Suppression to Commutation Failure of UHVDC in Hierarchical Connection Mode With Synchronous Condenser by Enhancing Post-Fault System Strength

The reactive power of synchronous condenser (SC) at the inverter of the ultra HVDC (UHVDC) in the hierarchical connection mode affects the system strength and the suppression effect on the commutation failure (CF). In this paper, a dynamic short-circuit ratio (DSCR) including the reactive power response of the SC is proposed to quantify the post-fault system strength. A coordinated control against the CF with the DSCR is proposed. The novelties are, (1) The analytical expression of the DSCR with the impedances of the Thevenin equivalent of both grids and the inter-layer coupling is newly derived. The recursive least-square method with the forgetting factor and the robust estimation is proposed to obtain the DSCR. (2) A multi-objective optimization model is proposed to tune the excitation parameters of SC and enhance the DSCR. (3) A coordinated control with the enhanced DSCR is proposed to adjust the dc current to suppress the CF. The simulation results verify the accuracy of the estimated DSCR and the effect of the coordinated control on enhancing the system strength and avoiding the CF under different fault conditions.