Classification

Types of Fault

Class A

This Covers all electrical protections for faults within the generator unit in which generator field breaker, generator and turbine should be tripped

Class B

This covers all mechanical protection of the turbine in which turbine will be tripped first and following this generator will trip on reverse power/low forward power protections.

Class C

This covers electrical protection for fault in the system in which generator will be unloaded by tripping of generator breaker only. The unit will come to house load operation and the UAT will be in service. Various protection of this class:

220 KV (HV Side of generator transformer bus bar protection)
Generator Transformer HV side breaker pole discrepancy.
Generator negative phase sequence protection.
Generator Transformer over current/Earth fault protection.
Reverse power protection without turbine trip.

NORMAL FAULTS IN GENERATOR

External Faults
Internal Faults

Over loading Short circuit, Earth fault, Phase to phase faults in winding, phase to Earth faults.

Faults related to Sub systems i.e., Stator, Rotor, Prime mover, Excitation, System, Voltage Regulator, Governor, Cooling System.

Under voltage, Over voltage, Under frequency, Reverse Power, Loss of excilation , Low - forwarded power.

GENERATOR PROTECTION SCHEMES

Functions

Steam & Gas Turbines

Hydro Turbines

Small

(< 10 MVA)

Medium

(10-100 MVA)

Large

(> 100 MVA)

Small

(< 10 MVA)

Medium

(10-100 MVA)

Large

(> 100 MVA)

Differential

Y

Y

Y

Y

Y

Y

95% Stator E/F

Y

Y

Y

Y

Y

Y

100% Stator E/F

N

Y/N

Y

N

Y/N

Y

Inter turn faults

Y

Y

Y

Y

Y

Y

Backup impedance

N

Y

Y

N

Y

Y

Voltage controlled O/C

Y

N

N

Y

N

N

Negative Sequence

Y

Y

Y

Y

Y

Y

Field laiure

Y

Y

Y

Y

Y

Y

Reverse power

Y

Y

Y

Y

Y

Y

Pole slipping

N

N

Y

N

N

Y

Over load

N

N

N

Y

Y

Y

Under load

Y

Y

Y

Y

Y

Y

Dead machine

N

N

Y

Y

Y

Y

Rotor Earth Fault

Y

Y

Y

Y

Y

Y

Over fluxing

N

Y

Y

N

Y

Y