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slaa517c single phase watt-hour meter - 2 current measurements ?

Ian Pratt
Intellectual 880 points

The TI design for a single phase watt-hour meter possesses a mechanism for acquiring line voltages through the MSP430 ADC. It also possesses 2 current transformer inputs also via ADC. Why do we need 2 current inputs ? surely one input is enough either for calculating single phase power through phasors {VI cos (phi)} or instantaneous power v(t) *i(t) which appears to be the method used in the TI code.

So my question is why do you need 2 current measurements ?

  • 0
    TI__Guru 56685 points

    Ian,

    The second CT is for neutral current monitoring. This is to enable some forms of tamper detection and line to neutral balance checks.

    --JH

  • 0 in reply to Jace H
    Intellectual 880 points

    JH,

    Thanks for your reply, much appreciated.

    I would have thought the trip current levels for an earth leakage breaker would be quite low and since they are derived through the series resistor / CT, the voltage levels must as a consequence be very low and I would suspect be below the quantisation levels of the ADC. I'm not sure this is what you mean by "line to neutral" balance checks though ? (ie say 1mA * 0.1 Ohm -> 100uV) compared with 3.3 V / 2^10 (10-bit ADC) = 3.2mV


    I think the alternative makes a little more sense to me "tamper detection" - you are using it as a form of redundancy measurement to measure current and if there is any reasonable difference in current you could argue that someone is fiddling with the meter ? I think this is the design intent  that you are outlining in your reply, rather than any form of earth leakage protection? Can you please confirm ?

  • 0 in reply to Ian Pratt
    TI__Guru 56685 points

    Ian,

    Your second point is correct. Detecting an imbalance between line and neutral is an easy way to detect tampering, but also keep in mind that earth leakage breakers, or residual current breakers, are not used in every installation.

    JH

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