A Novel MMC Topology And Its Control Strategy Research

The distribution of new energy in China is extremely uneven.Large-scale new energy power plants are generally built in areas far away from the load center.Flexible direct circuit Transmission Technology Based on Modular Multilevel Converter（MMC-HVDC）can efficiently and reliably deliver power to load centers on a large scale and at large distances.The MMC-HVDC system has the advantages of real power and reactive power Independent control,no commutation failure failure,power supply to the passive system,suitable for multi-terminal direct circuit transmission system and other conventional HVDC transmission unmatched advantages,so is widely used in new energy grid,to the island and the weak power grid,Urban power distribution and other fields.The main circuit topology of MMC is the core of MMC-HVDC transmission system.The main circuit topology and actual capacity,voltage level,loss,switch modulation mode,number of series IGBT and Switching frequency and other factors are closely related.Based on the analysis of the working principle,operation characteristics and mathematical model of MMC converter,this paper presents a new MMC sub-module topology,which can output three levels of 0,U_C11 and U_CS.Due to the increased voltage levels of the MMC-HVDC system,the number of sub-modules in the MMC will increase dramatically,potentially reaching hundreds of sub-modules in each leg.Aiming at the problem of large amount of operation and high switching frequency of MMC in order of modulation and sorting,according to the operating characteristic of MMC and the topology working method and the fast sorting method,a new method is proposed which can keep the sub-module capacitor voltage balance capacitor voltage Balance optimization method to reduce IGBT switching times effectively.The goal of MMC-HVDC system start-up control is to make the DC voltage of the system rapidly rise to the voltage level near normal working condition by controlling the way and auxiliary measures without generating excessive charging current and voltage overshoot.According to the operating characteristics of MMC,MMC-HVDC charging process is divided into three phases:uncontrolled rectification,half-controlled rectification and high-frequency rectification,so that the capacitor voltage reaches the voltage value at steady state operation.According to the characteristics of the MMC structure mentioned in this paper,a step-by-step unlocking method is proposed in the start-up phase of a half-controlled rectifier to suppress the DC voltage drop caused by the MMC2 unlock valve base instantaneously.Through the simulation,it is verified that the proposed topology and control method can realize the start-up of MMC-HVDC system and the smooth progress of the grid connection.