Generator Differential Protection with Flux Summation CT's 1

[Pitt03]

I'm having trouble seeing how the generator differential scheme on the attached One-Line will work. It uses flux summation CT's around the generator neutral and terminal leads inside the generator enclosure with the flux summation CT's connected to a relay with another set of CT's on the bus side of the generator breakers which are also connected to the relay. There is also a 3rd set of CT's connected between the generator and generator breaker.

The GE 489 relay only has two CT inputs so I'm not sure how three sets of CT's will even be connected. Even if they could be, how would this differential scheme work? The flux summation CT's in the generator terminal will produce a secondary current of 0A during normal operation however the CT's on the bus side of the generator breaker will measure the load current (connected in parallel to total load current from both breakers). Therefore the relay would see current on the breaker CT's and no current for the generator enclosure CT and thus operate on differential? Am I missing something?

Why would a 3rd set of CT's between generator and breaker be needed or relay?

 

[Parchie]

Pitt03,
There's nothing wrong with the diagram you attached. I see two sets of generator protection relay, GPR 1 and GPR 2, one for each feeder deriving power from your generator. [GCT(1) pairs with DCT(3)] Each GP relay gets its potential from separate feeder PTs.
GT-1 and GT-2 breakers, including a bus tie breaker BT-0 are supervised by your Master PLC.

 

[ScottyUK]

Parchie,

Take another look at the drawing. DCT appears to be a core balance CT on each phase of the stator winding, while GCT is a conventional earth fault CT. Those wouldn't normally pair up.

It is an odd looking scheme.

 

[Marmite]

Scotty, I think DCT is just a set of normal CT's, 1 on each phase, rather than a core balance CT. There is a 3 in brackets after DCT suggesting 3 of them. As I see it, there are 3 phase CT's and 1 neutral CT feeding into each protection relay.
Regards
Marmite

 

[Pitt03]

To me it appears that the DCT's are the CT's providing differential protection with flux summation or core balance CT's. Because the CT is around both the generators neutral and terminal leads that tells me that the actual CT will have a CT on each phase of the stator winding with both the neutral and terminal leads passing through the same CT on each phase with the CT summing to zero during normal operation.

I have seen this scheme discussed in various technical references such as Blackburn (figure 8.6) but have never seen it applied like this before. In these references it is referred to as an 87/50 element.

The GCT relay as I see it is simply a typical ground fault relay on the generator ground and has nothing to do with the differential scheme.

 

[ScottyUK]

Hi Marmite,

I thought that too at first, but the CT symbol for DCT has longer 'legs' to span the second line. My take is that it's either a lousy drawing - which I don't really believe because someone took the time to modify those CT symbols - or the CT has both winding ends passing through it.

As you describe it it would be a REF configuration rather than a true stator diff, but there's no ANSI 64 code to suggest that configuration.