Purpose:- To detect Voltage-Regulator malfunction.& 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.
Causes:- It mainly occur in start up, shut down & as a result of remote load rejection
Ø 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.
Ø Over Fluxing may also be occurring when the utility system frequency decreases to a low value by any reason.
Types of Relays used:- 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.
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.
Effect on Alternator:- It will cause over heating of cores of generator, main T/F& aux T/F associated
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.)
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.)
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.)
Technical Requirements:-
it need over Exaction sensing unit which has liner volts per hertz pickup characteristics & time unit provided a definite time before initiating alarms & trip
Note:- the settings of V/Hz shall be coordinated with exaction system of AVR because it acts as a primary protection in case of voltage regulator fails.
In put for relay:- protection PT
