Electrical Protection

Electrical Protection

Electrical Faults • Phase to phase • Phase to ground • Phase to to ph phase to to phase • Phase hase to phas phasee to to pha phase se to gro groun und d

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

Bus Zone

Circuit Breaker Y

Circuit Breaker X

CT CT CT CT 2 3 1 4

Bus

CT CT CT CT 3 2 1 4

"B" Bus Protection

"A" Bus Protection

Bus Zone H2

Bus Zone H3 Bus Zone H1

Bus H2

x

 Y

Bus H1

Zone T1

Transformer  T1

Zone SST1

SST1 LV Bus

Bus H3

Bus L1

Bus Zone L1

Transformer  SST1

G1

Zone G1

Protection Zones

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 CT

CT

1

2

Electrical bus

Electrical System

PT

System Input

A Protection

Initiate

Trip

Action Q

Protection Logic

B Protection

Close

Breaker X

Initiate

Breaker Y

Trip

Close

Device Z

Protection Output

Electrical Devices

Bus Protection

Bus Protection • Over-current • Differential • Back-up • Under voltage

Over-current Relay CT 2000/5 A I 1P I 1S

Differential Protection I = 1P I = 2P I CT1 2000/5 A

X

Bus H1

Y

CT2 2000/5 A

I 1P

I 2P I = 1S I = 2S I I 1S

I 2S I 1S

I = 1S I = 2S I

I 2S

Fault Conditions Fault

CT1 2000/5 A

X

Bus H1

Y

CT2 2000/5 A

I1P

I2 I 1S

I 2S I1S

I 2S

CT 2000/5 A

X

Y

Bus H1

CT 2000/5 A

I 1P

I 2P I 3P I 1S

I 3S

CT 2000/5 A T1

I3S

I 1S

I 2S

I 2S

Bus Protection Scheme Bus Differential

X T1

S

Backup

Zone L1 Load L1

Bus  Y

Zone L2 Load L2

Back-up relay Over Current Backup Relay

Differential Relay

X T1

S

Load L1

Bus  Y

Load L2

Bus Under Voltage Protection Armature shown in the non-energized (trip) position Normally Open Contacts

To Potential Transformer

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 Zone T1 CT 1

CT 2

CT 3

CT 4

CT 4

CT 3

"B" Transformer Protection

"A" Transformer Protection

CT 2

CT 1

Differential T1 CT1

CT2

I 1P

I 2P I 1S

Restraint Coil

I 1S

Operate Coil

I = 1S I = 2S I

I 2S

Restraint Coil

I 2S

Gas Relay

Winding Temperature

Ground Fault Protection T1

CT2

CT1 IN

CT3

Differential Relay

Transformer  Star Point

Ground CT

Ground Backup

IN

IF

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 Eleme nts Heat/Coo l

Load Indicating Pointer

Phase Unbalance Contacts Adjus table Overload Con tact

Heater (electrical connections not shown)

Overload Setting Pointer

Actuating Bimetal

Ambient Compen sating Bime tal Shaft Heat Shield

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 breakers

• Class B  – Trip generator output but leave it supplying station service  – Electrical faults in the switchyard switchyard

Classes of TG trips • Class C  – Over excitation high V/Hz  – Only used when generator is isolated isolated from grid

• Class D  – Trip turbine turbine  – Trip Generator after motoring motoring is detected  – Mechanical type turbine trips high high condenser 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 

Gene rato r Terminals R 

W

B Differential Relay

Protection Zones

Neutral

Connection

to Gro und a) He althy Phas e

b) Faulted Phas e

c ) Ope n Circ uit in the Phas e

Differential Generator Star Point IP = I1P = I2P

I 1P

G1 CT1

To Main Transformer

CT2

I 2P I 1S

Restraint Coil

I 1S

Operate Coil

Is = I1S = I2S

Restraint Coil

I 2S

I 2S

Ground Fault Protection T1

G1

GT1 Stator Ground Relay

IF

Rotor Ground Fault

Other Protections • Phase Unbalance • Loss of field • Under frequency • Out of Step

Reverse Power 

G

V.T. Reverse Power Realy

Generator Protection • • • • • • • • • • •

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