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High-Low Transformer Fault - What Evidence Would Remain?

High-Low Transformer Fault - What Evidence Would Remain?

High-Low Transformer Fault - What Evidence Would Remain?

(OP)
If there were a fault somehow between the high side and the low side of a medium voltage-low voltage transformer such that the high side voltage were to be seen on the low side, what evidence would be left?

The question is intentionally vague to not bias the answers, but if high side voltage appeared on the low side, what tell-tale signs would be left?  Could be in the transformer, what tests would show what?  Could be in relay records on the high side circuit that feeds the transformer, what would we see in currents and/or voltages?

Transformer is one of many on an underground distribution circuit.  Transformer is pad-mount type oil transformer with dead front bushings on the high side and spade lugs on the low side.  High side in loop through with a load-break switch under oil in the tank.

Thanks.

RE: High-Low Transformer Fault - What Evidence Would Remain?

Complaining customers on the transformer secondary? looking around

 

David Castor
www.cvoes.com

RE: High-Low Transformer Fault - What Evidence Would Remain?

I was going to go with the customers' TV sets blowing up as well.

But its possible that, for some point on a high side Wye winding close to the neutral point, the fault would just look like a shorted turn. There might not be enough fault current to trip high side protection or a high enough voltage placed on the secondary to cause problems. So the fault would just sit there and fizzle. Oil analysis could reveal its presence. Or you could take the transformer out of service and test winding insulation.

RE: High-Low Transformer Fault - What Evidence Would Remain?

(OP)
Transformers in question have been taken out of service and tested.  Ratio and insulation have tested OK.  Physical inspection has not turned up any tracking paths.  No TVs involved, wrong voltage, wrong application for that, but there has been damage on the low voltage side.  One faction says only way arcing could start is severe overvoltage due to high side voltage showing up on the low side.  Other faction contends that arcing fault started on low side due to means unknown, but not high side voltage, and relay event records show transformer impedance in the circuit during the fault.

RE: High-Low Transformer Fault - What Evidence Would Remain?

Are these grounded wye - grounded wye?  Any arrester or TVSS  failures on the low side?  No possibility of any resonant condition on the low side, I assume.

If the secondary side was not well grounded, then arcing fault could cause an elevated voltage via repetitive restrike, but if it is solidly grounded, that seem unlikely, at least theoretically.  

Quote:

relay event records show transformer impedance in the circuit during the fault

That's probably a logical inference, but might there be other scenarios that could explain the same fault data?  The substation relay is just giving current and (possibly) voltage data.   

David Castor
www.cvoes.com

RE: High-Low Transformer Fault - What Evidence Would Remain?

(OP)
Delta high side, grounded wye low side.

It's those other scenarios I'd like to find out about.  To settle it one way or the other conclusively, or even to find that there are multiple scenarios that work with the observed record without being contradicted by portions of the evidence.

RE: High-Low Transformer Fault - What Evidence Would Remain?

I cannot imagine a scenario with a temporary connection of primary to secondary internally without damage evident in the tests you performed.  I cannot imagine a scenario with temporary external connection with dead-front construction.
 

RE: High-Low Transformer Fault - What Evidence Would Remain?

Low side neutral-ground connection is good?  

David Castor
www.cvoes.com

RE: High-Low Transformer Fault - What Evidence Would Remain?

Were the transformers operating with normal voltage levels after the incident and only taken out of service for testing?  It sounds as if that is the case.  Did the equipment damage occur on more than one service?

Alan

RE: High-Low Transformer Fault - What Evidence Would Remain?

(OP)
3 incidents.  During the first incident it was discovered that if the transformer fuses attempt to clear the low side fault, they fail and flash over.  This complicates matters as this contaminates the oil and leaves some damage.  The windings still pass the ratio and insulation tests.  Two of the events have had this happen.  The third event was cleared at low voltage and the transformer was completely undamaged.

I can't answer about the low side neutral-ground connection, but these are new installations, the events happening fairly soon after initial energization.  Multiple tests, but I don't know exactly what, performed prior to energization.

RE: High-Low Transformer Fault - What Evidence Would Remain?

Delta-wye... Any chance of ferro-resonance due to single-phasing of primary during light loading?

I'm sure you've already thought of all this.  

 

David Castor
www.cvoes.com

RE: High-Low Transformer Fault - What Evidence Would Remain?

(OP)
No reason to suspect single-phasing, all event are well balanced 3-phase events.  

RE: High-Low Transformer Fault - What Evidence Would Remain?

Check IR ,C and tan delta between HV-LV and compare with bench marks.If there is a substantial reduction in values, HV to LV breakdown is to be suspected. There can be a chance of tracking from HV to LV at winding ends than through puncture of main insulation.But I doubt this failure mode.

Another scenario may be poor or no earthing of neutral and consequent  unbalanced overvoltage on any phase.

RE: High-Low Transformer Fault - What Evidence Would Remain?

What is the high side voltage?  What are the high side fuse voltage ratings?  You can have a recovery voltage as high as 1.73 pu when one or two fuses blow on a delta-grd Y transformer.

Under-rated fuses could explain the fuse failures.  Restrikes during fuse failures may cause high voltages.

RE: High-Low Transformer Fault - What Evidence Would Remain?

(OP)
High side is 34.5kV. Don't have the fuse information at hand at the moment and didn't know anything about the fuses until the first event. As I recall there are two sets of fuses, low range and current limiting. The low side fault when interrupted by the low range fuse causes the fuse to flash over. Low range fuses rated 20 something kV as there was nothing available at 34.5kV for a 2.5MVA transformer at the time. Had it been a wye connection on the high side the fuses would be fine.

Relay events clearly show a 3-phase low current event, followed by a bit of transition and then a phase-phase high side fault on the high side (for two events, one was cleared on the low side without fuse operation). The high side fuse failure seems to be the result of the low side event rather than a cause. All of the transformer damage found is around the fuses and no damage to the windings or insulation.  

RE: High-Low Transformer Fault - What Evidence Would Remain?

It sounds like the fuses need to have a higher voltage rating.  We ran into a similar situation with 25 kV transformers.  We ended up using a 35 kV expulsion fuse in series with a 23 kV rated current-limiting fuse.  We couldn't find current-limiting fuses with a full 25 kV rating.  For a through fault where the high recovery voltage is possible, the current limiting fuse would not operate.  We used ABB ProLink fuses.  
ABB
 

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