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Can you stack hybrid inverters in a large PV system?

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  • Can you stack hybrid inverters in a large PV system?

    For example, I am trying to model a 100 - 200 [kW] system with batteries. Either connected or not connected to the grid. Finding that large of a hybrid PV MPPT inverter has proven a challenge, so I was wondering if systems exist where you can stack the hybrid inverters. How would that affect the battery bank? Would multiple hybrid inverters mess up how the batteries are charged in a controlled manner?

  • #2
    A more important question is what type of building is the battery system going into?

    With 200kW of solar even at 48volt that is a friggin big battery system which will probably require some type of environmental permit.

    Where do you plan on building this system and why go with a hybrid inverter?

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    • #3
      Originally posted by SunEagle View Post
      A more important question is what type of building is the battery system going into?

      With 200kW of solar even at 48volt that is a friggin big battery system which will probably require some type of environmental permit.

      Where do you plan on building this system and why go with a hybrid inverter?
      It's a microgrid modeling project for school, so the cost of permits would be neglected for this. It's for "what-if" scenarios only as I know batteries are an expensive solution that does not make too much sense with currently priced technology.

      I've been spinning my wheels on this for a while now because I think there's a gap in my understanding on how companies create large systems. If it was a PV power plant, the answer would be simple because you could just use inverters with AC combiner boxes into a substation. The system gets messy with something with batteries though because I think the easiest method would be to use a hybrid inverter correct?

      I also thought about pooling all my energy into batteries with a solar charge controllers and then into an inverter, so I could probably get away with a smaller inverter, but there'd be a big loss of energy due to the round-trip efficiency of the batteries. And that still brings up the question of how multiple charge controllers would interact with a single battery bank.
      Last edited by TheMechE; 06-28-2017, 10:14 AM.

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      • #4
        Higher end hybrid equipment is stackable. Schneider Conext XW+, for example, advertises scalability to 100 kW.

        Why are you going with a single battery bank, instead of distributing the storage as well?
        CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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        • #5
          Most companies building large energy storage systems that are grid connected are using much higher battery voltages as well as Lithium chemistry batteries.

          I know that Schneider makes hybrid inverters that can accommodate up to 102kw but again that battery system will be huge and expensive even if you go with FLA type chemistry.

          There are probably other commercial grade hybrid inverters for large pv systems but I am not sure how they do it but probably do not simple "stack" a lot of smaller inverters connected to one battery system.

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          • #6
            FWIW, not so much for design at what sounds like preliminary design at this point, but additional info and source material, if you haven't done so yet, check out NREL's "SAM".

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            • #7
              Originally posted by sensij View Post
              Higher end hybrid equipment is stackable. Schneider Conext XW+, for example, advertises scalability to 100 kW.

              Why are you going with a single battery bank, instead of distributing the storage as well?
              This might represent a lack in my understanding as well, but I didn't want to run into a situation where a single hybrid inverter can't charge its assigned battery bank because the batteries are full yet inverter #2 has some batteries with spare capacity to fill. The inverter you mentioned looks good for up to 100 [kW]. Maybe for a large PV system I should just use multiple independent inverters into an AC combiner box? I hate the lose the efficiency when I have to hook up the batteries to a charge controller and change it back to DC though.

              I mean, I could use solar MPPT charge controllers to put all the power into batteries and then hook the batteries to inverter(s), but then I run into dealing with the round-trip efficiency of the batteries. I wanted hybrid inverters because I thought they decided whether to convert the input into AC or divert it to the batteries.

              Edit: Never mind, there's no way that's correct.
              Last edited by TheMechE; 06-19-2017, 02:45 PM.

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              • #8
                Originally posted by TheMechE View Post

                This might represent a lack in my understanding as well, but I didn't want to run into a situation where a single hybrid inverter can't charge its assigned battery bank because the batteries are full yet inverter #2 has some batteries with spare capacity to fill. The inverter you mentioned looks good for up to 100 [kW]. Maybe for a large PV system I should just use multiple independent inverters into an AC combiner box? I hate the lose the efficiency when I have to hook up the batteries to a charge controller and change it back to DC though.

