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  1. #1
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    Measuring leakage current in 3-phase system with current transformer

    Hi I want to measure precise number of leakage current in 3-phase trace heating system.
    The problem I faced is that typical current transformer gives a very high error at small currents. I found some solution either to use larger core or to apply some active compensation to pre-magnetize core.
    I think it is easier to pre-magnetize core because I have tight space requirements for the device, so larger core simply will not fit.
    But I can't find information about pre-magnetization. The only thing I know is I need to apply the voltage with same or 3 times frequency.
    In some industrial device, that does the same function I found two secondary windings, obviously one for pre-magnetization and one for measurement.

    Can somebody help me with this questions:

    1) What type of waveform do I need to apply to additional secondary winding?
    2) What parameters of additional secondary winding
    3) Can I use same core?

    Click image for larger version. 

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    •   Alt1st December 2016, 15:19

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  2. #2
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    Re: Measuring leakage current in 3-phase system with current transformer

    Hi,

    a typical current transformer is for some Amperes, but I assume you only need to know about milliamperes.

    --> Because the usefull input current range is very low, you should use a couple of primary windings to get higher secondary signal.
    Maybe increase the burden resistor value.

    Klaus



    •   Alt1st December 2016, 16:17

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  3. #3
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    Re: Measuring leakage current in 3-phase system with current transformer

    Quote Originally Posted by KlausST View Post
    Hi,

    a typical current transformer is for some Amperes, but I assume you only need to know about milliamperes.
    Yeah, something in range of 10-300 mA

    Quote Originally Posted by KlausST View Post
    --> Because the usefull input current range is very low, you should use a couple of primary windings to get higher secondary signal.
    Maybe increase the burden resistor value.
    Due to construction limitations, it is not possible to increase primary windings.
    And increasing burden resistor value doesn't help: insufficiently magnetized core has very poor current transfer performance :(



    •   Alt2nd December 2016, 12:55

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  4. #4
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    Re: Measuring leakage current in 3-phase system with current transformer

    Referring to you original question, a possible configuration to reduce errors by non-ideal core is a compensating feedback circuit that zeros the total flux.



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    Re: Measuring leakage current in 3-phase system with current transformer

    What kind of core material are you using ?
    Cheers, Tony.



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