Effect of waveform distortion on protective relays

The authors attempted to show, theoretically and by laboratory tests, the influence of harmonics on protective relays. Representative relays using various operating principles were tested using fundamental currents and/or voltages. They were then tested using single frequency inputs that were multiples of the fundamental frequency. Finally, tests were made using realistic combinations of fundamental and harmonics. From this series of tests, it is evident that the influence of harmonics (with magnitude decreasing with order) on the steady-state behavior of the protective relays is minor and insignificant. For faults containing a substantial DC component, the performance of the various relaying units will vary markedly depending on the duration of the DC, and more importantly on the nature of the measuring concept