Effect of waveform distortion on protective relays

The influence of harmonics on protective relays is described theoretically, and the results of laboratory tests are given. Representative relays using various operating principles were tested using fundamental currents and/or voltages, single-frequency inputs that were multiples of the fundamental frequency, realistic combinations of fundamental and harmonics. From the theoretical concepts and expectations discussed and the test results, it is shown that the influence of mixed-frequency harmonics (with magnitude decreasing with order) on the steady-state behavior of the protective relays studied is insignificant. Pure single-frequency inputs, above the fundamental, caused a distinct change in relay operations