Research On Negative Phase Sequence Problem Caused By Traction Power Supply System

With the rapid development of China's high speed electrified railway, traction power supply system will cause more severe power quality problems. These power quality problems mainly focus on two aspects: negative phase sequence (NPS) and harmonics. The NPS problem including its analysis, its transmission and its compensation is one of the most important topics for researchers.In order to evaluate the NPS problem caused by traction power supply system, this dissertation describes in detail how to design and develop integrative simulation software including traction calculation module and power system power flow calculation module firstly; provides the main structure of the simulation system; figures out how to realize the simulation calculation of traction substation's feeder. A comparative analysis on the existing methods of load flow computation for traction power systems is conducted. Considering the property of large R/X, the dissertation adopts PQ decoupled load flow algorithm to solve the traction power system via improving on the branch, and applies the algorithm in a practical system. The result of the calculation shows that this algorithm possesses good performance and can be effectively applied to solve NPS problem for AC electrified railway.In order to analyze NPS current caused by traction substation and reactive-power compensation capacity, a systematic method based on the magnetomotive force balance equations, winding connection equations, output-port equations and voltage transfer equations are adopted to analyze the operational performance of the single-phase, the wye-delta, the wye-prolonged delta, the Scott, connection transformers. The nodal admittance matrices of these transformers are set up. Meanwhile, unified mathematical model of non-symmetrical compensation is designed to balance NPS current caused by traction substation, which provides the theories leading for devising traction substation. Based on the wye-delta connection transformer, NPS current caused by traction substation and reactive-power compensation capacity are analyzed when reactive-power compensating devices are implemented in the primary or secondary side of the traction transformer respectively to correct power factor, which provides the theories leading for devising the compensation mode.Steinmetz circuit is effectively used for settling NPS problem caused by electrified railway, however which requires a quite large compensation capacity. Therefore proposed an optimal algorithm in this dissertation, its target is set to make three-phase compensation capacity lowest. By solving the mathematical problem with optimal method analytical results of the compensation capacity are obtained. Comparisons between the optimal results and that of the original Steinmetz circuit are made with MATLAB simulations, which demonstrate the advantages of the optimal design while meet the performance request. The calculation results show the system can be balanced by the optimal capacity with acceptable performance and less cost.