Wye Inverter on a Delta Service

[pv_n00b]

What brand/model is your example from, and are there others you are aware of that are similar?

I guess the existence of that manual settles the questions in my earlier posts today.

Cheers, Wayne

That's from the CPS 100kW-125kW inverter manual. Unfortunately, I don't have a list of inverters that can be connected to an ungrounded service.

100kW-480V Inverter - Chint Power Systems

Commercial String Inverters 100kW-480V String Inverters for North America Downloads CPS Datasheet 100kW-480V CPS 100kW-480V User Manual NRTL CSA CPS SCH100KTL-480V UL CERT The 100kW high power CPS three phase string inverters are designed for ground mount applications with 480Vac service...

www.chintpowersystems.com

 

[ggunn]

That's from the CPS 100kW-125kW inverter manual. Unfortunately, I don't have a list of inverters that can be connected to an ungrounded service.

100kW-480V Inverter - Chint Power Systems

Commercial String Inverters 100kW-480V String Inverters for North America Downloads CPS Datasheet 100kW-480V CPS 100kW-480V User Manual NRTL CSA CPS SCH100KTL-480V UL CERT The 100kW high power CPS three phase string inverters are designed for ground mount applications with 480Vac service...

www.chintpowersystems.com

A separate issue, but those inverters do not appear to support the SunSpec protocol for rapid shutdown.

 

[pv_n00b]

A separate issue, but those inverters do not appear to support the SunSpec protocol for rapid shutdown.

That's because these inverters are intended for ground mount arrays. I'm just offering an example of an inverter that can be connected to an ungrounded service, not specking an inverter for a particular application.

 

[pv_n00b]

Any suggestions on avoiding installing transformer? I called SMA (inverter tech support) and 1st tier guy I spoke to is stumped. He’s consulting with 2nd tier, while I listen to EuroTech elevator music and looking things up on the web and going back to my old text book reviewing the difference between Wye and Delta services.

Thanks in advance!

Just to put this to bed, I'm going to give you the bad news that with that inverter and an ungrounded service you will need a transformer. Typically when people find an ungrounded service, or a corner grounded service, or an open delta service, or a 3ph 240/120V service and want to use 3ph inverters they just get the delta-WYE transformer and that significantly opens up what inverter can be used. Next time check the service and don't make assumptions. It's a good learning experience.

 

[ggunn]

Just to put this to bed, I'm going to give you the bad news that with that inverter and an ungrounded service you will need a transformer.

As discussed in post #3 of this thread.

 

[tortuga]

they just get the delta-WYE transformer

Seems like a considerable performance hit and added cost to the job for a contractor mistake , two transformers a 112 and a 50kva, if I were the one signing the check I'd want the inverter to match the service voltage.

 

[ggunn]

Seems like a considerable performance hit and added cost to the job for a contractor mistake , two transformers a 112 and a 50kva, if I were the one signing the check I'd want the inverter to match the service voltage.

It could be done with a single transformer if both inverters are connected to the same service.

 

[pv_n00b]

Seems like a considerable performance hit and added cost to the job for a contractor mistake , two transformers a 112 and a 50kva, if I were the one signing the check I'd want the inverter to match the service voltage.

It sounds like the inverters will match the service voltage, that's not the issue. The transformers will be 480D:480/277Yg isolation transformers, just to provide a grounded neutral. It is a performance hit, but that's just what comes with the service. I'm sure it's going to be an interesting discussion as to who pays for the transformers due to the mistaken service.

 

[tortuga]

It sounds like the inverters will match the service voltage, that's not the issue.

Sorry make that I would not want to pay for inverters that required a transformer to operate correctly.
Nor would I want to pay for the monthly kwh vampire load. I mean thats gotta be what 3000kwh per year? Even if it cost an extra 20 grand to get the correct inverters it would probably pay for itself.

 

[wwhitney]

The transformers will be 480D:480/277Yg isolation transformers, just to provide a grounded neutral.

Since there are no actual L-N loads or sources, just a need to provide a neutral point for voltage stabilization and reference, what about using a grounding transformer?

I would assume that could be sized smaller, as it wouldn't actually need to carry the power produced. Either a small 480D:480/277Y or a zig-zag.

