Power Quality Compensation Technology In Electric Railways Based On 27.5 KV Straight Hanging Cascaded APF

Due to the electric railway's special power supply mode and locomotives' complex,time-varying operating conditions,the traction load leads massive reactive power and harmonics.This causes the serious power quality problems of traction power supply system,such as low power factor,waveform distortion and voltage fluctuation.The problems not only increase power loss,reduce power utilization rate,but also cause serious damage to the security and stability of electric railway supply power system and electric locomotives,even cause the equipment damage,power supply and traffic accidents,leading country great economic loss.Therefore it is necessary to take measures to compensate and treat the electric railway's power quality problems.This paper proposes active compensation based on 27.5 kV straight hanging cascaded APF,and study some related issues.Firstly the AC side electrical characteristics of typical DC and AC locomotives are analyzed in the electric railway.Based on massive field measured data,the reactive power and harmonic characteristics are analyzed.In addition,based on field data from the traction substation,the amount of reactive and harmonic current of traction busbars is analyzed.All the analyses reflect the urgency of power quality improvement in the electric railway and offer basic data for active compensation system's parameter design.Combining Beijing-Shanghai high-speed railway's pilot project,the the existing power quality improving methods are summarized,and the straight hanging cascaded active power filter(APF)is proposed to deal with the power quality problems dynamicly,such as reactive power and harmonics.In typical traction power supply modes,such as 27.5 kV voltage mode with YNd11,single-phase Vv transformers,and AT supply mode with 2x27.5 kV and 55-kV supply voltage,the operation principle of cascaded APF is analyzed.Some other analyses have been done,such as APF structure configuration,fundamental and harmonic equalival circuits and current flowing paths.Cascaded APF's complex main circuit topology,the corresponding control system structure is also complex and contains heavy tasks.Therefor some current 'detection methods,assuming a little space and time resource,are analyzed,such as ip-iq algorithm,sliding-window iterative DFT algorithm and two-weighter adaptive interference canceling algorithm.For load fluctuation,an improved ip-iq algorithm and single-phase harmonic detection algorithm without PLL are proposed in this paper,improving the detection precision,response speed and avoid PLL's detection errorFor the cascaded multilevel converter,the voltage and current control strateges have been analyzed.,mathematical model for DC voltage control is drived based on transient energy balance,and then analyzes the parameter tuning process.For current tracking control,two comprehensive control algorithms are proposed,one is dead-beat and quasi-resonance instantaneous current control,and another is dq decoupling direct current control.Dead-beat control has specific mathematical formula,and current reference forecast function.And quasi-resonance control enables AC signal's no-error tracking.The proposed dead-beat and quasi-resonance instantaneous current control conbines their advantages and is fit for high-power APF.The dq decoupling direct current control treats the source current as control object directly and reduces the amount of analog AD sampling,which is especially suitable for multi-feeder system in electric railway.For DC voltage balancing control,based on the principle of vector reconstruction and pulse exchange,two DC voltage balancing control algorithms and their implement method are analyzed.Software simulation results verify the proposed control strategs,correctness.Finally,software simulation models are built up,considerating traction power supply system,locomotives' characteristics and distord traction network voltage.Based on the simulation result,a small-power experimental prototype has been established,and structure of overall function of digital control is designed,and the results verify the control system's correction,which has valuable guidance to the engineering practice.And this paper designs the overall control system funcation's frame,centralized digital control system.For 27.5 kV engineering APF,plenty of debugging and experiments have been done,and the experimental results verify the correction of main circuit,control board,control strategies,and reflect that the active compersating system has good reactive power compensation and harmonic ellimation performance.