Transformer used as a DC link filter inductance in DC high power traction applications

For multi-system locomotives that have to be able to work under different supply conditions (e.g. a locomotive able to work under both the 3 kV DC supply in Italy and the 15 kV 16.7 Hz AC supply in Switzerland) both an AC transformer/filter and a DC filter inductance are normally required. This paper describes the use of the secondary winding of the transformer, normally used when the traction drive is supplied by an AC network, as a filter inductance when the traction drive is supplied by a DC network. This choice allows to reduce the on-board passive elements. The possible instability problems created by the nonlinear characteristics of this inductance together with the high dynamic torque response of the drive have been addressed. A stabilisation algorithm to increase the DC link voltage damping characteristics of the systems is proposed