# 3 phase power measurement

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Can anyone point me in the right direction on information as to how 3 phase power measurements are carried out ?.

TIA,

woodglass...

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Go to a book on alternating current circuits.

Bill

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I like this one->

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In what context? Do you want the general principles or specifics on the current practices in revenue metering?

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Just the general principles,

TIA,

woodglass...

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[snip]

OK. Here goes:

AC power (real power as opposed to volt-amps) flowing through a circuit (single phase) is the product of the voltage phasor magnitude (the rms value one would read with a meter), the current phasor magnitude, and the power factor. The power factor is the cosine of the phase angle between the above voltage and current phasors. This is valid for sinusoidal voltage and currents but has problems if the voltage and/or current contain harmonics.

Alternately, one can multiply the instantaneous current and voltage values of the circuit and sum the result over one cycle (or several cycles). This is (sort of) how a mechanical watt meter (or watt-hour meter) works. The meter is a special kind of AC motor whose torque is proportional to this product.

For a watt meter, this torque acts against a spring to move a pointer. For a watt hour meter, this torque acts against a known drag to produce a certain number of rotor revolutions per watt-hour. The disk revolutions per watt hour is known as Kh and can be seen on the nameplate of meters.

There are solid-state (analog and digital) methods of performing the above multiplication instead of the electromechanical method. These appear in most modern test equipment (where space and weight is at a premium), calibration equipment and large scale metering (where the cost of the precision circuitry can be absorbed).

For three phase systems, three 'elements' are used, one for each phase, to perform the above multiplication. These 'elements' might be three separate current/voltage winding sets in an electromechanical meter, or three sets of calculations performed and summed in a uP.

If you work out the vector math (maths for all our UK readers), the voltage phasors used in the above sum for any N-wire system can be a phasor measured from the phase conductor to _any_ common reference point in 'phasor space'. As long as the same reference point is used for all VI products, the sums of all the power flows will equal the total power flowing through the circuit at that point. By selecting this common point to be one of the circuit conductors, one can eliminate one metering 'element' and still get the total power. This is because the voltage phasor from that reference point to itself is always zero, so the product of that conductor's current and zero is ... zero.

For a two wire circuit, one line current and the voltage from that line to the other is measured.

For a three phase, four wire circuit, if the neutral is used as the reference, only the three phase currents are taken and multiplied by the phase to neutral voltages. For a three phase, three wire circuit, one phase is used as a metering common. Only the other two phase currents and the phase to phase voltages are measured to meter the power flow.

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With a 3 phase tape measure, silly boy!

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I thought that those were for measuring the imaginary power that make the difference between watts and vars.

Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz

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woodglass can try: