Research On The Optimal Control Strategy Of MMC-based DC Solid State Transformer

With the rapid development of new energy and distributed power and the increasing demand of urban DC load,DC distribution network has become a research hotspot.Modular multilevel converter direct current solid state transformer(MMC-DCSST)will play an important role in different voltage levels of DC bus interconnection system.The modular design of MMC-DCSST is easy to expand,and it is suitable for the occasion of high voltage level and high transmission power.This paper focuses on MMC-DCSST,as follows:1.This paper introduces the topology,working principle,mathematical model and the working state of the sub modules of MMC-DCSST,introduces the quasi square wave modulation and the nearest level approach modulation,calculates and compares the electrical quantities of the two methods,and points out that the quasi two level modulation is more suitable for the situation of DC solid state transformer,and carries out the pulse cycle voltage sharing method and the sub module sequence voltage sharing method commonly used in the system Introduction and comparison.The correctness of the above modulation method and voltage equalization strategy is verified by simulation.2.The problem of MMC-DCSST bias is studied.According to the mechanism of the system,the control strategy of the feed-forward control is analyzed.In view of the fact that the output voltage changes in the voltage mode,which leads to the unsatisfactory effect of feedforward control,a double closed-loop magnetic bias control strategy based on feedforward control is proposed.This method can eliminate the bias current quickly when the load changes by dynamically modifying the phase shift angle and the voltage duty cycle of the secondary side.The simulation results show that the method is correct and effective compared with single voltage closed-loop control,feedforward control and double closedloop control.3.The soft start problem of MMC-DCSST is studied.In order to reduce the charging overshoot current during the system start-up,the corresponding soft start strategy of MMCDCSST system is proposed.The original side charging strategy can be divided into uncontrolled charging stage and closed-loop controllable charging stage;the secondary side charging strategy is to increase the constant duty cycle,and finally realize the unlocking operation of the system.Various modes of the circuit under different control strategies are analyzed,and the corresponding power conduction formula is derived.The correctness and effectiveness of the soft start strategy are verified by simulation.4.Finally,the experimental platform used in this paper is introduced.The platform adopts DSP and FPGA dual frame control system,and the steady-state operation under q2 l modulation mode is verified by experiments based on the double closed-loop magnetic bias suppression strategy and soft start strategy of feedforward control.The experimental results are consistent with the simulation results to verify the correctness of the above c ontrol strategy.