Cheers, Wayne

 

[winnie]

Since there are no actual L-N loads or sources, just a need to provide a neutral point for voltage stabilization and reference, what about using a grounding transformer?

I would assume that could be sized smaller, as it wouldn't actually need to carry the power produced. Either a small 480D:480/277Y or a zig-zag.

Cheers, Wayne

That approach means either grounding the entire 480V system, or using a grounding transformer to derive an _ungrounded_ neutral.

The first approach is plausible, but means grounding the _entire_ building, not just the PV system. This could mean redesign of other portions of the building. Of course if the ungrounded system is a relic of previous use, this might be a significant improvement for the building.

The second approach is a bad idea; you are deriving a neutral which _isn't_ grounded. This will get the system working...but as soon as there is a ground fault anywhere on the building system, the faulted phase becomes the ground reference and the derived neutral operates at 277V to ground. Components which the inverter designer intended to be near ground potential will be at 277V, components intended to be at 277V will be at 480V.

-Jonathan

 

[ggunn]

Sorry make that I would not want to pay for inverters that required a transformer to operate correctly.

It is a virtual necessity for connecting a large-ish PV system to a 208/120V service.

 

[mac380abc]

First I know of no buildings that is fed with a Delta System, being most services need a center tap, that makes
the neutral. But if your sure this is a delta system, the fix is easy, just get a delta wye transformer rated at whatever
amperage is required for the equipment, problem solved. The delta configuration, is a corner grounded, although
haven't seen to

 

[jim dungar]

First I know of no buildings that is fed with a Delta System, being most services need a center tap, that makes
the neutral. But if your sure this is a delta system, the fix is easy, just get a delta wye transformer rated at whatever
amperage is required for the equipment, problem solved. The delta configuration, is a corner grounded, although
haven't seen to

Be careful using slang.
The word Delta alone is not sufficient to properly describe a system. These transformer connections can be open (using 2 transformers) or closed (using 3 transformers) and are used to supply:
3 phase 3 wire Ungrounded
3 phase 3 wire Corner (B Phase) grounded
3 phase 4 wire Center tapped (high leg)

Just because your local utility may not presently supply all of these possible connections on new construction doesn't mean they are not grandfathered in or are customer owned.

 

[electrofelon]

. But if your sure this is a delta system, the fix is easy, just get a delta wye transformer rated at whatever
amperage is required for the equipment, problem solved

It may be simple, but it is very costly. Say you have a 225 KW system, such a transformer will be maybe $7000 dollars, and $1500 per year just in no load losses - and that's not even counting load losses. As mentioned, there may be no other choice due to inverter availability, but it's certainly nice to avoid if possible.

 

[Flicker Index]

Guessing that inverter will be operating phase-to-phase for power delivery, could you synthesize the ground reference with a grounding transformer ? (zig zag)

 

[jaggedben]

Guessing that inverter will be operating phase-to-phase for power delivery, could you synthesize the ground reference with a grounding transformer ? (zig zag)

See post #71.

Also I think it's unclear that the inverters actually operate phase to phase. They could possibly operate phase to neutral but balanced. And that could be a problem if there isn't a low impedance ground reference.

 

[winnie]

See post #71.

Also I think it's unclear that the inverters actually operate phase to phase. They could possibly operate phase to neutral but balanced. And that could be a problem if there isn't a low impedance ground reference.

I find it unlikely (and a bad design) if a device that uses the EGC as a voltage reference requires it to be a low impedance (in the power distribution sense) reference. Because requiring a low impedance EGC implies that it injects significant current into the EGC.

A wye configured IGBT bridge can supply delta loads; this is the most common VFD/motor topology after all. And I suspect that a transformer derived (but not properly grounded) neutral would allow the inverter would function as a three phase source.

But for the reasons in post #71 I suspect this is a bad idea for an inverter which is _intended_ for connection to a wye service.

On the other hand, this exact technique might be used to derive the voltage references for a wye topology inverter intended for an ungrounded delta. The _huge_ difference is the design intent; such an inverter would be built with the expectation that any component could be at 480V to ground, rather than the expectation that some components would be near 0V and others 277V to ground.