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Thread: How does a ground fault affect the utility when the utility inverter fails to stop?

  1. #1
    Join Date
    Jan 2017
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    Watson, LA, USA
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    8

    How does a ground fault affect the utility when the utility inverter fails to stop?

    Site condition:

    -Three phase inverter
    -Negatively grounded
    -1500v array configuration with a VOC of about 1370 volts that under load is about 1185 volts
    -GFDI fuse has trigger pin but intended switch fails to see pin activated
    -Fault is a DC fault of about 890 volts constantly neg to ground
    -Inverter never tripped but would continue to produce power

    What would be the affects on the utility side of the pad transformer.
    high side is wye and low side is open delta (ground not tied to any phase for a reference), I believe is what the terminology is referencing.

  2. #2
    Join Date
    May 2011
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    4,261
    Presumably you're talking about an isolated inverter, i.e. there's a transformer in the inverter between the DC and the output. In that case I don't believe the grid would see any difference. You've just had a bunch of the DC current running on the EGCs in the
    arrays.

  3. #3
    Join Date
    Jan 2017
    Location
    Watson, LA, USA
    Posts
    8
    Just so the architecture is clear for the question.

    The central type utility inverters receive DC power from the array then that is applied to typically a DC Link Cap bank should stabilize the DC voltage ( some designs have boost circuits and some others that simply use the array voltage) but after the DC Link Cap bank you would find the power block consisting of the igbt packages configured to basically create a digital AC waveform (square waves) as in most PWM 3 phase products (the typical 3 phase scheme as most other products such as speed drives, soft starters would arrange 3 separate two series connected switches). After the igbt power block the coupling to an LCL filter to smooth out the stair step pulses should provide a relatively smooth sinusoidal wave shape of each AC phase ( so they say ). The outputs of the filtered AC current usually pass through some ACB (air circuit breaker) then on to the pad transformer with low sides ranging from 360 volts to 690 volts then high side to as much as 34.5kv

    It is these 1.2 mW to some 3 mW products that I would like to inquire about the DC ground fault voltage or amperage affect on utility protection relays or power quality.

  4. #4
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
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    4,401
    I agree with JB: the utility system doesn't care about a fault on the other side of an isolated system.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  5. #5
    Join Date
    Nov 2015
    Location
    CA, USA
    Posts
    561
    Quote Originally Posted by CajunTech View Post
    Site condition:

    -Three phase inverter
    -Negatively grounded
    -1500v array configuration with a VOC of about 1370 volts that under load is about 1185 volts
    -GFDI fuse has trigger pin but intended switch fails to see pin activated
    -Fault is a DC fault of about 890 volts constantly neg to ground
    -Inverter never tripped but would continue to produce power

    What would be the affects on the utility side of the pad transformer.
    high side is wye and low side is open delta (ground not tied to any phase for a reference), I believe is what the terminology is referencing.

    If you have a ground fault and the GFDI fuse opens you now have an ungrounded PV array with a ground fault on one leg. At this point, in theory, there is no ground fault current and the PV array is producing its rated power. If the inverter does not get the shutdown message from the GFDI and it has no other ground fault checks it will just keep doing what it does and converting DC to AC. On the HV side of the output transformer, there is no indication of a fault. On the output of the inverter you might be able to get a ground loop between a ground on the AC side and a ground on the DC side, that's why non-isolated inverters are only grounded on one side. It can mess up inverter operation.

    Now when the other DC leg faults to ground the fires start. That should get someone's attention.
    Last edited by pv_n00b; 04-16-18 at 08:32 PM.

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