Generator over Fluxing Relay (24G)

Generator over Fluxing Relay (24G)

-The relay measures the ratio of voltage to frequency (V/Hz) as this ratio is proportional to the flux-density level of the electromagnetic machine.

-Over Fluxing (Over Excitation) may be occur when a generator is on an isolated operation with the AVR in service and the generator speed decreasing from the synchronous speed.

-Over Fluxing may also be occurring when the utility system frequency decreases to a low value by any reason.

There are two sets of the V/Hz relays are provided for Generator Protection:-

A) Relay 24G (#2) for alarm

A definite time element #2 of the relay will be used to provide an alarm.

B) Relay 24G (#1 & INV) for trip

Operation of the relay 24G will trip the switchyard breaker.

An inverse time element of the relay will be used to provide a trip contact and a definite time element #1 is also used to further shape the protection curves.

Data for setting calculations

Short-time V/Hz withstand level at full load -----120%

Continuous V/Hz withstand level at full load-----105%

AVR V/Hz limiter setting -----------------------------105%

Setting of generator V/Hz alarm relay 24G (#2)

1)V/Hz alarm setting

Setting < continuous V/Hz withstand level at full load = 105 %

2)V/Hz alarm delay

Setting = 30 cycles (0.6 Sec.)

Setting of generator V/Hz alarm relay 24G (INV)

1) V/Hz trip setting for inverse time element

Setting = 107 %

2) V/Hz Reset rate for inverse time element

Reset rate = Practical recommendation is minimum setpoint of the relay

Setting = 1 Sec.

  

NOTE!! “If the generator is synchronized immediately after tripping by V/Hz relay operation, the generator will be damaged under V/Hz condition before limitation value. “

Setting of generator V/Hz alarm relay 24G (#1)

1)V/Hz alarm setting

Setting < short V/Hz withstand level at full load = 120 %

2)V/Hz alarm delay

Setting = 50 cycles (1 Sec.)