Study On Macro Thermal Management Strategy Of Photovoltaic Inverter

With the rapid development of solar photovoltaic energy,the application of photovoltaic inverters is more and more extensive,and its reliability is becoming more and more important.Industrial investigation shows that the converter is the weak link in the photovoltaic grid connected system,and the power semiconductor device is the most vulnerable component in the converter.At present,most power converters use insulated gate bipolar transistor(IGBT)as switching devices,so it is very important to study the reliability of IGBT.Previous studies have shown that temperature fluctuation is the key reason for gradual failure of IGBT module.In photovoltaic inverter,the fundamental junction temperature of IGBT module will fluctuate due to the sinusoidal variation of grid side current.In the long-term operation process,the external natural conditions such as irradiance and ambient temperature have the characteristics of random fluctuations,which will make the output power of photovoltaic system fluctuate greatly,leading to the change of IGBT module loss,resulting in low-frequency junction temperature fluctuations.Thermal management is an effective method to smooth junction temperature fluctuation and prolong device lifetime.In this paper,the IGBT module in the singlestage photovoltaic inverter is taken as the research object,and the historical data of one year’s irradiance and ambient temperature are taken as the operation mission profile.According to the relative size and distribution law of lifetime consumption of fundamental frequency and low-frequency thermal cycles,a macro thermal management strategy is proposed to smooth the junction temperature fluctuation,so as to extend the lifetime of IGBT module.The following work is mainly completed:First,a mathematical model which can reflect the physical characteristics of photovoltaic module is established,and the output related electrical parameters can be calculated analytically under different irradiance and ambient temperature.A simulation model of a single-stage grid-connected photovoltaic system is built to analyze the variation rule of electrical parameters on the DC side and AC side of the photovoltaic inverter when the operating conditions change.Then,evaluating the lifetime consumption of IGBT module in photovoltaic inverter in a long time scale.Taking one year’s irradiance and ambient temperature as the operation conditions of single-stage photovoltaic grid connected system,based on the mathematical model of photovoltaic module and the variation law of electrical parameters of photovoltaic inverter under different operation conditions,the electrical parameters of IGBT module for loss calculation under various operation conditions are obtained.The fundamental frequency junction temperature information of each operating condition is obtained by using the loss model and the thermal network model of IGBT module,including the maximum junction temperature,the average junction temperature and the minimum junction temperature.The extremum points of the average junction temperature under various operating conditions are extracted.The low-frequency junction temperature information is composed of the maximum junction temperature or the minimum junction temperature under different operating conditions,and the low-frequency thermal cycles is extracted by rainflow counting method.For two kinds of thermal cycles in long time scale,the analytical lifetime model of IGBT module is used to evaluate the lifetime consumption of fundamental frequency thermal cycles and low-frequency thermal cycles.Finally,based on the lifetime consumption distribution of IGBT modules,a thermal management strategy is proposed to smooth the large low-frequency junction temperature fluctuation.By adjusting the switching frequency in a specific operating range,the strategy can achieve effective smoothing of the large low-frequency junction temperature fluctuation in a specific range.And the thermal management strategy is verified by experiments.In the experiment,the power fluctuation of the photovoltaic inverter is simulated by changing the DC side voltage of the inverter,the low-frequency thermal cycle information under the fluctuating power is extracted,and a low-frequency junction temperature fluctuation amplitude is set as the control target.According to the proposed thermal management strategy,an appropriate switching frequency is selected and applied to the operation of the experiment.The control target is realized,which verifies the effectiveness of the proposed thermal management strategy.