| At present,due to the large available space and potential of offshore wind power,many well-developed wind power projects are located offshore.With the development of offshore wind power,the next generation of offshore wind farms are expected to be located 300km offshore,and it will be extremely important to solve the problems of wind power grid connection and long-distance large-capacity transmission.Fractional frequency transmission system(FFTS)technology,as a new type of power transmission method,has been considered by more and more researchers in recent years as one of the most promising solutions to the problem of offshore wind power transmission and grid connection.AC/AC converter is the core device of FFTS,and the modular multilevel matrix converter(M~3C)has the characteristics of low output harmonics,low switching frequency,good extension and redundancy,and can directly implement AC/AC conversion.In recent years,it has received widespread attention in the fields of variable frequency speed regulation,and researchers have gradually used it as an AC/AC converter for FFTS.This article takes M~3C as the main research object.First,it introduces the topology and working principle of M~3C in detail,and analyzes and summarizes the three working states of sub-modules.Then it analyzes the M~3C capacitor voltage ripple and its generation mechanism,the influencing factors of the ripple amplitude,and obtains two unstable operating conditions of the M~3C system.In this paper,a double??0 coordinate transformation is performed on the mathematical model of the M~3C system in a rectangular coordinate system to obtain the 8th-order AC decoupling mathematical model of the M~3C under steady state operation.By analyzing the power and capacitor voltage ripple of M~3C in the??0coordinate system,the spectral separation of capacitor voltage ripple is achieved.Based on the M~3C decoupling model,an equivalent mathematical model in the dq coordinate system is established for the input and output sides respectively,and the corresponding AC quantity is converted into DC quantity.The corresponding double closed-loop vector control scheme is designed respectively in accordance with the system operation requirements.In this scheme,the controlled quantities on the input/output side are all DC quantities,which is easy to implement without static error control.Capacitor voltage balance is the basis for the stable operation of the M~3C system.In this paper,the bridge arm capacitor voltage of M~3C is balanced by controlling the circulating current component.In order to balance the capacitor voltage between the sub-modules in the bridge arm,this paper gives the capacitor voltage balancing strategy of the sub-modules based on carrier phase-shifted PWM(CPS-PWM)and nearest level modulation(NLM)strategies,and for the problem of large computational complexity of the sorting algorithm when NLM is used,an improved sorting algorithm is proposed,which can accurately switch the submodules while reducing the amount of sorting calculation.In addition,in order to reduce the switching frequency of the switch tube,this paper optimizes the improved sorting algorithm again,so that it has a frequency reduction function.Finally,a 41-level M~3C-FFTS simulation model is built in Simulink.The simulation results verify the feasibility of the control strategy designed in this paper and the correctness of the improved submodule voltage balance strategy. |
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