winding differential relay

[powerjunx]

Anyone knows the compatibility features of 87GD (generator ground-differential relay) and 67N (ground-directional overcurrent)?

Base on the single line diagram, 87GD is connected to the generator; but on the panel installation it is a 67N instead of 87GD relay. This set up is operational.

I am just wondering how can this be? I keep on reviewing the manual about this relays and im convince that this relays are not alike, interms of protection scheme.

Or maybe im misleaded with my conviction, does anyone had an experienced like this?

 

[stevenal]

It's the CT connection that defines the 87GD function, but the relay used may be directional overcurrent. Phase CTs are paralleled and applied to one input, neutral CT to the other. See Blackburn's Protective Relaying Fig 9.12.

 

[stevenal]

An additional thought: The difference between 87GD and 67N is neutral polarity which affects direction. 67N would normally look down the line, 87GD would look back into the generator winding. 87GD would be fast, while 67N would be delayed to coordinate with down line protection.

 

[DiscoP]

I've seen applications where the 67N is seen to look in to the Stator Winding.
This is used where the Generator connects directly on to the Distribution network without a Delta-Star Step up transformer, and no differntial protection is available.
The directional element discriminates between System Earth Faults, and Faults on the generator, therefore eliminating the need for a delay.
If you have Differential protection, or a Delta/Star transformer this is not necessary.

 

[DiscoP]

Just to add something to my earlier post, it can also be used on directly connected generators with impedance earthing.
Differential protection might not cause an operation, however a 67N using a Voltage Operated Earth Fault relay could detect the faults not seen by the differential protection.