Low Pass Digital Filter Delay Compensation for Accurate Zero Cross Detection in Power Quality

Abstract

Zero crossing detection with a low pass filter is a common method used to estimate a signal frequency or period when the signal is affected by noise, harmonics or other parasitic frequencies. This method is often used in power quality measurements due to the typical disturbances of the electrical power grid signals and is for example, suggested in IEC 61000-4-30 for power quality frequency estimation. Zero crossing is also needed to evaluate RMS events since RMS evaluation must be synchronized with the input signal zero crossings. However, the filter introduces a delay which affects the accuracy of the RMS evaluation timestamp. High accuracy power quality measurement systems are expensive but their usage has significantly increased in recent years. To create low cost power quality analysers, a solution based on a digital low pass filter to detect zero crossings and measure signal characteristics such as the RMS value or distortions with accurate timestamps, is analysed in this paper.