Quantitative index to evaluate the impact of reactive power compensators on AC system's strength of UHVDC transmission under hierarchical infeed mode

Ultra high voltage direct current (UHVDC) transmission under hierarchical infeed mode (HIM) embedded into AC networks with diverse voltage levels (500 and 1000 kV) has been used by energy utilities for bulk power transfer. It has some dynamic characteristics challenges such as commutation failures under certain faults at inverter station. The installation of reactive power compensators (RPCs) such as synchronous condenser and static synchronous compensator at inverter's end can support the AC grid to mitigate commutation failures, which can ultimately result in the improved dynamic characteristics of UHVDC-HIM system. There is a need for analytical index that can accurately indicate the AC system's strength of UHVDC-HIM system with consideration of impact from RPCs and the interaction between AC networks. In this study, the resulting improvement in dynamic characteristics is quantified by proposed 'equivalent hierarchical mode short circuit ratio' (EHMSCR) index. An equivalent model of UHVDC-HIM system with AC system's strength modified via EHMSCR is established in PSCAD/EMTDC in order to assess the application of proposed index. Then, three different cases are carried out to further validate the accuracy of EHMSCR index. The results indicate that EHMSCR can accurately evaluate the AC system's strength of UHVDC-HIM system with RPCs.