Study On Quasi-online Monitoring Method For Thermal Parameters Of IGBT Module

The power converter has been widely used in a variety of applications,such as the wind turbines,photovoltaic generation systems,electric vehicles,railway tractions,smart grid,and so on.However,in these applications with high reliability,the designed and expected lifetimes of power converter are not same.Industrial experience indicates the power semiconductor device is one of the weakest components in the power converter.Thus,improving the reliability of power semiconductor device can reduce the downtime of power converter and corresponding maintenance costs.Further research shows that the failure mechanisms,junction temperature estimation,lifetime prediction,condition monitoring,diagnostics and thermal management of power semiconductor device are closely related to the thermal parameters(thermal resistance and thermal capacitance).Therefore,the identification,monitoring and application about thermal parameters are the focus of this paper.(1)In terms of thermal parameter identification,existing methods require measuring the junction temperature and power loss of the device and heating the device to thermal steady state.Since the devices in converter operating period are in the switching state and the power processed by devices are always time-varying,corresponding junction temperature fluctuates regularly and does not stay in a steady thermal equilibrium.Furthermore,the rapid change of the voltage and current of the converter makes the measurement of junction temperature and power loss more difficult.In order to overcome the limitations of existing methods,a quasi-online thermal parameters identification method based on the case temperature is proposed.This approach is applied during the cooling process of converter and belongs to the quasi-online type.Advantages of the proposed approach include easy implementation,no need to measure the power loss and no need to heat the module to thermal equilibrium.First,according to the electrical-thermal analogy,the thermal network model can be transformed into electrical network model.Then,based on the node voltage analysis and the mathematical connection between the roots and coefficients of the characteristic equation,the constraint equations about the time constants of case temperature cooling curve and the thermal parameters can be established.Finally,the thermal parameters can be identified by combining the constraint equations under different cooling conditions.Moreover,impacts of the delay of temperature measurement system,the junction temperature,case temperature sensing position and thermal coupling on the identified results of thermal parameters are analyzed.The results show that the proposed method can identify the thermal parameters of IGBT module in the converter accurately.(2)In terms of thermal parameter monitoring,existing researches often monitor the thermal parameters by other physical quantities which can be easily measured.These methods assumed that an increase in the thermal resistance of the device results in an increase in the junction temperature,which in turn causes a change in the monitored physical quantities.However,existing research shows that the aging of the cooling system will also bring about the rise of the junction temperature of the device,so the existing physical quantities are easily affected by the degradation of the cooling system.In order to overcome the limitation of existing methods,this paper takes the thermal time constant as a characteristic quantity to monitor the IGBT module and cooling system.This method can not only monitor the health status of the IGBT module,but also evaluate the health status of the cooling system.Based on the theory of heat transfer,the physical quantity of thermal time constant is first summarized.Then combining the aging condition,the feasibility of thermal time constant as a new indicator is analyzed.Finally,the procedure of finding the thermal time constant of monitored object from the cascaded thermal system is introduced.In order to verify the proposed method,a three-phase inverter with forced air cooling system is constructed.Meanwhile,the influence of operation condition of inverter on the monitored results is explored.The experimental results show that the proposed indicator can monitor the IGBT module and cooling system simultaneously.(3)In the aspect of thermal parameter application,taking the thermal management of IGBT module in wind power converter as an example.Previous studies show that the consumed lifetime due to the low frequency thermal cycling takes a great part in the total consumed lifetime of power devices in the wind power converters.Therefore,reducing the fluctuation of this thermal cycling can improve the reliability of power device significantly.Existing research methods have a good effect on smoothing the fundamental frequency thermal cycling but few works focus on reducing the low frequency thermal cycling.The reason can be attributed to the fuzzy control target of low frequency thermal cycling in that the maximum and minimum in each thermal cycle are decided by different wind speeds and ambient temperatures.Unlike the fundamental frequency thermal cycling,the fundamental frequency junction temperature fluctuation corresponds to a specified wind speed.In order to overcome the limitations of existing methods,this paper proposes a macroscopic thermal management method based on the distribution characteristics of consumed lifetime.This method can reduce the consumed lifetime due to low frequency thermal cycling specifically.The key of this approach is to determine the allowable range of junction temperature fluctuation based on the distribution characteristics of low frequency consumed lifetime and map it into the working range that the converter needs to be regulated.The variable switching frequency is chosen as the junction temperature control method.The feasibility of the variable switching frequency and its influence on the THD(total harmonic distortion)are analyzed in detail.The results show that the proposed method can not only effectively reduce the consumed lifetime due to low frequency thermal cycling but also meet the requirement of power quality of grid-connected converter.