Can you provide a one-line of the system? What is the collector string voltage? I assume that you have one or more 34.5 kV UG feeders to several 34.5 - ?? kV delta-wye padmounts feeding collector circuits. Were the failed jumpers on the primary of the 34.5 - ?? kV delta wye transformers? Did the jumpers fail faulted to ground or each other or fail open? Where were the failed arresters? Where were the currents measured? Was the failed breaker protecting a 34.5 kV feeder at the substation? When you say that the underground cables are going to multiple padmounts connected in series, I assume you mean that they have loop feeds on the primary side, not that they are electrically in series. It sounds like you could have a ferroresonance situation, but you would typically need to have the open point before cable feeding the delta wye transformers.

It's just a single feeder with 10 padmounts in parallel. (the series comment was a brain fart, fixed) The first padmount blew up internally, no sign of damage from the outside, we found it by a resistance check from H1A-H1B, H2A-H2B (Both were open) and H3A-H3B (shorted). H1A is incoming from the feeder, H1B is outgoing to the next padmount. The relay and breaker are in the substation, the under ground cable connecting the padmounts is likely about 2-3 miles. As they branch out and terminate, an arrestor is placed on each end of the circuit, about 4 arrestors per phase total.

The jumpers are internal to the padmount, just dead front externally. I don't know the state of the padmount, we didn't open it up, but no visual signs. We did see secondary voltage on the first padmount (600/400 nominal) of Ag/Bg/Cg; AB/BC/CA of 230/230/400; 240/600/600 (A phase fuse blown?) volts and downstream padmounts 230/2/230; 230/230/400 volts.

When we finally energized the circuit after jumpering out the first padmount the nominal current was 38 amps (due to the cable capacitance) which was the C phase current. When we energized the circuit after jumpering the first padmount 3 arrestors blew, after replacement the system energized, no issues.

The ground current is in phase with the C phase, the only undamaged circuit. Getting a 22 amp ground fault on a 35kV system is hard to do, without it blowing up. I expect it's in the failed padmount, but the current in C phase was normal.

With H1A-H1B and H2A-H2B open, you have a situation where the following transformer windings are in series with cable capacitances of the two phases that are isolated from the source. See the attached diagram. The transformer windings are non-linear reactances and when put in series with the cable capacitance can cause ferroresonance and high overvoltages. See https://electricalbaba.com/what-is-ferroresonance/

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