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Thread: Wye-Delta Grounding Transformer

  1. #11
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    Quote Originally Posted by jojo View Post
    Since the system is delta, the breakers won't trip. And there might be other issues since the system is ungrounded like prone to transient overvoltages as stated in IEEE Green Book and from other elec references (Beeman, etc).
    You can corner-ground the delta, or use a ground-detection set-up.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  2. #12
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    Ungrounded delta systems are commonly used where continuity of service is desired. The specific purpose is to prevent breakers from tripping in the event of a single ground fault.

    If you ground your delta system, you are specifically giving up this continuity of service feature. However you are correct about the risk of transient over-voltage in ungrounded systems.

    My understanding is that if you use a 'grounding transformer', you must then ground the derived neutral just as you would ground the neutral of a normal wye secondary.

    Your existing system is supposed to have ground fault detection, to inform you of a fault so you can fix it before a second fault occurs.

    Given the size and voltage of your system, if grounded I believe that ground fault protection is also required to prevent arcing faults from causing serious damage without tripping breakers. Basically in large 480/277V systems you can get arc faults which are sustained and dissipate lots of energy but low enough current to not trip the breakers.

    You might consider a resistance grounded system. In the event of a ground fault, only low current flows, so breakers don't trip. However enough current flows that ground fault detection can quickly identify the location of the fault, and for some loads you can use ground fault relays to shut them down in the event of a ground fault. Resistance grounded systems avoid the transient overvoltage problem of totally ungrounded systems.

    Both zig-zag transformers and wye:delta grounding transformers derive a good low impedance neutral. A simple wye transformer circuit does not have a low impedance neutral, meaning that the line to neutral voltage is not stable and only very limited line to neutral current will flow. Something external is needed to stabilize the line-neutral voltage and allow sufficient ground fault current to flow to trip a breaker. In a wye:delta grounding transformer that 'something external' is current circulating on the delta side.

    I don't have the background to properly size any of these options.

    -Jon

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    Quote Originally Posted by winnie View Post
    In a wye:delta grounding transformer that 'something external' is current circulating on the delta side.


    -Jon
    Jon,

    Is a "Wye-delta grounding transformer" a specific thing for the purpose or can one just use any "regular" (appropriately sized) wye-delta transformer?

    I hear a zig zag has a lower impedance neutral, but not sure if/when that matters
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Jon,

    Is a "Wye-delta grounding transformer" a specific thing for the purpose or can one just use any "regular" (appropriately sized) wye-delta transformer?

    I hear a zig zag has a lower impedance neutral, but not sure if/when that matters
    The issue with neutral impedance is what happens when current tries to flow from line to neutral.

    If the neutral impedance is high, then when line to neutral current flows, then the voltage across that phase coil drops quickly, limiting the current flow. If the goal is lots of short circuit current flow to trip a breaker, then a high impedance neutral won't provide this.

    I don't know if there are any real differences between a wye delta grounding transformer and regular transformer. Perhaps there is a difference of which coils are wound as the primary.

    -Jon

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    Jon, yes the purpose is to trip the breaker during line-to-ground fault. It would be better to give up that feature of delta ungrounded system because the in the event that the 2nd fault occurs, the voltage during fault might be 6 to 8 times its normal voltage. Putting too much stress on the cable insulation (usually 600V for low voltage systems) might lead to another fault which is a disaster to the system.
    If we should treat the grounding transformer like the normal wye, then i supposed that from the main switchboard ground bus, we will connect a wire to the center of the wye side of the grounding transformer. Please advise if my understanding is correct.
    Normally for 600V below, we are using solidly grounding system.
    Thank you for your comments/advises, I really appreciate it.

  6. #16
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    Quote Originally Posted by jojo View Post
    If we should treat the grounding transformer like the normal wye, then i supposed that from the main switchboard ground bus, we will connect a wire to the center of the wye side of the grounding transformer.
    Not if you're talking about the transformer's primary neutral - it should be left unterminated.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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    Quote Originally Posted by LarryFine View Post
    Not if you're talking about the transformer's primary neutral - it should be left unterminated.
    This is a _grounding transformer_, not a wye:delta transformer being used to change voltage or create a separate system.

    The specific purpose of a grounding transformer is to create a neutral point that can be grounded in an otherwise ungrounded 'delta' system.

    I believe (though this really should be confirmed by someone who regularly engineers such systems) that the derived neutral should be treated (and grounded) as if it were the _supply_ neutral of a 'wye' system.

    In other words the OP is trying to convert a delta system into a wye system without actually changing the supply transformer (2000kVA) but instead by adding a grounding transformer (significantly smaller).

    (Side note: if you have a normal grounded supply and are using a transformer with a wye primary, the reason that you don't connect the neutral on that wye primary is related to the reason that you _do_ ground the wye in a grounding transformer. In both cases the transformer is creating a low impedance derived neutral. If you have a grounded system, and you connect a wye primary and ground it, then your system will have two 'neutrals' fighting each other and you will see large circulating currents if there is any phase imbalance. If you start with an ungrounded system then that wye primary becomes the system neutral that you do ground.)

    Back to the OP: I know just enough about grounding transformers to get into trouble. I know the basic theory. In my _lab_ I've wired and used a _small_ zig-zag transformer setup (to make a 30 kVA transformer work). I have not been involved professionally in systems of the size you are talking about. I am happy to help with your initial survey of what you want to do, but I believe that you need significant engineering of the change you are proposing, specifically to determine what other changes to the over-all system would be required when you add grounding.

    Additionally, I believe you have a misunderstanding. If you have a ground fault in an ungrounded system, the second ground fault is not what causes transient over-voltage. The transient overvoltage can be caused by a _single_ ground fault of a specific type. A so called 'restriking' ground fault is one that is vibrating open and closed. For example a ground fault in a motor that is synchronized with the motor rotation. This sort of ground fault will interact with the capacitance to ground of the system and 'pump' the system-ground voltage up to above the normal line-line voltage.

    As Larry identified, you have other options to solve the problem of an ungrounded system. Rather than using a grounding transformer, you could 'corner ground' the system. This makes one of the phase conductors a grounded conductor, and makes the voltage of the other conductors 480V to ground. To do this you would need to evaluate that all the insulation and breakers are suitable for operation at 480V to ground. A corner grounded system prevents the transient overvoltage situation, and is perfectly fine if your loads are all transformers and directly connected motors. It is less desirable if you have any connected VFDs.

    -Jon

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    If you corner ground it, a ground fault is basically the same thing as a line to line fault, I don't see any problem.

    With gear built to NEMA standards that means you must have a 480 volt rated breaker and no't a 277/480 rated breaker however, but you also need that if you leave it as an ungrounded system.
    I live for today, I'm just a day behind.

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    Yes, i misunderstood the term transient over voltage, I stand corrected..thanks for the comments and suggestions. I'll consider also other options aside from using grounding transformer.

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