Input-series-output-parallel connected converter with single-phase UPF buck-rectifier

Traditionally, in ac-supplied traction systems the available catenary voltage, 25 kV/50 Hz; 16 kV/16.7 Hz is stepped down by a line-frequency transformer, before the final rectification stage. This results in large volume and weight, as well as poor efficiency and lack of redundancy in the system. To obviate this transformer, some medium rated converters are connected in input series and output parallel (ISOP). ISOP connection of converters not only provides a light weight solution, but also provides greater reliability through redundancy due to its modular nature. In spite of low output voltage requirement for traction drives (0.5-3 kV), conventionally source current waveshaping employs active boost-rectifiers. This results in higher module count. Considering the module count and requirement of high source current quality, a new converter topology based on buck-rectifier is proposed which is amenable to connection in ISOP configuration. The control objectives, viz. upf operation at input, transient stability, equal power and input voltage sharing are achieved through a single loop controller structure. Analytical results are validated by numerical simulation and experimental results with a 2 kW (2 × 1 kW), grid connected, laboratory prototype.