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Grounding Questions (Fuse panel, grounding rod, frame of panels)

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  • #46
    My inverters are isolated. The DC is not grounded. This version has a 10A fuse between DC neg and ground
    as a means of detecting an array fault, but no operational current flows through it. If the DC neg were directly
    grounded, the fuse would be useless. I think you mean, "one side of the 400V source is grounded", which has
    an entirely DIFFERENT meaning than "400V DC on the frames". Bruce Roe

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    • #47
      Originally posted by bcroe View Post
      My inverters are isolated. The DC is not grounded. This version has a 10A fuse between DC neg and ground
      as a means of detecting an array fault, but no operational current flows through it. If the DC neg were directly
      grounded, the fuse would be useless. I think you mean, "one side of the 400V source is grounded", which has
      an entirely DIFFERENT meaning than "400V DC on the frames". Bruce Roe
      what is the inverter type? I have 0 experience with any besides SMA SB latest series and I have that just because that's what I use for my system. Reading NEC led me to believe there're 2 types of inverters but yours falling out of this pattern, I'm just curious.

      please clarify what you mean by '400 DC on the frames'- do you mean panel developed DC fault to its frame and the frame bonding got broken (or never existed)? Yeah, that might be a problem.

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      • #48
        Max do not fall for the lie Transformerless Inverters do not have a Transformer because they do.
        MSEE, PE

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        • #49
          Originally posted by Sunking View Post
          Max do not fall for the lie Transformerless Inverters do not have a Transformer because they do.
          in the power processing path? I admit I haven't seen circuit diagram of any so it's hard for me to tell but if you get me curious enough I'll open one of my SBs just for laughs. And if I break it in the process as that joke goes I have another one . I'm like Bruce warranty free on any of my equipment- as long as I can get parts I can fix pretty much anything.

          I can see how one could design a circuit using MOSFETs and some inductors to directly convert input DC to L1/L2 without use of the transformer so I thought someone already did that.

          Found page which makes it clear IMO: http://www.elp.com/articles/powergri...ar-design.html

          I had designs #3 & #4 in mind.
          Last edited by max2k; 07-21-2017, 03:10 PM.

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          • #50
            Originally posted by max2k View Post
            in the power processing path? I admit I haven't seen circuit diagram of any so it's hard for me to tell but if you get me curious enough I'll open one of my SBs just for laughs. And if I break it in the process as that joke goes I have another one . I'm like Bruce warranty free on any of my equipment- as long as I can get parts I can fix pretty much anything.

            I can see how one could design a circuit using MOSFETs and some inductors to directly convert input DC to
            L1/L2 without use of the transformer so I thought someone already did that.

            Found page which makes it clear IMO[/URL]

            I had designs #3 & #4 in mind.
            I have Fronius inverters. I don't know about industrial size equipment, but the non isolated inverters
            aren't suitable for consumer use. The AC line can find its way onto the array, even with no sun. An
            inverter fault, and its pretty dangerous.

            If you look at the input cap in 3, operating the H bridge will cause it to be moving
            up and down hundreds of volts, at 50 KHZ. That, because the line isn't moving.
            This would put a heck of a strain on the EMC filters, since this kind of noise can't
            be allowed out of the inverter. I don't know if that is even a practical PV design.

            Bruce Roe
            Last edited by bcroe; 07-21-2017, 04:42 PM.

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            • #51
              Originally posted by bcroe View Post

              I have Fronius inverters. I don't know about industrial size equipment, but the non isolated inverters
              aren't suitable for consumer use. The AC line can find its way onto the array, even with no sun. An
              inverter fault, and its pretty dangerous.

              If you look at the input cap in 3, operating the H bridge will cause it to be moving
              up and down hundreds of volts, at 50 KHZ. That, because the line isn't moving.
              This would put a heck of a strain on the EMC filters, since this kind of noise can't
              be allowed out of the inverter. I don't know if that is even a practical PV design.

              Bruce Roe
              I thought it is not even practical it's how it is starting from TL series and alike. AC line is inside array already, just switching back and forth. In case of fault at array the fault current flows throw inverter and it closes its switches. In case of fault inside inverter breaker on AC side trips. I think it's more practical than transformer based design.

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              • #52
                Originally posted by max2k View Post
                in the power processing path?.
                Max they are Switch Mode Converters. They take the DC from the panels, convert to high frequency AC, convert back down to DC, then to 60Hz AC. 50 and 60Hz transformers are very large, heavy, and expensive for a given power level. High frequency transformers are very small and light. That is why Ships and planes tend to use 400 Hz power because the generators and transformers are much smaller and lighter.

                The downside to TL Inverters is there is no Galvanic Isolation between I/O, where a standard Transformer has Galvanic Isolation. That makes protection and circuit operation much more complex.Higher risk to electrocution and failures.

                The Upside to TL is smaller lighter, cheaper, and a small gain in efficiency. TL Inverters run around 95% efficient, and standard are around 93%. Most consumers do not give a crap about safety and failures. How much is the initial cost is the name of the game, screw long term and safety.
                MSEE, PE

                Comment


                • #53
                  Originally posted by max2k View Post

                  I thought it is not even practical it's how it is starting from TL series and alike. AC line is inside array already, just switching back and forth. In case of fault at array the fault current flows throw inverter and it closes its switches. In case of fault inside inverter breaker on AC side trips. I think it's more practical than transformer based design.
                  The AC isn't finding a way back to the array in the 2 classes I attempted to define; I think your 3 & 4
                  examples are a 3rd class which I didn't cover. There is no guarantee that a fault will trip a breaker.
                  Maybe yet the confusion will clear. Bruce Roe

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                  • #54
                    Originally posted by bcroe View Post

                    The AC isn't finding a way back to the array in the 2 classes I attempted to define; I think your 3 & 4
                    examples are a 3rd class which I didn't cover. There is no guarantee that a fault will trip a breaker.
                    Maybe yet the confusion will clear. Bruce Roe
                    the question is what class your inverters belong to? As I said earlier it significantly affects grounding requirements - 3 & 4 types don't provide galvanic isolation that is both blessing and curse.

                    Comment


                    • #55
                      Originally posted by max2k View Post
                      - 3 & 4 types don't provide galvanic isolation that is both blessing and curse.
                      There is no blessing or upside. Just a curse. Galvanic Isolation isolates electrical systems and prevents current flow. It is used to stop current flow where two or more systems are at different ground potentials. It breaks ground loops. Its main purpose is life safety preventing accidental current from reaching ground through a human body, or through different electrical systems.

                      You give all that up to save a few bucks and a couple extra percentage points in efficiency. Not sure that counts as a blessing.
                      Last edited by Sunking; 07-21-2017, 11:58 PM.
                      MSEE, PE

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                      • #56
                        Originally posted by Sunking View Post
                        There is no blessing or upside. Just a curse.
                        true, I just want to feel better

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                        • #57
                          Originally posted by max2k View Post

                          the question is what class your inverters belong to? As I said earlier it significantly affects grounding requirements - 3 & 4 types don't provide galvanic isolation that is both blessing and curse.
                          My inverters are isolated, high frequency switchers. Bruce Roe

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