Harmonic currents in switch-gear will increase heating and losses in switch-gear in the same way as has been discussed for power cables above. Similarly, voltage distortions can cause problems for voltage transformers (VT) and connected relays, while current distortion can do the same for current transformers (CT) and current-operated relays.
In particular, electromechanical over-current relays exhibit a tendency to operate more slowly, while static under-frequency relays that use zero crossing for frequency measurement are susceptible to substantial changes in operating characteristics in the same way as noted above for electronic control circuits.
Harmonics can also impair the speed of operation of electromagnetic-type differential relays.
🔺 Modern digital relays use filtering techniques to produce the fundamental frequency only so that the fault current and voltage measurement circuits are not significantly affected by harmonics. Correct filtering requires the relay algorithm to track the system frequency, and most relays have a limited frequency range over which they are designed to operate.
Although the fault-measuring capability may not be compromised, its ability to correctly measure and detect overloading conditions depends on its ability to measure the heating effect accurately, i.e., the full rms values.
Although corrections can be made in the settings when the harmonic mix is known, it is important to know the cut-off frequency of the relay to be able to fully compensate and protect devices, such as high-frequency filters.