Research On The Junction Temperature Balance Of Three-level Neutral Point Clamped Grid-connected Inverter

The Neutral Point Clamped(NPC)three-level converter based on 600 V IGBT devices has been widely used in the grid-connected inverters of distributed energy generation.Compared with the two-level converters,the three-level NPC converters have advantages in reducing the number of passive filters on the network side.However,there are imbalance of the loss distribution,junction temperature and life-time among the switching devices.It is feasible to solve the problem by customizing the devices.On the other hand,adding active power and capacitive wattless power into the grid can realize the function of the reactive power compensator simultaneously.However,there are concerns about the overloading of the power modules.The thesis focuses on the loss distribution and junction temperature balance of the three-level NPC grid-connected inverter,investigates the effects of injected reactive power and operation parameters on the conduction and switching losses of each switching devices under a constant grid-connected power.Based on the analysis,a new method is proposed to balance the loss distribution and junction temperature between switching devices.To be specific,a calculation model with multi-parameter input is constructed to obtain the capacitive reactive power online or offline under a certain active power.This strategy ensures the dynamic equilibrium of the junction temperature of the switching devices inverters under the frequent fluctuations of the grid-connected power in the distributed generation.Meanwhile,the improved system provides a considerable capacity of capacitive reactive power compensation while avoiding the overload of the power module.Firstly,the causes of the disproportionation of the loss distribution among the devices in the three-level NPC inverter are analyzed.The general term formulas of the device loss of each semiconductor under different modulation strategies are derived.Based on the theoretical analysis,the influence of the switching frequency,power-factor angle,modulation strategy and modulation ratio on the loss distribution among switching devices are systematically investigated.It is found that the power-factor angle and the switching frequency have a major influence on the loss distribution while the effect of the modulation strategy and modulation ratio can be ignored.Moreover,changing the switching frequency can only realize the wear-leveling under specific operating conditions.For the purpose of balancing the device loss and the junction temperature under two different operating conditions,a method of adding the capacitive reactive power is proposed and verified by the practicality.Additionally,a dynamic balance tracking control scheme is designed to recognize the change of the active power to ensure the dynamic balance of the device loss in the grid-connected scenarios where the active power fluctuates greatly.Finally,a three-level NPC half-bridge IGBT module-based test platform is built.The algorithm codes for the self-equalization of NPC three-level grid-connected inverter are developed and debugged to realize the flexible control of the active power and reactive power.The junction temperature of the IGBT module is measured by the infrared thermal imaging method in real-time.The steady-state junction temperature of the IGBT under different operating conditions is consistent with the calculated result,verifying the feasibility of adjusting the capacitive reactive power outputting to realize the wear-leveling.