Research On Energy Storage-type Power Quality Control System For Electrified Railway

Nowadays,due to the continuous increase of railway networks layout and traction power in China,railway energy consumption increases sharply and power quality problems become serious.To improve the energy supply-demand imbalance and the power quality problems,how to availably solve the problems of negative sequence(NS),considerable regenerative braking energy(RBE)and high load peak power in traction power supply system(TPSS)has become the focus of current research.Traditional solutions have shortcomings such as large compensation capacity and complex control strategy.At the same time,different from DC TPSS such as subway and light rail,AC TPSS has its own power quality problems,which adds difficulty to the recovery of RBE.For the problems and characteristics of traction load and its power supply system,this paper makes a comprehensive exploration in principle analysis,parameter design,control method and energy storage system(ESS)configuration,so as to provide research ideas for achieving low-cost improvement and high efficiency recovery of RBE.Firstly,the evolution history of electrified railway is briefly described,and the power quality problems and hazards existing in TPSS are illustrated.Then the existing power quality improvement technologies are summarized.Finally,the research focus and significance of an energy storage-type power quality control system(ES-PQCS)for electrified railway are studied broadly.Based on the measured data of a substation in Hunan and the dynamic adjustment test data of single train EMU,the basic characteristics and internal laws of traction load and its TPSS are fully excavated and summarized.Then the unbalance degree of voltage/current on the primary side of traction transformer(TT)is analyzed,and the negative sequence inhibition ability of balance TT and non-balance TT(Y/Δ,V/v)is compared.Besides,the different degree of harmonic pollution of electric locomotive under different locomotive types and driving conditions is introduced.Furthermore,the power factor of the primary side,the voltage,the peak power and the RBE of the traction side are counted and evaluated.Finally,the intuitive trend chart is given,which points out the direction for the safety prevention and control of the power quality of the substation,and provides a realistic basis for the system simulation.Aiming at the problems of large design capacity,complex control strategy and low energy utilization rate of traditional power quality improvement technologies,an ES-PQCS with partial compensation strategy was proposed.Based on the typical topologies of railway power conditioner(RPC)and bidirectional DC converter(BDC),the system topological structure and mathematical model of the system are analyzed,revealing the relationship of compensation current with the primary power factor(PF)and the load current.And the mathematical descriptions of active power and reactive power compensation are given.Besides,the working principle of full compensation and partial compensation are compared,the active power flow and power compensation under railway operation scenarios are discussed in detail,and the power-flow management scheme with 11 classic cases is listed.Meanwhile,the negative sequence inhibition ability and RPC volume effect of the strategy are evaluated.Finally,the common control strategies of RPC and BDC are summarized,and the integrated control strategy of ESS-RPC based on passive control is proposed.In view of the singleness and poor economy of capacity configuration of ESS,a capacity configuration of ESS based on economic benefit optimization was proposed.Starting from the working mechanism of ESS-RPC,the paper describes the load change by monthly load power curve,and reveals the interaction between ESS capacity and load power.Taking initial investment,productivity benefit,operation benefit and other economic factors into consideration,an optimization model with peak power regulation and recovery RBE as the leading scenarios was established with the objective function of maximizing comprehensive economic benefit.At the same time,the solution method and algorithm flow of ESS capacity optimization based on particle swarm optimization algorithm are presented,which provides a more economical scheme and theoretical basis for the engineering application of energy storage-type electric railway.Finally,the MATLAB simulation model is established by referring to the measured data of traction substation,and the feasibility and effectiveness of the proposed compensation strategy and power-flow management scheme are verified.The simulation results show that the ES-PQCS can carry out the scenarios conversion,mode optimization,compensation for power quality problems and braking power recovery,showing the capacity-saving effect and utilization rate of the RBE.