Analysis of Harmonic Amplification in AC Traction Systems Using LCL Filter with Active Damping

An AC train traction drive generates harmonics which are injected into the traction supply lines and can be greatly amplified if they are near a resonant frequency of the supply network. The paper analyses the line resonant impedance. The most important are series resonances defined by admittance poles whose frequencies, and residues, vary according to the pantograph distance from the end of the traction supply line. An LCL filter, supplemented with a virtual notch filter for active damping, is designed for harmonic current suppression, and applied in traction drives using either a two interlaced H-bridge rectifier or a three-level NPC converter. Simulation results for a traction drive moving along a realistic overheadline are presented showing that the scheme is most effective for the NPC-based train drive.