A I_2P CT 2000/5 A Bus H1 X Y I_1S _I 2S I_1S I_3S CT 2000/5 A I_3P T1 I_2S I_3S

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Bus Protection Scheme

X Y Load L2 Load L1 T1 S Bus

Bus Differential Backup

Zone L1

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Back-up relay

X Y Load L2 Load L1 T1 S Bus Over Current

Backup Relay Differential Relay

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Bus Under Voltage Protection

Armature shown in the non-energized (trip) position

Normally Open Contacts

To Potential Transformer

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Bus Protection

• Over-current • Differential • Back-up

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Transformer Protection

• Instantaneous • Differential • Gas • Thermal Overload • Ground

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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

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Transformer Zone

T1 CT 2 CT 3 CT 4 CT 1 T1 Zone CT 3 CT 1 CT 4 CT 2

"A" Transformer Protection "B" Transformer Protection

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Differential

I_1P I_1S CT1 I_2P I_2S CT2 I = I_1S= I_2S T1 Operate Coil Restraint Coil Restraint Coil I_1S _I 2S

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Ground Fault Protection

CT1 T1 Ground Backup Differential Relay CT2 Transformer Star Point Ground CT I_N I_N CT3 I_F

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Transformer Protection

• Instantaneous • Differential • Gas • Thermal Overload • Ground

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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

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Motor Protection Summary

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

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Inverse

Time

Relay

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Single Phase to Ground

Protection

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Thermal Overload & Phase

Unbalance

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Diagram of the Unit

Rotation as Eleme nts Heat/Coo l Phase Unbalance Contacts

Load Indicating Pointer

Adjus table Overload Con tact

Overload Setting Pointer Shaft Heater (electrical connections not shown) Actuating Bimetal

Ambient Compen sating Bime tal

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Motor Protection Summary

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

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Generator Protection

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Classes of TG trips

•• CCllaasss s AA

–

– Trip generator breaker, field breaker andTrip generator breaker, field breaker and

turbine turbine –

– Electrical trips before the output breakElectrical trips before the output breakersers

•• CCllaasss s BB

–

– Trip generator output but leave it supplyingTrip generator output but leave it supplying

station service station service –

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Classes of TG trips

•• CCllaasss s CC –

– Over excitation Over excitation high V/Hzhigh V/Hz –

– Only used when Only used when generator is isolatgenerator is isolated from grided from grid •• CCllaasss s DD

–

– Trip turbinTrip turbinee –

– Trip Generator Trip Generator after motorinafter motoring is detectedg is detected –

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Generator Protection

• Differential

• Split phase differential • Ground • Rotor ground • Phase Unbalance • Low field • Under frequency • Out of Step • Reverse power

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R Protection Zones W B Differential Relay Neutral Connection to Gro und

a) He althy Phas e b) Faulted Phas e c ) Ope n Circ uit in the Phas e Gene rato r Terminals

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Differential

G1 Operate Coil Restraint Coil Restraint Coil I_1S I_2S I_2P I_2S CT2 I_1S CT1 I_1P Generator Star Point I_s= I_1S= I_2S I_P= I_1P= I_2P To Main Transformer

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Ground Fault Protection

T1 G1 GT1 Stator Ground Relay I_F

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Other Protections

• Phase Unbalance • Loss of field

• Under frequency • Out of Step

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Reverse Power

G Reverse Po we r Realy V.T.

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Generator Protection

• Differential

• Split phase differential • Ground • Rotor ground • Phase Unbalance • Low field • Under frequency • Out of Step • Reverse power