Study On Redundancy Fault-tolerated Control Strategy Of Modular Multilevel Converters

Compared with AC,high voltage direct current(HVDC)has shown great advantages in the long-distance transmission of power,and its application is more and more widely,which promoted the development of high-voltage DC transmission converter components.Modular Multilevel Converter(MMC),with its simple topology,high output level,modular design,transformerless and filterless configurations,etc.,has recently become the most popular high-voltage direct current transmission converter.The higher the required output level,the more the number of sub-modules,and thus greatly increases the chance of sub-module failure,so MMC fault-tolerant control strategy is one of the most popular research.This paper first discusses the research background and significance of MMC,compares and analyzes several kinds of multi-level topologies and their advantages and disadvantages.The development path of DC transmission,the research status of MMC,the latest research findings and engineering application cases are involved.This paper introduces the modulation strategy of flexible DC transmission system using MMC and its existing fault-tolerant control method.Secondly,combined with the MMC topology and mathematical model,this paper explores the working principle of MMC,analyzes the possible causes and hazards of sub-module failure,and compares the difference between the redundancy-free fault-tolerant control scheme and the redundant fault-tolerant control scheme;Thirdly,aiming at the shortcomings of the existing fault-tolerant control scheme,this paper proposes an improved redundant hot stand-by fault-tolerant control scheme,which puts the hot stand-by sub-module into the normal switching,participating in the balance of the DC side voltage,using the maximum voltage deviation pressure control method to reduce the number of sub-module switching,and the methods named lack of level operating scheme can be used as the emergency scheme when the standby sub-module is not enough.By comparing the sub-module capacitor voltage waveform and loss value of this hot stand-by scheme and the conventional hot stand-by scheme in the sub-module failure,this paper confirms the feasibility of the proposed scheme;In addition,this paper describes a control method to reduce the voltage sensor in MMC system,that is,using capacitor voltage calculation module to optimize the entire converter hardware structure.On the basis of this,the fault-tolerant control strategy in the case of reducing the sensor is studied:Based on the hot stand-by scheme proposed in this paper,the N+m mode grouping is used to control the bridge arm sub-module,While suppressing the sub-module capacitor voltage fluctuations to ensure that the bridge arm voltage is stable,so that the capacitor voltage calculation module is still able to maintain accuracy in the sub-module failure.Finally,in order to verify the correctness of the proposed control strategy,the simulation model is built on Matlab/Simulink simulation platform.The results show that the improved hot stand-by scheme proposed in this paper can make MMC system run normally when sub-module fails,without affecting the performance of the system.The fault-tolerant control strategy proposed in this paper,in the case of reducing the sensor,can ensure the stability of the capacitor voltage calculation module and the stable operation of the system when sub-module fails,so that the MMC can pass the fault period smoothly.