The Research On Sub-module Fault Diagnosis And Redundant Operation Of MMC Based On Grouping Detection

Modular Multilevel Converter(MMC)has the advantages of Modular design,strong fault tolerance,low device switching loss and so on.The MMC is considerd as the most mainstream VSC Converter topology in the VSC HVDC transmission applications at present.When MMC is used in high voltage and high power situation,flexible DC transmission and other fields,there are many sub modules(SM)in each arm.Traditional detection methods need to use a large number of voltage sensors.The system structure is complex and the cost is high,and each sub module is a potential failure point.A large number of sub modules lead to an increase in the probability of system failure.Power switching device is the most prone to failure among submodules.The fault diagnosis of power switching device and redundant operation of submodules are important methods to improve the reliability of MMC.This article mainly focuses on the open-circuit fault diagnosis and positioning and redundant operation of MMC sub-modules based on sub-module capacitance voltage grouping detection.Firstly,explained the topology and operating principle of MMC,the working state of MMC sub-modules is analyzed,the mathematical model of MMC is established,and the DC side current and bridge arm current are analyzed when the MMC is in steady state operation,and the commonly used modulation strategy and voltage equalizing strategy and the method to improve the stability of the system are introduced.Secondly,aiming at the problem that a large number of voltage sensors need to be configured when there are too many submodules in MMC,the capacitance voltage grouping detection method of submodules is introduced in detail.The measurement error of capacitance voltage of submodule during grouping detection is analyzed quantitatively to provide the basis for voltage grouping of submodule.The simulation is carried out under the condition of the offset of the capacitance value,and the simulation result shows that the detection error calculation method of the proposed group detection can obtain the detection error range.Thirdly,this paper analyzes the working characteristics of MMC sub-module when IGBT open-circuit fault occurs,and proposes a method of MMC sub-module IGBT open-circuit fault diagnosis and location based on sub-module voltage grouping detection,which can simplify the system and realize the open-circuit fault diagnosis and location of sub-module IGBT.In the case of capacitance offset,different types of faults are simulated,the simulation results indicate that the proposed failure judgement and location method can realize the open-circuit fault judgement and location of the sub-module under the condition of reducing the sensors.Finally,the commonly used traditional MMC redundancy control methods are analyzed,and introduces the advantages and disadvantages of various methods.In the group detection,the influence of the charging process of the cold standby sub module on the capacitance voltage detection of the remaining sub modules is analyzed,and the capacitance voltage detection method of the charging process of MMC spare submodule based on packet detection is designed.On the basis of the redundancy control strategy,the capacitor voltage correction link is added to eliminate the accumulated detection error in the charging process of the sub-module,and realize the replacement of the faulty sub-module by the spare sub-module under the group detection.The simulation is carried out under the condition of normal and offset of the spare submodule capacitance.The simulation results show that the proposed detection method can realize the detection of the spare sub-module capacitance voltage while reducing the voltage sensor,and complete the redundancy protection.