GT w/battery backup AC vs DC coupling

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  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: GT w/battery backup AC vs DC coupling
    BB. wrote: »
    I think we ran into a poster here that wanted to use a Hybrid Inverter system with a utility that wanted to "pay" for all power generated and charge for all power used.

    With the XW Hybrid--there is no simple place (that I am aware of) to install the power generated billing meter(s) and the power used meter(s).

    -Bill

    This sounds like it needs a line side tap on the grid side of the use meter to then separate the metering.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: GT w/battery backup AC vs DC coupling
    I got the attached file from Xantrex. I am not sure about the UL1741 Solar Guppy talked being a problem. I will have to look into finding a complaint ATS. I would think that it would be available if Xantrex had this drawing made unless it was for use outside the US.

    I had not thought of having the ATS on the XW's output ... interesting configuration. Not sure how AC coupling and parallel WX-6048's will like this. Not shown in the diagram would be breakers and the GT inverters if you choose that route vs a gang of 6 DC charge controllers.

    But yes, as the diagram is shown, that is code compliant and addresses another issue not brought up is how one would connect a 20kw solar to a residential 200 amp panel ( which you can't ), that's why this application is shown for the line side tap, which many a locality won't allow I have read. I should also note all the required breakers for the Charge controllers ( input and output ) and the Xw6048's ( input and output ) are not shown in the diagram and will require the BOS boxes. And if DC charge controllers are required you'll need fused combiners for each string of solar panels, allot of extra hardware
  • wisementradingwisementrading Registered Users Posts: 20
    Re: GT w/battery backup AC vs DC coupling
    I had not thought of having the ATS on the XW's output ... interesting configuration. Not sure how AC coupling and parallel WX-6048's will like this. Not shown in the diagram would be breakers and the GT inverters if you choose that route vs a gang of 6 DC charge controllers.

    But yes, as the diagram is shown, that is code compliant and addresses another issue not brought up is how one would connect a 20kw solar to a residential 200 amp panel ( which you can't ), that's why this application is shown for the line side tap, which many a locality won't allow I have read. I should also note all the required breakers for the Charge controllers ( input and output ) and the Xw6048's ( input and output ) are not shown in the diagram and will require the BOS boxes. And if DC charge controllers are required you'll need fused combiners for each string of solar panels, allot of extra hardware

    Each mppt80-600 will only have one string attached to it. They come with fuse holders so only DC disconnects will be needed on the DC side. What makes you say 6 controllers? That would only give about 250V per string which on a hot day might stop harvest and at the very lest would drop the efficiency. Also, the mppt80-600 doesn't need de-rating up to 45C (113F). I have provided the diagram to my utilities, but they have not agreed to anything yet. Are there any national laws requiring them to allow grid connection?
  • wisementradingwisementrading Registered Users Posts: 20
    Re: GT w/battery backup AC vs DC coupling

    addresses another issue not brought up is how one would connect a 20kw solar to a residential 200 amp panel ( which you can't ),

    What is the issue with 20KW into a 200 amp panel? 20kw is only like 83.3 amps at 240v. Also the panels will never be producing 20KW. 18kw would be the top after the loss from the scc and inverters and that is in a perfect world.
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: GT w/battery backup AC vs DC coupling
    What is the issue with 20KW into a 200 amp panel? 20kw is only like 83.3 amps at 240v. Also the panels will never be producing 20KW. 18kw would be the top after the loss from the scc and inverters and that is in a perfect world.

    The total input to the panel can only be 120% of the bus bar rating. On a 200 amp panel that is 240 amps. The combination of main breaker plus solar breaker can't exceed that total. If you are back feeding 80+ amps you will need a 100 amp back feed. This means the main breaker will need to be 140 amps or less.

    Most likely the code guys are going to say that a 125 amp main breaker is not sufficient.
  • wisementradingwisementrading Registered Users Posts: 20
    Re: GT w/battery backup AC vs DC coupling
    solar_dave wrote: »
    The total input to the panel can only be 120% of the bus bar rating. On a 200 amp panel that is 240 amps. The combination of main breaker plus solar breaker can't exceed that total. If you are back feeding 80+ amps you will need a 100 amp back feed. This means the main breaker will need to be 140 amps or less.

    Most likely the code guys are going to say that a 125 amp main breaker is not sufficient.

    Can't I feed the inverter output into a breaker in another box before connecting it to the main panel?
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: GT w/battery backup AC vs DC coupling

    I know this one all to well. I had to back feed with a 60amp breaker which would require a derate of my main breaker to 175 amps. Nix'd by the code guys because of my loads. Solution was to upgrade the service entrance to a 400 amp with dual 200 amp panels, one derated to 175 amps. This required a new wire pull from the utility transformer.

