Service available on site --Electrical Panel 120/240 volt High Leg

[inforaj]

The service (Electrical Panel 120/240 volt High Leg) available on-site is an open-delta configuration which only utilizes 2 transformers and provides a 3-phase service (L1 to L2=240V, L2 to L3=240V, L1 to L3=240V, and L1 to Ground=120V, L2 to Ground=208V, L3 to Ground=120V). The FRONIUS -Symo advance (12.0-3, 208-240) has a provision for a High Leg grid connection.
But now the Utility company is saying - The inverter that was selected is a 3-phase 240V connection, but the service transformer is an open delta (2 phase). A 3-phase inverter is not allowed to be on an open-delta transformer. If you choose to install a 3-phase inverter, the open delta bank will need to be closed, and the report will include the cost to upgrade it to a 3-phase closed delta bank with the 3rd transformer can being large enough to support this application. If you choose to install a single-phase inverter, the existing transformer can will need to be upgraded to support this application unless this application is downsized enough to avoid the upgrade.

Could you please advise on this situation? Any suggestion will be highly appreciated. Thanks in advance.

 

[electrofelon]

The service (Electrical Panel 120/240 volt High Leg) available on-site is an open-delta configuration which only utilizes 2 transformers and provides a 3-phase service (L1 to L2=240V, L2 to L3=240V, L1 to L3=240V, and L1 to Ground=120V, L2 to Ground=208V, L3 to Ground=120V). The FRONIUS -Symo advance (12.0-3, 208-240) has a provision for a High Leg grid connection.
But now the Utility company is saying - The inverter that was selected is a 3-phase 240V connection, but the service transformer is an open delta (2 phase). A 3-phase inverter is not allowed to be on an open-delta transformer. If you choose to install a 3-phase inverter, the open delta bank will need to be closed, and the report will include the cost to upgrade it to a 3-phase closed delta bank with the 3rd transformer can being large enough to support this application. If you choose to install a single-phase inverter, the existing transformer can will need to be upgraded to support this application unless this application is downsized enough to avoid the upgrade.

Could you please advise on this situation? Any suggestion will be highly appreciated. Thanks in advance.

View attachment 2562109

It appears the POCO is giving you two options, I guess you have to weigh the costs of each. IF not significantly more, I would probably have them close the delta.

 

[pv_n00b]

I could tell you that there are several options that would work, but the POCO has told you what they will accept so that's pretty much it. I will say I choose to only use single-phase inverters on the phases that have XFMRs in these open delta systems.

 

[inforaj]

It appears the POCO is giving you two options, I guess you have to weigh the costs of each. IF not significantly more, I would probably have them close the delta.

But the inverter has a provision for connecting the High-leg, and also the Inverter technical engineer confirmed that this inverter is suitable for this kind of grids.

 

[electrofelon]

But the inverter has a provision for connecting the High-leg, and also the Inverter technical engineer confirmed that this inverter is suitable for this kind of grids.

The POCO doesnt care what the inverter can do, they have given you two options, and one is to close the delta for three phase.

 

[electrofelon]

I could tell you that there are several options that would work, but the POCO has told you what they will accept so that's pretty much it. I will say I choose to only use single-phase inverters on the phases that have XFMRs in these open delta systems.

I think the size of the PV system would be the major deciding factor. For a relatively small system, single phase would be fine. In fact, at least for the fronius line since that what the OP mentioned, their largest single phase inverter is larger than their largest 240V three phase inverter (15 vs 12KW). Note the single phase inverter does indeed do 1kv strings even though they often say 600v on the spec sheet. For a large system where it is worth using a transformer for larger 480V inverters, you would of course have to go the closed route with three phase.

 

[inforaj]

I think the size of the PV system would be the major deciding factor. For a relatively small system, single phase would be fine. In fact, at least for the fronius line since that what the OP mentioned, their largest single phase inverter is larger than their largest 240V three phase inverter (15 vs 12KW). Note the single phase inverter does indeed do 1kv strings even though they often say 600v on the spec sheet. For a large system where it is worth using a transformer for larger 480V inverters, you would of course have to go the closed route with three phase.

The PV size is 92 KW DC.

 

[electrofelon]

The PV size is 92 KW DC.

that doesnt seem that big. Have you looked at the cost of the inverters both ways? I dont have access to my wholesale pricing on those models at the moment for some reason. Is there a long wire run between the interconnection point and the combiner that would make looking at a transformer to 480 an option?

 

[ggunn]

I have run into this myself. Perhaps the kVA rating of the transformer powering the B (high leg) phase is less than what the inverter would feed it? It is typical that it is rated less than the other one. Whatever the reason, the POCO is telling you what will work for them, and IMO arguing with them about it isn't going to get you anywhere.

