Transient Frequency Temporal and Spatial Distribution Characteristics of MMC-HVDC Sending-end AC Subsystem for Wind Power Transmission During Symmetric Fault

The frequency characteristics of the modular multilevel converter based high voltage direct current (MMC-HVDC) sending-end ac subsystem for wind power transmission are totally different from traditional as system. When there is a fault, transient frequency fluctuation may cause the disintegration of the transmission system. However, the frequency loop control technology is still immature and cannot be used in actual systems at present. Hence, the paper is aimed to explore the transient frequency fluctuation suppression method without altering the traditional U/f dual closed-loop control structure. For the purpose, considering the dynamic processes of both the MMC and the wind turbine, a detailed mathematical model based on the control structure is initially established. And then transient frequency temporal and spatial distribution characteristics are analyzed through simulation based on Matlab/Simulink. Simulation results show that the transient frequency spatial distribution difference is mainly caused by transient current changing ratio, and the dominant factor of the transient frequency temporal characteristics is the PI controller parameters of the MMC.