Frequency Computation During Events and Point-on-wave Measurement

Point-on-wave (POW) technique features high sampling rate and GPS-synchronized sampling, which enable it to record transient waveforms conveniently with accurate time stamp. During a transmission line fault, the resulted fast oscillation and voltage sag bring troubles to the monitoring devices which compute the frequency in real time. It is demanding to improve the accuracy of existing frequency calculation algorithms. This paper focuses on comparing the performance of four frequency computing algorithms by two fault conditions including a phase-to-ground fault and a phase-to-phase fault happened near peak. The result shows that all the algorithms have large fluctuation during a phase-to-phase fault due to the existence of phase shift. Two characteristic points on a waveform, namely the point-of-inception (POI) and point-of-recovery (POR) is identified by the algorithms on the phasor complex plane, which represent the start and the end of an event respectively. The waveform between POI and POR is effectively smoothed by a Kalman filter, which enhances the computation stability.