Optimal impedance termination of 25-kV electrified railway systems for improved power quality

Many single-phase 25-kV electrified railway systems today are loaded with conventional ac-dc thyristor-based locomotives, which draw current rich in harmonic content. The flow of this harmonic current through the feeder impedance can result in a distorted pantograph voltage waveform, which, in turn, can constrain the amount of power delivered to the locomotives. A possible solution to improve the supply quality is to connect a hybrid power-quality conditioner consisting of active and passive elements at the end of the traction feeder. However, if not appropriately designed, the installation of a power-quality conditioner can result in harmonic amplification at some points along the feeder. This paper now presents an analysis to study how the power-quality conditioner must function to implement an optimal impedance termination at the end of the feeder to achieve uniform harmonic suppression along the whole feeder length. The theoretical concepts presented have been confirmed in simulation and experimentally using a model traction system.