Design optimization of power electronic transformers in traction applications

To increase efficiency and reduce weight of low frequency traction transformers (LFTs), many studies have been dedicated to the study and design of power electronic transformers (PETs). Most of these studies have focussed on 16.7 Hz traction systems and a few on 50/60 Hz systems. PETs offer several degrees of freedom to the designer such as configuration and modulation of power stages, switching frequency, magnetic material etc., which are evaluated based on given specifications to meet the objectives of maximum power density and efficiency. This paper presents a detailed analysis of efficiency and power density of each power stage in a PET with respect to its design parameters. A design methodology is outlined to replace an LFT with an optimally designed PET. To elucidate the design process, the considered case is a 4.16 MW transformer supplied by a 25 KV, 60 Hz catenary for a Shinkansen series-700. The configuration of the PET considered is of the indirect type with a cascaded H-bridge as the AC/DC stage interfacing the catenary and a set of parallelly connected DC/DC converters at the output. The best obtained efficiency is 97.7% with a weight of 1.69 tonnes which is significantly higher than that of the LFT.