protn-desc3

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 Bus Zone

Circuit Breaker Y

Circuit Breaker X

CT CT CT CT 1 2 3 4

Bus

CT CT CT CT 2 1 4 3

"B" Bus Protection "A" Bus Protection

Bus Zone H2

Bus Zone H3 Bus Zone H1

Bus H2

x

Bus H1

Zone T1

Bus H3

Transformer T1

Zone SST1

SST1 LV Bus

Y

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 2

CT 1

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

I2P I1S

I2S I1S

I2S

CT 2000/5 A

X

Y

Bus H1

I 1P

CT 2000/5 A 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 Elements Heat/Co ol

Load Indicating Po inter

Phase Unbalance Contacts Adjus table Overload Contact

Heater (electrical connectio ns no t s ho wn)

Overload Setting Po inter

Actuating Bimetal

Ambient Co mpensating Bimetal 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

• 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

Ge ne rator Terminals R

W

B Diffe re ntial Relay

Prote ction Zo ne s

Ne utral Co nne c tion 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

Re ve rse Powe r Re 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

For You To Do