Research On The Negative Sequence Characteristics And Compensation Method Of Different Traction Transformers

Electric railway is a heavily fluctuating, high-power and single-Phase unbalanced load. Up to date, there are still some issues remained to be solved, such as low power factor, a great amount of harmonic and negative sequence generated in the power system and so on. With the rapid development of electrified railway in recent years, more attention has been focused on the problems of low power quality due to electric traction. Despite the heavy use of AC-DC-AC electric locomotive significantly improves the harmonic and power factor, the issue of negative sequence becomes more and more severe with the sharply increase in traction power.In this study, we systematically investigate several equations about non-loaded voltage, load current, negative sequence currents and the primary or secondary side current relationship, and voltage outputs, which are related to several kinds of traction transformer commonly used in our country at present. Through our analysis, we show the negative-sequence influence of different connection traction transformer on the three-phase power system.Based on the measured data of Shenshuo heavy haul electric railway, the three-phase V/v traction transformer and Scott traction transformer are compared and discussed on the character of negative sequence current, which shows the actual effects of adopting balance transformers on mountainous heavy haul electric railway. In addition, we develop a method to estimate the system impedance and short-circuit capacity of the incoming line of substation based on the measured data, which depends on the traction load fluctuation remarkably and causes the large change of current and voltage in a short time. Along the Hangzhou-Ningbo passenger dedicated Xin-Shangyu substation adopts the new type with the secondary side of Scott connection transformer which extract in the midpoint. This new type transformer’s electrical characteristics are derived and do the authentication. By comparing with the adjacent Niu-Huiqiao substation, that uses the V/X connection type, we show the effect of this new type of transformer on the suppression of the negative sequence currents.In view of the negative current governance technology at present, the basic principle of static var compensator(SVC) and static var generator(SVG) are introduced, and the concept of cophase power supply is put forward. On the basis of reactive power compensation, we propose and develop a method of installing compensation device on system side that has been verified. It shows the necessity of compensation in order to reduce the negative sequence current caused by traction load.