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# Electrical riddle no.13 – Electrical energy meter errors

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• #151
Hamid

A three phase and three wire energy meter has installed in a medium voltage network with a lot of third harmonic current and unbalanced load (approximately %10).
As you know, any distortion in sine wave and deviation of rated frequency in AC electrical system may be cause the increasing of measuring errors.
According to a common technical practice, the third harmonic of measuring current can be trapped by using of suitable interposing current transformer with star-delta connection.

This recommendation applied in above mentioned system and accuracy of measuring system improved, but the system was not disencumbered of third harmonic nuisance completely.

How can you explain the reason of nuisance continuity?

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• #1311
Hamid

AC three-phase system can have positive, negative and zero sequence currents when the phases are not balanced. Likewise, the third harmonic currents can have positive, negative and zero sequence currents. When the third harmonic current in each phase is balanced and displaced by 120o, the current is positive sequence. When the third harmonic currents are balanced and in phase with each other, the current is zero sequence. In most three phase systems the third harmonic current produced are closer to being in-phase, but with a mentioned slight unbalanced (%10) in magnitude and phase angle, whole third harmonic of currents are not zero sequence and can not trapped by interposing delta connection completely.

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