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Thread: PV & Battery System Ground Reference

  1. #1
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    PV & Battery System Ground Reference

    I’m interested in getting some comments regarding about and upcoming project.

    I went to a meeting yesterday with a builder to discuss the power supply for a new single-family dwelling. This project will be off the grid due to the cost of getting utility power to the site and the owner would prefer not having any utility power.

    I have worked on off grid projects before that used traditional batteries and multiple parallel Outback inverters that were designed for the purpose. This projects is a little different since they will be using a battery system and inverter that is normally used for grid tie systems. The PV system that will supply power will utilizes a grid tie inverter.

    There are 3 companies involved with this system.

    (1) The company designing the battery system with its inverter.

    (2) The PV company that will supply the PV system and its inverter. They will also supply all the other required equipment to complete the system.

    (3) My company taking the power to the home.


    The PV company and I were discussing where their work should end and my work should start. The Battery inverter and PV inverter for this system are only available in 3 phase 480. There will be a transformer to step the voltage down. The PV contractor agreed that I should supply the transformer and all wiring past it.

    The engineer for the battery company said the I must use a 480 wye primary and delta secondary. He said the primary must be wye since this system is not tied to the grid and needs the neutral point for the inverters to work correctly. I said that is fine but I need a wye secondary to supply the house. He responded that that system will not work with a wye secondary. I asked if we could use a transformer with a wye primary and wye secondary and he said that will not work. If I need a wye secondary 2 transformers must be used.

    Essentially, we have an ungrounded 480 source that needs a ground reference but at the same time we also need to reduce the voltage. I know there are zig-zag transformers that can be used for the ground reference, but wouldn’t that be the same as using a wye primary? If so why couldn’t the secondary of the transformer also be wye to avoid using a second transformer?

    Since this is going way beyond my expertise the PV company and Battery supplier said they will work on a solution but I though I would ask here how a system like this should be designed to satisfy my own curiosity.
    Curt Swartz
    Electrical Contractor

  2. #2
    Join Date
    May 2018
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    Jamaica and london
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    that is one of the problems of using grid tied inverters rather than stand alone inverters. It almost sounds like the customer is trying to use one of the Tesla AC systems including the AC batteries... and without a 240V with neutral signal to the inverters or 208V with neutral signal to the inverters at a certain amperage, you cannot fool the system into believing it is at a grid tied location.

    Many of the newer grid tied inverters at large sizes are actually designed with sensing circuitry to make the old ways of defeating the sensors not work. Which is why the person is calling for the wye / delta rather than delta wye...

    the nice thing is that all you have to do to shut down the system is stop the supply current.. everything else automatically shuts off, including the battery back up. But, the battery back up only runs for a certain amount of time and basically is there to supply excess loads... unlike the normal off grid battery systems.

    Not gonna say too much about it but not the system I would choose for my own home to jeep it off grid.
    Student of electrical codes. Please Take others advice first.

  3. #3
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    Jan 2018
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    SE Michigan
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    Have a look at this post, for a link to a paper that describes what type problems can occur on a Y-Y system.

  4. #4
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    Oct 2009
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    Austin, TX, USA
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    Quote Originally Posted by curt swartz View Post
    I’m interested in getting some comments regarding about and upcoming project.

    I went to a meeting yesterday with a builder to discuss the power supply for a new single-family dwelling. This project will be off the grid due to the cost of getting utility power to the site and the owner would prefer not having any utility power.

    I have worked on off grid projects before that used traditional batteries and multiple parallel Outback inverters that were designed for the purpose. This projects is a little different since they will be using a battery system and inverter that is normally used for grid tie systems. The PV system that will supply power will utilizes a grid tie inverter.

    There are 3 companies involved with this system.

    (1) The company designing the battery system with its inverter.

    (2) The PV company that will supply the PV system and its inverter. They will also supply all the other required equipment to complete the system.

    (3) My company taking the power to the home.


    The PV company and I were discussing where their work should end and my work should start. The Battery inverter and PV inverter for this system are only available in 3 phase 480. There will be a transformer to step the voltage down. The PV contractor agreed that I should supply the transformer and all wiring past it.

    The engineer for the battery company said the I must use a 480 wye primary and delta secondary. He said the primary must be wye since this system is not tied to the grid and needs the neutral point for the inverters to work correctly. I said that is fine but I need a wye secondary to supply the house. He responded that that system will not work with a wye secondary. I asked if we could use a transformer with a wye primary and wye secondary and he said that will not work. If I need a wye secondary 2 transformers must be used.

    Essentially, we have an ungrounded 480 source that needs a ground reference but at the same time we also need to reduce the voltage. I know there are zig-zag transformers that can be used for the ground reference, but wouldn’t that be the same as using a wye primary? If so why couldn’t the secondary of the transformer also be wye to avoid using a second transformer?

