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)
-
V/Hz trip setting for inverse time element
Setting = 107 % -
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.)