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Calculation Method of Secondary Circulation Current for MMC-UPFC Based on Simulated Annealing Optimization

The modular multilevel converter (MMC) based unified power flow controller (UPFC) is the newest flexible AC transmission system technology at present, which can realize the adjustment of active power and reactive power with voltage regulation, series compensation and shift equalization. With secondary circulation in the MMC bridge arm during UPFC running, a method for calculating secondary circulation current of modular multilevel converters is proposed, which establishes a calculation method of secondary circulation current by simplifying averaging control to be double proportional regulator, based on capacitance volt-ampere characteristic and equivalent switching function. The topological structure of MMC-UPFC is analyzed, and the specific process of the above model is displayed based on understanding the reason of producing secondary circulation current. In order to improve calculation accuracy, correction model of secondary circulation current is established, and the optimal parameters are obtained by using simulated annealing based on least squares. An important conclusion that secondary circulation current is associated with KV1 and KI1, and it increase with the increase of KV1 and decrease with the increase of KI1, is obtained by analyzing the above model, which provides a theoretical basis for suppressing secondary circulation current and selecting power device for UPFC. Finally PSCAD simulation results show that the method can accurately calculate secondary circulation current, which verifies its validity and feasibility.