Lightning arrester failures 1

[podobing]

Hello,
Is this possible:
At one of our customer's mining substations, they operate 2 -10 MVA transformers in parallel. The transformers are 69 kV primary (delta) to 12.47 kV (resistance grounded wye). At some point in time, the center fuse on one of the transformers (T1) blew. The fuse size is 100 E. These stations are rather remote, and it was not known on what day the fuse blew or what the weather was like when it happened.
As the underground mining activity has progressed, this substation is rather lightly loaded, and only supplies power to several underground conveyor belts, so the blown fuse was not realized until a weekly inspection. The plan was to take transformer T1 out of service on the weekend.

Before the weekend arrived, the center fuse on the other transformer (T2) blew. That fuse was replaced and it blew again. Running out of options, they replaced the center fuse on transformer T1, isolated transformer T2, and that is how they are currently running. They want to test and possibly replace T2 during the next planned power outage in a few months.

Both transformers have lightning arresters (MOV type - not porcelain) mounted next to their high voltage bushings. From ground level, the arresters LOOK okay. Is it possible that the center arresters are damaged internally, caused the fuse(s) to blow, but later allowed the power to be re-applied?

Just wandering if anyone has seen anything like this before, or if there is another explanation out there.

Thanks.
Regards,
Dave

 

[jghrist]

When one phase blows on the primary of a delta-wye transformer, two of the primary windings have Ø-Ø voltage across them, with each winding having half of the voltage, 180° out of phase from the winding with two intact fuses. This makes two secondary Ø-n voltages 50% of normal with the phases 180° from the intact phase. If these are put in parallel with a transformer without blown fuses, heavy circulating current will be produced. This is what probably caused the fuse to blow on the parallel transformer.

 

[podobing]

jghrist,
I understand what you are saying, but does that apply to two transformers in parallel? I was thinking that the transformer with all of the fuses intact would have produced legitimate three phase voltage on its secondary. By legitimate I mean they are the proper magnitude (12470 volts)and phase angle. That voltage would then back feed the transformer with the blown fuse, and create legitmate three phase voltage (69 kV) on its primary. The blown fuse would basically not allow load sharing between the two transformers. Your response could explain why the other fuse blew, my reasoning does not explain why that happened.
Dave

 

[jghrist]

Dave,
I agree with your point about the intact transformer backfeeding the one with the blown fuse and creating full voltages. I'd have to analyze it further, but I think that there could still be heavy circulating currents.

 

[Power0020]

I think with jghrist analysis that four fuses will need to blow not only one as the circulting current will be between two wye phases corresponding to the two affected phases.

The only other option I think is that since the wye is resistance earthed, I don't think the circulating current will be that big when divided by twice the wye resistance