generator differential 1

[kobavr]

hi,

we have a problem about current angles for generator differential protection-REG670 relay.
Between generator and transformer there is no CB. CB is after transformer.
And for synchronizing with grid, they did replace B and C phases on the primary side(grid side) of transformer.
Does this effect generator differential protection.
I can not see the angles reverse, between two side of generator.
Can you help me?

 

[dpc]

A couple of things:

The transformer phase shift, if present, must be accounted for. In a delta-wye transformer, there will be 30 degree phase shift one way or the other.

I'm not sure what was replaced, but if the CTs or CT wiring was involved, the CT polarity could be off, which would cause the current to be 180 degrees from where it should be, or the phasing could be off, or both.

By looking at the current phasors coming into the relay, you should be able to get an idea about where the problem might be.

 

[kobavr]

The transformer is Ynd11

 

[rasevskii]

kobavr:

Please see what dpc has said. If the CTs were replaced (is that true) then the polarities of the new CTs are probably reversed or otherwise wrong and the relay will not balance when load current is flowing. You may be getting a false trip.

You may have to run a new primary test to sort this out, that means running the unit on short-circuit on the HV side. This was probably done on the original commissioning, and now may have to be repeated.

Since the CTs will have each several socondaries (likely) then possibly other circuits, protections, and metering may now also be wrong.

rasevskii

 

[dpc]

Also, if this a new or re-programmed relay, the settings in the relay must match the physical wiring and transformer connection.

Traditionally, CTs on the Y side were connected delta and on the Delta side were connected wye to correct phase shift and handle the zero sequence current. With new digital relays, both sides are normally connected wye and the relay does the math. But you have to tell the relay how the CTs are physically wired.

But if you replaced CTs, my first guess is that the polarity is backwards.

 

[slavag]

Sorry, Im not understand something.
generator differntial is generator differntial, no phase shift and is not suitble for block protection.
what is a code of your REG670?
C30 or A20, or B30. C30 and B30 types are included transformer differntial protection, you can used it.
why only phases B and C was replaced?

 

[dpc]

I assumed a unit differential relay encompassing the generator and the generator step-up transformer. This is a common application, at least in this part of the world. Obviously if the differential zone covers only the generator itself, the phase shift of the transformer is not relevant.

 

[PHovnanian]

What dpc said. A backwards CT is a backwards CT, transformer or not. The Ynd11 30 degree (or 330 degree shift) is just going to throw another level of confusion into figuring out what's going on.

 

[slavag]

David, you are right.
But REG670 is special relay, is included a few differntial protections in one box with lot of current inputs ( for example, version C30 is included 87G, 87T and 87UT).

 

[dpc]

Slava,

Thanks for the info - I am not really familiar with this relay. It's hard to keep up with all of the relays these days.

Cheers,

David

 

[kobavr]

The relay model is A20. (just generator diff)
I solved the problem today.
Polarities has changed, so I do the correct wiring.
Now relay sees the correct values and angles.
thanks all of you