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Phase Differential on Ground Faults
2

Phase Differential on Ground Faults

Phase Differential on Ground Faults

(OP)
With the new relay technology nowadays (i.e. GE SR735, Areva 634), are these phase digital differential relays capable of protecting Delta-wye (400A resistor grounded) power transformers on ground faults on the secondary (within its differential zone)?

RE: Phase Differential on Ground Faults

Probably, almost certainly if the relay has an REF element.

RE: Phase Differential on Ground Faults

Nightfox.
What David saied. REF is covered ground fault in such appliaction with resistive grounding.


Best Regards.
Slava

RE: Phase Differential on Ground Faults

(OP)
If anyone is practically familiar with GE SR 745 transformer differential relay.

We have a three winding transformer wherein, the two secondaries are connected to the SR745. That leaves us one more CT input for the primary. Our primary is connected to an upstream tee with two power circuit breakers (which are part of the ring bus system). Is it possible to connect the relay in two primary CTs (on the two circuit breakers) where the CTs will be connected in "summation" (or parallel).

Will this affect the settings of the two CTs?

RE: Phase Differential on Ground Faults

CTs can be summed, but have to be as identical as possible.  Certainly must have the same ratio and that is the ratio that goes into the relay.

RE: Phase Differential on Ground Faults

(OP)
What if the primary CTs that will be summed are in a ring bus connection where the flow of current will be either be in one direction or opposing each other (in case all switches and breakers in the ring are closed.

RE: Phase Differential on Ground Faults

The problem with paralleling the CTs is that during a heavy through fault, one CT may saturate more than the other, causing a false differential current.  It is better to use a 4 restraint differential relay in this case.

RE: Phase Differential on Ground Faults

What jghrist says is absolutely true.

CTs on a ring being paralleled is quite common.  Both sets of CTs have their wye point toward the transformer.  Current into the transformer is summed, current around the ring is positive in one CT and negative in the other CT and they cancel.  Always better to have the right number of restraints.  Four restraint relays are available from many manufacturers; I know of one five restraint transformer differential.

RE: Phase Differential on Ground Faults

(OP)
I looked at the GE SR 745 relay and it only has three CT inputs (and three restraints, inherently). I contacted a GE representative about this and he says its ok to do the CT summation with our present TEE connection set-up at the primary ring bus. I will post an SLD for clarity. I am cautious due to the fact of possible CT saturation as flagged by davidbeach and jighrist.

I look into this matter seriously and look at the relay manual and come back with further queries as needed.

Do I need to to do CT saturation calculations for this?

RE: Phase Differential on Ground Faults

We've only paralleled CTs on Ring Buses when there were no other options available.

For transformer differential, there are relays on the market that can have up to 4 separate inputs.

Companies that we have used are SEL (387), Basler (BE1-CDS240) & Areva (P634). We haven't needed to parallel CTs for transformer differential.

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