Research On A Novel Power Quality Conditioner With DC Microgrid For Traction Network

Traction network is one of the core components of electrified railway.On the one hand,due to the single-phase power supply mode and the non-linear,intermittent and impact characteristics of electric locomotive,the traction network has serious power quality problems,and effective compensation measures must be taken to realize comprehensive control.On the other hand,renewable energy generation,energy storage and microgrid have prominent advantages in energy saving and consumption reduction and have made great progress,but their application in electrified railway traction network is rarely reported.it is significant to combine the power quality regulator with dc microgrid and apply it to traction network.For the characteristics of high voltage,single phase and large capacity of electrified railway traction network,a novel power quality regulator with DC microgrid is proposed in this paper.DC microgrid converters all adopt three-level structure,and the DC bus side is connected to the power quality regulator.In order to realize the high voltage and large capacity of the power quality regulator,a hybrid cascade 81 level structure is proposed.In order to improve the dynamic performance of compensation,the PWM converter and 81 level converter are used in series.The working mechanism of each component is described in detail at first.For DC microgrid,four modes of three-level converter and the cause of unbalanced neutral-point potential are analyzed.The modulation strategy of three-level converter and the suppression method of neutral-point potential imbalance are studied.For distributed generation and distributed energy storage,their control schemes are proposed.The optimal scheduling model of energy management during DC grid-connected operation is analyzed.For hybrid cascaded 81-level converters,a stepped-wave modulation algorithm for calculating conduction angle by sinusoidal wave division is proposed.The relationship between the output 81-level step fundamental wave voltage and the DC bus voltage is deduced.By controlling the DC bus voltage,the fundamental voltage of the traction network is offset.Then,the power distribution of four H-bridges are deduced,and the advantages of the topology and modulation algorithm in improving power conversion efficiency are analyzed.For PWM converter,short-circuit impedance of transformer is used as output filter.The advantages of low voltage across the converter and improving dynamic characteristics of power quality regulator are analyzed.The operation method of instruction current based on phase-shifting construction method and the basic strategies of active power optimization and reactive power optimization are deduced.The command current tracking control method of PWM converter is determined.Finally,the simulation model of the novel power quality regulator with DC microgrid is built by using MATLAB.Simulations confirm that the scheme has not only excellent harmonic suppression and reactive power compensation ability,but also renewable energy power generation access and traction network peak-shaving and valley-filling ability when applied to electrified railway.