Current distortion evaluation in traction 4Q constant switching frequency converters

The four quadrant converter (4Q) is the actual best choice to supply the DC voltage link from AC power contact line. A typology of distributed power high speed electric trains having as input stages more 4Q converters [1] has been considered. The presence of more converters is necessary to guarantee a good redundancy in case of failure and gives the opportunity, using dedicated control logics, to interlace them in order to reduce the harmonic content of the absorbed current. The high power requested by the train for its acceleration in starting phase and for the auxiliary services needs high power converters that do not allow high switching frequencies. Consequently the absorbed current presents a high ripple value characterized by high harmonic current components that cannot be tolerated by the system. Indeed the track circuit used for signaling and communication for the traffic management and safety employs signal currents overlapped with the power ones. These currents can have low frequencies (50 Hz and 178 Hz) in the traditional signaling system, or they are in the audio frequencies range for the new European ERTMS/ETCS one. Therefore, the harmonics produced by the 4Q converter can disturb the communications of the track circuit, degrading the safety of the trains circulation. On the other hand the 4Q converter has the main benefit to give a nearly sinusoidal line current in both directions of energy flow and the mitigation of reactive power drawn from the line. In fact the 4Q converter is based on the use of forced commutation switches (GTO, IGBT) and presents a sinusoidal current absorption in phase with the contact line voltage. Moreover, this converter is intrinsically bidirectional and then it can be used both for traction and regenerative braking phases. The aim of this paper is an analysis of the current absorbed by the high speed trains through a suitable model of more 4Q converters. Thanks to a control logic applied in this work, it is possible to interlace two or more converters in order to evaluate the harmonic contribution of both single converter and the interlaced configuration. The simulation results obtained with an electromagnetic analysis will be presented