 

[inforaj]

I have run into this myself. Perhaps the kVA rating of the transformer powering the B (high leg) phase is less than what the inverter would feed it? It is typical that it is rated less than the other one. Whatever the reason, the POCO is telling you what will work for them, and IMO arguing with them about it isn't going to get you anywhere.

Could you please explain a little bit more about this?
I have run into this myself. Perhaps the kVA rating of the transformer powering the B (high leg) phase is less than what the inverter would feed it? It is typical that it is rated less than the other one.

The service for the utility fees is very high (around 25K). Is there any transformer we may use and convert it 3-phase, 240v into 120/240V? The procurement department already issues the PO of the inverter.

 

[electrofelon]

The service for the utility fees is very high (around 25K). Is there any transformer we may use and convert it 3-phase, 240v into 120/240V? The procurement department already issues the PO of the inverter.

Did I understand your OP correctly that whether you go with single phase or three phase inverters, either way requires upgrading transformers? If so is one option cheaper than the other?

 

[ggunn]

Could you please explain a little bit more about this?
I have run into this myself. Perhaps the kVA rating of the transformer powering the B (high leg) phase is less than what the inverter would feed it? It is typical that it is rated less than the other one.

The service for the utility fees is very high (around 25K). Is there any transformer we may use and convert it 3-phase, 240v into 120/240V? The procurement department already issues the PO of the inverter.

On an open delta 240/120 three phase service there is one center tapped transformer that supplies the 240/120V single phase phase L1 and L2 and another that supplies the 208V to neutral L3, and the L3 transformer is often much smaller. The portion of your 240V delta inverter's output that would be applied to L3 may exceed the rating of the L3 transformer.

You cannot convert a three phase inverter to single phase with a transformer.

 

[wwhitney]

On an open delta 240/120 three phase service there is one center tapped transformer that supplies the 240/120V single phase phase L1 and L2 and another that supplies the 208V to neutral L3, and the L3 transformer is often much smaller.

Right, and furthermore the L3 transformer is also 240V and connected say L1-L3.

So for such a service, to specify the capacity, you need to know both transformer ratings, right? E.g. if the L1-N-L2 transformer is 75 kVA, and the L1-L3 transformer is 25 kVA, and I assume 240V is really 250V for ease of calculations, then the service could supply:

A 100A 3 phase delta load (which is 250V * 100A * sqrt(3) =43.3 kVA) , which uses the full 25 kVA capacity of the L1-L3 transformer plus 25 kVA of the capacity of the L1-N-L2 transformer, along with up to 50 kVA of L1-N-L2 single phase loads (the remaining L1-N-L2 transformer capacity).

Or if the L1-N-L2 loading exceeds 50 kVA, then your allowable 3 phase delta load would go down accordingly. E.g. with 62.5 kVA of L1-N-L2 loading, you could only supply a 50A 3 phase delta load. [You could also supply a bonus 50A 250V L1-L3 load, since the 3 phase delta load isn't using the full capacity of the L1-L3 transformer.]

And then for PV backfeed, if you aren't going to mix inverter types, you can either use an up to 43.3 kVA 3 phase delta inverter, or an up to 75 kVA single phase inverter. At least power wise; the POCO might have a rule like "no 3 phase inverters on open delta services" in which case you'd only have the single phase option.

Cheers, Wayne

 

[electrofelon]

.the POCO might have a rule like "no 3 phase inverters on open delta services" in which case you'd only have the single phase option.

Cheers, Wayne

Right which seems to be the case for the OP. Do you know off hand if inverter manufacturers say anything about connection to open deltas?

 

[inforaj]

Right which seems to be the case for the OP. Do you know off hand if inverter manufacturers say anything about connection to open deltas?

Yes, they said that Inverter terminals L1 and L2 are connected to lines, and L3 will connect to High-leg terminals.

 

[electrofelon]

Yes, they said that Inverter terminals L1 and L2 are connected to lines, and L3 will connect to High-leg terminals.

Yes but do they specifically say anything about whether the serving transformers must be a closed Delta? That type of service can be fed with either an open or closed Delta. (This is an academic question, the power company already said it must be closed).

 

[jaggedben]

The high leg is almost beside the point. The utility has told you that the existing transformers can't handle the backfeed. It is not particularly surprising that on an open delta you can't backfeed equal current on all three legs or that the split-phase transformer is too small for a large PV system.

If you choose the closed delta option, then you won't have to replace an inverter you might have already bought, or re-wire an array you might have already installed. If you are still in the planning stage perhaps you don't have to account for those sunk costs and can choose whichever option the utility charges less for, or downsize the system.

Regardless, the utility is not wrong and you'll have to add up the costs and/or savings for each option and decide with the client how to move forward.