We are looking at a solar project for a farm that has a 240/480V service. Currently it is a 3-wire configuration and is fed by two 15 kVA transformers. The utility is doing some work on the property now and is replacing the two 15 kVA transformers with two 25 kVA transformers, while also supplying a neutral on the secondary during this work.

I'm trying to find an inverter that plays nice with this configuration (customer wants about a 35 kW-DC array). Any ideas? We typically use SolarEdge for most of our installations so I'm not as familiar with what Fronius, SMA, or other manufacturers may have that works with this configuration.


Hello. Are there two separate services now? 240 Delta and 480 Delta? Will the new services be a 120/240 split phase and a 480/277 3 phase? Either way, for a 35kW DC array which will be about 28-25kW AC, that is too much to push through a 25kVA transformer. Can you split the array onto 2 different inverters and back feed both transformers? If you have a N on the the inverter side of the transformer, you can use almost any inverter that you like.

The utility got back to me and says it's a 3-Phase 4-Wire Delta 240/415/480, Wild Phase. Currently, it's fed by two 15 kVA transformers, but they are replacing those with two 25 kVA transformers and adding a neutral on the secondary (currently no neutral with the 15 kVA transformers). 

Ok, that makes sense. It's a 480 High (or Wild) leg Delta. Different parts of the country use different terms to describe the Phase to (N)eutral leg that will surprise an unwary electrician - in your case it's 415V. There is nothing we can do with that leg in terms of PV interconnection.

So now we know we have 480 3 phase service created by two transformers - that's fine, but I don't recommend more that 50% of the total transformer rating for back feeding. About 25kW here; you might be able to squeeze the 35kW into that depending on where you are and what inverter you are using - or just trim it to 32.5kW DC and use any 480 3phase inverter you want.

MOST IMPORTANT: Draw up a quick 1-line showing your inverter AC rating (or have your PV supplier do it) and give it to the utility to approve. They will have to make the final call on the amount of power you can back-feed into those two transformers on all three phase (two plus one virtual).

JethTran commented 2 years 8 months ago

HI....I've never seen a 240V Three-Phase framework introduced ungrounded. I've worked with 480V ungrounded frameworks a considerable amount and just know about one catastrofic disappointment.

Administration coherence is the solitary advantage of an ungrounded framework. The weaknesses are: transient overvoltages are not controlled, framework voltages over the ground are not controlled. The ungrounded framework is entirely coupled to ground. Were administration progression is required an ungrounded framework is protected as I would like to think, if the appropriate strategies are utilized, for example ground locator lights, find and right ground-blames when useful, and so on

Does your client have a real requirement for an ungrounded framework? Assuming not, I would suggest a grounded framework. Corner establishing presents different issues, for example no breakers whenever grounded stage is interuppted (should open all stages). You could utilize a crisscross transformer to ground the framework however it is smarter to simply supplant the 75 kVA xformer with a 240V 3-Phase 4-Wire xformer.

5 years 7 months ago
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