Grid-connection Control For Multiple Traction Substations Based On Five-level Three-phase To Single-phase Converter

With the increase of engine load capacity,the problems of reactive power,harmonic and three-phase unbalanced increasingly prominent in the current railway traction power supply system.And,the existence of crossing neutral section makes the locomotive running speed and power quality reduced.The advanced co-phase traction power supply system based on three-phase to single-phase converter as a new traction power supply mode,the three-phase to single phase transformation was achieved by controllable power electronic devices.This power supply system can cancel the neutral sections thoroughly and the traction network can be linked up completely.Therefore advanced co-phase traction power supply system has received the widespread attention.This thesis mainly studies grid-connection control of traction substations based on diode clamped five-level three-phase to single-phase converter,the feasibility of control strategy is verified from the aspects of theory,simulation and experiment.Firstly of all,the paper studied the topology structure and the mathematical model of three-phase five level diode clamping rectifier,and based on the mathematical model of the port voltage,three-phase dq decoupling control strategy is adopted;Besides,the topology structure and mathematical mode of single phase five level diode clamping rectifier is analysised,the single-phase pq decomposition control strategy is adopted according to the control expression,the control can regulate the output of ac-side reactive power;the control strategies of dc-side capacitor voltage balancing and the modulation strategy of three-phase to single-phase converter are studied.Secondly,the paper researches the grid-connected topology structure,the hybrid grid-connected system based on the controlled voltage source inverter substations and controlled power source substations is presented.Traction network impedance is calculated and the influence of traction network impedance on power source output voltage is also analysised;The circulation characteristics between two substations are researched;What's more,An improved grid-connected control strategy applies to this system is presented base on the thought of droop control,and the controller is designed.Compared with the traditional droop control scheme is limited by its fixed coefficient and unable to adjust the power output of substation quickly,the scheme can real-time monitoring voltage phase and amplitude in the junction of the traction substation and traction network according to the change of locomotive load,the required reference value of the active current and reactive current are getted,so as to control the power output of traction substationsOn the basis of research mentioned above,Two and three substations are simulated respectively based on Matlab/Simulink.The simulation results show that this control strategy can real-time control power output of each substation and keep the output voltage of each substation has the same amplitude,phase,frequency no matter the locomotive load putted into or removed,traction substations realize grid-connected completely,while,the system has good steady-state and dynamic performance.Finally,a low-power,low-voltage experiment platform based on Field Programmable Gate Array controller is designed,completing the three-phase and single-phase five level diode-clamping rectifier experiment,three-phase to single-phase system grid-connected experiment,two substations grid-connection experiment respectively.The experimental results verify the rationality and feasibility of main circuit topology structure,three-phase and single-phase control strategy and grid-connection control strategy.