A screening technique for anticipating network instabilities in AC-DC systems using sequence impedances obtained by frequency scanning

In this paper, we present further applications of the time-domain simulation based frequency scanning technique which has been proposed in the earlier literature. The behavior of the complete HVDC system is modelled as the closed loop action of two independent systems (AC system and DC system). The separately obtained frequency scan results of these independent systems are used to get the frequency response of the complete HVDC system. A heuristic screening method, based on the evaluation of frequency dependent positive or negative sequence resistance seen at the converter terminals, is utilized to anticipate unstable AC-DC interaction. We present a methodology to incorporate frequency domain criteria into the cost function of a simulation based optimization procedure so as to obtain a robust controller which avoids adverse interaction.