padmount dist xfmrs & lightning

[Jay27]

We know the cause was lightning, I just want to understand why...

Single phase underground line with a lightning arrestor and line fuse at the TP pole (terminal pole - where the line goes from OH to UG). The line has 15 or so padmount transformers on it, serving 20 or so customers. All tubs are 120/240V, 15 - 25 kVA. The last tub on the line also has a lightning arrestor; the line ends at this tub. All the transformers have 2 ground rods - one in the basement, driven 7.5' and one 6' in-front of the tub (for step-potential).

We had a bit of a storm... got 4"+ of rain and lots of lightning. We suspect lightning struck the ground (as in the physical Earth) in the vacinity. Of the 15 transformers, 9 had blown bay-o-net fuses. The line fuse did not blow, both arrestors were good. All of the transformers held when re-fused. I assume, the lightning strike to the ground caused a rise in ground potential, question is: how did this result in the transformer bay-o-nets blowing? All tubs are fed in-and-out. De-energizing a single tub (pulling elbow) does not de-energize tubs further down the line. The tubs with blown fuses were not in series... it would skip 1 or 2, hit 3, skip 1, hit 2, etc. It is possible the conductor is bare-concentric (I'm not sure if that contributes).

you can't fix stupid

 

[PHovnanian]

Its possible that the ground potential rise in the middle of the string of transformers caused internal flashover inside the transformer, initiating a power fault which the fuses interrupted. Proper fuse coordination would result in the transformer fuses blowing before the line fuse. Although multiple parallel faults could sum up to a high enough current at the pole fuse to take that one out first under other conditions.

All tubs are fed in-and-out. De-energizing a single tub (pulling elbow) does not de-energize tubs further down the line.

Hmm. Tub in this context being the handhole upon which the padmount sits? I've not heard this jargon before. For feed-through padmounts, pulling one elbow will de-energize all downstream units. However, if the 'tub' contains a 3 way 'junction box', its is possible to pull the one elbow feeding that unit without affecting downstream transformers.

 

[jghrist]

Another possibility is that the potential rise caused the windings to saturate and the primary current increased enough to blow the fuses.

 

[Jay27]

@PHovnanian
'tub' is the slang we use for just about any transformer. All our transformers are feed-thru... It's probably just how we spec them.

@jghrist
I like the over-excitation theory in terms of an explanation

you can't fix stupid