3 phase 3 wire installation

[Aile16]

My customer has a 3 phase 3 wire service, and I'm not terribly sure how to interconnect onto it. I'm stuck using Solectria inverters that require a neutral. My EE recommends a 480 / 480/277 transformer, but another EE recommends a grounding transformer.

What's my best course of action?

 

[kwired]

My customer has a 3 phase 3 wire service, and I'm not terribly sure how to interconnect onto it. I'm stuck using Solectria inverters that require a neutral. My EE recommends a 480 / 480/277 transformer, but another EE recommends a grounding transformer.

What's my best course of action?

Sounds to me like they are both asking for the same thing, sort of. More details are necessary to know exactly what you need and how it might need configured. Is one of the lines of the existing grounded or is it supplying an ungrounded system. Assuming it is a 480 volt system, but you didn't exactly confirm that either.

 

[Aile16]

Sounds to me like they are both asking for the same thing, sort of. More details are necessary to know exactly what you need and how it might need configured. Is one of the lines of the existing grounded or is it supplying an ungrounded system. Assuming it is a 480 volt system, but you didn't exactly confirm that either.

Yes, it's 480, good catch.

I don't know how to answer your ground question, is there something I can look for at the site?

 

[pv_n00b]

My customer has a 3 phase 3 wire service, and I'm not terribly sure how to interconnect onto it. I'm stuck using Solectria inverters that require a neutral. My EE recommends a 480 / 480/277 transformer, but another EE recommends a grounding transformer.

What's my best course of action?

You don't indicate what size the PV system is. Typically a grounding transformer can be smaller than an in-line transformer. But there are minimum sizes for grounding transformer systems that make sense so if you are installing a small PV system then it's probably a wash. The 1:1 transformer to provide a neutral is the easiest choice, the grounding transformer might be the less expensive option if the PV system is large.

 

[Aile16]

You don't indicate what size the PV system is. Typically a grounding transformer can be smaller than an in-line transformer. But there are minimum sizes for grounding transformer systems that make sense so if you are installing a small PV system then it's probably a wash. The 1:1 transformer to provide a neutral is the easiest choice, the grounding transformer might be the less expensive option if the PV system is large.

My bad, wasn't sure if the size was relevant yet.

The system is 86kWdc, 72kWac through 2 Solectria PVI36TL

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[Garrison]

My bad, wasn't sure if the size was relevant yet.

The system is 86kWdc, 72kWac through 2 Solectria PVI36TL

Sent from my SM-G920V using Tapatalk

Not sure if you can switch inverters, but Solectria does have a 480V 3Ph 3W inverter that might fit your application. The PVI line comes in 50, 60, 75, 85, and 100kW. 3W is standard.

 

[Aile16]

Not sure if you can switch inverters, but Solectria does have a 480V 3Ph 3W inverter that might fit your application. The PVI line comes in 50, 60, 75, 85, and 100kW. 3W is standard.

Unfortunately the utility would want a new interconnection agreement, which is basically impossible at this point.

I certainly don't mind getting a transformer, but I'm not sure where to find someone to spec what I need.

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[pv_n00b]

I would go with a 100kVA 480/480/277V matching transformer. You could use a smaller grounding transformer but after you add the protection and shunt trip breaker you are probably over the cost of a 100KVA transformer.

 

[Aile16]

Thank you very much for the advice. I was leaning towards the 480 - 480/277 transformer but I was worried that it's a bit much compared to the grounding transformer.

Last question - will there be any production losses associated with the transformer?

 

[GoldDigger]

Thank you very much for the advice. I was leaning towards the 480 - 480/277 transformer but I was worried that it's a bit much compared to the grounding transformer.

Last question - will there be any production losses associated with the transformer?

Yes, but in the low single digit percentages of rated power.
The biggest loss may well be the core and copper loss of the transformer running idle for the 16+ hours daily that the inverter is not producing. But the hassles of switching the transformer off and the consequences for the inverter may argue against that cost reduction.

 

[PWDickerson]

I recently worked up a financial analysis for a system to disconnect a 75 kW PV system at night to avoid transformer losses. It was not cost effective. Not even close actually.

 

[electrofelon]

Yes, but in the low single digit percentages of rated power.
The biggest loss may well be the core and copper loss of the transformer running idle for the 16+ hours daily that the inverter is not producing. But the hassles of switching the transformer off and the consequences for the inverter may argue against that cost reduction.

I Get about $35 per month for a 75 KVA transformer. Sound in the ballpark?

 

[electrofelon]

You could use a smaller grounding transformer but after you add the protection and shunt trip breaker you are probably over the cost of a 100KVA transformer.

Can you elaborate on why that is required and where it would go?

 

[pv_n00b]

Can you elaborate on why that is required and where it would go?

You need to protect the transformer itself with an OCPD and if that trips or someone manually opens it that action needs to disconnect the PV system from the utility or shut the inverters down. You don't want them operating without a ground reference. This is the basic setup for a small system.

 

[Garrison]

Unfortunately the utility would want a new interconnection agreement, which is basically impossible at this point.

I certainly don't mind getting a transformer, but I'm not sure where to find someone to spec what I need.

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Try Deepak Gupta at Dongan Transformers out of Michigan. They will build to your exact specifications and are very familiar with the requirements of PV inverters.

 

[electrofelon]

You need to protect the transformer itself with an OCPD and if that trips or someone manually opens it that action needs to disconnect the PV system from the utility or shut the inverters down. You don't want them operating without a ground reference. This is the basic setup for a small system.

But doesnt that action itself disconnect the PV system :?

 

[GoldDigger]

But doesnt that action itself disconnect the PV system :?

It would definitely shut down if an interface transformer failed.
It is not as obvious whether it would shut down immediately if the grounding transformer failed.

 

[pv_n00b]

But doesnt that action itself disconnect the PV system :?

No, there is not "loss of grounded conductor" test in 1741. If the inverter picked up a voltage to neutral that is out of specifications because it was no longer grounded it would shut down but floating the neutral would not necessarily result in this happening. That's why loss of the grounding transformer need to initiate a positive action to shut down the PV system.

 

[dionysius]

My customer has a 3 phase 3 wire service, and I'm not terribly sure how to interconnect onto it. I'm stuck using Solectria inverters that require a neutral. My EE recommends a 480 / 480/277 transformer, but another EE recommends a grounding transformer.

What's my best course of action?

Use a Delta to Wye 1:1 480 - 480/277 xfr to derive a new service. Ground the neutral of the secondary Wye. I am probably saying what others have said but using different terms.

 

[electrofelon]

No, there is not "loss of grounded conductor" test in 1741. If the inverter picked up a voltage to neutral that is out of specifications because it was no longer grounded it would shut down but floating the neutral would not necessarily result in this happening. That's why loss of the grounding transformer need to initiate a positive action to shut down the PV system.

I am still not following you. I am not saying you are wrong - I admit I have never used, or even thought much about using a grounding transformer so maybe I just dont see it yet. Lets back up and make sure we are on the same page. I believe you stated that a zig-zag transformer requires a shunt trip breaker to be used, while a 2 winding transformer does not. What actuates the shunt trip and under what conditions? Couldn't we have the loss of the neutral on a isolation transformer as well?