a power quality monitoring apparatus is developed for a railway power system . the apparatus is calculating the energy flow and the power consumption and recording the by using the voltage and current sensors at ac and dc power source in the various industrial fields . the voltage and current phases are detected for the ac power . the direction of current is detected to calculate the power consumption for the dc power . then , the detected data is analyzed . it is possible to monitor the variation of the voltages and current values of the railway vehicle in real time . it is possible to display the variations of the vehicle loads , the ac power output , the voltage and current value fluctuations , the power consumption and power flows in a form of the graphic or text format . thereby , it is possible to easily understand the overall status of the overhead line &# 39 ; s environment .