Effect of waveform distortion on protective relays

The authors attempt 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 the theoretical concepts and expectations discussed and the series of tests performed, it is evident that the influence of mixed frequency harmonics (with magnitude decreasing with order) on the steady state behavior of the protective relays studied is minor and insignificant. The influence of pure single-frequency inputs, above the fundamental, on various protective relays was also evaluated. Results showed a distinct change in relay operation.<>