Simulation Analysis of Control and Protection Model and Operation Mode of a 500kV Four-Terminal Flexible DC System

The tide analysis is studied in an AC-DC hybrid operation mode is conducted for the control system and true bipolar wiring method of 500kV four-terminal flexible DC system. The AC active power, AC reactive power and DC voltage were calculated with zero sequence component-removed AC voltage and current as the reference quantity at the converter transformer valve side. The selection of reference quantity and model of internal and external rings control were studied; the droop control strategy for ensuring stable system operation was explored; and a PSCAD simulation model in line with actual site conditions was established. The trigger-level control adopts the latest level modulation method, and the rounded voltage which is output by a receiving internal ring control is taken as the number of output levels. The simulation result indicates that all stations of the four-terminal system can be used as the voltage control ends of the flexible DC system. The tide distribution of the fault state in an island operation mode and AC-DC hybrid mode were studied, and a reasonable operation mode and countermeasures were provided.