Study of Analogue THD-meters with Notch Filter for Voltage and Current

Electricity is the most important product used in industry, commerce, education, and residential sector, whose quality must be maintained at the levels required by the EMC standards. Total harmonic distortion (THD) is an important indicator of power quality. The importance of issues of electrical power quality has recently increased due to the intensive use of electrical equipment. The article analyses an analogue THD-meter with notch filter, relatively simple and inexpensive, which can be used to measure the THD of voltage and current, made with linear integrated circuits and RC components. The analogue THD-meter is based on a notch filter that eliminates the frequency of 50 Hz, and signals with a frequency different from 50 Hz will be processed. The output of the analogue THD-meter is a voltage (in mV) proportional to the THD value. Simulations and experiments with a THD meter for voltage and current are presented and the errors are smaller when measuring THD for voltage (up to 4%) compared to measuring THD for current (up to 7%).