This article deals with the determination voltage of one of the most complicated to measure the quality index of electric power, State Standard 13109-97 normalized , – harmonic distortion coefficient of the voltage curve for phase to phase (phase) voltages. Analog-to-digital determination of this quality index used sampling time (adaptive to variable period Tm of the fundamental frequency of the power supply), analog-to-digital conversion of the instantaneous values of the electric voltages. In this case, the instantaneous values obtained codes contain uninformative dominant components of the fundamental frequency voltages. From these elements, it is desirable to get rid of the ADC. One possible way - using the differential method of measurement  is characterized by increased complexity. In this paper, for the significant weakening of the fundamental frequency voltage is proposed to use a cascade connection of filters symmetrical components negative and positive sequence. The investigations and modeling of dynamic properties of this circuit have shown that when connected to the its inputs of a jump positive sequence voltages of fundamental frequency (the dominant component of the three-phase voltage ) transitional process is finished on its output almost completely after 1.2Tm .
harmonic distortion coefficient of voltage phase to phase (phase) voltages, power quality indexes