Can we use same CT for high impedance as well as low impedance

[arnabanindita]

Dear All,
We want to use same CT core for low impedance transformer differential protection as well as high impedance REF protection. That is, both low impedance transformer differential relay and high impedance REF relay is in series with phase CT core.
Does it work?

 

[RRaghunath]

No, it won't work. High impedance protection always calls for a dedicated CT with stringent specifications to avoid saturation and consequent maloperation during through fault conditions. Clubbing of biased differential and high impedance REF protection on the same CT core is not recommended.

The relay manuals generally give detailed recommendations in this regard and should help.

 

[jghrist]

You might be able to. We are using a single CT for both directional overcurrent and high impedance bus differential on a project. The CT leads from one bus terminal go through a Schweitzer SEL-351S directional overcurrent relay before they are paralleled with other CTs for connection to the bus differential relay (SEL-587Z). Schweitzer has reviewed the scheme and says it is workable. The high impedance bus differential has a high voltage during internal bus faults, but is limited by MOVs to values within the withstand of the o/c relay.

This would not work with an electromechanical transformer differential like an ABB HU because the CT circuits are not accessible after they go through the restraint transformer and before they are internally paralleled with other restraint circuits. Microprocessor transformer differential relays such as the Schweitzer SEL-387 have the CT circuits go in and out, so it might be possible. I would check with the relay manufacturer first.

 

[davidbeach]

rraghunath, in high impedance differential schemes, the impedance is intended to force the CTs into saturation. This is to remove the difference in CT performance when conditions are such that some CTs might saturate and others might not. The relay then uses the properties of saturated CTs to make its evaluation.

jghrist, true, but let's make that "might be able to." The CTs are going to be forced into saturation at currents far lower than they would normally saturate (saturation being a function of secondary voltage). If the protective scheme of the other relay (not the high impedance bus diff) can deal with the CT saturation, then yes, it might work. If the saturation could cause problems for the other relay, then it should be provided with separate CTs and the high impedance diff should have its own CTs.

 

[jghrist]

davidbeach,

If there is an internal fault that forces the CTs into saturation, the bus differential will trip. If the other relay is an overcurrent relay on one line into the bus, then it may not see the full current, but the breaker will trip anyway on bus differential.

For a transformer differential relay, if the transformer is a source of fault current to the internal bus fault, the fault is a transformer thru fault and you do not want the transformer differential to operate (although in most cases, it wouldn't cause additional outages). With one transformer winding CT saturated, the transformer differential may operate.

I would certainly agree that dedicated CTs are desirable in any case.

I just noticed that the original post refers to the high impedance differential being for REF protection, not for bus differential. This might pose different problems. The solution here would be to use a transformer protection relay such as a Schweitzer SEL-387 or G.E. T60 that has REF built-in using only one CT per winding.

 

[DiscoP]

It can be done - provided the correct CTs are used. In the Areva NPAG (available on the Areva-td.com website) chapter 16, section 16.10 is about this topic.

 

[RRaghunath]

I agree that most of the numerical transformer differential protection relays (irrespective of the make) include REF protection element that uses same set of phase CTs as the biased differential as stated DiscoP and jghrist.

Here, the REF is complementing the differential protection to improve the protection sensitivity in case of earth faults (NPAG/AREVA). The possibility of maloperation of REF element during heavy current phase faults exists as explained by NPAG.

The numerical, biased diffrential protection relays, in most cases, include overcurrent and earth fault elements as well.

All the processing in the numerical relays being software based, the need for dedicated CTs (maloperation of protection if not) may not be relevant with numerical relays, I guess.