Research On Power Quality Management Of Electrified Railway

Electrified railway is an indispensable means of transport in the new era of transportation system.The railway network of our country is spread all over the country.Compared with traditional diesel locomotives,electric locomotives cause much less pollution to the environment.However,with the continuous increase of our country’s electrified railway operating miles,the accompanied harmonic and reactive power problems have brought different degrees of influences to people’s production and life,as well as social development.In this paper,based on the grid operation,maintenance,and diagnosis analysis projects of the Gansu Qingyang branch of the State Grid in 2019,we propose to adopt Activate Power Filter(APF)and State Var Generator(SVG)to deal with the harmonic and reactive problems of the electrified railway comprehensively.Based on the mathematical models,we analyze the principles of these two problems,and carefully study the parameter design method for their DC side capacitance and various components connected to the system.Besides,we also focus on analyzing and researching on the harmonic and reactive reference current detection and compensation current tracking control.In terms of reference instruction current detection,the traditional i_p-i_q detection algorithms require delay to construct virtualβ-phase.This kind of operation,together with coordinate transformation,will lead to large time delay and memory requirements.In addition,the inherent defects of the low power filter(LPF)always lead to low detection accuracy.To solve the above problems,we propose an improved i_p-i_q detection algorithm,which needs no coordinate transformation,and adopt a notch filter to filter out the double frequency fluctuations in the output DC signal of LPF.In addition,the phase-locked loop in traditional i_p-i_q detection algorithms has poor phase-locked performance when the system voltage fluctuates or distorts,or the system frequency shifts.To solve this problem,we adopt a non-phase-locked loop reference command current detection algorithm,and exploit the integral method to obtain the low-frequency signal required in the detection method,to improve the accuracy of the reference command current as much as possible.The simulation tests demonstrate that the non-phase-locked detection algorithm has more superior performance indexes than the i_p-i_q detection method,such as shorter time delay and higher detection accuracy.In the aspects of current tracking and control,in APF,the traditional proportional integral(PI)controller cannot track alternating current(AC)harmonic signals without static error.What’s worse,the quasi-proportional resonance(QPR)controller must be used in parallel if it wants to control multiple AC signals simultaneously,which always makes the control system very complicated.To solve the above problems,we propose to use QPR to control the third harmonic current separately,while use the PI controller to control other harmonic currents uniformly.In this way,the difficulty of control will not be increased while improving the accuracy of the compensation current.As for SVG,we adopt the hysteresis control in DC current,which is easy to understand and implement.In addition,to make reasonable use of the compensation capacity,we exploit a cooperative control method to optimize the configuration of the reference instruction current,and obtain the compensation capacity needed to suppress harmonics and control reactive power by controlling the main circuits of APF and SVG.We establish the system model in the Matlab/Simulink platform and set four different working conditions,including taking no measures,only investing in APF,only investing in SVG,and investing both APF and SVG in cooperative control.The simulation results demonstrate the feasibility and superiority of APF and SVG in solving the harmonic and reactive power problems under cooperative control mode,as well as the effectiveness of the control strategy proposed in this paper.