Common-mode voltage limits for the transformerless design of MV drives to prevent bearing current issues

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. 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 VSD systems. Both mechanisms are caused by the converter-generated CM voltage applied to the machine terminals. Their physical understanding is used to define thresholds for the peak CM voltage and the maximum CM voltage slope to ensure the sufficient mitigation of bearing currents. The newly introduced CM voltage limits allow the development of a generalized CM filter design for transformerless VSDs which is the scope of another publication. By this, a reliable transformerless operation of general purpose drives and retrofit applications is feasible.