Analysis of AC Failure of New Energy Transmission System via Flexible DC Grid under Island and Grid-connected Operation Mode

The new energy transmission system through flexible DC grid needs to have two operation modes–island mode and parallel operation conditions with the AC grid. Therefore, it is necessary to study the similarities and differences of the failure mechanism of the system under the two operation modes. Analyzing the risk of overvoltage and overcurrent during failure is also of vital importance. This study focuses on clarifying the differences in the main circuit and control strategies between the two modes of new energy delivery system through modular multilevel converter high voltage direct current (MMC-HVDC). The mechanism of AC three-phase short circuit at a sending terminal is analyzed. Under the two modes, the characteristics of short circuit current output by converter station are different. Besides, we find that the value of grounding resistance has an effect on failure characteristics. The PSCAD/EMTDC is used to establish a new energy transmission system through a flexible DC grid for simulation verification.