Research On Control Strategy Of Modular Multi-level Matrix Solid-state Transformer For Port Power Supply

In recent years,with the development of China’s economy and the continuous improvement of the level of opening up,the scale of foreign is also increasing.At present,90% of China’s total export freight is transported by sea.However,the discharge of large amounts of pollutants from ports and ships has also severely damaged the environment of port cities.Therefore,it is urgent to promote port shore power technology.Due to the climate characteristics of our country,the annual average wind speed of coastal ports is between 5.5～ 8.0 m/s,which has the inherent conditions for the development of wind power.This dissertation combines offshore wind power generation with port power supply.Based on the research of modular multilevel matrix converter(M3C),a new modular multilevel matrix solid state transformer(M3C-SST)is proposed,and the control strategy of the input stage(M3C)is deeply studied,and the function of M3C-SST is realized under different working conditions.The main work of this paper is as follows:(1)A three-level topology of M3C-SST is proposed.The SST can realize the functions of offshore wind power grid connection and port power supply at the same time.Offshore wind power and shore power grid are standby for each other.The structure of the input stage,intermediate stage and output stage of the proposed M3C-SST are described in detail,and the working principle of the three-stage structure of the M3C-SST is also analyzed respectively,and the mathematical model of each part is established.(2)The control method of injecting the output frequency circulating current into the arm bridge of M3 C is described in detail.The simulation results using Matlab/Simulink software shows that when the transmission power among nine arms of M3 C is unbalanced,the injected circulating current of the output frequency will flow into the input current of the converter,resulting in the distortion of the input current.(3)In the case of unbalanced transmission power of M3 C bridge arms,the traditional control method of output frequency circulating current injection will have an impact on the input current.The specific causes for the problem are analyzed.In addition,on the basis of the conventional control method,a dual frequency circulating current(DFCC)injection control method is proposed.In this method,some restrictions are added to the injected circulating currents,which ensures that the injected circulating currents only flow inside the bridge arms.(4)The overall control method of M3C-SST is proposed,and the effectiveness of the topology and control method proposed in this paper is verified by simulating three typical working conditions: the power supply of offshore wind power plant and shore power grid at the same time,the power supply of offshore wind power plant alone and the power supply of offshore wind power plant to port and shore power grid connection at the same time.