REF Trip on a 22 MW Transformer with OLTC

[eaaf]

Hi everyone.
I have a 22 MW transformer 69KV to 4.16 KV Delta-Star resistance earthed, with a On Load Tap Changer. It has a SR745 Multilin Relay (GE) for protection.
It had two Restricted Earth Fault trips on a month. I made the following test to the transformer in order to look for a insulation failure: physicist - chemistry tests, Chromatography for gas disolved in oil in the principal tank and in the OLTC tank. Power factor to the oil, power factor to the windings, power factor to the bushings, XFMR Excitation Test. I have the test made on may of 2006, and the tests made yesterday. I found no difference between the values of the both test to the principal tank. The only problem that I have is in the OLTC, this one have a problem in the fixed and mobile contacts. I have a lot of gases disolved in the oil, and a visible quantity of coal (carbon) suspended in the oil, while the maintenance on the OLTC could be made, I am filtering and changing the oil. It is posible that a fault in the OLTC maybe get reflected in the current of the neutral, so the REF are triping. Because I can found an insolation problem. Have any idea.
Thanks a lot.
EAAF

 

[RRaghunath]

EAAF,
It is normal to have gas and carbon particles in OLTC oil. This is because the arcing is allowed in the OLTC diverter switch contacts when ever tap change is done. Refer the OLTC manual for further info and maintenance instructions.
The REF trip, probably is because of problem in CT wiring. You better check the CT secondary wiring and currents in the relay through primary injection. Also, perform the REF protection stability check. I think you will be able to identify the problem.
Further, when the transformer energised and loaded, do not forget to check the spill current in the relay and confirm that there is no spill.
Is the REF protection high impedance type. What are the REF protection settings and the stabilising resistor type.
If the REF trip has been taking place during faults in one of the outgoing circuits on secondary of the transformer, it could be that the protection is not stable for through-faults and it is better to verify the calculation and correctness of stabilising resistor value.

 

[ScottyUK]

Very strong possibility that the neutral CT is reversed which will allow normal operation when there is little to no neutral current but which will cause the REF scheme to operate when an earth fault occurs out on the HV network.

Primary injection is awkward to arrange but if you have no confidence in the CT markings then may be your only option.


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Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[slavag]

Hi.
As usally, very hard add some comments to Raghun and ScottyUK recommendation ( 99% of mistake in CT wiring).
But I would like add some possible reason:
Not right choice of CT , REF function have very high requerements to CT and CT's must be same.
Regards.
Slava

 

[DiscoP]

To add to ScottyUK's post, in one of their version changes, the SR745 relay changed the polarity for the Neatral CT connection. Please check to ensure you have the correct connection diagram for your relay version.

 

[eaaf]

Hi.
Thanks for all your answers.
This transformer is in service for 10 years ago. The last two month that I have problems with it. First the OLTC, with the problem with the fixed and mobile contacts, I just waiting for the ABB for the replacement of all the internal parts and the engineer who will made the service on the OLTC. Two years ago we did the dehydrated and filtering of the oil of the transformer. And the adjustment of the inside connections of HV and LV. This due to the detection of dissolved gases. This problem was solved. The disolved gases in the principal tank are stable, and under the limits.
I will check the CT wiring. I think the REF protection on the SR745 is low impedance. The setting are 0.07xCT (CT 300:5), the resistor that is conected fron the center of the star to the ground is 10 ohms. The REF trip hasn't been taking place during a fault in the outgoing circuits in the secundary. As you said, if the CT is reversed, it could work normal until something happens in the HV Network, I will check if in moment of the trip something had been happened in the network.
The CT must be X type or it could be a normal CT with the SR745.
I didn't make an upgrade in the relay since it was installed.
The phase CT are 3000:5 and the REF CT is 300:5
Regards.

 

[slavag]

Hi Eaaf.
1. As I understand LV is Star (Y) of xrf, that means HV ground faults are not influence on the REF installed on LV, but ground faults on outgouing feeders influence. Maybe something happend on the last time in the MV.
2. According to SR745 manual : REF is low impedance, you are right.
3. What is fault value recordered into relay?
4. I'm also recommend visual check of all isolators/bushings in REF protected zone and you must clean
them.
Regards.
Slava

 

[PUNGLU]

Check the tap-changer neutral rings for any damage, seems there is mechanical stress...May seem obvious but I have experienced such!Also check the switch contacts for uniformity and any signs of letting up

 

[eaaf]

Hello.
Slavag:
Your right, the LV is a star and it is connected to ground by a 10 ohms resistor, where is the CT for REF is located. I double check for an event in the LV side. But I couldn't find something. The fault value was 21 amps.
I checked everthing in the transformer, visual and electrical tests.

Punglu:
I have a mayor problem in the OLTC. The contacts are damage. I will made a mayor maintenance in November.

I think these REF trips was due to the problems in the OLTC. But I am not sure. So I start this thread.
I really thanks all yours comments, I found all these very usefull, becase, step by step, I will clean all the possible causes.

Thanks.
Regards

 

[isquaredr]

Is the oltc on the HV or the lv winding ? I have only ever seen oltc on the HV side in which case the lv REF would not see such a fault.

 

[eaaf]

Isquaredr: The OLTC is the LV winding. The transformer is a 69KV/4.16KV.

Regards.