Research On Effect Evaluation And Governance Of The Power Quality In Electrified Railways Accessed To The Power Grid

The electrified railway has many advantages, such as large transport capacity, low energy consumption, and high speed. With the rapid development of electrified railway, its impact on the power grid is becoming more and more concerned. The safe operation of power system is always affected by some productions of electrified railway such as harmonic currents, load fluctuation, negative sequence currents. Aiming to meet the relevant national standards in the regulation of electrified railway power quality, some research should be done on the the harmonics and negative sequences caused by the electrified railway and their corresponding influences on the power quality of power system. It’s of vital importance for both the railway and the power department to study the management schemes and measures to be taken.In this paper, the significance of electrified railway research is analyzed; the development status of electrified railway power quality research, and current governance methods and cases at home and abroad are summarized; the assessment program of electrified railway power quality is introduced; the related national regulations of electrified railway harmonics and negative sequences are listed. As the main components of electrified railway, traction transformers and electric locomotives are analyzed and simulated respectively. Their characteristics and harmonics and negative sequences caused by them are analyzed. The paper lays a foundation for assessing and managing electrified railway power quality.Based on the project "Research on evaluation and treatment technology of large shock loads and electrified railway access power system" of a power supply company, in this paper, we analyze power quality of five electrified railways and fourteen traction substations in a city. Since the city has a large-scale power grid, the simulation time is quite long. So the equivalent simplification of the outer part of the boundary nodes is needed, so as to reduce the simulation scale and improve the simulation efficiency. Software PSCAD/EMTDC is used for building circuits of traction systems with symmetric loads and asymmetric loads; power quality problems such as harmonics and negative sequences caused by traction systems are researched. The assessments of power quality show that when there are large loads in the electrified railway, the grid currents are quite unbalanced, especially when the loads are asymmetric. The corresponding negative sequence currents cause great loss in electrical equipments, and damage equipments. As a result, Harmonic currents and unbalancd voltage in a particular case exceed the national standard limits. Power quality problems in case are very serious, so the measures are needed to be taken.Static var compensator (SVC) can provide adjustable reactive power by dynamically controlling the angle of thyristors, so it can decrease the unbalanced degrees of current and voltage, correct power factor, and improve static and dynamic stability. In this paper, SVC is adopted to manage harmonics and negative sequences in the power system. In the electrified railway, SVC can be installed in the low-voltage side of traction substation, the high-voltage side of traction substation, or the electric railway power supply side of substation. Different installation positions result in different compensation effects. The installation position of SVC should be determined according to actual conditions and compensation objectives. This paper elaborates management effects on harmonics and negative sequences when using SVC from the theoretical research and simulation. Then four schemes for SVC are proposed, by comparing the governance effects of four schemes simulations, we come to the conclusions that when SVC is installed in the traction arms, the governance effect is the best, and FC accessing voltage level is low, insulation requirements are low, voltage loss of traction system is more conducive to compensate. When power grid is disturbed, with the ability of regulating reactive power fast and continuously, SVC can effectively improve the stability of power grid increase stability of the voltage, improve transient stability of power-angle, reduce the voltage deviation and increase the transmission capacity of the system. As a result, the electrified railway is becoming more stable and secure.