                I mean, I could use solar MPPT charge controllers to put all the power into batteries and then hook the batteries to inverter(s), but then I run into dealing with the round-trip efficiency of the batteries. I wanted hybrid inverters because I thought they decided whether to convert the input into AC or divert it to the batteries.

                Edit: Never mind, there's no way that's correct.
                that is not a hybrid inverter. It is a combined hybrid inverter with built in MPPT Charge controller.

                It doesn't matter if it is combined or not though. For example mine is Outback and not combined but still hybrid grid tied.
                So the power comes from PV, to MPPT charge controller to DC BUS. On the DC bus is the battery AND the inverter. So the power does not have to be converter to chemical power in the battery and then back, it can go directly from the Charge controller to the inverter. This is the usual path for my system as it is grid tied for backup so the batteries are charge most of the time.


                Most smaller systems use a single battery bank for multiple inverters that stack.
                OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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                • #9
                  Originally posted by ButchDeal View Post

                  that is not a hybrid inverter. It is a combined hybrid inverter with built in MPPT Charge controller.

                  It doesn't matter if it is combined or not though. For example mine is Outback and not combined but still hybrid grid tied.
                  So the power comes from PV, to MPPT charge controller to DC BUS. On the DC bus is the battery AND the inverter. So the power does not have to be converter to chemical power in the battery and then back, it can go directly from the Charge controller to the inverter. This is the usual path for my system as it is grid tied for backup so the batteries are charge most of the time.


                  Most smaller systems use a single battery bank for multiple inverters that stack.
                  Thank you for the information. Just to make sure I understand, so when the hybrid inverter is in parallel with the charge controllers, will the system be smart enough to know not to charge the batteries when the inverter needs all the PV output for the loads?

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                  • #10
                    Originally posted by TheMechE View Post

                    Thank you for the information. Just to make sure I understand, so when the hybrid inverter is in parallel with the charge controllers, will the system be smart enough to know not to charge the batteries when the inverter needs all the PV output for the loads?
                    This is called DC coupled.

                    This is governed by ohms law.
                    The MPPT charge controllers put the power out to the battery (all they can, unless some special programming is set up).
                    Hybrid inverter generally means that you have a grid tied battery backup system like mine. So assuming you don't have some export limiting configuration set up (rare). Then the inverter will inverter any extra power beyond what is needed for charging and dump it on the grid/load terminals (the two outputs are usually tied together until the grid goes down).


                    The cpu only controls things when it is in off grid mode. And when it is in off grid mode the inverter is the only source of power so if you need 3kW on the load side, are only producing 2kw on the PV side and the batteries want to be charged at 3kW, well what happens.....
                    Since it is off grid at the time, the inverter sucks the 2kw of solar and another 1kw from battery to keep things running.

                    Now if you have rated limited or feed in limitations on the grid side like in Hawaii then the inverter will not inverter any more than is needed.
                    So in this case if the grid is up and batteries are say fully charged, load is 1kw but solar has 6kw then the MPPT CC will adjust the solar such that it is only getting 1kW and inverter will invert just the 1kw. This type of configuration is not the norm though.
                    OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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                    • #11
                      Originally posted by ButchDeal View Post

                      This is called DC coupled.

                      This is governed by ohms law.
                      The MPPT charge controllers put the power out to the battery (all they can, unless some special programming is set up).
                      Hybrid inverter generally means that you have a grid tied battery backup system like mine. So assuming you don't have some export limiting configuration set up (rare). Then the inverter will inverter any extra power beyond what is needed for charging and dump it on the grid/load terminals (the two outputs are usually tied together until the grid goes down).


                      The cpu only controls things when it is in off grid mode. And when it is in off grid mode the inverter is the only source of power so if you need 3kW on the load side, are only producing 2kw on the PV side and the batteries want to be charged at 3kW, well what happens.....
                      Since it is off grid at the time, the inverter sucks the 2kw of solar and another 1kw from battery to keep things running.

