Resonant link inverters for traction applications

The replacement of traction motor-alternator (MA) sets by the static power converter as a source of auxiliary power meets many of the requirements in terms of reduced maintenance and increased flexibility. However, these power converters are usually based on conventional `hard-switching´ technology and tend to be larger than equivalently rated MA sets. Further, the quality of the generated waveforms is limited by the low switching frequencies. In traction applications, harmonic generation is not just a problem of technical refinement, it is also safety related because of the effect of traction generated harmonics on signalling systems. The constraint of having to use low switching frequencies can be traced to the high switching loss of `hard-switching´ power converters. `Soft switching´ is an emerging technology which addresses some of the problems of conventional power converters by reducing the switching loss to very low values. Here, the two methods of switching are compared by the authors