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Grounding Concept and Common-Mode Filter Design Methodology for Transformerless MV Drives to Prevent Bearing Current Issues

Recently, the transformerless design of medium-voltage 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 reduced total volume, weight, losses, and system costs. However, without a dedicated feed transformer, the converter-generated common-mode (CM) voltage may generate harmful bearing currents that could yield serious bearing failures within a short time. For this reason, the suppression of bearing currents is a fundamental design aspect of VSD systems. In this publication, a comprehensive bearing current study highlights the two dominating bearing current mechanisms in transformerless VSDs. Both mechanisms are caused by the converter-generated CM voltage applied to the machine terminals. Their physical understanding is used to define thresholds that ensure a sufficient mitigation of bearing currents. The newly introduced CM voltage limits allow the development of a generalized CM filter design for transformerless VSDs. Accordingly, a suitable filter grounding concept including a CM filter design methodology for transformerless high-power medium-voltage converters is presented for the first time. Here, both the CM voltage injection to the grid and the CM voltage applied to the machine terminals is considered. At the machine-side, the novel design reference to prevent bearing currents is applied. By this, the reliable transformerless operation of general purpose drives and retrofit applications is feasible.