Thermal Loss Analysis Of Semi-bridge Submodule For Modular Multilevel Converter

Modular Multilevel Converter(MMC)has attracted more and more scholars’ attention due to its outstanding advantages and engineering application value,which is widely used in flexible HVDC and Solid State Transformer(SST).At present,scholars at home and abroad have done a lot of research and analysis on control methods,modulation methods,voltage equalization methods and loss characteristics for MMC.In recent years,the research on device junction temperature has attracted more attention in the industry.This thesis took the half-bridge rectifier circuit of MMC topology as the research object,established the heat loss mathematical model of MMC submodule,calculated the power device loss and junction temperature;build the Matlab/Simulink simulation model to compare the junction temperature fluctuation under the strategy of NLM(Nearest Level Modulation)and CPS-PWM(Carrier Phase Shift Pulse Width Modulation),and to study the junction temperature fluctuation under the strategy of circulation suppression control and other interference factors of MMC.The main works of this thesis are as follows:Firstly,the thesis introduced the topology structure of MMC,analyzed the switching operation mode of MMC half-bridge submodule,and established the mathematical model of MMC topology structure.The thesis introduced the control strategy of MMC,and gave the function of the corresponding control strategy.Secondly,the thesis established the thermal impedance model of the IGBT and its anti-parallel diode,and combined the Cauer model and the Foster model.It calculated the conduction loss and switching loss of the switching device respectively,and derived the junction temperature expression of the device.Then,the thesis introduced the basic principle of MMC different modulation strategy,build the NLM strategy and the CPS-PWM strategy simulation models under this two different modulation strategy.Under the condition of keeping the main circuit parameters consistent,the junction temperature fluctuations under the two modulation strategies were compared and analyzed.The simulation results show that the junction temperature fluctuation of submodules in the NLM control strategy is small,and the junction temperature fluctuation consistency between submodules is high,and the overall junction temperature is lower than that under the CPS-PWM control strategy.Finally,the thesis analyzed the junction temperature fluctuation with and without circulation current suppression control strategy,and build the simulation model.The simulation results show that the circulating current will distort the bridge arm current and affect the power consumption and junction temperature without the circulation current suppression control strategy;the junction temperature of the upper and lower arms reached an agreement and the fluctuation of the junction temperature of the internal components of the submodule was significantly reduced with the circulating current suppression control strategy.In addition,the thesis studied the effect of the fluctuation of switching frequency and capacitance voltage on junction temperature.