Research On Loss And Temperature Optimization Of Modular Multilevel Converter

Modular multilevel converter(MMC)has been proposed since it was first proposed in 2001.The number of modules can be adjusted,which can realize flexible changes in voltage and power levels,multi-level voltage output,and high output power quality.The low switching frequency can reduce the switching loss,and the advantages of redundancy,fault tolerance,and high reliability have become the most potential topology in the field of medium and high voltage and high power applications.Since the topology was proposed,it has received extensive attention and research in academia and industry and has been widely promoted and applied in the fields of flexible HVDC transmission,power quality management,rail transit,energy storage,and new energy grid connection.In order to ensure the low-cost and high-reliability operation based on MMC,it is of great value and significance to research reducing the total power loss of the MMC system,optimizing the power loss of power devices,and optimizing the junction temperature of power devices.This paper takes MMC as the research object,and studies the control of reducing the total loss of MMC system,the optimal control of sub-module power device loss,the optimal control of sub-module power device temperature,the loss balance control under sub-module fault,and the three-phase power device temperature balance control under grid voltage unbalance.The simulation and experimental verification are carried out to provide theoretical support and technical support for low-cost and high-reliability operation in the field of MMC applications.The main research contents of this paper include the following aspects:1.A control method to reduce the total power loss of MMC is studied.A Power Loss Reduction Control(PLRC)strategy is proposed to further reduce MMC by injecting optimal second and fourth harmonic circulating currents into the bridge arm currents of MMC,compared to the conventional injection of only second harmonic circulating currents.of power loss.The relationship between the sub-module(SM)power loss and the second harmonic circulating current(SHCC)and fourth harmonic circulating current(FHCC)is revealed.The optimal current reference value can be determined according to the SHCC and FHCC corresponding to the minimum value of the SM total power loss under the given operating conditions.The effectiveness of the proposed PLRC strategy is verified by simulation and experiments.2.The optimal control method of power device loss in the sub-module is studied.The power loss distribution of insulated gate bipolar transistor(IGBT)/diode in SM of MMC under various power factors is analyzed,and it is found that the upper IGBT/diode loss is smaller and the lower IGBT/diode loss is larger.In order to improve the power loss distribution of each power device in the SM,a power loss optimization control(PLOC)is proposed,which improves the loss distribution of the power devices in the SM by injecting the optimal second harmonic circulating current into the bridge arm current.The effectiveness of the proposed loss optimization strategy is verified by simulation and experiments.3.The optimal control method of power device temperature in the sub-module is studied.The junction temperature distribution of power devices in MMC SM under different operating conditions is analyzed,and it is found that the temperature of the lower IGBT/diode is usually higher than that of the upper IGBT/diode.In order to improve the temperature distribution of the power devices in the SM,a temperature optimization control by injecting the optimal second harmonic circulating current into the SM is proposed to improve the temperature distribution of the power devices in the SM.Simulation and experiments verify the effectiveness of the proposed temperature optimization control strategy.4.The method of power device loss balance between bridge arms under sub-module fault is studied.The SM loss distribution between the MMC faulty arm and the healthy arm is analyzed under the SM fault.The SM fault will cause the SM capacitor voltage and switching frequency in the faulty arm to increase,which will lead to the increase of the SM capacitor voltage and the switching frequency between the faulty arm and the healthy arm.SM power loss distribution is unbalanced.A variable sampling frequency power loss balance control(VSF-PLBC)of MMC under SM fault is proposed.By adjusting the sampling frequency of the capacitor voltage balance control of the faulty bridge arm,the power devices between the faulty bridge arm and the healthy bridge arm can be effectively controlled.power loss to balance it.Simulation and experiments verify the effectiveness of the proposed VSF-PLBC.5.The temperature balance control method under unbalanced grid voltage is studied.The junction temperature distribution of power devices in the three-phase MMC under the unbalanced grid voltage is analyzed.The unbalanced grid voltage will cause the current unbalance of the three-phase bridge arm of the MMC,which will lead to the unbalanced temperature distribution between the three phases of the MMC.In order to improve the problem of uneven temperature distribution of MMC three-phase under unbalanced grid voltage,a temperature balance control based on variable carrier frequency is proposed,and the temperature balance of MMC three-phase is achieved by adjusting the carrier frequency of each phase.Simulation and experiments verify the effectiveness of the proposed temperature balance control strategy.