345kv/138kv wye connected autotrans

[oneninefivenine]

345kv/138kv wye connected autotransformer with 34kv delta tertiary.
34kv tertiary has 3 wye connected 300/1 pt's and 2 34kv/240-120v open delta station service transformers.
Transformer was energized for the first time from the 345kv gcb with open 138kv breaker.
Approx 10 sec later, xfmr tripped via 87T.
Found LA's on 34kv phase b and phase c destroyed. Further inspection found that phase b and phase c of the 34kv pt secondary were shorted via an errant jumper. Jumper was upstream of the sec fuses.
Question: Presuming an overvoltage caused the arrestor failures, why was there an overvoltage to begin with as a result of the pt secondary being shorted between two phases. As noted above arrestors on phase b and phase c were destroyed, and phase b and phase c pt secondary were shorted.
Any and all thoughts are appreciated.

 

[Mbrooke]

I will take a wild guess, but my hunch is ferroresonance. If those transformers were single phased (as most likely happened when one or two of your fuses blew from the shorted secondary), then it certainly is possible, at least where long over head lines or underground cables are present. I have never heard of it on a station service transformer, but I can not think why it would not be possible under a single phase condition.


Your question is a good one btw. If single phaseing leads to ferroresonance it might be wroth having your utility look into mitigation. Heck now that I think about it I am concerned about my designs lol.

 

[prc]

Why we should look for ferroresonance? The LA failures are a consequence of PT secondary phase to phase short circuiting. PT primary and secondary star neutral earthed? Voltage class of LA?

 

[Mbrooke]

How does a shorting secondary produce voltages to blow out primary LAs?

 

[krisys]

I presume that the PT secondary is wye connected with neutral solidly earthed. When the PT circuit is open there is a sudden capacitive voltage build up, which might have cause the Ferro resonance and damaged the LAs.

 

[Mbrooke]

I assumed wye on the secondary only... my mistake.

 

[oneninefivenine]

LA's were sized wrong. 20kv rather than 36kv.

 

[prc]

Still it is not explaining the reason for failure. Was the secondary shorting only LL Fault or LLG fault? Was neutral on primary earthed or not? Normally in such situation, in case a phase is grounded, voltages in other phases go to LL voltage and LA fails. But in this case LAs in the faulted phases were damaged.