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Single phasing delta Y transformers

Single phasing delta Y transformers

Single phasing delta Y transformers

This is at a wind farm site where the cables are underground going to multiple padmounts connected in parallel, standard setup. The main power transformer is Wye connected on the 34.5kV side. The collector string is connected delta-Wye. We had a padmount nearest to the substation (connected delta-wye) where the internal jumpers on A and B phase failed. Now we have a padmount with normal voltages, and the rest (about 8-10?) have only one phase connected (20kV to ground, 34.5kV nominal). When we energized (not knowing the internal jumpers had failed, that took a while to figure out) we had currents of (A/B/C/G)17/17/38/22 amps. The arrestors started failing on A and B phase and were replaced. We found the faulted padmount, have isolated that.

When we energized we blew more arrestors. They didn't really cause any issues, but the ground lead would pop off. The initial fault caused one phase of the substation breaker to fail, all the porcelain cracked, we caught that before it failed catastrophically. (the breaker bushing failure has nothing to do with the issue, was just an noted item, it was cracked before and the magnetic forces (6kA) caused the damage, but interesting)

It makes intuitive sense to me that they were over voltaged though the padmount, but I'm not sure the path. If only 1 phase is energized I don't see the path to over voltage the other phases.

Could you give me your take on why the arrestors saw over voltage?

RE: Single phasing delta Y transformers

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.

RE: Single phasing delta Y 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.

RE: Single phasing delta Y transformers

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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