Transformerless multi-level converter based medium voltage drives

A new concept for transformerless medium voltage drives is presented, which avoids any common-mode inductors. By introducing a small capacitive filter on the grid side with a low-impedance ground connection at the star-point, a dominant common-mode voltage capacitive coupling to ground is achieved. No significant common-mode voltages or currents are reaching the grid in low- and high-impedance grounded systems. It is shown, that the additional common-mode stress on the medium voltage machine windings and related bearing current effects are well under control even for standard electrical machines and retrofit applications. The demonstrated concept can be applied to any kind of VSI based multi-level converter, as for example for ANPCML and MMLC converter solutions.