Mission Profile Based Reliability Evaluation For New Power Decoupling PV Grid-connected Inverter

As a key link of distributed power generation system under 10 k W,the reliability of single-phase inverter determines the safety and efficiency of the whole power generation system.Because of the working characteristics of single-phase inverters,the problem of double power frequency fluctuation inherent in its system is determined.The traditional solution is to buffer pulsating energy by parallel with large electrolytic capacitors at the DC-link.However,the electrolytic capacitor has a short life,low reliability and mismatch with the life of other components of the photovoltaic system,which will inevitably affect the overall life and reliability of the system,resulting in energy loss and increased maintenance costs.Therefore,the active power decoupling scheme has become the hot spot to solve this problem by reducing the decoupling capacitance and replacing the electrolytic capacitor with the large capacitance and short life by using the thin film capacitor with small capacitance and high reliability.However,active power decoupling generally requires the addition of additional power electronics,which will inevitably lead to changes in system reliability.In view of this problem,this paper takes the new voltage source inverter with voltage-boosting and power decoupling capabilities(VSI-BPD)as the research object,establishes a life prediction statistical model based on mission profile,and obtains the prediction life of photovoltaic grid inverter with a certain confidence level by Monte Carlo statistical calculation.The main contents of this paper are as follows:1.Based on the single-phase VSI-BPD,the topology and power decoupling principle are analyzed in detail,and a dual closed-loop control strategy is proposed on the basis of open-loop decoupling control to realize the dynamic simulation of the topology under the task profile.At the same time,the derived topology of single-phase VSI-BPD(active power decoupling topology based on the front-end Buck-boost converter)and the traditional two-stage single-phase photovoltaic grid-connected inverter topology are studied,and the PLECS thermal model simulation circuit is built,lays a foundation for the reliability comparison evaluation based on mission profile.2.This paper analyzes the main failure mechanism of IGBT,builds the IGBT equivalent thermal resistance model and the IGBT fatigue injury cumulative life model,calculates the junction temperature information of IGBT under mission profile through PLECS thermal simulation and Lookup Table interpolation simulation,lays a foundation for the reliability evaluation of photovoltaic grid-connected inverter systems.3.This paper analyzes the impedance characteristics of capacitors,builds the equivalent thermal network model of capacitors and the fatigue damage cumulative life model of capacitors,obtains the hot temperature information of capacitors under mission profile through PLECS simulation and Lookup Table interpolation simulation,and lays a foundation for the reliability evaluation of photovoltaic grid-connected inverter systems.4.Based on the life prediction results of IGBT and capacitors,this paper establishes a reliability evaluation model based on Monte Carlo statistics,and evaluates the reliability of the entire power decoupling inverter system by the system reliability block diagram method.The reliability of active power decoupling and traditional passive power decoupling inverters are evaluated by simulation comparison,and the new active power decoupling topology proposed by this research group is verified,which can not only realize the function of power decoupling,but also improve the reliability of the traditional two-stage single-phase photovoltaic grid-connected inverter.