Comparison of Voltage Distortion Impacts from High-Speed Railway Systems Connected to Electrical Grid Under Different Special Transformers

High-speed trains in Thailand use AC traction system with 2×25 kV and 50 Hz power system. In traction system, it consists of power electronic devices; therefore, trains can be considered as moveable sources of harmonics in power system. For supplying electrical power to trains, three-phase high voltage (HV) to two-phase medium voltage (MV) by special transformers has been used. This paper focuses on developing 2×25 kV highspeed railway (HSR) system consisting of catenary system, highspeed rail loads and various kinds of specially connected transformer models. Three types of specially connected transformers, i.e. V-V transformer, Scott transformer and Modified Woodbridge transformer, are modeled and verified by DIgSILENT PowerFactory software. The impacts on voltage distortion from HSR system connected to utility grid at point-of-common coupling (PCC) with different types of special transformers are analyzed and compared. It is found that harmonics currents can have different impacts on utility grid by different specially connected transformers. Selecting an appropriate range of short-circuit power from an early stage of construction of high-speed railway until future expansion of high-speed rail loads is crucial to evaluate the total life cycle cost of the developed HSR project.