Research On Fault-Tolerated Control Strategy For Sub-Module Faults Of Modular Multilevel Converters

In recent years,modular multilevel converter(MMC)has been widely used in the field of high-power energy conversion systems,especially in HVDC transmission applications,due to its low power device switching frequency,high equivalent switching frequency,superior harmonic performance,high modularity,etc.But the large number of sub-modules cascaded on the arm also has brought security risks for the operation of the entire system.Therefore,in this thesis,the MMC topology with half-bridge sub-module structure is taken as research object,and the operation characteristics and related control strategy of the MMC under sub-module failure are analyzed and discussed.First,the steady-state mathematical model of MMC under the sub-module failure is discussed.In this section,the basic topology and working principle of MMC are expounded.On this basis,several common reasons that may induce sub-module failure during normal operation and its possible influence on the whole system after the sub-module failure are analyzed.Then,based on the average switch state model,by introducing the sub-module faults number of each leg as variables,the mathematic model which can be used for analyzing steady-state characteristics of the system under different sub-modules fault types is established.At last,the steady-state operating characteristics of MMC under different sub-module fault types are systematically analyzed and summarized.Secondly,the sub-module isolation schemes and the fault-tolerant control strategy of the system under sub-module failure are discussed.The several existing sub-module isolation schemes after sub-module failure are summarized and compared in the first.Then,in order to reduce the number of SM bypassed and the complexity of the architecture of whole fault-tolerant control system after sub-module failure,a fault-tolerant control strategy based on partial symmetrical-bypass method is proposed.At the same time,the application advantage of traditional circulating current suppressor based on proportional resonance(PR)control in this fault-tolerant control strategy is analyzed.At last,the effectiveness of the control strategy is verified by simulation and experiment.Thirdly,the neutral point offset control strategy and its application in the field of the MMC fault-tolerant control under sub-module failure are discussed.In the first,the advantages of traditional neutral point offset control in fault tolerant control of multilevel converters are discussed,and through the analysis of its basic mathematical principles and implementation,the defects in the implementation of MMC system are pointed out.Then,a neutral-point shift control(NPSC)fault-tolerant control strategy for MMC sub-module fault is proposed in order to avoid the tedious calculation of the offset table in the traditional neutral point offset control.It similarly uses the balanced line voltage as a control target.Also,the impact of MMC system operating characteristic under the NPSC fault-tolerant control is analyzed,and the distribution characteristics of fundamental frequency component and DC component of bridge current is pointed out.At last,simulation and experiment results verified feasibility of the theoretical analysis and the proposed scheme.Finally,the circulating current suppression control strategy for MMC under sub-module failure is discussed.According to the MMC steady-state mathematical model of the sub-module failure,the harmonic characteristics are combed and the control target is pointed out in the first.Then,for the problem of the steady-state circulating current suppression after sub-module failure,a general fault-tolerant circulating current suppressor(GFCCS)based on quasi-PR control is proposed.It can be used for different fault types and also can reduce the design burden of the MMC fault-tolerant control system under the sub-module fault.And simulations and experiments also verify the effectiveness of GFCCS.At last,for the problem of transient response of circulating current suppression controller in the control system at the sub-module failure time,a fault tolerant circulating current suppressing controller with a virtual resistor is proposed.It can improve the response speed of the entire controller and limit the peak value of the DC and circulating current when the sub-module faulting.And simulation and experiments are verified,they show the effectiveness of the control strategy.