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Thread: Solar Output to Grid Elementary Question

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    Solar Output to Grid Elementary Question

    There may not be a simple answer to this but the question was asked at a recent IAEI meeting on Solar and the only answer was "magic".

    Typical residential solar arrangement interactive with the grid.
    Assume a solar available output of 100 amps at a particular time of day..
    Residence load happens to be zero.
    Solar company states the 100 amp available solar power will flow back to the grid through the solar meter.
    When load is on the grid why will the full capacity of the solar flow into the grid rather than the load being satisfied by the other utility generation units ?
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

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    1) The solar inverter is configured to output the full available power. That current has to go somewhere. The only thing that will stop the inverter from trying to send it somewhere is if the voltage and/or frequency varies out of the proscribed range. 100A is not generally going to disturb the grid voltage or frequency enough to do that, especially not if the local transformer is appropriately sized for it.

    2) The other grid generators simply see it as decreased load. (Or, the grid voltage increases until grid operators reduce the output of other generators. Same thing, really.)

    Check out the 'net demand' graph at California ISO's website to get an idea of what this looks like at a large scale. Check out the supply page too to understand why it looks like that. (Note: What you're seeing in their graphs doesn't include 'behind the meter' solar systems. It only includes solar and wind facilities that are utility generators and which the balancing authority therefore has data from. Part of the mid-day sag in the total demand is due to residential and commercial solar systems that the ISO doesn't track in real time.) Grid balancing authorities have always had to make sure that they don't have too much or too little power. It turns out not to be as hard to balance the intermittency of solar and wind as some people feared a couple decades ago.

    3) There are some other issues that matter, such as power factor. But you said your question was 'elementary'.

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    Quote Originally Posted by augie47 View Post
    There may not be a simple answer to this but the question was asked at a recent IAEI meeting on Solar and the only answer was "magic".

    Typical residential solar arrangement interactive with the grid.
    Assume a solar available output of 100 amps at a particular time of day..
    Residence load happens to be zero.
    Solar company states the 100 amp available solar power will flow back to the grid through the solar meter.
    When load is on the grid why will the full capacity of the solar flow into the grid rather than the load being satisfied by the other utility generation units ?
    I was asking similar questions. See:
    https://forums.mikeholt.com/showthread.php?t=195364

    Start about post 13

    If I understood correctly, consider the solar power inverters as current sources - they will put out the power no matter what.

    Perhaps another way to look at this is to consider the solar inverters as generation, and the grid as parallel generation. The two sources are separated by an impedance (distribution lines and transformers - a very small impedance). The solar inverters being a current source, the inverter voltage will rise a bit - enough to overcome the vd across the impedance and push the power into the grid.

    All dependent on if I understood the physics lesson on the solar inverters. I know about generators, and a bit about distribution - not so much about solar inverters.

    the worm
    Without data you’re just another person with an opinion – Edwards Deming

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    Quote Originally Posted by iceworm View Post
    I was asking similar questions. See:
    https://forums.mikeholt.com/showthread.php?t=195364

    Start about post 13

    If I understood correctly, consider the solar power inverters as current sources - they will put out the power no matter what.

    Perhaps another way to look at this is to consider the solar inverters as generation, and the grid as parallel generation. The two sources are separated by an impedance (distribution lines and transformers - a very small impedance). The solar inverters being a current source, the inverter voltage will rise a bit - enough to overcome the vd across the impedance and push the power into the grid.

    All dependent on if I understood the physics lesson on the solar inverters. I know about generators, and a bit about distribution - not so much about solar inverters.

    the worm


    A couple comments:

    Don't just 'consider' the solar as parallel generation. Understand that's what it is. Loads on the grid can't tell if they're getting power from someone's solar inverter any more than they can tell which conventional power plant they are getting it from.

    Solar inverters are programmed to crudely replicate what other grid generators are required to do to within a balancing authority. If the load drops, and the voltage rises, the authority will take a generator off line to keep the voltage from getting too high. If the load rises, the voltage drops, and the authority will bring a generator back in to raise the voltage back up to serve the load.

    From a regulatory point of view, the utilities and balancing authorities are required to keep grid voltages and frequencies within proscribed ranges at the customers' end of things. (This would be required regardless to give the right voltage and frequency to the loads.) So small solar systems are not supposed to have to go on and offline to balance the grid. (Large solar and wind systems that serve as grid generators are more likely to get 'curtailed' if their power is not needed.) However, it won't exactly go on that way forever. The more solar (and wind) you get, the more features have to be put in place to keep the grid from being overpowered on a sunny day. In fact, new net-metered solar systems in California are now required to adjust their power factor if voltage goes high or low. And there are deadlines on the horizon for communications capabilities with the utilities that would allow solar inverters to be 'throttled' back from full output if there was too much power going into the grid.

    There is no reason a solar inverter has to be programmed to output full power as a current source. That's just the way that it's been so far for various practical and political/regulatory reasons.

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