Research On The Fault Diagnosis And Fault-Redundancy Operation Of Modular Multilevel Converter

Modular multi-level converter(MMC)has a high degree of modularity,easy to achieve redundant operation and maintenance and many other advantages,in recent years,it has gradually become the mainstream converter topology in the application of high-voltage direct current(HVDC)transmission.In addition,MMC can flexibly expand the number of submodules according to the different voltage levels,the sub-module can also constitute a distributed photovoltaic cell access port,to achieve functional integration,so it also has great potential applications in the medium voltage system.However,compared with the high-voltage system,voltage level in medium voltage system is low,the number of MMC modules required less,usually only dozens,each sub-module has an important effect on the operating characteristics of the MMC,so a single sub-module failure will have a significant adverse effect on the converter performance,the converter will not work properly or even exit the operation,greatly reducing the operational reliability of the system.For this reason,the accurate and rapid fault diagnosis and simple and effective redundant control for the sub-module fault are necessary in the medium voltage system.In this paper,three-phase DC/AC MMC in the medium voltage system is studied,and its sub-module fault diagnosis and redundancy control are studied.The main contents of this paper are as follows:1)The design of the modulation and sorting equalization algorithm and the analysis of the basic operation state of the MMC are carried out.Because of the small number of MMC sub-modules in the medium voltage system,the in-phase phase disposition pulse width modulation(PDPWM)strategy is selected to reduce the harmonic content of the output voltage and current.The classical sorting pressure equalization algorithm is selected to achieve a good dynamic balance of the capacitor voltage.The principle of the MMC converter under the modulation and pressure equalization strategy is analyzed,and the theoretical analysis is given for the following fault diagnosis and fault redundancy control.2)The design of MMC sub-module fault diagnosis scheme is carried out.For the feature of the sub-module number of MMC in medium voltage system is small and sub-module failure has significantly affect on the system.This paper presents a sub-module fault diagnosis scheme based on sliding mode observer(SMO).The scheme builds the sliding mode observer with the sub-module capacitor voltage as the object without adding additional sampling circuit.In contrast to the traditional two-step diagnostic scheme of fault detection and fault location,this scheme can realize fault detection and location only through one step.Therefore,the diagnosis process is more rapid and the sub-module fault can be found in the 1/4 fundamental cycle.3)The design of MMC sub-module redundancy control scheme is carried out.For the features of the MMC control system is more complex,this scheme do not change the existing modulation and sorting pressure equalization algorithm,only by adding a redundant control unit to achieve redundant operation.And further design the hierarchical redundant operation scheme and give its influence on system loss,device capacity and so on.4)In order to validate the effectiveness of the above-mentioned detection scheme and control strategy design,the Matlab/Simulink simulation model and low-power experimental platform are also established.The simulation and experimental results demonstrate the effectiveness of the above design scheme.