Locomotive drive harmonic pollution in AC traction systems

AC railway systems operating at high voltage (20, 25 or 50 kV) and network frequency (50 or 60 Hz) are currently the best choice for new track design, thanks to their reliability and efficiency. However, AC railway systems are connected to the national power grid by single phase power transformers and consequently the three phase voltage vectors on the public system become unbalanced. Moreover, AC locomotive electronic drives generate a current harmonic distortion which heavily affects the network voltage waveform. A study of the harmonic evaluation methods has been carried out to choose the best methodology for this application. On the basis of this study, a deterministic approach has been carried out: a time domain analysis of an AC traction system has been developed in order to evaluate the amount of unbalance and of harmonic pollution in the mains power supply network; this analysis allows one to verify the observance of the IEEE 519 and IEC 1000 EM recommendations and standards. Simulation results are presented and discussed and some considerations for optimized harmonic reduction techniques are proposed.