Differential relay operation on transformer

[alexandergr8]

Recently, on our 12.5 MVA, 15.5 / 6.9 KV Dyn1 transformer, differential relay operated while changing the taps.

The transformer has 17 taps position i.e., +8, .., 0, ..., -8 with a regulation of 10 % voltage.

The incident happened when the operator was changing the tap from -7 to -8 position which is the last one. During TTR, we found out that in a recent maintenance job on OLTC, the drive mechanism was not correctly aligned with the tap position. -7 at local indicator was hooked upto to -8 inside.

The Alstom KBCH120 relay showed 874 A on HV side where a 600/5 CT was used. Load was around 10 % on the transformer. while on LV side it showed 274 A on 1200/5 CT. This mismatch is huge. Esp when we checked the transformer; megger results were excellent, TTR showed one posistion mismatch only, REF relay showed no fault current.

BLS550 is the type of OLTC. Can sb explain what might have happened inside the transformer?

Thanks in advance.

 

[electricpete]

If I did my math right the rated primary current is 481A.

So the 874A primary is the abnormality, either real or indicated.

Maybe that much was obvious. I'm just trying to work through it. Can't get much farther though.

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[electricpete]

I find it hard to conceive of a tapchanger malfunction which would create a dramatically higher primary current.

All the LTC is doing is picking out different points on the windings. Three separate phase units. No chance of shorting between phases. Perhaps a malfunciton which does not place proper inductance between taps in a bridging position creates high circulating currents, but not seen at the bushings and ct's.

The only things I can beging to think of (swag) are ground fault or open circuit. Ground fault normally I would think would cause higher current, unless there is some system neutral grounding resistance.

Is the ltc on high side or low side? Was the LTC malfunction you mentioned detected before or after the incident. If before it would certainly seem a TTR on each tap position would be in order.

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[electricpete]

and if not already done you should check dga and sample gas space if applicable.

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[alexandergr8]

TTR was carried out after the operation of diff relay. The most disrubing thing to me is the HV side in excess of rated FLC. Yes, the LV current you calculated is ok.

I am collecting a sample for DGA. But it will take time before i get the results of the analysis.

Tap changer is on HV side.

One more thing to tell you guys. The operators reset the diff after 8 min without informing maintenance people and energised the tripped transformer with the downstream breaker open i.e., at no load . The diff again tripped with 874 A on HV side and this time 0 A on LV side. The tap was found to be stuck between position -7 and -8 from local indicator.

Now the transformer is in service and we have temporarily inhibited the operation of tap to -7 and above position. We will rectify the tap mismatch at the next available oppurtunity.

 

[RalphChristie]

If your trsf is running from tap 1 to tap 7, your windings are fine. (for phase-phase or phase-earth faults) For interturn-faults in the top-part of the winding the current will not be that high. I would suspect a problem in your tap-change meganism. (Short or open circuit during tapping, I would not think there is an earth-fault because REF-protection is much more sensitive than diff-protection.)

Do the OLTC have intertaps? Normally, for a 17-tap transformer, the intertap will be at tap 9, and it is almost where you have your problem.

Can you give the fault-values on all the phases?

Regards
Ralph

 

[ScottyUK]

Is the tapping winding a separate buck / boost winding, connected in series or anti-series by the tapchanger, or are the taps on the main winding?

Is the tapchanger specifically designed for 17 taps, or is there an over-travel position beyond position -8? Since your operators managed to drive it while actually in position 8, it would seem that there may well be an overtravel position. Investigate what this overtravel position does electrically. Also check how the diverter switch behaves in the overtravel position.



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