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Thread: 240 delta with high leg solar installation

  1. #11
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    Quote Originally Posted by GoldDigger View Post
    If a fault to ground occurs on any of the ungrounded primary wires (call them A, B and C), then the fault current will flow through the fault to ground and and from that ground/EGC connection back to the grounded neutral (middle of the A-C POCO secondary winding). The excess current in A, B, C or more than one, depending on the fault, will trip the service OCPD, typically a three pole breaker.

    If a fault occurs on the secondary side of your delta-wye transformer, fault current will flow through that fault to grounded metal which is both connected to the GEC and the solidly grounded wye point of the secondary. It will probably trip the breaker on the primary side of the transformer as a result.
    If the fault is past the secondary side protection on your transformer, it will most likely trip that OCPD and possibly the primary side OCPD too.

    If there is a fault between any secondary ungrounded conductor and any primary ungrounded conductor, fault current will flow in both sets of windings and through the common (metallically connected) wye point of your secondary winding and the POCO secondary (your service). Either your secondary OCPD or your primary ODPC or the service OCPD would then open.
    In your explanation ,it sounds like, you are counting on the service OCPD to protect the transformers output. I believe you could still have an active fault on the transformer output side until the PV system shut itself down and stopped feeding the transformer. Unless the transformer is right next to the service OCPD this could still be a problem.

  2. #12
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    Ok does it matter that th inverter is rapid shutdown compliant. The xfmr is 15 ft from. The service OCPD. If this is still a problem, would you happen to know of a sollution to it. I am being told that a zig zag xfmr could create the neutral I am missing.

  3. #13
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    Also need to mention that there is a fused disconnect right next to the xfmr on the service side in addition to the breaker that feeds it.

  4. #14
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    Quote Originally Posted by BenJChaney View Post
    I guess after talking to the Inspector more I need to make a correction to my post. On the service side of the xfmr, there is no neutral connection. There is one on the inverter side, but not service side. The GEC is tied continuously from the array, through the inverter, through the disconnect,through the xfmr which is also tied to the neutral on the inverter side of xfmr, through the disconnect, and tied the the ground/neutral bar at the service. I'm including a photo of the label on the transformer. I just cannot wrap my head around what he is saying, but iI am inexperienced when it comes to this and fully ready to admit I'm wrong. I just wanna make sure the install is correct and safe.
    The inspector is still wrong. GoldDigger's post above regarding how fault current would flow still applies. Rapid shutdown is irrelevant. You still have not shared any explanation of the inspector's objection that makes any sense.

    I beleive we can't know if you've properly protected the primary and secondary conductors without knowing specs. But that would not be a problem that requires changing the inverter, transformer, or anything about the installation the you've given details on so far. The concept of your install is sound.

    Perhaps it would help to make this point: the fact that a solar system would backfeed power under normal operation does not mean that the utility would not supply fault current in the case of a ground fault or ahort circuit. The utility will still trip a breaker as on a load circuit.

  5. #15
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    Quote Originally Posted by BenJChaney View Post
    I guess after talking to the Inspector more I need to make a correction to my post. On the service side of the xfmr, there is no neutral connection. ...
    Is there a grounded [neutral] conductor run from the PoCo transformer(s) to the service disconnecting means (required for a grounded system)?

    Is there a main bonding jumper (required for a grounded system)?

    If yes to both, your system is grounded. The neutral does not have to be run to the utility side of your transformer. A ground fault will send current through the equipment ground, through the main bonding jumper, and through the service neutral conductor back to the PoCo transformer(s) [and trip the breaker].

    If no to either of the above questions, you have a different problem.
    Last edited by Smart $; 05-14-17 at 01:42 PM.
    I'll never get there. No matter where I go, I'm always here.

  6. #16
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    Quote Originally Posted by Smart $ View Post
    Is there a grounded [neutral] conductor run from the PoCo transformer(s) to the service disconnecting means (required for a grounded system)?
    ...
    I can't believe there wouldn't be, unless the description of it being a high-leg delta is incorrect. .

  7. #17
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    Quote Originally Posted by jaggedben View Post
    The inspector is still wrong. GoldDigger's post above regarding how fault current would flow still applies. Rapid shutdown is irrelevant. You still have not shared any explanation of the inspector's objection that makes any sense.

    I beleive we can't know if you've properly protected the primary and secondary conductors without knowing specs. But that would not be a problem that requires changing the inverter, transformer, or anything about the installation the you've given details on so far. The concept of your install is sound.

    Perhaps it would help to make this point: the fact that a solar system would backfeed power under normal operation does not mean that the utility would not supply fault current in the case of a ground fault or ahort circuit. The utility will still trip a breaker as on a load circuit.
    In that case your counting on the utility side OCPD to protect the transformer output. Isn't it still possible to have fault current on the output of the transformer until the PV inverter shuts down? The inverter will see a power loss and kick off after that utility OCPD trips, but how long would that take before it denergizes ?

  8. #18
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    Quote Originally Posted by jaggedben View Post
    I can't believe there wouldn't be, unless the description of it being a high-leg delta is incorrect. .
    Well, there are a lot of people that are unfamiliar with delta high-leg. Just trying to make sure...
    I'll never get there. No matter where I go, I'm always here.

  9. #19
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    Yes there is a neutral conductor from the POCOs xrmr(s) to the MSP. Also in the event of a fault the inverter will shut down within 10seconds.

  10. #20
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    Quote Originally Posted by acrwc10 View Post
    In that case your counting on the utility side OCPD to protect the transformer output. Isn't it still possible to have fault current on the output of the transformer until the PV inverter shuts down? The inverter will see a power loss and kick off after that utility OCPD trips, but how long would that take before it denergizes ?
    Short answers: of course I am, not really, and very quickly.

    Long answers: Inverter output conductors are sized for more current than the inverter could possibly ouptut, so any OCPD is always to prevent an overcurrent from the utility. Any serious fault will draw many times the OCPD rating at trip the breaker's magnetic sensing device, with any fault current contributed by the inverter being unnecessary and irrelevant.

    It might be theoretically possible to have a 'resistive fault' that steadily draws less than enough current to trip the utility side OCPD. However, anaccidental fault whose resistance stays in the window where utility sourced current isn't enough to trip the breaker because there's a contribution from the inverter... that's a very small window of ohms values and just isn't a typical real world problem. It's also quite likely that an inverter would see such a fault as an unacceptable voltage imbalance o fluctuation and shut down, in which case the utility side OCPD either does or doesn't trip just as on any load circuit.

    When utility inverters see problems like these they typically cease outputting power in a fraction of a second. I think Ben's last post refers mistakenly to a Rapid Shutdown spec, which is DC side and completely irrelevant to any other details mentioned so far.

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