Extremely confused how to connect an inverter to 240highleg delta service via transformer

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Adam308

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Location
Sacramento
Occupation
Solar contractor
Hello, everybody, this may be a long shot but I have been trying to research why a project isn't working.

The inverter outputs 277/480V 4 wire WYE. The building's power is 240 High Leg DELTA.
Inverter spec: https://www.solaredge.com/sites/def...erters_for_the_277_480V_grid_datasheet_na.pdf

After consulting several electricians, we placed a "480D -240D/120 CT" 75kva transformer in between.
Transformer spec: https://www.mgmtransformer.com/wp-content/uploads/2016/04/Bulletin10-DOE2016.pdf

Since then, the inverter in unable to function and constantly reads error codes.

I'm completely stumped. Perhaps the transformer is incompatible with the inverters WYE output?

Does anybody have any insight? Much appreciated!! Thank you for reading!
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Agreed. You need a 240V delta : 480/277V wye transformer, _or_ you need to add a zig-zag transformer to your existing delta:delta to derive a neutral.

At 75kVA you are much better off simply getting the proper delta: wye transformer.

-Jon
 
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zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
Another couple of thoughts-
How much power are you planning to stuff back into the 240 delta system? Quite often, they're built with one larger transformer for the two-leg-plus-center-tap and a single smaller one for the third "high" leg (e.g. a 75kva for for the main and a single 25kva for the high leg, in an "open delta" configuration).
Has this been approved by the power company? (Sac'to Muni?)
What does SolarEdge say about feeding a high-leg system?

(There are lots of threads on Mike Holt about high-leg delta systems; they're one of the most misunderstood systems out there. also check https://en.wikipedia.org/wiki/High-leg_delta.)
 
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paulengr

Senior Member
Read the VFD manual carefully. There are surge arresters and often a filter connected to the input that need 4 wire wye to work correctly. However many VFDs have a jumper or a screw that dissbles these and allows delta operation. If yours doesn’t there are several that do,

One issue you may have now is you might have damaged the front end powering it up delta without pulling the jumper/screw, if it has one. So it might not be fixable.
 
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Adam308

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Location
Sacramento
Occupation
Solar contractor
Hello, everyone thanks so much for the replies.

oldsparky52, the inverter would hang up, showing AC voltage too high error, then standby.

zbang, our PV breaker is 175A, realistically the inverter output would rarely even come close to that. And yes, we'll have to deal with PG&E (this is in Hayward, CA), and I've reached out to Solaredge without much luck.

Also, Winnie, why do you think we're better off replacing the transformer vs using a zig-zag transformer?

Paulengr, I'll need to research this, I'm not too familiar with transformers.
Thank you
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I am not well versed with actually buying zig-zag transformers, but as a relatively uncommon type I suspect it would be at a significant cost premium.

I don't actually know if code would permit you to use a zig-zag transformer to derive a neutral that you would actually use as a circuit conductor; generally they are used to ground large existing ungrounded systems.

If you do use a zig-zag transformer you would have a more complex system with more to go wrong.

The correct delta:wye transformer is perhaps $3K https://store.maddoxtransformer.com...se-480v-delta-208-y-120?variant=2102327345161

On the other hand, _in theory_ a relatively small zig-zag transformer could derive a neutral for a much larger delta transformer, certainly worth a call to a manufacturer to ask.

But cheaper than all of that is to make sure you have someone with the proper experience looking at the project (not me; I do electric motor research and have a good theoretical understanding of the electrical field, but I am not a solar power electrician).

I believe that Paulengr's comments were related to 'variable speed drive' units for running motors; they are also often called inverters.

-Jon
 
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Adam308

Member
Location
Sacramento
Occupation
Solar contractor
Ahh ok, understood! I am floored how well versed you guys are. I had never even encountered a high leg delta service, likewise for a few other solar installers I spoke with.
 
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Carultch

Senior Member
Location
Massachusetts
winnie said:
Whoops.

What about: https://store.maddoxtransformer.com...277-step-up-transformer?variant=2194321440777
Click to expand...

