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Thread: Utility Transformers.

  1. #1
    Join Date
    Jan 2005
    Location
    Massachusetts
    Posts
    117

    Utility Transformers.

    How are Ground faults and shorts cleared on the secondary side of a utility transformer?

    When a ground fault developes on the secondary side of a utility transformer on the line side of the main circuit breaker what happens?

    Will it blow itself clear?

    How is a short circuit cleared in the same situation?

    Does the utility transformer have secondary overcurrent protection?

    If not, will this short circuit current pass through the transformer to the primary side?

    If so,will it open the primary overcurrent device
    protecting the transformer?

    Is the secondary fault current large enough to accomplish this or will it just blow the secondary winding?

    Thanks, I look forward to your comments.

  2. #2
    Join Date
    Feb 2003
    Location
    Indianapolis
    Posts
    6,102

    Re: Utility Transformers.

    How are Ground faults and shorts cleared on the secondary side of a utility transformer?

    I am answering these questions for pole mounted transformers and overhead lines. We actually do a good job of protecting the equipment for underground lines and pad mounted equipment.

    We prefer for them to burn free. The outage will only be the customers that are downstream from the fault and the rest will remain in service. We may be able to repair the fault without taking the rest of the customers out of service.

    When a ground fault develops on the secondary side of a utility transformer on the line side of the main circuit breaker what happens?

    It depends on the size of the transformer and service. On large 3Ø service, the transformer fuse will let go, on small 1Ø services, it will burn open. We will fuse a 25 kVA transformer at 15T. A T rated medium voltage fuse will carry 150% forever without blowing. Calculate the kVA.

    Other electric utilities have different protection schemes but we attempt to keep customers in service. The concept is not to protect the equipment but to protect the line.


    Will it blow itself clear?

    We hope it will. If it doesn't clear itself, the line may burn in the street or the transformer may explode.

    How is a short circuit cleared in the same situation?

    It depends. If it is a good primary fault, it will take out the overcurrent device. If a tree limb is on the line, it may not clear. We are hoping for the tree limb to burn off the line. If it is a secondary fault, it will probably not take out the primary device. It will have to burn free or destroy the transformer.

    Does the utility transformer have secondary overcurrent protection?

    We used to use CSP (completely self protected) transformers. They had a secondary circuit breaker, a primary fuse link, a LA, and a red light to let you know that the transformer has been overloaded.

    We have bypassed all of the secondary circuit breakers, removed the primary fuse links, taken out the light, and removed the LAs. Our protection is now a good LA and a primary fuse.


    If not, will this short circuit current pass through the transformer to the primary side?

    Yes

    If so,will it open the primary overcurrent device protecting the transformer?

    No

    Is the secondary fault current large enough to accomplish this or will it just blow the secondary winding?

    See my answers above.
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  3. #3
    Join Date
    Sep 2004
    Location
    Midwest
    Posts
    154

    Re: Utility Transformers.

    I watched a utility secondary fault one time. To my amazement, the overhead drop to the neighbor's house started smoking. Soon, the wire insulation caught fire, and a sparking fire burned the wire up just like a fuse, starting from the house end and moving to the transformer just like a fuse in a cartoon movie! When the fuse-like fire reached the single phase transformer, it blew up in a shower of sparks. By that time I had collected my wits and called 911, but I believe a primary fuse may have cleared the fault by that time because the show was over. It was a great lesson in what can happen.
    Lawrence Lile, P.E.

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