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Thread: 12kV PT Acting Strange

  1. #1
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    12kV PT Acting Strange

    I'm managing a 2MW battery energy storage system tied to a 12kV grid and recently had the main breaker open on a voltage discrepancy. The AB voltage is 3500, BC is 12kV (grid voltage) and AC is 15kV. Seems to me like the A phase PT may be having issues? Seems like a phase shift where the A phase is being reported as slightly delayed, which could be due to damaged coils in the PT?

    A few weeks ago we had a similar issue and one of the high side fuses was blown in one of the PTs.

    Does anyone have experience with these types of issues and what may be causing them?

    I have screenshots of voltages, etc. if that would help.

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    Quote Originally Posted by stephanw View Post
    I'm managing a 2MW battery energy storage system tied to a 12kV grid and recently had the main breaker open on a voltage discrepancy. The AB voltage is 3500, BC is 12kV (grid voltage) and AC is 15kV. Seems to me like the A phase PT may be having issues? Seems like a phase shift where the A phase is being reported as slightly delayed, which could be due to damaged coils in the PT?

    A few weeks ago we had a similar issue and one of the high side fuses was blown in one of the PTs.

    Does anyone have experience with these types of issues and what may be causing them?

    I have screenshots of voltages, etc. if that would help.
    Check insulation resistance of PT with a meggar and also its historic insulation resistance tests to ascertain any trending.

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    Ratio test on the PT would reveal any inter-turn shorts.

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    Do you have access to hot sticks? If you do check the input to the PT.

    That said the fact a fuse has blown would have me looking for a replacement PT. I will say that in 40 years of working on MV gear I’ve never had a PT fail.
    The reports of my death are greatly exaggerated.

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    Fuse was blown, PT tested fine. Insulation was good and the A Phase PT matched very well with the C Phase PT. We're replacing the fuse and will look for a replacement A Phase PT when/if this happens again.

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    Maybe ferroresonance occurring. Are PT's lightly loaded? Any MV VCB switching taking place also?

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    Quote Originally Posted by Sahib View Post
    Maybe ferroresonance occurring. Are PT's lightly loaded? Any MV VCB switching taking place also?
    Ferroresonance in a PT? Please educate me; never heard of such.

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    Have had PT fuses blow on 3MW 11kV gen sets on shutdown due to field control staying on while rotor slows, saturated cores.

    What is your tie in to the battery bank consist of?

  9. #9
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    Quote Originally Posted by Sahib View Post
    Maybe ferroresonance occurring. Are PT's lightly loaded? Any MV VCB switching taking place also?
    Ferroresonance can occur in PTs that use a capacitive voltage divider followed by a wound transformer, aka Capacitive Voltage Transformers (CVTs). They are mostly used in HV but also in MV.
    The resonance can be at a subharmonic (typically 1/3), and the light loading Sahib mentioned makes such a resonance more pronounced.

    https://electrical-engineering-porta...ers-essentials

  10. #10
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    Quote Originally Posted by Sahib View Post
    Maybe ferroresonance occurring. Are PT's lightly loaded? Any MV VCB switching taking place also?
    What would make them lightly loaded? No MV switching taking place.

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