A hybrid compensation method for electric railway´s power quality improvement

Electric railway brings many power quality problems to the public power system. In this paper, a new power quality improvement method is proposed which can alleviate the voltage unbalance and low power factor problem of electric systems. The method is composed of a specially connected three phase to two phase transformer and a phase balancing device. The specially connected transformer has the following common characteristic: secondary voltages are equal in magnitude and differ 90 degrees in phase angle. Such transformers include Scott Transformer, Le-Blanc Transformer, and modified Wood-bridge Trans former and so on. The phase balancing device consists of two parts: one is used to regulate the power factor to unity; the other part is to transfer power flow between the two feeders, thus balancing their loads. With the proposed scheme, negative current can be eliminated and power factor can be regulated to unity. The advantage of the new method is that it is very simple and cost-effective. Validity of the proposed method is verified by simulation results. The capacity of the compensation device is also analyzed in the paper.