Voltage form factor control and reactive power compensation in a 25-kV electrified railway system using a shunt active filter based on voltage detection

AC railway traction systems are particularly susceptible to harmonic distortion. They are weak single-phase networks often loaded with severely distorting SCR-based locomotive drives which continually change their point of physical connection to the network. For a public distribution system the total harmonic distortion is a clear measure of supply quality. However, for a traction system the more important parameter is the voltage form factor, which is directly related to the maximum power a locomotive can deliver. To minimize this form factor, this paper proposes the use of a shunt active filter that minimizes the third, fifth, and seventh harmonic voltage components at the point of filter coupling. It is shown that the filter is effective in both compensating the pantograph voltage form factor, and providing reactive power to maintain the feeder voltage. The results are confirmed by detailed simulation using the EMTDC package.

I. Introduction

Many MAIN LINE electrified railway systems operate at 25 kV 50/60 Hz. Locomotives obtain their power from a single-phase overhead contact feeder, connected via feed transformers to the public utility system. Although the latest generation of locomotives uses ac traction motors powered through sophisticated pulsewidth modulation (PWM) ac drive control systems, many of the locomotives still in service today are based on dc traction motors. Fig. 1. Typical pantograph voltage waveform at the end of a 30-km contact feeder. These older types of locomotive, which are expected to still be in service for many years to come, use thyristor-based rectifier converters for speed control and, hence, not only draw a significant amount of lagging load current at fundamental frequency but also inject significant levels of harmonic current back into the overhead feeder.