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The energy markets deregulation holds a new contractual perspective between customers and utilities, in which energy price can depend on voltage quality and load characteristics, as well as on responsibility for disturbances caused on supply voltage in power systems. In this contest, harmonic sources detection is one of the most urgent problems, because of the proliferation of loads which draw nonsinusoidal currents. In this paper, the authors present a new instrument based on a time domain method for harmonic active power detection in three phase systems. Amplitude and sign of harmonic powers can be measured directly and no spectral analyses are required for the evaluation of amplitudes and phase angles of currents. Moreover, the instrument is able to synchronize itself to the input signal, to measure voltage and current total distortion factors and harmonic voltages amplitudes. Theoretical aspects are discussed, measurement accuracy is evaluated and experimental results are presented.