| Compared with high-voltage alternating current(HVAC)and high-voltage direct current(HVDC),low-frequency transmission technology has some advantages in solving long-distance and large capacity power transmission in China.Frequency converter is one of the most important equipments in low frequency transmission system.Modular multilevel matrix converter(M3C)has a series of performance advantages as a new type of converter,but its application in low-frequency transmission system is still in the initial stage.In this paper,the parameters of M3C are analyzed firstly,and then the steady-state control strategy of low-frequency transmission system based on M3C is studied.For parameter analysis,this paper focuses on the relationship between Power/Low frequency system and converter voltage and current,reveals the mechanism of DC side capacitance voltage fluctuation of M3C submodule;secondly,according to the voltage wave momentum of module capacitance under Power/Low frequency power coupling,a general method for capacitance calculation is proposed,pointing out that the selection of capacitance value is related to the voltage amplitude ratio,frequency and power of Power/Low frequency system The factors are closely related;finally,from the economic point of view,the paper analyzes the voltage amplitude ratio of Power/Low frequency system and the influence of frequency on the selection of module capacitance value,and then from the perspective of system stability,explores the relationship between the system power factor at both ends and the selection of module capacitance value.Parameter analysis lays a theoretical foundation for the study of M3C control strategy.According to the steady-state control strategy of M3C two kinds of mathematical models are established from the Power/Low-frequency side.On the basis of the mathematical model of double dq transformation,the hierarchical DC voltage stabilizing control strategy is proposed,which are the global DC voltage stabilizing control strategy,the DC voltage stabilizing control strategy of converter chain and the DC voltage control strategy of chain link.RTDS simulation verifies the necessity of the hierarchical DC voltage stabilizing control strategy Necessity and correctness.On this basis,three scenarios of low-frequency transmission system are built,which are offshore wind power grid connected scenario,urban transmission scenario and island power supply scenario.The simulation results show that the model can realize the power transmission and flexible control of flexible low-frequency transmission system,and has good steady-state characteristics. |
