Research On Reliability Of Photovoltaic Grid Connected Inverter

With the increasing depletion of fossil fuel reserves and more pressing environmental problems such as air pollution and greenhouse effect,development and utilization of high-efficient,clean and economical energy sources has become a priority.However,industry surveys show that the failure of new energy power generation system is largely ascribed to the failure of grid-connected inverters,and the reliability of inverters has become a common and urgent practical problem to be solved.In order to accurately evaluate the reliability of grid-connected inverters,this thesis takes grid-connected PV power systems as an example.To begin with,a PV grid-connected inverter simulation model is built by using MATLAB/Simulink software to obtain the inverter’s direct voltage and alternating current through measuring solar irradiance and environmental temperature,which can provide data support for reliability modeling and calculation of the device.Then the loss and thermal network simulation model of power devices is built.Afterwards,the damage calculation model for IGBT modules is constructed by using the LESIT life prediction model based on physics of failure,rain-flow counting method and theory of cumulative fatigue damage.In addition,this thesis analyzes the impact of such factors as power fluctuation,ambient temperature,and switching frequency on the reliability of the device.The results show that:(1)the impact of various factors on the device is first reflected on the junction temperature of the device,and it meets the rule of the higher the temperature,the lower the reliability;(2)low-frequency junction temperature fluctuations caused by power fluctuation are the main factor that affects the reliability of the device;(3)the change in switching frequency can be used to improve the reliability of the device.Herein,this thesis proposes a strategy of reducing the switching frequency in inverters at the full-running stage to heighten the reliability of power devices.The strategy is simple and practicable,and can enormously improve the reliability of power devices.Based on the reliability data of each component,reliability block diagrams and the Monte Carlo method are used to calculate the mean-time-to-first-failure(MTTF)of grid-connected inverters.The results of the two methods corroborate each other.When conducting a reliability analysis of traditional power distribution systems,the failure rate is constant for each device.However,inverters as a repairable system composed of multiple components should change failure rates with the maintenance and updating of internal components.Aiming at this problem,this thesis considers the calculation method of the equipment reliability of the season,the aging of the equipment and the equipment maintenance,and gives the corresponding conclusion and the data of the grid inverter failure rate..The approach is valuable in engineering,whether it is to study the reliability of active distribution network in electric power grid or to determine the maintenance period of grid-connected inverters.