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Confused about utility transformer sizing for PV system

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    #16
    The harmonic distortion of solar inverters can cause a very small but apparently not negligible amount of heating in a distribution transformer when the photovoltaic backfeed is an appreciable part of its rating:

    https://www.researchgate.net/profile...ansformers.pdf

    A small increase in temperature to an already marginal transformer could effect reliability because the expected lifetime has an exponential relationship with temperature.

    Whether this effect is really that significant or not, it's possible it could be one of the factors in play from the the POCO's point of view. I thought it might be good to be aware of this issue just in case the POCO brings it up, and so you can have a response ready to give them if they do.

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      #17
      Originally posted by ggunn View Post
      He said it was line side interconnected. In that case, per the NEC he is limited only by "the size of the service", but how that is defined is not exactly clear.
      Would it also be limited by the size of the service conductors? I see this as a blindspot in the NEC. Consider a 200A service built with #2/0Cu conductors, and suppose you wanted to fully utilize this with 200A of generation. You'd be doing a supply side tap (in #3/0) that needs more ampacity than the #2/0 service conductors themselves. It logically doesn't make sense to be allowed to do this, because the #2/0 service conductors are a chokepoint that don't have the capacity for the full 200A.

      The 83% service conductor ampacity rule existing in the first place is another matter entirely. I get that there is load diversity and you rarely draw the full 200A on a 200A service, but in concept that is already accounted for in the load calculations that size the 200A service in the first pace. 175A service conductor ampacity for a 200A service creates a theoretical point of failure in the event that you ever do draw 176A or more, for a serious amount of time. The conductors exceed their ampacity, and the OCPD will never trip.

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        #18
        Originally posted by Carultch View Post
        Would it also be limited by the size of the service conductors? I see this as a blindspot in the NEC. Consider a 200A service built with #2/0Cu conductors, and suppose you wanted to fully utilize this with 200A of generation. You'd be doing a supply side tap (in #3/0) that needs more ampacity than the #2/0 service conductors themselves. It logically doesn't make sense to be allowed to do this, because the #2/0 service conductors are a chokepoint that don't have the capacity for the full 200A.

        The 83% service conductor ampacity rule existing in the first place is another matter entirely. I get that there is load diversity and you rarely draw the full 200A on a 200A service, but in concept that is already accounted for in the load calculations that size the 200A service in the first pace. 175A service conductor ampacity for a 200A service creates a theoretical point of failure in the event that you ever do draw 176A or more, for a serious amount of time. The conductors exceed their ampacity, and the OCPD will never trip.
        That's the kind of thing I was alluding to when I said that the "size of the service" isn't clearly defined.

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          #19
          190721-2237 EDT

          pendragon:

          You are working with a rule based system which may not relate to logic. You need to know what are the legal rules, probably public service commission defined, and how those rules relate to you.

          If there is voltage drop from your supply transformer to your home, then the power company is supplying you power no matter how much power your solar system produces.

          If the voltage drop is from your home to the power transformer, then you are supplying the grid with power.

          If your solar system can never supply more current to the power transformer than it is rated for, then there should be no problem. However, this statement is not correct unless you know more about how the transformer is really rated. Additionally is the distortion that might exist in the current waveform mentioned by synchro.

          If you powered your equipment from a battery bank and fed this battery from both your PV and the grid, then you would feed nothing to the grid. Your PV will only provide substantial power for approximately 6 to 8 hours per day. If on average you produce more solar energy than energy you use, then you get no payback from the power company. They don't really like net metering because they lose money on net metering. But if you have a major power outage, then with such a battery system you may be able to at least partially run your equipment during such outage.

          Note that only about 1/3 of your electric bill goes to buying energy. Thus, net metering costs the power company to buy power from you. About 3 times what it costs for energy from their primary source. There are other factors in the cost equation, but this is a simple view.

          .
          Last edited by gar; 07-22-19, 12:18 AM.

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            #20
            I can handle a rule-based system, particularly when I can see the rules. I read through the PoCo's electrical standards document again, and went through the state's statutes and public utilities commission's regulations over the weekend. There's a lot there, but not much that applies to my situation. I'm assuming the PoCo also has internal policies, engineering documents, and cultures that are not necessarily exposed for public consumption. To understand how these apply I will likely need to engage an outside expert familiar with them.

            I can well understand why the PoCo could want to push back here. Not only would this be a large residential installation, but if I elect the full net metering option (banking credits forever), this would be a binding contract that would apply as long as I live here. Furthermore as this would be a > 25 KW PV system, the PoCo would have to pay me for RECs at a considerably higher rate than a system < 25 KW (the rate this year is $0.0375/KWh). Assuming the PV system generates at least as much energy as I use, that will cover many times over what I will have to pay in connection and administrative fees.

            So I will be a money bleeding proposition for them from the get go, and now even more so with our new geothermal system's electrical requirements, replacing our boiler's gas usage (PoCo also provides gas).

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