Study on inter-harmonics in short-circuit current of hybrid AC/DC electric power systems

With the widespread application of HVDC technology, power system harmonics generated by power electronic equipments can no longer be neglected. For example, the real-time current signal processing for protective relays or PMUs might be deeply affected by harmonics. In order to tackle with the accuracy for relays and PMUs, the inter-harmonic source for short-circuit current is theoretically analyzed, based on the interactions between low frequency oscillations in the power system electromechanical stability area and the electromagnetic transient process during faults. A typical hybrid AC/DC system is modeled and simulated, quantitative analyses on the inter-harmonic components in short-circuit current waveform are conducted with 51Hz and 52.5Hz simulated low frequency oscillation AC sources separately. The inter-harmonic phenomenon in transient short-circuit current is demonstrated by the simulation results, and it is suggested that the influence of inter-harmonic should not be neglected during online real-time signal processing of protective relays or PMUs.