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Thread: Solar Inverter with weird voltage

  1. #1
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    Solar Inverter with weird voltage

    Hi there

    Looked at an existing install for a friend of client of mine. They had solar inverter installed with with mains supply for when the sun isn't shining. Basically when the sun is shining the solar panels charge the batteries, if the sun is not out the mains charges the batteries (after load shedding). The power goes to a sub DB (from the man DB) in 6mm flat twin and earth, through 30 amp breaker to the inverter and back to the sub DB through an earth leakage before going to 4 x 20 amp circuit breakers feeding 2 sockets each. The earth leakage sometimes trips, even when nothing is plugged in. The socket circuits were tested (earth, ins res, polarity) and all good, I tested the earth leakage to see if was oversensitive and it didn't trip. I found that the voltage coming out of the inverter to the earth leakage is 230 L-N, 180 volt(+/-) L to E and 50 volt(+/-) N - E. Even when I completely disconnect the out going cables from the inverter I still get these unusual voltages at the out put terminal of the inverter. I have triple checked with 3 different meters (2 major techs and a fluke 1653B) and still the same. The neutral bars are separate in the sub DB, one for the supply into the DB and supply out to the DB the other for the earth leakage protected circuits.

    If I do a Earth leakage test it doesn't trip when in the sub DB, which is because of these strange voltages. If i take the same earth leakage and test at my own home, it trips at 24mA perfectly.

    Contacted suppler and suggested that I link Neutral and Earth of the out going terminal and install an earth spike on the outgoing side. This sounds like a very bad idea to me. I get linking N and E on a generator but that operates when the the supply is completely disconnected via change over switch

    I have never had this before from an inverter. Surely should be 230 L - E, 240 L - N and pretty much 0V N -E

    What do you guys think, faulty inverter?

  2. #2
    Diamond Member Justloadit's Avatar
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    Note that an inverter is a supply on its own, and usually isolated from the batteries. If the inverter is a pure sine wave inverter, they use high frequencies to generate the sine wave, these high frequencies are in a realm of their own which causes certain components to react in different ways, and hence the voltages you have measured between L and Earth and Neutral and Earth. What leads me to think that the inverter manufacturers have inserted a very small value capacitor between N and Earth, to feed this high frequency voltage to earth. This is common practice in switch mode power supplies, and with personal PCs I am sure that you have felt this tingling feeling when you touch your PC case and earth, this is voltage that is residual and is created by the high frequency electronics.

    If the supplier has suggested that you connect the inverter N and earth, then this is a good suggestion, as it now brings the inverter output floating supply to a known reference potential. If you feel concerned, then use a 1Amp fuse to test your system, between N and earth. If the fuse does not blow, and they pass your earth tests, then connect a wire instead of the fuse.

    We tend to forget that the ESKOM supply, transformer centre tap is connected to earth at the substation transformer, and that all connections connected to this are now referenced to earth by this connection. The inverters are a mini substation transformer, and therefor require that one of the floating wires is connected to earth.
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  3. #3
    Diamond Member AndyD's Avatar
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    The inverter suppliers suggestion of knocking in an earth spike could be dangerous if you inadvertantly PME the Eskom supply when it's not a suitable existing earthing arrangement.

    Just a thought but check the connections at the inverter in case they've connected the cpc of the final circuit to the chassis of the inverter instead of the correct earth point or maybe there should have been a link between the earth from the incoming supply and a bonding terminal on inverter chassis.
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  4. #4
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    It is a requirement to do a earth neutral bridge at supply side. Only on supply side and not at consumer side.

    This is the same for generators and PV solar / inverters used as alternative supply.

    The requirement for earth spike is due to the fact that should the supplier's supply be disconnected the possibility of supplier earth also being disconnected is very high and thus using an alternative supply the possibility could be that no earth being available and thus be very dangerous as earth leakafe will not trip and floating voltages could be found on neutral.

    With earth spiking be attentive to the soil resistivity by conducting an earth resistance "earth spike" test.

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