Research On Control Strategy Of Modular Multilevel Co-phase Compensation Device

In order to solve the problems of harmonic,negative sequence,reactive power quality and electric phase separation in electrified railway,a new type of co-phase compensation device is adopted to realize co-phase traction power supply system.As an active compensation device for electrified railway,co-phase compensation device(CPD)is an important device to realize cophase traction power supply system.The application of modular multilevel converter(MMC)in the CPD can not only improve the system efficiency,but also reduce the volume and the cost of CPD.It is suitable for applications of high voltage and high power.Therefore,the control strategy of modular multilevel converter and co-phase compensation device(MMC-CPD)is analyzed in this thesis.(1)Based on the analysis of common active compensation devices in the co-phase power supply system,a new compensation device using three-phase two-arm MMC-CPD is proposed.The mathematical models of three-phase two-arm MMC-CPD are established in abc stationary coordinate system and dq rotating coordinate system respectively based on new topological structure.The working principle of the models is analyzed and the switching function is deduced.(2)The traction load current characteristics of AC-DC locomotive and AC-DC-AC locomotive are discussed.In order to compensate the power quality of cophase power supply system,the current of load and CPD must be measured accurately and punctually.A current detection method based on dq synchronous fundamental rotating coordinate system is adopted in this thesis.(3)Three modulation strategies are analyzed based on three-phase two-arm MMC-CPD topology which are carrier phase shift,carrier disposition and nearest level modulation(NLM)respectively.The control principle of each modulation strategy is analyzed,and the output voltage waveforms of AC side obtained by different modulation strategies are compared in the condition of open-loop.Finally,the appropriate modulation strategy is selected for closed-loop control of co-phase power supply system.(4)The control strategy of three-phase two-arm MMC-CPD is researched.In dq rotating coordinate system,the current tracking control of three-phase two-arm MMC-CPD is carried out by using vector proportional integral(VPI)controller.The selective harmonic compensation function of VPI controller can compensate the special harmonic of electrified railway.In order to simplify the controller with the increase of equivalent switching frequency in the converter,a direct current controller is used to track and control the current of three-phase twoarm MMC-CPD in abc static coordinate system.The direct current controller can control the voltage of two-phase in static coordinate system directly.In order to balance the capacitance voltage of the sub-module and restrain the circulating current of the arm,the corresponding control links are added to the two current tracking control strategies.Finally,two load models of AC-DC and AC-DC-AC are built in Matlab/simulink.The applicability of various control strategies for three-phase two-arm MMC-CPD and the effectiveness of power quality compensation are verified.