The Effect of Nonsinusoidal Current Waveforms on Electromechanical and Solid-State Overcurrent Relay Operation

Modern day power systems create harmonics within the electrical network that can have an impact upon the associated protection system. This paper has investigated how both an electromechanical and a solid-state protection relay are affected by the nonsinusoidal resulting current within the electrical system. The work was carried out using a commonly used GEC CDG11 electromechanical overcurrent relay and a Multilin 369 Plus digital Motor Management Relay. The testing was carried out using a three-phase Omicron 256 Plus secondary injection test set. The test results showed that harmonic currents are not filtered out by either of the two relays and that both of the relays tested will operate faster than they are otherwise seen to do due to nonsinusoidal currents. The work further demonstrated that neither of the relays responded to the simple equivalent rms summation of the harmonic current in the circuit.