Research On The Theory And Application Of Large-Capacity Power Quality Conditioning System For High-speed Electric Railways

Electric railway traction supply system features much power quality problems such as negative sequence unbalance,harmonic distortion,voltage fluctuation etc.Though PWM rectifier equipped trains are characterized by good output current waveforms,high power factor,et al.,and are being used widely in electric railway industries,the high-speed large-capacity trend of electric trains makes some prominent problems,such as three-phase unbalance,voltage fluctuation,et al.,much worse.To relieve the bad influence of those problems,the grid voltage and capacity for new electric railways have to be enhanced.But this measure can not eliminate the negative sequence current.So,it is necessary to full study the prominent power quality problems in the high-speed large-capacity traction systems,and offer effective and applicable solutions.With the support of the National Natural Science Foundation of China under Grant 51477046 and 51377001,the Science and Technology Program of China State Grid Corp.under Grant 5216A014002,some new power flow controlling facility and systems are innovatively proposed,with the trend of the development of high-speed electric railway finely matched and the requirement of the large-capacity traction compensation properly satisfied.Some key technology issues,including system topology and structure,compensation strategy and method,current tracking and controlling scheme,system stability and compensation capacity,model simulation and experiment,and so on are analyzed and studied broadly.Specifically,the following aspects are emphasized:(1)Based on numerous measured data on two different real traction substations,the features and changing rules of some characteristic quantities in traction system like negative sequence unbalance,harmonic distortion,power factor and even regeneration energy,are comparatively analyze,which lays the base for the establish of power quality problems compensation schemes and the movement of system simulatin and experiment study.(2)A quadrature characteristic of YNvd connected balance transformer based partial compensation strategy,with the compensation goals of primary-side total power factor not less than 0.9 and negative sequence current unbalance ratio not lower than 10%,is proposed.This strategy features specific physical meaning,no complex optimization,and simple implementation.In this paper,the related mathematical expressions are deduced in detail,the implementation process is offered,and the capacity advantage of this compensation strategy is analyzed.Since the quadrature characteristic is shared by common balance traction transformers,so the proposed strategy is applicable to all kinds of balance transformer,and shows relatively high practical value.(3)A YNvd connected balance transformer based parallel + cascade type large-capacity power conditioning system is proposed.This system explores a low voltage taps from the secondary side delta-connected winding and offers the required interfaces for multiplex back-to-back H-bridge converters from two different sides with the help of a single-phase multi-winding step-down transformer.To coupling correctly,this system employs a new asymmetric structure with an interface of multiplex back-to-back H-bridge converter cascade connected and another series connected.The topology innovations and advatanges of this proposed system is analyzed in detail,and the concrete implementation process of current compensation method under this system structure is clearly illustrated.(4)A passive dissipation characteristics of general circuit based output current tracking method for converter controlling is proposed.Taking the YNvd connected balance transformer based parallel + cascade type large-capacity compensation system as a research object,the physical essence and implementation of this control method are discussed in detail,the stability issue of this controlling method considering the system parameter deviation and the LPF delaying characteristic in the real application is analyzed.Numerous simulation and experiment work is carried out for the study and validatation of the system circuit topology,partial compensation strategy,and the current tracking performance of this controlling method.(5)A shunt active + series hybrid type static reactive power generation system for high-speed electric railway co-phase power supply system is proposed.Different from the traditional back-to-back H-bridge converter based co-phase compensation systems,the proposed system can compensation reactive power of each feeder and balance the active power of traction transformer merely by reactive power generation,with the help of the unique series and parallel connection of converter circuit.Since no active power is coupled,the control of two converters can be implemented independently.Inspired by the hybrid compensation strategy,an ac capacitor is added in the coupling branch of hybrid converter to share the reactive power compensation capacity,which greatly reduces the compensation burden of the active part of this converter and much improves the cost-efficiency of the proposed system.In this paper,a detailed analysis on the compensation strategy and controlling method of this system is implemented,and the general designing method of ac capacitor is discussed.Based on the proposed topoloty and strategy,the system compensation effect on primay-side and the capacities of converters are evaluated.Simulation model is established and small-capacity prototype is designed.Based on the obtained experimental results,the compensation performance and operation characteristics of this system on different loading conditions are verified and analyzed.(6)Combining with the practical engineering requirement,a small-capacity principle prototype of Unified Power Manipulate Supply(UPMS)is developed.In this paper,the basic system structure and control method are introduced first.And then,the implementation of main function modules of system software is prominently illustrated.At last,the corresponding test results are discussed and analyzed.Based on the measured data and recorded waveforms of one real large-capacity UPMS engineering prototype,the compensation performance of UPMS is analyzed,which shows that after the adoption of UPMS,the power factor is improved,the negative sequence voltage and current unbalance ratio is suppressed,the main low-order harmonics are filtered,and the voltage fluctuation range of feeders are reduced.So the engineering application prospects of UPMS are bright.