Capacitor Voltage Balancing Control Strategy Based On The Predictive Algorithm For MMC-HVDC

Modular multilevel converter (MMC) is one kind of topology of voltage source converters.Compared with the traditional converter topology, MMC does not need to series connect IGBT directly,but to series connect sub module which contains IGBT in improving the voltage level,in order to avoid the problem of dynamic voltage balancing when IGBT devices are connected directly, and greatly reducing the difficulty of the production. So, it has become a hotspot of research in recent years. In this paper, researching has been done in three aspects: the steady state control strategy, the circulating current suppression method and the sub module capacitor voltage balancing under the MMC control system of the H half bridge structure:(1) The mathematical model of the MMC-HVDC system is deduced after analyzing the operation principle of MMC-HVDC system. Double closed loop control is used in the system.The PI control is used in the outer loop and current controller based on predictive algorithm is used in the inner loop. Principle and parameters designing of predictive current control has been analysed.(2) Considering with the circulation in system and adding AC component circulation in the prediction algorithm, it can predict the inner current and the circulation in the same time. The results above can be used to calculate the modulation wave. Circulating current suppression is achieved while tracking the inner loop current.(3) Several strategies of sub module voltage balancing have been analysed. One sequencing strategy has been given to reduce the switching frequency. It combines this strategy with the algorithm of predictive current control. A seven level MMC-HVDC simulation model is built in the simulation software. The outer loop uses PI controller and the inner loop respectively uses the traditional QPR controller and the predictive current controller. The result of current loop tracking a given instruction, DC voltage control performance and sub module capacitor voltage balancing are compared.(4) Experiments are taken on an equipment of 20kVA capacity of MMC-HVDC, verified the result of strategy in the practical application in this paper. MMC single ended rectifier...