Simple active power filter

Abstract

Railway systems may be one of the best solutions for gas pollution. However, due to large motor load in the railway, power quality is badly affected. The reactive power increases the requirement of transmission line. Transformers need to be derated. Besides, loads with discontinuous current generate high frequency current harmonics, which interferes other loads. To solve this problem, the active power filter is introduced. It is connected in parallel with the load to shape the line current. The goal is to shape the line current to be in phase with, and the same shape as the line voltage. The active power filter is constructed by a half bridge inverter. The inverter acts as a boost converter to charge up the capacitor to a voltage that is higher than the peak line voltage. When performing current injection, the inverter acts as a buck converter and the current flows through an inductor to shape the line current. After measurement, the shaped line current’s THD is found to be about 4%. The efficiency of the filter is about 90%.