| More and more renewable energy sources as a distributed generation (DG) system access to large power grids, the grid converter work capacity requirements are getting higher and higher. In order to reduce the cost of governance of power quality in distributed generation systems and improve the cost-effectiveness of grid-connected inverters,multifunction converters are designed and started to be widely used. The multi-function converter also has the function of output active, reactive power compensation and harmonic suppression. The power margin is usually used to compensate for reactive power or harmonics on the basis of active output. However, different types of compensation for the power device caused by the temperature rise is different from the control, it is likely to cause the temperature overheating and burn the device, thus affecting the reliability of the system. Aiming at this problem,the junction temperature of the multifunctional power device in different working conditions is analyzed, and the optimal control strategy is proposed. Specific research work is as follows:At the beginning, this paper analyzed the various working condition of converter.Then, author described the current research situation of thermal analysis of power module under various working conditions.On the basis of analyzing the working principle and running characteristics of IGBT,the reason of loss in conduction and switching process is pointed out. Considering the fact that power loss produce heat and increase the junction temperature, the electric coupling model framework is established. This article proposed the calculation method for on state and switching loss of IGBT and anti-parallel diode. As for the thermal model,the heat transfer and influencing factors on thermal impedance are introduced. Then, this research chosen an appropriate Foster fourth-order structure as an equivalent model of electric conversion on the basis of two kind of RC equivalent heat networks.This paper uses MATLAB/Simulink to establish the electrothermal simulation model on the basis of thermocouple model. In the premise of ensuring that the converter is operating at rated current, the loss and junction temperature of IGBT and diode are simulated and analyzed in a variety of working conditions.The results show that the output current is the largest when the current is harmonic, also have the highest junction temperature. The active command current is the second, and the reactive current is the smallest.Finally, the loss and junction temperature caused by the converter working in the mixed condition are compared and analyzed. The experimental result verified the conclusion that the higher the IGBT loss, the higher the junction temperature, the smaller the diode loss, and the lower the junction temperature. It is further deduced that the change of junction temperature when the two kinds of command current simultaneously affect the change of the junction temperature. Converter rated power under a variety of command current mixing control, the junction temperature rise and fall affected by the larger proportion of the command current changes in the decision. According to this, the optimization strategy is proposed to ensure that the junction temperature is below the limited temperature and the reliability of the system operation is improved for the current situation of the traditional multifunctional converter power margin without considering the compensation of the temperature rise. |
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