Grounding Concept and Common-Mode Filter Design Methodology for Transformerless MV Drives

Recently, the transformerless design of medium-voltage (MV) drives is an emerging design concept of variable-speed drives (VSD). Here, no dedicated feed transformer is used. The VSD is directly connected to the on-site facility transformer, where multiple other loads are connected as well. The benefits of such a transformerless VSD are the reduced total volume, weight, losses and system costs. In this publication, a suitable grounding concept and a generalized CM filter design methodology for transformerless MV drives is presented for the first time. Both the CM voltage injection to the grid and the CM voltage applied to the three machine terminals is considered. At the Point-of-Common-Coupling (PCC), the CM part of the grid harmonic standards can be met through a proper filter design. Since the converter-generated CM voltage could also cause issues on the machine-side, a novel design reference for the mitigation of bearing currents is applied. The introduced CM filter design methodology provides a suitable CM filter design for transformerless high-power medium-voltage converters. By this, the reliable transformerless operation of general purpose drives and retrofit applications is feasible.