DIRCTIONAL EARTHFAULT RELAY FOR GEN.STATOR EARTHFAULT

[appunni]

Sir,
I have seen in some power stations that the directional earth fault relays are used for generator stator earth fault protection. I did not see any such protection scheme in IEEE standards for generator protection. What is the purpose for giving derectional sensitivity for stator earth fault relays? In most of the generating stations, these type of protection is not used. So what is the necessity of using such protection? Please give a detailed explanation.
With regards,
appunni

 

[RRaghunath]

appunni,

It is described as sensitive directional earth fault protection applied for motors (with earthed neutral) or for the generators. The relay is set to look in to the machine and set sensitive so that any earth fault is detected and the machine isolated quickly.

 

[peterb]

Where there are several generators connected on a common bus, one method of system earthing would be to provide a separate earthing transformer and resistor. This ensures that the system earth fault current is the same no matter how many generators are on line.
In order to protect the generators for internal earth faults, each generator circuit is equipped with a directional earth fault (DEF) relay. This is to provide selective operation for individual generator earth faults. The earthing transformer is equipped with a time delayed earth fault relay as a backup and to detect bus earth faults.

 

[vinnerk]

Normally if a number of generators are paralleled at generation voltage without a step up transformer, then only one Neutral grounding resistance is kept in service to prevent circulating currents between generators. In such cases a restricted e/f scheme is necessary which should be functional whether NGR is in service or not.

In such systems, the directional earth fault realay may be used by providing the open delta voltage of bus bars and zero sequence currents of generator CTs. the direction is such that if the generator has internal fault, then earth fault current will flow from bus bars (i.e., other generators) to the fault and circuit will be completed by the NGR which is in service. In case the fault is outside the generator, the directional element will prevent false tripping of the generator.

This is useful even if there is only one generator because a direction insensitive relay will operate if current flows through NGR even for an fault outside the generator.