    I didn't fight this too hard as I knew that within a couple months I was going to add another 125 amp sub panel for the office/shop building addition. It also allowed me to make the upgrade part of the solar add.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: GT w/battery backup AC vs DC coupling
    Each mppt80-600 will only have one string attached to it. They come with fuse holders so only DC disconnects will be needed on the DC side. What makes you say 6 controllers?

    The 6 controllers was for the lower voltage 60-150. That's about $3000.00 of controllers

    If you go with the HV unit, you still will need 5 of those, that's $6250.00 of controllers, based on Google/internet shown pricing of $1250 each

    You need to design for about 350 amps on the batter side for a 20kw array
    What is the issue with 20KW into a 200 amp panel? 20kw is only like 83.3 amps at 240v. Also the panels will never be producing 20KW. 18kw would be the top after the loss from the scc and inverters and that is in a perfect world.

    The XW-6048's have a 12kw surge, so that's 36kw possible if you have three. Also, it not uncommon for solar to exceed name plate, either in cold weather or edge of cloud events. NEC requires a 25% buffer over nameplate rating. Then there is the 120% of the bus bar rating, which limits a typical 200 amp main panel to 40 amps backfeed
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: GT w/battery backup AC vs DC coupling
    Can't I feed the inverter output into a breaker in another box before connecting it to the main panel?

    It still will need to have a breaker in the panel as its entrance point. you will have 83amps to backfeed, that will require at least a 100 amp breaker, NEC required a 25% buffer but I suspect the 17 amps may be close enough for the code guys, but only a permit pull will confirm this. The only way to get the increase you need if you are backfeeding to a main panel is a service entrance upgrade. That will consist of either a 400 amp meter head with dual 200 amp panels with one panel derated to 150 (or maybe even a 125) amp grid side main breaker or a 400 amp panel with the breaker derated to some size under the 400 amp main breaker depending on what is available. You will need an AC combiner to join the 3 inverter outputs together with three breakers, one for each inverter then a backfeed line to the panel, usually only one backfeed breaker is allowed per panel. Generally they required a external meter head and a single box main disconnect as well in the line.

    Sounds like the details here need to be worked out, a line side tap is much easier in this situation but many code jurisdictions will simply not allow them in a residential install. NEC does clearly spell out what this entails. This would simplify your metering requirements as well for measuring PV vs Usage by separating the meters completely. This is not trivial to obtain or get the utility to accept. It has its own set of hurdles to jump.

    here is a link to a document from my utility about line side taps.
    http://www.aps.com/_files/solarRenewable/TapGuide.pdf

    And a page with all kinds of interconnection rules and requirements
    http://www.aps.com/main/green/choice/choice_79.html
  • wisementradingwisementrading Registered Users Posts: 20
    Re: GT w/battery backup AC vs DC coupling
    solar_dave wrote: »

    Sounds like the details here need to be worked out, a line side tap is much easier in this situation but many code jurisdictions will simply not allow them in a residential install. NEC does clearly spell out what this entails. This would simplify your metering requirements as well for measuring PV vs Usage by separating the meters completely. This is not trivial to obtain or get the utility to accept. It has its own set of hurdles to jump.

    here is a link to a document from my utility about line side taps.
    http://www.aps.com/_files/solarRenewable/TapGuide.pdf

    And a page with all kinds of interconnection rules and requirements
    http://www.aps.com/main/green/choice/choice_79.html

    I think that a system with two meters has to have the supply side connection. I have attached a sample one line I got from TVA. Since this is the system they are using in my area I would think this will not be a problem in my case. However what I mean by a separate box would be a box with 2X 200 amp breakers up stream from the main panel. Would that not work?
  • wisementradingwisementrading Registered Users Posts: 20
    Re: GT w/battery backup AC vs DC coupling
    The 6 controllers was for the lower voltage 60-150. That's about $3000.00 of controllers

    If you go with the HV unit, you still will need 5 of those, that's $6250.00 of controllers, based on Google/internet shown pricing of $1250 each

    You need to design for about 350 amps on the batter side for a 20kw array

    Ok that sounds right. I know HV SCCs are pricy, but they will reduce both the wire size and the line loss, plus I will not have to have any string combiners. Can the lightning arresters be in the DC disconnect or would I have to have a box for those anyway?
  • solar_davesolar_dave Solar Expert Posts: 2,343 ✭✭✭✭
    Re: GT w/battery backup AC vs DC coupling
    I think that a system with two meters has to have the supply side connection. I have attached a sample one line I got from TVA. Since this is the system they are using in my area I would think this will not be a problem in my case. However what I mean by a separate box would be a box with 2X 200 amp breakers up stream from the main panel. Would that not work?

    Yeah since you are going to have 3 inverters you will need a combiner before the disconnect, a panel with like three 30-40amp breakers, one for each inverter. That drawing is correct.
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