Electrically safe variable speed drive for underground mining applications

Common-mode-voltage (CMV) is a well-known source of bearing currents and electromagnetic interference (EMI) in industrial applications. In underground earthing and protection environment, CMV is associated with potentially undetectable fault currents, due to their diversion away from the protection circuit. An ideal solution to these problems would be such a VSD that creates minimum or zero CMV and removes the alternative paths for the fault currents. Previous study performed by the authors has suggested that, amongst the existing drive topologies used by the industry, a 2-level Inverter with Active Front End (AFE) would be ideally suited for CMV elimination. This paper develops a control strategy for the VSD based on AFE Inverter such that it eliminates CMV, minimises EMI and optimizes harmonic distortion at the drive input and output sides. To validate the proposed strategy, a laboratory prototype VSD has been developed and the test environment has been set up to represent an underground mining electrical system. Experiments have confirmed that the prototype VSD generates practically no CMV, thus eliminating the EMI problem. Under an earth fault, the fault current flows into the designated protection circuit, thus providing the required electrical safety of the proposed VSD.