Reliability Evaluation Of Isolated Microgrid Based On Converter Real-Time Failure Rate Model

In recent years,environmental pollution,energy shortage,temperature warming and other issues have become increasingly prominent,while the demand for electricity is growing.On this premise,renewable energy has been widely used because of its advantages of environmental protection and energy saving.Microgrid provides possibilities for renewable energy to be integrated to power grid.It solves the problem of long-distance transmission of power and effectively guarantees the electricity reliability of local residents.However,due to the intermittent of renewable generation output power,the power supply reliability of power grid has been adversely affected.Especially when the micro-grid operates on an isolated island and loses the energy exchange with the main power grid,the reliability has been challenged severely.Meanwhile,a large number of power electronic converters are used to realize power conversion in microgrid.The failure of converters also brings new problems to the reliability of microgrid.Therefore,considering all the factors which affect the reliability of microgrid,including the failure of power electronic converters,perfecting the reliability evaluation model of islanded microgrid is of great significance for accurately evaluating the reliability of microgrid and improving its reliability,which paves the way for the planning and design of microgrid system.In this paper,the reliability evaluation of islanded microgrid is studied through the following aspects:Firstly,a converter real-time failure rate model is established at the component level.Through literature analysis,we can know how operation conditions affect the power electronic device failure,that is,the failure probability of converter is affected by the output power of micro-source and the junction temperature of power electronic devices as well.Then,based on the output power model of micro-sources and the charge/discharge model of energy storage device,the junction,board temperature and its fluctuation are calculated from the power loss and thermal model of the device.A real-time failure rate model based on operating conditions is established for the converter.Then results of example analysis show that the failure of the converter is obviously affected by the operating conditions,microgrid parameters and converter parameters.The converter real-time failure rate model can better reflect the influence of operating conditions,working conditions and device parameters on converter failure rate in microgrid compared to traditional model.Secondly,considering the influence of converter failure on the reliability of microgrid power generation system,an improved algorithm for reliability evaluation of microgrid power generation system based on the converter real-time failure rate model is proposed.Within a given simulation period,combined with the converter real-time failure rate model,the downtime-operation sequence of all components is sampled by sequential Monte Carlo simulation method,and then the state duration of the system is calculated.Considering the output adjustment of micro-source caused by converter failure,the power balance of the system is analyzed by means of peer-to-peer control strategy,and the reliability indices of the system are obtained by statistics.The influence of converter and operation conditions on the reliability of microgrid power generation system is analyzed in case study.The effects of configuration of micro-source capacity on the reliability are also analyzed.The results show that the reliability level of microgrid power generation system is affected by converter failure,operation conditions,micro-source capacity ratio and energy battery parameters.Moreover,the improved reliability evaluation algorithm takes into account the effects of converter failure and operating conditions on system reliability,making the evaluation results more accurate.In addition to the micro-sources with different output characteristics,the isolated micro-grid also has the corresponding grid structure and load.The load reduction method is determined according to the importance of load points.Then,the component failure is enumerated in turn.The repair time matrix of load points is obtained by power redistribution.Combining with the converter real-time failure rate model,the operational reliability indices of load points are calculated.The effects of converter failure,micro-grid operating conditions,the number and types of micro-sources,and energy battery on the operational reliability level of the system are analyzed in case study.The result shows that the values of SAIFI and SAIDI indices are positively correlated with converter failure rate and inversely correlated with system generation capacity.The value of ENS index is inversely correlated with the system generation capacity,that is,both converter failure and system generation capacity affect the operation reliability of the system.Energy battery can improve the reliability of load points,thus improving the island operation ability of the microgrid itself.Finally,based on the above analysis,some suggestions are put forward to improve the operational reliability of islanded microgrid.