The length of CT lead of differential relay

[ciy]

There is a differential relay of generator placed on GCP.

The both CTs of differential relay, one in the 13.8kV SWGR and the other one located at secondary side of generator.

There is a question that the length of CT lead from 13.8kV SWGR to GCP (eg. 1000M) is far longer than from Generator to GCP (eg. 20M).

Have any impact to the relay performance when the length of CT lead of differential relay is different?

Have any practice solution for this subject, if any?

Thanks for your advice in advance.

 

[LSpark]

The increased lead length will mean a higher burden on the CT, the ratio errors on a CT vary (within the allowable accuracy limits) according to the burden amongst other things.

For instance on the test certificate sat on my desk at the moment at 25% rated burden the ratio error is -0.15% and at rated burden it is 0.16% so approx 0.3% difference. This is for a metering class CT but protection CTs are likely to be similar. Have a look on the test certificate to see what the variation is and whether it is acceptable.

Different cross sectional areas of CT lead have different resistance or you can parallel two CT leads to halve the resistance.

 

[mgtrp]

... noting of courxe that with a differential relay, you are not really concerned with the difference at normal load conditions, but during fault conditions. You will end up with the CT's saturating at different points, which may or may not be an issue depending on the CT ratings and your generator.

 

[OliverJanke]

Hi,

As mgtrp already said, the problem you might run into is the saturation. The CT will not transmit the full secondary current during external fault conditions. This will look like a differential current to the relay. Depending on the characteristic it will now trip, or not.

The relay manufacturer normally offers formulas for the dimensioning of the CTs. You might find them in the relay manual or on the web page. You should measure or calculate the burden (resistance) of the 1000m cable including the protection device and check back the dimensioning of the CTs.

A solution would always be a higher diameter for the cable or bigger CTs. Both might be expensive. Another solution could be using a line differential protection system with one device at each side and a fiber optical connection in between, which is also not so easy to handle.

Best regards,
Oliver Janke

 

[rcw retired EE]

LSpark's suggestion is a good one if your calculations show that less circuit impedance would be desirable. If you have spare wires, use parallel wires for the CT circuit to halve the wire resistance.

 

[ciy]

Thanks for your great advice.

So I know that keypoint is the burden of CT.

 

[veritas]

CT burden will not be your only problem. Due to the fact that the diff relay is not located at the electrical centre of the differential system, there will be a spill current in the relay which is directly proportional to the difference in magnetisation currents of the CT’s.

Thus it is important to use matched CT’s and if possible to get the lead burdens as equal as possible. The suggestion of doubling up on the lead cores is one practical consideration.

In any event, I would recommend that detailed calcs be done to ensure that the minimum differential current required for relay operation (usually one of the first settings) be set well above any anticipated spill current due to difference in mag currents.

Regards.

 

[zer0sequence]

One other question: high impedance differential or low impedance differential?

Either way, you will have some serious issues with CT lead lengths of over half a mile... Like the earlier post said: I think you need more differential zones. If you need maximum selectivity (which is best, because it makes for extremely fast fault determination, and when set up correctly, is also very secure) go with a full overlapping differential scheme: Generator diff, Bus Diff, Line Diff, Transformer Diff, etc, etc...