Reliability Assessment And Preventive Maintenance Method Of MMC In Large-scale Wind Power Transmission System

Voltage source converter-based high-voltage dc(VSC-HVDC)transmission is an attractive technology for large-scale wind power integration into power grid.As a key component in VSC-HVDC system,the operational reliability of modular multilevel converter(MMC)is significant for the stable operation of the system.Preventive maintenance based on reliability assessment is an important way to ensure the normal operation of the MMC.However,the power of MMC for wind power transmission changes greatly with the wind speed fluctuating.Moreover,because of the variation of wind power output and air temperature,environmental factors affect the reliability of MMC seriously.In addition,as the installed wind power capacity increases,MMC needs more components to fulfill higher voltage requirement.This leads electrical parameters have more influence on the operational reliability of MMC.Therefore,it becomes quite meaningful to describe the impact of environmental and electrical factors on MMC reliability,thereby coordinating the reliability and economy of the MMC during preventive maintenance process.This paper deals with these problems in the following three aspects:(1)The current reliability assement model of MMC fails to describe the impact of environmental and electrical factors.A reliability assessment model of MMC with multi-term thermal damage effects is proposed.First,in order to take the effects of wind speed,temperature and electrical parameters into account,a reliability evaluation model of power devices with multi-time scales is established,based on the operating characteristics of MMC and the failure mechanism of power devices.Second,a MMC reliability evaluation model considering the impact of environmental and electrical factors is deployed based on the topology of MMC.Third,reliability of Hybrid MMC is limited due to the failure of sub-module caused by high loss components.The bypass switch is added to high-loss power module in the sub-module with DC fault ride-through capability,so that the sub-module can still support the voltage when it fails.Then,a hybrid MMC reliability evaluation model with multi-state sub-modules is constructed.Finally,a MMC simulation model in the large-scale wind power transmission system is constructed.The impact of environmental and electrical factors(e.g.wind speed,air temperature and switching frequency)on the reliability of MMC and the reliability of the front/rear hybrid MMC are analyzed by using real meteorological data,to verify the validity of proposed model.(2)Current preventive maintenance method of MMC with constant failure rate can not coordinate reliability and economy with the variations of different wind farm flexibly.A preventive maintenance method of MMC with dynamic redundancy is proposed.In this method,first,a reliability model of MMC considering the renewal process in full life cycle and impact of operational condition is constructed,based on the failure mechanism and maintenance strategy of MMC components and topology of MMC.Second,a MMC preventive maintenance model is established.In this model,in order to minimize the preventive maintenance cost(e.g.redundant sub-modules and basic maintenance),redundant sub-modules on every stage,maintenance frequency,reliability of MMC and limited working time are taken into account.Then,redundant sub-modules on every stage and maintenance frequency are optimalized by using non-sequential Monte Carlo estimation and genetic algorithm,a preventive maintenance method of MMC with dynamic redundancy is proposed.Finally,to verify necessity and effectiveness of the proposed method,the proposed method and traditional preventive maintenance methods are analyzed in terms of reliability and maintenance economy,in different wind farms.