Research Of The AC Fault Ride-through Control Of VSC-MTDC

As the transmission capacity and voltage level are growing and new energy demand continues to increase,high-voltage flexible multi-terminal HVDC are now beginning to show great advantages in economical efficiency,interconnection and controllability etc..With the development of MTDC,the formation of DC power grid is imperative.When faults occurred to the AC system connected to the MTDC system,in order to maintain the stability of the system,a certain fault ride-through capability of the DC power system should be required.According to this demand,focused on the MMC-MTDC system,based on the transient modeling,the main influence of AC faults to the system is analyzed,and the faults characteristics are simulated.By adding a proportional integral loop of the modulation ratio to the valve control,the amplitude of AC current is kept steady.By improving the inner loop decoupling control of MMC-MTDC system,the negative sequence component and negative sequence component are controlled separately,so that the negative sequence of the AC current is suppressed under asymmetric fault.In the case of serious AC faults,when the converter needs to be quitted,the voltage droop control strategy is analyzed and optimized.By adding an active power feedback,it's possible to make the other converters allocate power flexibly and quickly reach a new stable state with a smaller voltage rise.A four-terminal MMC-MTDC system is built in PSCAD/EMTDC,and the simulation results show that the rise of the amplitude and the asymmetry of the AC current are eliminated,and when a terminal is removed the droop control can offer a better allocation of the active power compared with voltage margin control,decreasing the rise of the DC voltage,thus proving that through the design of the control strategy,thus the ability of the MMC-MTDC system to pass through the fault of the AC system can be improved to a certain extent.