Research On Submodule Condition Monitroing Technologies For Modular Multilevel Converters

The Modular multilevel converter(MMC)was proposed by Rainer Marquardt in 2002 and it has unique advantages in terms of scalability,power quality,fault-tolerant control,and control flexibility.The MMC has attracted a lot of attention in the field of direct current transmission and it was widely studied and adopted for practical application.With more and more flexible DC transmission systems put into operation,the reliability of the system has gradually attracted the attention of the industry and academia.Submodule(SM)is an important part of MMC.In order to ensure the reliable operation of MMC,it is of great value and significance to study the condition monitoring technologies of the SMs in the MMC.The thesis of this dissertation focuses on the SM condition monitoring technologies of MMCs,which studies the SM power device open-circuit fault detection,SM capacitance monitoring and SM capacitor voltage estimation.The simulation studies and experimental studies are carried out for verification.Theoretical basis and practical support is provided for achieving reliable operation of the MMC.The main research contents of this dissertation include the following parts.1.An open-circuit fault detection method of SM power devices is studied based on the peak difference of capacitor voltage.The mechanism that the capacitor voltage of the faulty SM with lower switch opencircuited is close to the capacitor voltages of the normal SMs when the MMC works in rectifier mode is revealed.And the SM capacitor voltages in the MMC with the faulty SM is analyzed,where the MMC works in rectifier mode.A SM power device open-circuit fault detection method is proposed based on the peak difference of the SM capacitor voltages.The effectiveness of the proposed SM power device open-circuit fault detection method is verified with simulation results and experimental results.2.An open-circuit fault detection method of SM power devices is studied based on the proposed rearranged bleeding resistor circuit(RBRC).The fault characteristics of MMC under light load condition is analyzed and the mechanism that the capacitor voltage of the faulty SM is almost consistent with the capacitor voltages of the normal SMs is revealed when the MMC operates under light-load condition.A SM power device open-circuit fault detection method is proposed for rapid diagnosis of the SM power device fault based on the rearranged bleeding resistor circuit,which compares the digital output of the optocoupler of the RBRC with the switching state of the SM.Experimental results verify the effectiveness of the proposed fault detection method based on the RBRC.3.A capacitance monitoring method is studied based on the fundamental frequency Fourier coefficient.The magnitude of SM capacitor voltage and capacitor current in different frequency components is studied,and the relationship among SM capacitor voltage,capacitor current and capacitor value is analyzed.A capacitance monitoring method is proposed based on the fundamental frequency Fourier coefficient.The immunity ability to sensor noise is analyzed for the proposed method.The effectiveness of the proposed capacitance monitoring method is verified by simulation results and experimental results.4.A capacitance monitoring method is studied based on the reference discharging time.The effects of the SM auxiliary power supply and the sensors’ system error on the SM discharging time are respectively analyzed.A capacitance monitoring method based on the reference discharging time is proposed,where the capacitance is monitorted based on the SM discharging time and SM capacitance measured in the reference state and the SM discharging time measured in the online state.The effectiveness of the proposed capacitance monitoring method based on the reference discharging time is verified with simulation results and experimental results.5.A dual half-bridge SMs(DHB-SMs)based MMC and its capacitor voltage estimation method are studied.The topology of the DHB-SMs and the DHB-SMs based MMC are proposed,where each DHBSM has two half-bridge units but only one voltage sensor.The relationship between the sensor voltage and the switching state of the DHB-SM is analyzed,and the corresponding capacitor voltage estimation method is proposed.The sampling of the sensor voltage is synchronized with the carrier of the DHBSM.And the voltages of the two capacitors in the DHB-SM are estimated based on the sensor voltage sampled at the top and bottom of the carrier.Compared with the conventional half-bridge SMs based MMC,the DHB-SMs based MMC can reduce the number of the voltage sensors by half.The effectiveness of the proposed capacitor voltage estimation method is verified with simulation results and experimental results.