The Effect of Non-Sinusoidal Current Waveforms on Electro-Mechanical & 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 electro mechanical and a solid state protection relay are affected by the non-sinusoidal resulting current within the electrical system. The work was carried out using a commonly used GEC CDG11 overcurrent relay and a Multilin 369 Plus 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 non-sinusoidal currents. The work further demonstrated that neither of the relays responded to the simple equivalent RMS summation of the harmonic current in the circuit.