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Power flow in solar hybrid system

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  • #16
    Originally posted by lamagra View Post
    Fair enough but this is the behavior of an inverter acting as a voltage source and not a current source as the grid tied is supposed to act. Does this mean that the hybrid inverter acts as a voltage source even when the inverter is grid tied?
    Anything that is grid tied and therefore properly synchronized to the grid will be acting as a current source in some sense.
    A standalone inverter or standalone generator will supply a constant (within reason) voltage and will supply current when there is a load to consume it.
    A generator connected in parallel with the grid will behave differently to some extent. The control system will apply some logic to decide how much load the generator will carry but typically the generator will be running at the full power output of its prime mover (engine) and will supply power to local loads and possibly feed power back to the grid. In that mode is it also acting more as a current source than a voltage source.
    If it is configured to not feed power back into the grid, then as the local loads decrease the controls will throttle back the prime mover to reduce the generator output.

    A pure grid tie inverter will always deliver its maximum power output based on the available DC input. It counts on the grid being a infinite current sink and is acting as a pure current source.
    A hybrid inverter will also be acting as a current source when paralleled to the grid, with the option of reducing its output to prevent back feed to the grid. In that case the excess PV power is either left unused or is diverted to battery charging.
    When the hybrid inverter is in off grid mode it supplies its own AC reference voltage waveform and it is acting as a voltage source.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #17
      Originally posted by lamagra View Post
      Fair enough but this is the behavior of an inverter acting as a voltage source and not a current source as the grid tied is supposed to act. Does this mean that the hybrid inverter acts as a voltage source even when the inverter is grid tied?
      When a hybrid inverter is connected to the grid it is effectively a current source. When a hybrid inverter is powering an independent load it is effectively a voltage source. Note that the power conversion circuitry is identical in both cases - just the control law (i.e. the intelligence) changes.

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      • #18
        Your hybrid Inverter is Two Inverters. Battery Inverter which is from a Voltage Source. Batteries only go On-Line when AC fails, your ATS operates, and connects yourl loads to the Battery Inverter. Otherwise it operates as a GT Inverter which is now a Current Source. If you panels are producing more than you are using, you sell it to the POCO. If your panels fall short you are buying from the POCO. Power has to go somewhere and come from something. If it has no where to go, there is no power. Power cannot be created or destroyed. It just moves around in various forms.

        Depending on how your system is programmed and set up, your batteries do nothing except Float waiting for a day to do something when power goes out. They require no significant amount of power. The primary Charge Source for the batteries is commercial AC Power from the POCO via your Hybrid Inverter Built-In Battery Charger. Panels are only used to charge the batteries if the grid is down, no generator Input is active, and the batteries or load demand power.
        MSEE, PE

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        • #19
          Wow, I really dig this thread, I might turn it into a hybrid PV sticky what do you think peeps?

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          • #20
            The Outback Radian and FXR Series can manipulate the Solar, Grid and stored battery power with the " Grid Zero " concept with it's bi-directional I/O since 2013. The Stored Battery power can be used whenever and how ever you want, it can be voltage based, time based or load based with grid support or battery.

            http://www.outbackpower.com/download...BC_2-18-14.pdf

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            • #21
              So just to conclude can we confirm the below:

              - If grid is on, PV production>demand load and battery needs charging then PV power will feed loads and use (PV production - demand load) to recharge the battery
              - If grid is on, PV production>demand load and battery is full then PV power will feed loads, inject the small amount of current needed to keep the battery charged and export the remaining to the grid
              - If grid is on and PV production<demand load then the inverter can be programmed to feed loads with PV+battery or PV+grid

              There are other scenarios as well but let's stick to the above.

              Please give your comments on the above.

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              • #22
                Originally posted by lamagra View Post
                So just to conclude can we confirm the below:

                - If grid is on, PV production>demand load and battery needs charging then PV power will feed loads and use (PV production - demand load) to recharge the battery
                - If grid is on, PV production>demand load and battery is full then PV power will feed loads, inject the small amount of current needed to keep the battery charged and export the remaining to the grid
                - If grid is on and PV production<demand load then the inverter can be programmed to feed loads with PV+battery or PV+grid

                There are other scenarios as well but let's stick to the above.

                Please give your comments on the above.
                That is the most common set of decision points for a hybrid system, but not all hybrid systems can do that and for those which can there are other choices that can be made instead.
                A DC coupled system which uses a separate PV to CC system and a separate grid-powered charge to feed the batteries and the DC bus will not be able to control all of those choices.
                Only a tightly integrated DC coupled hybrid system will be able to make all of the choices you list.
                SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                • #23
                  Originally posted by lamagra View Post
                  - If grid is on, PV production>demand load and battery needs charging then PV power will feed loads and use (PV production - demand load) to recharge the battery
                  Yes, although you can program the inverter to sell at lower and higher battery levels. Thus in some cases the battery will not be fully recharged while the system is selling.
                  - If grid is on, PV production>demand load and battery is full then PV power will feed loads, inject the small amount of current needed to keep the battery charged and export the remaining to the grid
                  Yes, same caveat as above.
                  - If grid is on and PV production<demand load then the inverter can be programmed to feed loads with PV+battery or PV+grid
                  Sort of. In grid tied mode the inverter functions as a pass-through from the grid to the loads. So the grid is always powering the loads. The inverter feeds excess power back to the grid. You can think of the system in this mode as a completely independent system; the loads run off the grid, the solar power system is charging batteries and feeding back. (If the grid drops the inverter very quickly switches over to voltage-source inverter and powers the loads.)

                  So the system isn't making the decision "how do I power the loads?" The answer to that is always "the grid" when it is present.

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                  • #24
                    Originally posted by jflorey2 View Post
                    So the system isn't making the decision "how do I power the loads?" The answer to that is always "the grid" when it is present.
                    Right, the inverter in grid on mode is just deciding how much to invert generally based on how much PV is available and in better system also on how much charge is required. So with mine (OutBack Flexpower 1) the inverter comes on to inverter amount based on : power available from PV - charge wanted to batteries

                    It does, as others have mentioned, have a different mode where it would then look at the AC load and try to feed that as priority till the batteries are low, but that would only be useful in TOU areas or non net metering areas.
                    OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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                    • #25
                      It looks like most of the posted comments about Hybrid Inverters come from what people have learned from the Schneider / Xantrex XW series designed in 2003-2004 long before AC coupling, Mini grid and Grid Zero concept's were even dreamed of. Even the XW can do Grid Support and parallel the inverter output to the AC grid input. The decision on how much grid power and how much Inverter support ( battery ) is determined by the ACin amp limit. The modern designed Hybrid Inverters are much more sophisticated than the above and have many more settings and perimeters built in.

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