Sub-module Fault Analysis And Fault-tolerant Control Strategy For Modular Multilevel Converter

With the application of VSC-HVDC,voltage source converter(VSC-HVDC)is widely used in the fields of HVDC transmission,grid-connected renewable energy generation and islanding.Among them,the voltage source converter Technology is an important part.The Modular Multilevel Converter(MMC)has become the mainstream of flexible DC transmission projects due to its low switching frequency,low harmonic content and low operating loss.In the system operation,the transient stability of the HVDC system based on the modular multi-level converter can be stabilized and restored as soon as possible under fault conditions is an important factor to ensure the stable operation of the HVDC transmission system.To study the operation mechanism of the modular multi-level converter sub-module in different situations under unbalanced and unbalanced conditions,it is proposed that there is an important analytical value to improve the stable operation of the system.In this paper,multi-level converter is the research object,mainly discussing its topology,sub-module capacitor voltage balance problem,how to restrain circulating current reasonably and control strategy in unbalanced state,and verify the proposed Validity of control strategy:First of all,starting from the basic principle of modular multi-level converter,the working principle,mathematical model and operating characteristics of multi-level converter are introduced in detail.The average switching model of sub-module is established.The hierarchical MMC-HVDC control system.Secondly,the principle and scope of the commonly used modulation strategy are analyzed.Two key technologies of capacitor-voltage balance control and loop current suppression in the modular multi-level converter sub-module are described.The working principle of recent level-approximation modulation is analyzed in detail.The problems of traditional voltage-sharing strategy based on NLC,such as large switching loss and excessive occupation of resources,are pointed out.Based on this,an improved capacitor-voltage balancing control strategy is proposed,The optimization and design of control parameters are completed.This strategy can significantly reduce the number of sub-modules to be sorted and reduce the switching loss.Finally,the failure of the modular multilevel sub-module is analyzed,and the unbalanced energy of the bridge arm caused by the fault is analyzed.The operation characteristics of the MMC asymmetric bridge arm are analyzed.In the case of asymmetrical operation.In this paper,we propose a cyclic current controller with sub-module fault tolerance and fault tolerance,and introduce the control of the fundamental frequency components in the circulating current in the double frequency loop controller.At the same time,a method to reduce the voltage ripple of MMC capacitor is proposed.To the bridge arm into the double frequency current to suppress the capacitor voltage fluctuations.The double frequency harmonic injection controller is designed.By controlling the double loop current in the bridge arm,the sub-module capacitor voltage can be optimized and reduced.The correctness of the proposed method is validated.