A novel numeric relay algorithm for extracting negative phase sequence quantities from three-phase measurements

Extracting negative phase sequence components from the three-phase AC voltages and currents has been a long standing challenge for protection engineers. This paper presents a novel numeric, three-phase filtering algorithm for extracting the negative phase sequence components from sampled three phase measurements. The algorithm has been formulated to be suitable for use in a numeric relay to determine either the absolute value of the negative phase quantities or the relative value of the negative phase sequence to the positive phase sequence measured at the operating point. To demonstrate the capabilities of the algorithm, the paper presents results that show that the level of negative phase sequence components can typically be extracted within 30 milliseconds following a disturbance using a basic post-extraction smoothing filter. This demonstration used a nominal sampling rate of 1200 samples per second and an automatic frequency tracking algorithm to maintain synchronisation between the sampling clock and the power system frequency.