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Thread: photovoltaic 101

  1. #11
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    Quote Originally Posted by jaggedben View Post
    Didn't we already answer that?
    Oh, right, that was this thread.
    Thank you !!! I'm still not with it, but it did help a lot ..............
    " I'm at a crucial part of my painting "...........Monika Danneman

  2. #12
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    Quote Originally Posted by ritelec View Post
    BUT, nowhere did I see the theory of how two power sources use the same conductors to the main and the main magically knows which one to use ???
    this ^^^

    iI follow a few solar guys around, hook 'em up . I'm always asking here, it usually ends in a 705 ref

    but it doesn't help me understand it all at an atomic level...or sub-atomic....?

    I'm a simple spark....not a physicist.....


    ~RJ~

  3. #13
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    Quote Originally Posted by romex jockey View Post
    this ^^^

    iI follow a few solar guys around, hook 'em up . I'm always asking here, it usually ends in a 705 ref

    but it doesn't help me understand it all at an atomic level...or sub-atomic....?

    I'm a simple spark....not a physicist.....


    ~RJ~
    It's a parallel circuit of sources and loads, but the sources are a current source (the PV inverter) in parallel with a voltage source (the grid). This is what gives rise to the notion that your loads "preferentially" receive your PV output over being serviced by the grid. The PV pushes out whatever it can irrespective of the local loading and the grid either makes up the difference or takes the surplus.

  4. #14
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    Quote Originally Posted by romex jockey View Post
    this ^^^

    iI follow a few solar guys around, hook 'em up . I'm always asking here, it usually ends in a 705 ref

    but it doesn't help me understand it all at an atomic level...or sub-atomic....?

    I'm a simple spark....not a physicist.....


    ~RJ~
    You never really know what load draws power from what source. The energy just flows "downhill". The sources set up voltages at the point where connected to the circuit, and then enough current will flow out of each one, in order that Ohm's and Kirchhoff's laws will be satisfied. If you can understand the microscopic views of Ohm's and Kirchhoff's laws, you can understand how a grid of conductors distributes power and "knows" how much to take from each source.

    Consider this example. R3 represents the total loads. If we were dealing with on-site loads and neighborhood grid loads, we'd connect another resistor in parallel with R3 to represent the neighborhood grid loads, and put a meter between the two, representing your service meter. R1 and R2 represent the wire resistance from both sources. V1 and V2 represent the two sources themselves, and the output voltages. You can think of them as batteries for a simple example, but the concept can also extend to AC sources as well (generators, utility services, inverters, etc). The currents must balance at every point, thus I3 = I1 + I2. The voltage across the load due to each source, will also have to be the same. So if V1 and V2 are not equal, the voltage drops across R1 and R2 will be as much as necessary, until they are equal. The current through each resistor will be what is needed to make this so.

    This means that the currents, I1 and I2, will be what is needed to satisfy the following conditions:
    V1 - I1*R1 = V2 - I2*R2
    I3 = I1 + I2
    I3*R3 = (V1 - I1*R1)

    R1, R2, R3, V1, and V2 are given.




    In the case of an inverter, there is a feedback loop that continuously monitors grid voltage, and generates a slighty higher voltage to "push out" whatever power it can. It functions as a current source, rather than a voltage source, because its output voltage is flexible to adapt to the grid.

  5. #15
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    Think in terms of an automobile. It has two sources of energy, the battery and the alternator, connected in parallel with the loads. The battery voltage is 12.6 at rest, the alternator is around 14 while running. When the alternator is putting out 14 volts, it is supplying energy to all the loads, including the battery. When the alternator shuts off or dies, the battery will be the source of energy and will no longer be a load. The other loads don't 'care' where the energy comes from, they just use it.
    Cheers and Stay Safe,

    Marky the Sparky

  6. #16
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    Quote Originally Posted by Carultch View Post

    In the case of an inverter, there is a feedback loop that continuously monitors grid voltage, and generates a slighty higher voltage to "push out" whatever power it can. It functions as a current source, rather than a voltage source, because its output voltage is flexible to adapt to the grid.
    That's simplified, of course. A grid tied PV inverter is a current source because PV modules are current sources. It doesn't need to raise the voltage to "push out" current; it could just as easily deliver current into a superconductor where there would be no voltage drop.

  7. #17
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    Quote Originally Posted by ggunn View Post
    That's simplified, of course. A grid tied PV inverter is a current source because PV modules are current sources. It doesn't need to raise the voltage to "push out" current; it could just as easily deliver current into a superconductor where there would be no voltage drop.
    Speaking of oversimplification....
    A PV module is by its nature a current source.
    A grid interactive inverter is best characterized as a current source because it is designed that way, and would be whatever provided its input power.

    Hybrid inverters in grid interactive mode are current sources even when running off battery and the SMA Secure Power Supply is a voltage source even though it is powered by PV.

    Sent from my XT1585 using Tapatalk

  8. #18
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    I appreciate the responses fellas , thankfully there's no law against educating a redneck spark

    But here's one that turns my head around ......

    the PV guys come in and do their thing, then determine the panel is too small, by too small i mean the buss

    they'll have me place a 200A MLO into a 100A (meter/100A main) system for this

    So we end up with a 200A MLO off a 100A service , i just can't 'get' why the focus is on the panel being inadequate when the the rest of the serive is ok?

    ~R (brain fried in vermont)J~

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    Quote Originally Posted by romex jockey View Post
    I appreciate the responses fellas , thankfully there's no law against educating a redneck spark

    But here's one that turns my head around ......

    the PV guys come in and do their thing, then determine the panel is too small, by too small i mean the buss

    they'll have me place a 200A MLO into a 100A (meter/100A main) system for this

    So we end up with a 200A MLO off a 100A service , i just can't 'get' why the focus is on the panel being inadequate when the the rest of the serive is ok?

    ~R (brain fried in vermont)J~
    How often it is likely to happen? IDK, on a dwelling seems not so likely, but I think they are afraid of drawing 100 amps from the utility plus whatever the PV is able to deliver - so that 100 amp bus is capable of having more than 100 amp on it without tripping anything.
    I live for today, I'm just a day behind.

  10. #20
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    Quote Originally Posted by GoldDigger View Post
    Speaking of oversimplification....
    A PV module is by its nature a current source.
    A grid interactive inverter is best characterized as a current source because it is designed that way, and would be whatever provided its input power.

    Hybrid inverters in grid interactive mode are current sources even when running off battery and the SMA Secure Power Supply is a voltage source even though it is powered by PV.

    Sent from my XT1585 using Tapatalk
    Well, OK, but a PV module is only mostly a current source. When V gets to be between Vmp and Voc, I comes down. Inverters powered by PV modules can behave as voltage sources by tweaking MPPT but only in that very narrow slice of the DC voltage between Vmp and Voc. It seems to me that making a battery inverter behave as a current source would be easier than the other way round.

    Ideal voltage and current sources do not exist in the real world, just like ideal resistors, capacitors, and inductors. The devil is in the details. But this is beyond the scope of PV 101, innit?

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