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How do you ratio current transformers located inside an oil filled transformer?

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  1. williamwood9's Avatar
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    How do you ratio current transformers located inside an oil filled transformer?

    What method would you use to ratio test CTs inside of an oil filled transformer?

    I thought it would be via excitation method, but I was incorrect. Does anyone know the answer, and why? Thank you.

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    Quote Originally Posted by williamwood9 View Post
    What method would you use to ratio test CTs inside of an oil filled transformer?

    I thought it would be via excitation method, but I was incorrect. Does anyone know the answer, and why? Thank you.
    As far as I have ever been aware, Secondary voltage injection Turns Ratio test. IEEE C57.13.1

    Excitation would be to verify accuracy of the CT installed matches what was intended by the manufacturer. Whether the CT is installed inside a transformer or bus should make no difference in determining Ratio of the CT.

    IEEE C57.13.1
    6.1 Ratio test
    This test is not intended to prove the accuracy of the ratio, but simply to prove that the ratio, as installed, is
    as specified, and if taps are available, that they also have the correct ratio and have been wired to the
    correct terminals (see Clause 8).

    6.5 Excitation test
    This test confirms that the CT, as supplied, is of the correct accuracy rating, has no shorted turns in the CT
    and no wiring or physical short circuits have developed in the primary or secondary windings of the CT
    after installation. Manufacturer’s design curves for the CT should be available so that the actual results can
    be compared with those curves (see Clause 12).

    16.1 CT in a closed-delta transformer connection
    Ratio and polarity tests must be made prior to assembly if the delta winding terminals are not brought out.
    Ratio tests must be made by the voltage method of 8.1. Main power transformer excitation requirements
    and impedance would require a test set with much higher capacity than is normally found in order to use
    the current method.
    The tester should be made aware that it is necessary to short circuit the unused winding of the affected
    phase of the power transformer when making the polarity test of 9.1.

    16.2 Generator CTs
    High-ratio generator CTs present a special type of problem. The voltage method affords the only practical
    method of performing a ratio test. A convenient method of checking both ratio and polarity is to use a dual
    channel or dual trace oscilloscope to measure the magnitudes and phase relationships. The procedure is
    outlined in 9.2.

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    RedBrick is offline Junior Member Pro Subscriber
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    Quote Originally Posted by williamwood9 View Post
    What method would you use to ratio test CTs inside of an oil filled transformer?

    I thought it would be via excitation method, but I was incorrect. Does anyone know the answer, and why? Thank you.
    Also remember that the CT primary is in series with the transformer winding, this may cause a turns-ratio measurement error. Depending on your situation you will have to short the opposite winding.

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