A Smooth Operating-state-switching Method Used in Multi-terminal Flexible Interconnected Device

In order to maintain the DC voltage stability and the power balance between converter stations in the control mode switching of the modular multilevel converter multi-terminal DC transmission system When the feeder fails, a smooth switching strategy based on direct current control is proposed. The state compensation method is adopted to realize the smooth switching of the control mode of the three-terminal MFID, and compared with the conventional switching method, the advantages of this method of smooth adjustment and stable output are explained. In order to verify the practicability and effectiveness of the switching method selection, a three-terminal MFID simulation model was built in the PLECS environment to simulate the transient and steady-state characteristics of the system under the new smooth switching mode. The results show that during the switching process under different conditions, the DC side voltage fluctuation is less than 2.3% of the rated voltage, and the fluctuation time is less than 8ms. Therefore, the strategy proposed in this article is conducive to the stability of DC voltage and the balance of power.