Differential Trip

[sanchezjl]

Hi..

I have a question for the relay engineers..

Recently we had a trip in one of our substations transformers where we are using a SEL-387 relay with an external single phase to ground fault.

One the event was downloaded; we sent it to SEL because it was not clear for us (operations put the transformer in service without diff protection and it worked, but today we took it out of service for inspections).

The thing is that they realize that the hi side C phase and the low side C phase (the faulted phase) where in phase and around the same magnitude before the fault, but during the fault it was a 50 degrees shift between those two.

Have any of you guys had something similar in the past?

I will appreciate your answers.

Thanks!

 

[ScottyUK]

Hi Sanchezjl,

Have a look at faq559-1100 for posting images.


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Sometimes I only open my mouth to swap feet...

 

[oldfieldguy]

sanchezjl--

Per my previous post, once you get your CT inputs reconfigured and the transformer energized, the SEL 387 relay has a command which will allow you to read the relay's computation of the differential model it sees in light of its programming and the CT inputs.

If your measured differential current differs significantly from zero, there is likely a problem with either the CT connections or the settings for the relay. I have seen both. It is not uncommon for the CT's to be connected to the relay incorrectly, the high side being connected to the inputs expecting the low side CT's. The differential current metering function will show this.

old field guy

 

[dpc]

I'm too lazy to read this entire thread, but I assume that the delta winding has a phasor shift setting of 12 and not 0? This is necessary in a delta-wye transformer in the SEL relays, I think.

Also, I'm not clear - has this relay been in service for a while and this is the first (apparently) mis-operation?

 

[waross]

A single phase to ground fault, right. The fault current would return ultimately via the neutral. A neutral current implies a voltage drop in the neutral. The voltage drop in the neutral has the effect of moving the wye point out of the geometric center of the vector diagram. This results in altered phase angles. It is quite possible that the relay is operating correctly and you are seeing a real phase shift that has resulted from a severe voltage drop on the neutral.
I have seen phase shifts at utilization equipment caused by unevenly loaded primary phases and excessive neutral current.
There would certainly be phase shifts at the location of the ground fault.
Could this reflect back to the relay? I know that it will happen in the field, but I don't know that much about relays.

respectfully

 

[stevenal]

Waross,

You are speaking of voltages, but this is a current relay. It is also intended to be secure for through faults.

 

[jghrist]

Try looking at unfiltered current data to see if there is distortion in one of the winding currents, indicating a saturated CT. I think that the phase angle of filtered (fundamental) current from a saturated CT would have a phase shift.

 

[sanchezjl]

Finally, we found the problem in the CT is located in the low side of the transformer.

Even do we tested the CT’s (CTR & saturation) and it was Ok, but we did not test the insulation of those CT (at that time we did not have a 500V megger).

CTs located on X1 and XO bushings are zero ohms to ground (secondary of CT’s of course) with the megger test (they showed open with a “continuity” test with a regular Fluke).

Now, with the fluke (volmeter) tester, there is a 0.1 ohms resistance between the CT located on X1 on the one located on X2; but X2 is fine with the megger test (why?, I don’t know).

Now, if you ground the secondary of X3 CT, then the X2 CT has zero insulation with the megger test (again, I don’t know why).

Today we are opening the “feedtrough” located on the top of the transformer trying to find something abnormal (the wires from there to the cabinet are Ok).

I will keep you informed.

Thanks for all your recommendations.