Research On Control Strategies Of Modular Multilevel Converter

The modular multilevel converter(MMC)is a new type of voltage source converter,which is a major breakthrough in the topology of multilevel converter.Compared with traditional two-level or three-level voltage converter,MMC has many advantages,such as high output voltage,low voltage stress of power device,no problem of voltage equalization among devices in series and so on.MMC has been widely used in high-pressure commutation system.This paper mainly focuses on the problem deterioration of voltage balancing for sub-modules(SM),caused by traditional voltage measure method,when the quantity of the voltage sensor is decreased,and the problem high complexity of the traditional control strategy of MMC,this paper conducts an in-depth study.In view of the problem deterioration of the SM's voltage balancing,caused by traditional voltage measurement method,when the quantity of the voltage sensor is decreased,a voltage measure method based on sample delay compensation is proposed.First,the basic principle of the voltage measurement is analyzed.Then,according to the principle,a method of capacitor parameter correction is studied.After that,the equivalent transfer function of the sampling system is established considering the influence of the intrinsic delay characteristic of the sampling system.Then,the step-wave signal sampled by each sensor is equivalent to the superposition of the pulse signals,with pulse width T_s,and the influence of the pulse signals on the current sampling value is analyzed.Finally,the effect of the first h pulse signals which has great influence on the current sampling value is eliminated by the voltage compensation.Simulation and experimental results show that,compared with traditional voltage measurement method,the voltage measure method based on sample delay compensation can reduce the voltage measurement error significantly,and the SMs have good performance of voltage balancing,which verifies the correctness and validity of the voltage measurement technology based on the sampling voltage delay compensation.In view of the problem high complexity of the traditional control strategy of MMC,a dead-beat control strategy for MMC is proposed.Firstly,starting from the basic topology of MMC,the deadbeat discrete model of MMC is deduced.Then,according to the analysis of discrete model,the circulation reference value and the load curre nt reference value are used as input of the controller,which is different from the traditional control strategy,with the output voltage reference value as the input of the controller,effectively reducing the number of PI controller and reducing the complexity of the system.After that,the inner loop transfer function of MMC is obtained according to the dead-beat control block diagram,and the stability analysis is carried out by using the Routh criterion,the stable error is analyzed by using the termin al value theorem.Finally,in order to verify the control performance of the deadbeat control method,simulation and experiment were carried out,the simulation and experimental results show that,compared with the traditional control method,the deadbeat control method can not only reduce the PI parameters,simplify the control but also have better quality of electricity.