Research On Fault Diagnosis And Location Strategies Of Sub-module Switching Device In A Modular Multilevel Converter

Modular Multilevel Converter(MMC)has the advantages of high output power quality and modular structure for easy redundancy control,and it has been widely recognized in high voltage and high power applications such as flexible DC power transmission and power electronic transformers.However,the MMC circuit topology usually requires a relatively large number of cascaded sub-modules,and the operation status of the sub-modules plays an important role in the normal operation of the system.Once a sub-module fails,if it is not detected in time,it will have a serious impact on the overall performance of the converter.Therefore,the three-phase halfbridge MMC in power electronic transformer is taken as the research object in this thesis,and a more in-depth study on the fault detection and location of its sub-module switching devices is conducted.The main research contents are as follows:Firstly,the working principle,modulation strategy and sub-module capacitance voltage equalization method of the MMC are analyzed.Considering the application scenarios of the MMC and the universality of the subsequent fault detection and location methods,the carrierphase-shifted modulation strategy combined with the classical sequential voltage equalization algorithm is finally chosen to realize the modulation and capacitance voltage equalization control of the converter..Secondly,the behavioral characteristics of the MMC in the fault state of sub-module switching devices are analyzed,the principles of open-circuit and short-circuit faults of switching devices are described,and the effects of different capacitance voltage equalization algorithms on bridge arm currents and sub-module capacitance voltages during open-circuit faults of submodule switching devices are studied.For the submodule lower tube VT2 open-circuit fault,an improved sliding mode observer(SMO)for fault detection and localization is studied,which can effectively suppress the system jitter and improve the accuracy of fault diagnosis.For the opencircuit fault of sub-module upper tube VT1,when the sequencing voltage equalization algorithm is used,there is usually a problem that the capacitance voltage of normal sub-module in the same bridge arm rises synchronously with the capacitance voltage of the faulty sub-module,so a method based on sub-module discharge energy accumulation is studied to achieve fault detection and location.Finally,the Matlab/simulink simulation model and the three-phase low-voltage low-power MMC experimental platform are built.The simulation and experimental results show the effectiveness and correctness of the fault detection and localization method studied in this thesis,The method is simple and easy to implement,and has good reference value for engineering applications.