                      Now if you have rated limited or feed in limitations on the grid side like in Hawaii then the inverter will not inverter any more than is needed.
                      So in this case if the grid is up and batteries are say fully charged, load is 1kw but solar has 6kw then the MPPT CC will adjust the solar such that it is only getting 1kW and inverter will invert just the 1kw. This type of configuration is not the norm though.
                      Wait, so how do your batteries stay charged if your inverter is grid tied? Wouldn't your inverter sap all the charge and pass it to the grid?

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                      • #12
                        Originally posted by TheMechE View Post

                        Wait, so how do your batteries stay charged if your inverter is grid tied? Wouldn't your inverter sap all the charge and pass it to the grid?
                        My inverter inverters what comes off the CC unless the battery monitor calls for some charging.
                        CC puts power on buss inverters pulls it off or less off If charging is called for. Batteries stay charged till grid is down
                        OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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                        • #13
                          When two or more inverters are operating in grid interactive mode, no special software or hardware is needed to synchronize their output. They all independently synchronize to the grid.
                          Similarly two or more charge controllers with their own PV or line power sources can connect independently to the same battery bank. Each regulates its output based on the battery voltage is senses.

                          But when there are overall decisions to be made whether to charge or power loads, operate grid tied or islanded, and buy or sell grid power, there needs to be communication and coordination among the components.
                          SunnyBoy 3000 US, 18 BP Solar 175B panels.

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                          • #14
                            Originally posted by inetdog View Post
                            When two or more inverters are operating in grid interactive mode, no special software or hardware is needed to synchronize their output. They all independently synchronize to the grid.
                            Similarly two or more charge controllers with their own PV or line power sources can connect independently to the same battery bank. Each regulates its output based on the battery voltage is senses.

                            But when there are overall decisions to be made whether to charge or power loads, operate grid tied or islanded, and buy or sell grid power, there needs to be communication and coordination among the components.
                            That is good to know! So I can connect as many grid-tie inverters that I want to the grid (given the proper power purchase agreements, line limitations, etc)? There are absolutely no limits even if it's 200 inverters all rated for 100 watts each?

                            What about if you aren't grid-tied? How do you sync multiple AC inverter lines into a single AC line? They'd be out of phase I believe, so how would someone overcome this problem?

                            Originally posted by ButchDeal View Post

                            My inverter inverters what comes off the CC unless the battery monitor calls for some charging.
                            CC puts power on buss inverters pulls it off or less off If charging is called for. Batteries stay charged till grid is down
                            From what I read, you can't put both a CC and a grid-tie inverter on the same PV panel array line unless they are designed for it. I think I read your explanation wrong. So you have your PV panels directly connect to the MPPT CC and then in parallel are actually the batteries and the inverter. I am still confused on how your batteries don't get sapped by the inverter then. What stops the inverter from drawing power from the batteries to put on the grid?

                            Is it a large zener diode to ensure energy only goes into the batteries and not out unless the grid goes offline? And then your hybrid inverter flips a switch when the grid goes offline and connects the batteries to a circuit where they can power loads?
                            Last edited by TheMechE; 06-21-2017, 05:42 PM.

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                            • #15
                              Why do people try so hard to over complicate things

                              my system and all D.C. Coupled systems have

                              Code:
                                                                                Backup load
                                                                                         I
                              solar ------- CC------- DC bus-------inverter-------grid
                                                                       I
                                                                 Battery


                              the inverter can regulate how much int inverts and if it is a charger or inverter, thus it controls the flow of power
                              you can have multiple inverters and multiple CC on the system
                              ouback inverters can sync with each other and the grid
                              they can sync either in phase or opposite phase or with three of them for 3 phase.
                              if they are configured for in phase or out of phase with an auto-transformer then one can shut down if the load is less than the other can handle by itself.
                              Last edited by ButchDeal; 06-21-2017, 08:22 PM.
                              OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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