Electrically Safe Variable Speed Drives for Underground Mining Applications

Common-mode-voltage (CMV) is known for creating bearing currents and electromagnetic interference (EMI) in industrial applications. Additionally, in underground earthing and protection environment, CMV has important safety implications. These are associated with potentially undetected 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. To find the best solution, this project performed a comparative evaluation of several VSD topology alternatives. They were extensively studied by simulation and then the most appropriate topology was selected, namely, the Inverter with Active Front End (AFE).

This is one of the standard topologies, already actively used by the industry. However, CMV suppression is not included in any of the options available on the market. To fill this gap, a CMV suppression strategy is developed in this project.

The approach taken in the project was to not require any hardware changes and to only modify the software code that controls the switches. To allow for an easy upgrade of the existing drive controllers, the new code was developed in the form of an optional Add-on to the standard code. To validate the strategy, a laboratory prototype VSD was developed and the test environment was set up to represent an underground mining electrical system. Experiments were conducted, including normal operation and earth fault conditions.

Experiments confirmed that the prototype VSD generates practically no CMV thereby eliminating the EMI problem. Also, under an earth fault, the fault current flows into the designated protection circuit, as required.

A plan for future developments leading to commercialisation of the Electrically Safe Drive has been outlined.