Analysis And Research On Electric-Thermal Coupling Characteristics Of High Power Three-Level Converter

With the wide application of large capacity power electronic converters in China and the increasing capacity demand,the reliability requirement of converters is becoming higher and higher.Power electronic devices work in high-frequency switching mode with high voltage and large current,and the power loss will inevitably lead to the increase of device temperature,which will lead to device overheating failure in severe case.Thermal failure has become the main form of high-power converter system's failure.High-power three-level converter is widely used in megawatt highpower applications due to its advantages such as high-voltage level,low output harmonic and relatively simple structure.Therefore,this paper takes three-level converter as the research object,and analyzes its electro-thermal coupling characteristics.By establishing the electro-thermal coupling model of converter,the power loss and junction temperature of IGBT under different working conditions are studied as well.To begin with,the transient process and switching loss calculation method of highpower IGBT module are analyzed theoretically.Aiming at the problem that loss of IGBT is difficult to calculate online,the loss data of IGBT and diode under different conditions is obtained through a large number of offline experiments,and the loss mathematical model of IGBT module is established by mathematical fitting method.Next,taking high-power three-level inverter as example,the commutating circuit under different working mode of three-level converter is analyzed deeply.Considering the problems of traditional loss calculation models,such as strong parameter dependence,difficult calculation of complex working conditions,and inability of online calculation,two real-time loss estimation models of converters are proposed: The first calculation method of converter loss based on IGBT driving pulse can be simulated to estimate the real time loss of converter under complex working conditions.The other calculation model of converter loss based on field data can realize on-line calculation of converter loss by using IGBT duty cycle information.In addition,aiming at an air-cooled cooling system of three-level converter with a specific radiator structure,the overall thermal network model of the converter is established.The thermal coupling effect of chips inside the IGBT module and the thermal coupling effect between IGBT modules are comprehensively considered,and the thermal network parameters are extracted by using ANSYS finite-element analysis software.Finally,the electrical-thermal coupling model of three-level converter is established by employing the loss calculation model and thermal network model of three-level converter.Simulation analysis of converter loss and IGBT module junction temperature under different working conditions shows that load current,base wave frequency,switching frequency and so on can affect the junction temperature and fluctuation amplitude of IGBT module,and the loss distribution of converter is related to the load power factor.The thermal coupling effect between IGBT module's internal chips is small,however when multiple IGBT modules share one radiator,the thermal coupling effect between different IGBT modules is more serious.The high-power double-threelevel mutual feed experiment proves that the electric-thermal coupling model established in this paper is consistent with the actual situation and can reflect the electric-thermal coupling law of the converter accurately.