Yes. That is what would be needed in this situation, to adapt 277/480V to the 240V delta. That transformer would work for any 240V delta grid, whether it is high leg or corner grounded.

Another way to connect to a 240V high leg grid is to use single phase 240V inverters, and stagger them among the 3-phases. You would need to verify that the inverter manufacturer allows this, as there is a good chance the inverter requires 120V line to neutral. Or if your utility allows it, simply connect all inverters to the A & C phases, that would work just like a 120/240V single phase system as far as the inverters are concerned. Utilities often require balancing, so that may be a reason why you couldn't do an A-C phase exclusive system.

One pitfall of high leg services, is that the transformers that supply them may be disproportionately smaller for the units that connect to the B-phase. All 120V loads are limited to the A & C phases that uses the centertapped transformer, so it is a possibility that you have smaller AB & BC phase transformers, than the A-N-C transformer. If you connect a balanced 3 phase system, its kVA would be limited to smallest of the 3 service transformers, multiplied by 3. In otherwords, assume all three service transformers are worst-case scenario in their kVA rating. This kind of service is intended for services where 240V loads are the majority, and 120V loads are still needed, but a small fraction of the total load.
 
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Adam308

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Location
Sacramento
Occupation
Solar contractor
Thank you Carultch, we just went out today to try to locate that transformer to see how small it really is.
This is fantastic, very thorough information, thank you so much!
That's a good point about using several smaller inverters, which also may be more trouble/$$$ than its worth.
Based off all the advice, as well as practical concerns including speed and permitting, we've got to simply get a new transformer. Which is ideal, since the project's already approved for the pre-existing transformer.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Before you make any physical changes I would get the interconnection approved by PG&E. Perhaps that's been done but I'm a bit doubtful they would have approved a design with an incorrect transformer. If it hasn't been done, they may not even allow you to backfeed all three phases. On systems this size you need to get the interconnection approved before you build.
 
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Adam308

Member
Location
Sacramento
Occupation
Solar contractor
Hey everybody, sorry this is a bit late. Carultch, for that transformer, why doesn't the 240V delta side need a center tap, and what difference would it make? Also, wouldn't the 480V wye side need to be grounded? Thanks
 
synchro

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Adam308 said:
... for that transformer, why doesn't the 240V delta side need a center tap, and what difference would it make?
Click to expand...
The center tap on a utility transformer supplying a high-leg delta service is there to provide a neutral for conducting the unbalanced current from 120V loads placed across each side of the center-taped winding. ... Just like on a common 240V/120V split-phase service.

A center tap on the delta of your transformer would serve no useful purpose, and in fact it could cause significant issues if it was connected to the neutral of the high-leg service. For example, if you connected only one half of the center tap winding then an approximately equal voltage which is phase shifted by 180 degrees would appear across the other half-winding. If this voltage does not precisely match the voltage provided by the utility then when that leg is connected high currents will flow through the two half-windings (because of the relatively low impedance of the transformer).
Adam308 said:
... wouldn't the 480V wye side need to be grounded? Thanks
Click to expand...
The center connection of the wye should be connected to the grounded conductor (neutral) of the high-leg service.
 
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Carultch

Senior Member
Location
Massachusetts
Adam308 said:
Why doesn't the 240V delta side need a center tap, and what difference would it make? Also, wouldn't the 480V wye side need to be grounded? Thanks
Click to expand...

The 240V side is connected to the premises wiring grid, as if it were any other 3-phase load. The reference to ground is already established at the centertap of the A-C winding of the service transformer. There is no need to bond any of the winding connections on this side to the EGC system, when it is a sub-600V transformer connected to a service that is already grounded. It would be redundant, and counterproductive to system safety, as it means the potential for current on the EGC/housing/conduit, where it doesn't belong. You still have to bond the transformer housing to the EGC, but not any one of the winding terminals on this side.

The 277/480V side connected to the inverters, becomes a separately derived system (SDS). As such, it is isolated from the 240V side. Its neutral terminal therefore needs to be bonded to the EGC, and bonded to a grounding electrode, in order for phase-to-ground voltage to be no greater than 277V nominal.
 
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