Railway industry has developed steadily to accommodate increasing transport capacity and demands for safety, speed, and eco-friendly transportation. Therefore, power supply capacity for electrical railway has increased to meet the increased traffic. Power supply must satisfy reliability and improved power quality.
However, high speed technologies, expansion and increased demand of electrical railway systems cause problems such as voltage drop, increase of voltage fluctuation and so on. The rating of electrical equipment should be considered in designing power supply system of electrical railway. In terms of voltage stability, minimum and maximum voltages at catenary line or at the pantogragh in cocmpliance with standardised criteria has determined whether system operation is possible at all.
Power conversion for AC electrical railway is performed by scott transformer that is located in railway substation. It has been changed three phases to two single phases with phase difference of 90 degrees. This system has some disadvantages. One of those, it causes voltage unbalance on three phases side in case of different loads and power factors on two single phases. Also, it has neutral sections to separate power supply sections because of two single phases with phase difference, so it effects on restriction of speed improvement and application of regenerative energy.
Recently, the use of power electronic converters instead of transformers or autotransformes has been increased due to developments of power electronic technologies. There is typically voltage sourced converter(VSC) for three phases to single phase. It enables to control active power and reactive power respectively. Also, it reduces harmonics by operating high speed switching and effects on downsizing of harmonics filter.
In this paper, as one kind of VSC topology, Modular Multilevel Converter(MMC) that is being studied for high power systems applies to AC electrical railway power supply system in a high speed railway substation. Also, this paper study voltage drop on catenary line using scott transformer and MMC. The modeling and simulation about two systems are performed using Matlab/Simulink.
The MMC can be achieved the voltage drop compensation in contrast with scott transformer and extension of substation spacing. Simulation results are shown to demonstrate the effectiveness of MMC.