We have a utility we are trying to interconnect with that currently is supplying three phase power to the customer with an open delta transformer. They are requiring that we upgrade the transformer configuration to a closed delta and are saying to do that they will need to run the third phase over a mile to the site. Is this correct? I thought open delta three phase power required that all three primary lines be run to the transformers whether it is open or closed. How are they providing three phase/4 wire supply to the customer from the transformer secondary if they only have two phases/hots supplying the transformer primary side?
Open delta three phase from only two primary lines
- Thread starter SunFish
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winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The installation is called 'open wye/open delta' and uses two primary lines and the primary neutral to get the _two_ different phase angles needed for open delta.
-Jon
-Jon
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
We just had a thread on this:
I posted a link to SCL's construction document for this bank.
I am curious about the implications of a three phase interconnection to this type of bank. What size is the PV system?
Possibility of 120/208 3 phase open delta?
Hi there. I am an electrical engineer who has seem 120/240V, 3 phase, 4 wire open delta (high leg) systems in the past. Question: is 120/208V 3 phase open delta a possibility as well? don't think so, but looking for feedback. Thanks in advance.
forums.mikeholt.com
I posted a link to SCL's construction document for this bank.
I am curious about the implications of a three phase interconnection to this type of bank. What size is the PV system?
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Why. What happens if I interconnect all three phases?
What about the high leg?
does your invertors work on 240 delta? Or 120 single phase?
I’m not sure...
we have refused some for the same reason, they were served with an open wye, open delta. They needed to pay for us to run the third phase.
does your invertors work on 240 delta? Or 120 single phase?
I’m not sure...
we have refused some for the same reason, they were served with an open wye, open delta. They needed to pay for us to run the third phase.
Thanks all! I discovered the open wye to open delta after posting this.
So I have successfully backfed solar onto open delta - open delta transformer configurations before by using single phase inverters connected to only the two low legs (not connecting to the high leg). Never had any issues with this before and I don't see any difference between backfeeding the two low legs whether it is open delta - open delta or open wye - open wye. As long as we're backfeeding only the low legs transformer (usually the larger of the two) I don't see how this would be any different from backfeeding a normal residential single phase service.
Also, they are saying that "there will be Ferroresonance issues, especially on the open phases". I am not familiar with this, can anyone validate this claim? Will this be a concern with a single phase inverter only connected to the two low legs?
So I have successfully backfed solar onto open delta - open delta transformer configurations before by using single phase inverters connected to only the two low legs (not connecting to the high leg). Never had any issues with this before and I don't see any difference between backfeeding the two low legs whether it is open delta - open delta or open wye - open wye. As long as we're backfeeding only the low legs transformer (usually the larger of the two) I don't see how this would be any different from backfeeding a normal residential single phase service.
Also, they are saying that "there will be Ferroresonance issues, especially on the open phases". I am not familiar with this, can anyone validate this claim? Will this be a concern with a single phase inverter only connected to the two low legs?
We would be using a standard single phase inverter only connected to the two "low legs" and not interconnect to the high leg
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
On a high-leg delta I would initially look at backfeeding only the A-C phase and avoid the high-leg.
Sunfish, I hope you are doing the interconnection application before installing the system.
Hv&Lv, there are few inverters out there that would backeed 240 delta without looking the neutral, and there's the more common strategy of splitting the system among multiple single phase inverters. Everything might look fine until you're actually pushing significant amps out and the open-delta impedance starts to matter.
Edit: for the record I was typing this while Sunfish was posting his last replies.
Sunfish, I hope you are doing the interconnection application before installing the system.
Hv&Lv, there are few inverters out there that would backeed 240 delta without looking the neutral, and there's the more common strategy of splitting the system among multiple single phase inverters. Everything might look fine until you're actually pushing significant amps out and the open-delta impedance starts to matter.
Edit: for the record I was typing this while Sunfish was posting his last replies.
I have been told that the issue with the high leg is there is limited capacity with this on open delta transformer configs and the voltage can swing pretty wildly when it is heavily loaded. I've discussed with inverter manufacturers and they say that if you connect to the high leg (assuming you have an inverter that can do three phase 240 delta) the voltage on that leg will swing out of the inverters allowable IEEE range and the inverter will throw errors and shut down.
Hahaha! Yup, definitely doing the interconnection app before we started installingOn a high-leg delta I would initially look at backfeeding only the A-C phase and avoid the high-leg.
Sunfish, I hope you are doing the interconnection application before installing the system.
Hv&Lv, there are few inverters out there that would backeed 240 delta without looking the neutral, and there's the more common strategy of splitting the system among multiple single phase inverters. Everything might look fine until you're actually pushing significant amps out and the open-delta impedance starts to matter.
Still in negotiation with the utility. The want us to pay to close the delta, but this will require running the third phase about a mile... trying to convince them to let us connect the system to the two low legs with a single phase inverter as we've done this before with the same utility but now they have changed their stance on the subject.that depends on the loading of the transformer. Lightly or no load transformers with open fuses create essentially a series LC circuit.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Makes sense, although it seems like this could be resolved with appropriate transformer sizing.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Ferroresonance is a condition which basically ruins the transformers ability to produce a counter EMF. I am no expert on ferroresonance, but I am skeptical of their claims that this situation is prone to it. In fact, this source says an open wye is one of the most unlikely transformer configurations to have it.
FERRORESONANCE AND TRANSFORMER CONNECTIONS
Ferroresonance and Transformer Connections are related as such that the latter is a significant factor to the occurrence of the former du...
PWDickerson
Senior Member
- Location
- Clinton, WA
- Occupation
- Solar Contractor
What is the rating of the transformer(s), and what is the rating of the inverter(s)?
The total inverter capacity is 30 kW. The transformer for the two low legs that we want to connect to is 50 kVA and the high leg transformer is 25 kVA.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Oh, only 30kw. Will they just let you use single phase inverters? I know that was mentioned already. I was thinking a larger system, perhaps making it worth a fight with POCO( not that you would win anyway
)Yeah, we originally designed it with single phase inverters already knowing that it was an open delta supply before we even submitted our interconnection app. They are still fighting us on itOh, only 30kw. Will they just let you use single phase inverters? I know that was mentioned already. I was thinking a larger system, perhaps making it worth a fight with POCO( not that you would win anyway
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