Research On The Condition Maintenance Strategy Of MMC-HVDC Converter Station

MMC-HVDC is characterised by flexible control,low harmonic levels and high fault handling capability and has been widely used since its introduction.Reliability assessment work and maintenance strategy development is an important part of power system operation.At present,research work related to MMC-HVDC is still extended to LCC-HVDC,which are no longer able to meet the increasingly complex requirements of multilevel and real-time.Therefore,it is necessary to carry out adaptation studies on reliability calculations and maintenance decisions for MMC-HVDC.Firstly,considering the multi-state and time-varying characteristics of electrical equipment,a multi-state reliability analysis method for key equipment in MMC-HVDC converter stations is proposed,and the analysis is carried out using MMC-HVDC converter valve and hybrid DC circuit breakers as examples.For the MMC-HVDC converter valve,a half-bridge sub-module reliability model is established using the block diagram method,the remaining number of redundant sub-modules is defined to obtain the multi-state transfer process of the converter valve bridge arm,and the Markov process theory is applied to obtain the relevant reliability parameters;for hybrid DC circuit breakers,the equipment multi-state transfer process is obtained by simulating the equipment multi-level degradation state,preventive maintenance and fault repair state,and the failure rate model considering state differences is obtained by introducing the health index theory based on the Weibull distribution function,and the multi-state transfer rate model is obtained by incorporating the inspection rate and maintenance rate,and then reliability analysis is carried out based on the Markov process.Secondly,considering the large number of devices and variable states in a MMC-HVDC converter station,a method of calculating the overall reliability of the converter station based on a universal generating function method is proposed,which can effectively reflect the differences in the performance states of the converter station by introducing a combined operator of phase multiplication.Using a domestic MMC-HVDC demonstration project as background,the reliability indexes of the MMC-HVDC converter valve,hybrid DC circuit breaker,converter transformer,DC circuit breakers based on coupled negative voltage commutation,mechanical DC circuit breaker and the MMC-HVDC converter station as a whole are calculated.Finally,the optimization plan is proposed at the equipment and converter station levels,using availability as an optimisation target,and providing an optimization strategy for the equipment maintenance method,timing and coordination with the overall maintenance plan of the converter station.At the equipment level,advice on the design and maintenance of equipment parameters through sensitivity analysis of the factors influencing equipment availability;at the converter station level,the concept of incomplete maintenance is introduced to optimize the equipment maintenance method,and the idea of setting the equipment maintenance threshold and the coordination strategy between equipment maintenance and converter station maintenance is proposed under the logic of opportunistic maintenance.The research results of the paper can provide strategic reference for the reliability assessment and maintenance decision-making of MMC-HVDC converter stations.