The Research To Improve Power Quality Of 220kV Direct Traction Supply System

220 kV direct traction supply system has many advantages, such as high reliability, big capacity of system short circuit, steady voltage, less phase of railroad contact network, safe, etc. It is an important development direction of our country electrified railway. However, electric locomotive is the high-power single-phase rectifier load which has the problems of harmonics, negative sequence current and low power factor. The problems are seriously hindering the development of electric railway and influencing the power quality of ordinary users, and are unfavorable to the development of society economy.Although some of the traditional measures to improve power quality of traction supply system have certain effects to harmonic control and reactive power compensation, but they have drawbacks which make us work hard to develop more effective method to improve power quality. This paper is for this purpose.First, this paper analyzes the traction supply system and electric locomotives. This paper introduces the 220kV direct traction supply system detailedly and analyses its characteristics and shortages based on the principle and components of traction supply systems. It also analyses the reasons that why the problem of power quality are caused, and introduces harms and improve measures of power quality.Secondly, this paper proposes a new scheme to improve power quality of traction supply system based on the technique of active power filter to improve power quality. Active power filter that its theory has been developed for a short time will be widely used in the future along with the rapid progress of electric technique seen from its principle, circuit topology and control mode. Thus, this paper proposes a new scheme to improve power quality, introduces its circuit designed method and control method detailedly.At last, this paper builds the model of the traction supply system and other electric devices, simulates and analyses them in the use of power system block set of MATLAB software. The parameters of different model are compared after simulation to testify that the new scheme has obviously effect to improve power quality of traction supply system.It has a significant sense to improve power quality of traction supply system, is helpful to the safe, stable and economical operation of traction supply system and power system.