    Since this is going way beyond my expertise the PV company and Battery supplier said they will work on a solution but I though I would ask here how a system like this should be designed to satisfy my own curiosity.
    It sounds like a Rube Goldberg setup to me. I wouldn't want to have anything to do with it. DSFDF and YMMV.

  5. #5
    Join Date
    May 2011
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    4,639
    Quote Originally Posted by curt swartz View Post
    I’m interested in getting some comments regarding about and upcoming project.

    I went to a meeting yesterday with a builder to discuss the power supply for a new single-family dwelling. This project will be off the grid due to the cost of getting utility power to the site and the owner would prefer not having any utility power.

    I have worked on off grid projects before that used traditional batteries and multiple parallel Outback inverters that were designed for the purpose. This projects is a little different since they will be using a battery system and inverter that is normally used for grid tie systems. The PV system that will supply power will utilizes a grid tie inverter.

    There are 3 companies involved with this system.

    (1) The company designing the battery system with its inverter.

    (2) The PV company that will supply the PV system and its inverter. They will also supply all the other required equipment to complete the system.

    (3) My company taking the power to the home.


    The PV company and I were discussing where their work should end and my work should start. The Battery inverter and PV inverter for this system are only available in 3 phase 480. There will be a transformer to step the voltage down. The PV contractor agreed that I should supply the transformer and all wiring past it.

    The engineer for the battery company said the I must use a 480 wye primary and delta secondary. He said the primary must be wye since this system is not tied to the grid and needs the neutral point for the inverters to work correctly. I said that is fine but I need a wye secondary to supply the house. He responded that that system will not work with a wye secondary. I asked if we could use a transformer with a wye primary and wye secondary and he said that will not work. If I need a wye secondary 2 transformers must be used.

    Essentially, we have an ungrounded 480 source that needs a ground reference but at the same time we also need to reduce the voltage. I know there are zig-zag transformers that can be used for the ground reference, but wouldn’t that be the same as using a wye primary? If so why couldn’t the secondary of the transformer also be wye to avoid using a second transformer?

    Since this is going way beyond my expertise the PV company and Battery supplier said they will work on a solution but I though I would ask here how a system like this should be designed to satisfy my own curiosity.
    It's really beyond my expertise and take the following with a big grain of salt. But I would wager that he is in some way wrong about the part in red.

    My take is that battery-inverter company is responsible for supplying the rest of you with a 3-phase 480/277 system, and they are responsible for figuring out where to create that grounded neutral reference. But your side of the transformer should be, well, whatever you need it to be. I don't know why he has an opinion on what your side of the transformer should be. Perhaps his confusion involves the fact that most often their systems would connect to a utility transformer that would be delta-wye with their equipment on the wye side. When backfeeding a service like that you want to backfeed delta on the utility side. (see MTW's post). But your project doesn't involve feeding a utility supplied system that is grounded elsewhere than the transformer you mention; so it's appropriate in this case to ground your secondary side at the transformer, regardless of what's found on the primary side.

    Put another way: If you were connecting your transformer to a 480V utility service, you would probably use a delta-wye transformer. You wouldn't connect a neutral on the primary side because that system is already grounded somewhere and grounding it at the transformer primary would cause circulating currents. (See MTW's post, again) So why can't these other companies set-up a 480/277 system, with a ground-referenced neutral somewhere on their side, and let you connect the same delta-wye transformer? If they can't do that it seems to call into question whether they can fulfill their part of the project. From what you've described him saying, I'd be a little concerned over whether the battery inverter is actually capable of providing a primary source off-grid reference. I'd want to know exactly how it works in a power outage when normally grid-tied.

    If the other companies can't come up with their promised solution, perhaps an independent electrical engineering review is called for, by someone with experience with inverters.

    ...

    By the way, for my curiosity, what 480V battery inverter is being considered for a single family home? I haven't heard of any such options intended for residential use.

  6. #6
    Join Date
    May 2011
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    4,639
    To put it more succinctly: I believe in this case the battery inverter is responsible, more or less, for mimicking a utility service. They provide 3L+N to a disconnect, bond N-G and land a GEC on terminal bar in the disconnect, and promise it will supply the loads connected.

  7. #7
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    Thanks for the replies. I really don't think this is a bad setup. It actually a very expensive battery system that is primarily used for utility and large commercial projects. It's just in this case it is being use for off the grid so the design is a little different. The battery company engineers are working with the PV contractor so I'm sure they will find a solution.

    It may very well end up that 2 transformers will be required. PV/Battery to a wye-delta transformer, then a delta-wye transformer for our final distribution. It just seems like there should be a simpler way but I'm not an engineer so this is way above my pay grade to design. It more curiosity for me.
    Curt Swartz
    Electrical Contractor

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