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Another Trany Q, Y to open delta?

Discussion in 'Electrical Engineering' started by uray, May 26, 2004.

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  1. uray

    uray Guest


    I'm still having transformer problems. I've got a 480 to 575 open delta
    transformer, it is fed from a 480Y panel. The neutral does not come into
    the transformer at all. I'm getting my 600 volts OK, but I am also showing
    a 150 amp load on the ground line. The casing of the transformer is also
    heating up, although the actual coils are not. I am still checking all the
    wiring, it seems OK though. I still want to take a megger to it.

    My question is regarding the 480Y service that is feeding it. Is the fact
    that it is 480 Y and not 480Delta significant?

    Thanx for any help,

  2. uray

    uray Guest

    The transformer started out as a 480 to 240 volt 500kva transformer. It was
    modified by an electrical house into a 480-575(600) volt open delta using
    two of the windings. The load on the ground is precisely that, the
    transformer is grounded and the ground wire heading back to the main panel
    is carrying 150 amps. The transformer is wired with a separate disconnect,
    the ground wire is bonded there, and continues back to the main feeder. The
    load only shows up to the bonding point in the disconnect switch, the wire
    going back to the main shows no load. I did not do the actual wiring and I
    am not sure if the neutral is bonded to ground at the main panel or not.

    I'm not sure what to make of it. I do have 480 in, and 600 out. Why the
    case ground is carrying so much current escapes me. Although I did not have
    a chance to check the windings with a megger yet, a standard VOM shows no
    shorts or grounds.

    I ran out of time to devote to it today and will continue checking it out
    tomorrow. I am suspicious of the ground and may need to pull the covers on
    the main and see exactly where the electricians made their connections. The
    transformer may have developed an internal problem as well, a megger will
    help me determine that.

  3. uray

    uray Guest

    Yes, it is a common core with the center winding unused.
    The 150 amps is with no load on the transformer. I am reading that 150 on
    the ground wire, since the case is also grounded through the conduits the
    total current may much higher.
    I megged the center winding and it seems to be isolated. When energized I
    am reading about 700 volts on one side and about 180 volts on the other
    side. I've been wondering if I should do something with that center
    winding, ground it perhaps?
    With the breaker off I see no load on any of the wires. This thing is wired
    up with multiple conductors, three per phase, which makes it difficult to
    measure with the equipment I have here.
    I will see if I can post a copy of the hookup on the web somewhere. Right
    now I am still talking to the company that modified it. They seem to be
    about as confused as I am. It is starting to look like there is a problem
    in the transformer somewhere. They may be able to come by tomorrow with
    some better test equipment. They would like to do a thermal image of it to
    see where exactly it is heating up.

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