Research On Improving The Reliability Of Modular Multilevel Converters

The modular multilevel converters(MMCs)overcome the shortcomings of the traditional two level converters and cascade multilevel converters because of their outstanding structure and technical advantages.The MMCs have been widely used in the high-voltage high-power applications and have been considered as the most promising high-voltage high-power converters.The reliability of MMC is limited by the large number of devices.This dissertation takes the half bridge submodule(SM)of MMC as the research object.In order to improve its operation reliability,the SM fault diagnosis method,the SM fault tolerant control strategy and the high reliability SM topology are deeply studied.The main works in this dissertation are as follows:1.The robustness of the fault diagnosis method based on the model was analyzed for the Modular Multilevel converters.The faults were detected by means of comparing the average value of the difference between the predictive value and the measured value with the threshold value using the incremental predictive model,and the faults were located by comparing the difference between the capacitor voltage of submodule and the average voltage of submodules with the threshold value.The incremental predictive model of the circulating current was established.In order to improve the robustness of fault detecting,correction method,multi-step prediction method and three-stage compensation method were proposed to compensate the uncertainties of the parameter and the measurement errors and eliminate disturbances.The principle of parameter design for the fault diagnosis method was given.The simulation and experimental results verify that the proposed method is characterized by strong robustness.2.The speed characteristics of fault diagnosis were analyzed while submodule failures occurred in the Modular Multilevel converter.A new fast fault diagnosis method was proposed.The diagnosis is implemented with two steps,one is fault detecting and the other is fault locating.The fault detection step found failures by means of using increment predictive model of arm current which detected the fault by comparing difference between the predictive value and the measured value with the threshold value,and then the failures were located rapidly by comparing slopes of submodule voltage.The proposed method could locate multiple submodule failures without additional sensors and can be easily implemented with the using of microprocessor.The proposed method has been validated by the simulation and experimental results.3.A fault tolerant control method for modular multilevel converters(MMCs)based on model predictive control with fixed calculation times is proposed.The proposed control is sequenced by three steps including the AC current optimal control,the multi-objective optimal control and the submodule(SM)capacitor voltage balancing control.First,the AC current was conducted with optimal control.Secondly,multi-objective optimal control was carried out for circulating current and AC current.Moreover,a sorting method based on level difference and direction of arm currents was used to balance the SM capacitor voltages.The proposed fault tolerant control method only needs 8 predictive iteration to find the optimal solution,which reduces the amount of calculation greatly.And the method can ensure stable operation and good output characteristics of MMC not only under normal condition,but also under faulty condition that the arm is asymmetric.The feasibility and advantages of the proposed method had been validated by the simulation and experimental results.4.The upper and lower power switches of submodules(SMs)in Modular Multilevel Converter(MMC)could be switched on simultaneously by disturbances which could destroy the SM circuit and cause damage to MMC,a new half-bridge submodule(SM)topology with the additional clamping circuit and energy transfer circuit is proposed.The clamping circuit ensures the stability of the SM voltage,and the energy transfer circuit guarantees the balance between the energy flowing into the SM and that flowing out the SM.The new topology has the shoot-through ability and the self-balancing ability.The SM capacitor voltage information is not needed to be upload to the control processer for balancing.It greatly improves the reliability,and simplifies the system structure,and reduces the calculation.Moreover,the AC component of the circulating current can be small even no circulating suppression strategy is adopted.The circuit topology,the operation principle and the control method of the novel SM are presented and the feasibility of the proposed SM topology has been validated with simulation and experimental results.It has been demonstrated that the MMC prototype can run well while some SMs are shoot-through and the AC circulate current is small while no suppression strategy is used.