Studies Of The Comprehensive Compensation Schemes For Traction Substations

Nowadays, the electrified locomotives have an extremely rapid development all over the world because of its great advantage compared to internal-combustion engine. But this type of load during its operation generates tremendous negative sequence and harmonic currents which flow in the power system causing voltage, current asymmetric and wave distortion. This makes the power quality become worse. At present, with the development of electrified railway, the pollution engendered by the electrified locomotive is becoming more and more serious, even threats the safe and economic operation of the power system. Hence, it is a significative subject to study the comprehensive compensation of traction substation.Firstly, the harm to the power system and customers caused by electrified locomotives due to their characteristics of lower power factor, negative sequence current and harmonic currents is analyzed in the thesis. Secondly, the main several comprehensive compensation schemes of traction substation are introduced and evaluated. Finally, based on the widely used transformer with Yndll connection in traction substation, intensive observation is made on several compensation plans according to different conditions.In order to deal with the pollution caused by electrified locomotive, its characteristics and rule of change must be mastered. In this thesis, the configuration of several main types of electric locomotive and harmonic currents generated by them are illustrated. Based on the analysis of the loading currents distributed in each phase winding of traction transformer, the corresponding formulas are deduced, these formulas can define how many negative sequence and harmonic currents swarm into power system from the transformer's high voltage side and how these change versus the loads. In order to assay the result of different methods, the relevant parts of National Standards of Power Quality are given, too.Apart from filtering, filters can provide some fundamental reactive power to the power system. Hence, a filter could be designed to increase power factor of traction substation and decrease the harmonic currents. The configuration and principle of single-tuned passive filters are introduced, the basic parameters and relevant formulas are listed for the filter determination. Then a mathematic model for the filter optimization design is set up. The objective function is to minimize the total investment with qualified power factor and filtering rate. This is a nonlinear optimization problem. In this thesis, the extended Lagrange multiplication technique is applied. More over, the step-acceleration method is introduced to deal with nonlinear problem without constraints. As an example, the filter optimization design for traction substation is carried out using the C++ Builder 5.0. The designed filter works well. With the power factor satisfied, the harmonic currents are reduced to be lower than the required limits in the National Standards of Power Quality.With the installation of filter on locomotives and the development of frequency converter, harmonic currents caused by the electric locomotive have been relieved. With the loads of locomotives increasing, the problems of negative sequence are becoming more and more noticeable that are ignored when traction substation is comprehensively compensated. In order todeal with the problem, an two-phase optimization compensation plan is put forward in this thesis. It is to increase power factor and minimize the injected negative sequence currents. After the analysis of the characteristics of loads in the two power supplying arms, power factor, as well as the voltage loss in each phase of traction transformer, a conclusion is achieved that it is reasonable to over-compensate the lagging phase and less-compensate the leading phase. According to this analysis a mathematic model is set up aimed at minimizing the negative sequence currents with the constraints for power factor and each phase voltage considered, and the compensation capacity of each phase is determined. F...