Microcontroller based true RMS current measurement under harmonic conditions

Root mean square is the fundamental measure of magnitude of AC signals. Increased use of power electronics and switching devices having non linear voltage-current characteristics produces harmonic currents. With total harmonic current distortion more than 100%, the analog reading type instruments used for RMS measurement introduce appreciable error since these instruments are calibrated for sinusoidal signals. The semiconductor based RMS converters require some digital processor to provide complete metering solution. In this paper, new approach for true rms measurement of electrical parameters using single digital processor is proposed. In the proposed approach, bipolar input signal is converted to unipolar using fast ac to dc converter. The unipolar ac signal is synchronously sampled by successive approximation type A/D Converter. Input signal is over sampled to obtain higher precision and incremental delay type sampling avoids the repeated sampling of the input signal at same points and increases the bandwidth of measurement. The sampled data is digitally processed to calculate the true RMS value of the signal. A prototype for current measurement is developed and tested in the laboratory. Laboratory test results are given which compares the performance of the prototype and standard meter under non sinusoidal condition. The meter has accuracy up to 0.1%.