Evaluation of IGBT multilevel converters for transformerless traction applications

The low-frequency line transformer in todays ac rail vehicles suffers from poor efficiency and high weight. Future traction drives may operate directly from the mains without this transformer. A feasible concept for a transformerless drive system consists of series connected medium voltage converters applying modern HV-IGBTs. In a first design step, the switching characteristics and losses of the new 6.5 kV IGBTs are compared to 3.3 kV and 4.5 kV IGBTs which are already commercially in use for some time. Based on the considered HV-IGBTs, the paper analyzes the properties of multilevel converters and evaluates their applicability to the transformerless system. This includes a loss analysis, reliability aspects and harmonic spectra. Due to the reduced effort for additional passive components, the 3-leveI NPC converter turns out to be a reasonable solution to realize line and motor converter modules.