| The conventional railway traction power system adopts single-phase power frequency AC power,which brings power quality problem dominated by negative sequence,power supply safety issues and train speed loss caused by neutral section.These problems limit the development of high-speed and heavy-load railways.In order to solve the problems above effectively,the 24 k V DC electrified railway is proposed as an alternative solution.In this article,the topology and control strategy of modular multilevel converter(MMC)type DC traction substation(TSS),the topology and control strategy of the traction drive system of DC electric locomotive,and the coordinated control strategy among the multiple parallelconnected TSSs are three issues that mainly focused on.Firstly,the topology structure of DC traction substation is given,which is mainly composed of step-down transformer and MMC.By analyzing of MMC’s mathematical model,control strategy,modulation strategy,suppression strategy of circulating current,and submodule voltage balance algorithm,the control scheme of DC TSS is realized.The simulation model of DC TSS is built in Matlab/Simulink,and the simulation results show that the DC TSS can output stable DC voltage,and the circulating current suppressing controller can suppress MMC internal circulation effectively,and the voltage of each sub-module can be stabilized near its rated value under rated operating condition,sudden loading changes condition and regenerative braking condition.Secondly,the topology and control strategy of the traction drive system of DC electric locomotive is analyzed.With reference to the structures of the existing traction drive system of various electric locomotives,a traction drive system composed of N dual active bridge(DAB)DC-DC converters with input-series and output-parallel,traction inverter and traction motor is proposed.The topology,operating rules,control strategy of DAB converter and the control strategy of N DAB converters with input-series and output-parallel are studied.And the simulation model of electric locomotive is built,the simulation results show that the locomotive can run stably under traction condition and regenerative braking condition.Finally,in order to realize the coordinated control among multiple parallel-connected DC TSSs in the system,a hierarchical control strategy based on the dynamic consensus algorithm(DCA)is proposed.The state space model of the system is established to analyze the stability of the system when the parameters of control system change or the locomotives are moving.The simulation model of 24 k V DC traction power supply system is built,and the simulation results demonstrate that the proposed hierarchical control strategy can achieve stable output voltage of each TSS and traction load power can be proportionally shared among the TSSs under the condition of load movement,sudden change,any fault of a TSS,and when trains are in regenerative braking status.The hierarchical control strategy enables each TSS to participate in the voltage regulation,and realizes that each TSS shares the load power in proportion to its rated capacity,thereby reducing the demand for the rated capacity of the traction substation,reducing investment costs.In addition,TSSs can realize "plug and play",which makes the system more flexible and reliable. |
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