Multilevel Traction Converter Topology With Medium Frequency Isolation

This paper presents a converter configuration for railway traction application with the bulky grid frequency transformer replaced by a medium frequency transformer. The conversion stage on the primary side of the transformer is based on an existing modular multilevel converter (MMC) based topology. For the secondary side, an ac-ac conversion stage is proposed for interfacing the transformer with the traction motor. This stage is obtained by connecting two three-phase two-level inverters and it generates multilevel voltage waveform on the transformer as well as the motor side, thus ensuring low dv/dt stress on the windings. The motor side voltage of the proposed secondary side converter is generated using the voltage references from the motor controller. However, for generating the transformer side voltage, no additional voltage reference is required; it is generated by utilising the switching common mode voltage of the two inverters. The topology and the control scheme are explained and are supported by simulation results.