| The Sichuan-Tibet Railway is a world’s first electrified railway built on the plateau with an altitude of more than 4000 meters,the demand of huge traction power makes a strict requirement on the power supply ability for traction power supply system(TPSS).However,due to the fragile power system condition,the severe power quality problems represented by voltage unbalance(VU)have attracted widespread attention,which have interfered with the safety,effectiveness,reliability of railway and power grid.Therefore,to address the VU problem aroused by the TPSS of Sichuan-Tibet railway,based on the existing structure of power system,the main objective of this paper is focuses on the propagation mechanism,assessment method and compensation technique.The thesis includes the following.At first,based on the port general conversion approach of TPSS,the positive-and negative-sequence equivalent impedance model considering the exchange phase connection and the power supply mode of traction network is established,which uses to analyze the impact of TPSS’s special structure and operation characteristic on the impedance distribution.Moreover,selecting the VUF/CUF,equivalent impedance as the input variables,the VUF calculation model is built to research the impact of various external grid connection access methods and operating characteristics of traction load on VUF of PCC,as well as illustrate the transmit characteristic of unbalanced power flow aroused by TPSS in the network.Secondly,taking the weak segment of power grid for Sichuan-Tibet railway section as an example,a general simulation model ”power grid-TPSS-traction load” is established by the DIg SILENT/Power Factory platform,and the based on the VU assessment standard GB/T15543-2008 and DL/T 1375-2014,the distribution feature of VUF for the traction substation group(TSSG)is quantitatively analyzed,which considers the different external grid connection access methods.At the same time,the VUF contribution of each traction substations for PCC are analyzed.Moreover,we set up the target of the lowest compensation capacity and number of neutral sections,a multi-objective optimization method constrained by VU limit is presented,and then a optimal scheme is obtained.Finally,based on the co-phase power supply technique,a novel VU compensation system is proposed which adopts to the special structure of TSSG for Sichuan-Tibet railway,the working principle and the power flow distribution relationship for co-phase TPSS is illustrated which utilizes the IEEE Std.1459-2010 power theory.Adopting the Instantaneous power theory to establish a VU compensation strategy in which set VU limit as the compensation target,and the calculation approach of compensation current of each port for co-phase TPSS is deduced.Taking the technical scheme of the typical section for Sichuan-Tibet Railway as an example,based on the MATLAB/SIMULINK platform,the correctness and validity of the proposed strategy is demonstrated by the static and dynamic load simulation. |
