REF protection trip for out-of-zone fault

[vijaypandiyan]

Dear engineers,
We have 6.6kV switchgear which feeds 250kW motors. One day, one of the 250kW motor got tripped on Inst OC protection which is fed from section B. As per DR, fault currents are 0.79kA, 0.21kA, 0.03kA. At same time, REF protection which was provided in the 6.6kV incomer for the section B transformer got tripped for out-of-zone fault. Trip current as per DR in REF relay was 1.01kA. Trip current seems to be sum of three phase fault currents recorded in motor feeder protection relay.
Switchgear is in service since 2010. REF protection is high impedance type. Neutral of the transformer has NGR.
Stability test was conducted in last shutdown and found OK. Transformer IR testing was found OK.
REF relay model is Siemens 7SJ611. One thing I observed was, no stabilizing resistor was provided for the REF protection.

Kindly help me to understand the reason for REF trip.

Thanks,
Vijay

Thanks,
Vijay

 

[RRaghunath]

When NGR is provided, the coverage of the REF protection becomes an issue making the protection less useful. It is unlikely the protection will maloperate on CT saturation due to stab resistor not being there.
In my opinion there is CT polarity issue and most likely that of neutral CT.
You mentioned that Stability test was conducted during last shutdown - was it for Transformer differential protection only or was it done for REF protection as well?
If yes, was current through neutral CT simulated during REF stability test!
You may like to review.

 

[stevenal]

REF should be summing the three phase currents with neutral, summing to ~0 for through faults. With a neutral resistor and balanced load, there is no way to check it with load. Testing with secondary injection only verifies the test set was connected correctly. I agree the most likely problem is neutral CT polarity.

 

[Kiribanda]

Stabilizing resister is used to avoid out of zone tripping (through fault). It should be properly sized at the
beginning so that it is in series with the relay coil. The existing voltage setting of your High Imp Ref relay may not be high enough.
In my opinion the stability test confirms only the CT connections.
Therefore, if a stabilizing resister is not available at all or undersized then there is a possibility for the relay to trip
for a through fault. Knowing all parameters you should be able to size it now.

 

[vijaypandiyan]

I understand neutral CT polarity might be the issue based on replies so far. But one thing I would like to understand is, if polarity is reversed, it should trip during normal loading conditions also right? Or atleast some current should flow in the neutral CT secondary which might be less than pickup value.
In same Incomer, we also have another 7SJ64 relay for 50/51, 50N/51N protection earth fault was not sensed in this relay.

Thanks,
Vijay

 

[stevenal]

NGRs are not generally used where unbalanced line to neutral loads are involved since they will affect the voltage and increase losses. I expect your loading is balanced and will not initiate a trip regardless of polarity.

 

[RRaghunath]

It is 6.6kV system where there will not be any Phase to Neutral connected single phase loads.
So, during healthy conditions no current is expected in the neutral.
Thus, there will be no current in neutral CT connected Standby earth fault protection.
Hope this answers your query.

 

[Kiribanda]

1) The CTs of 7SJ64 relay for 50/51, 50N/51N protection are connected in residual connection. Therefore, depending
on how the relay is programmed and the method of connection, the resultant zero seq. current I0 is either calculated by the relay
or measured by the IN winding.
2) 7SJ611 is a voltage relay. Therefore, if there is no allowance given to the voltage setting of relay, it will trip for
an out of zone fault.
You may have to re-visit the voltage setting of 7SJ11 knowing all CT parameters.