Considerations for ground-fault protection for low-voltage variable-frequency drive circuits supplied by high-resistance-grounded power-distribution systems

Many low-voltage (LV) adjustable speed drives (ASD´s) have a built-in ground-fault function that is not capable of detecting a low-level ground fault and may require the use of a suitable supplemental protection relay for reliable detection. While high resistance grounding (HRG) has become the norm, over recent decades, in many industrial facilities the surge in application of ASD´s has challenged designers and end users to manage the attendant risks associated with achieving the intended safety and reliability of LV HRG systems, while managing the inevitable growth of loads that incorporate high-speed electronic switching equipment. Using a zero-sequence current transformer (ZSCT), connected to a ground-overcurrent relay, to detect small values of fault current in ASD circuits is accepted as a reliable approach; but where to locate the CT, upstream or downstream of the drive, is a matter of some debate. This paper reports the unsettling discovery in an industrial installation that the ground-fault-protection of low-voltage ASD´s was incompatible with the HRG systems, discusses appropriate HRG design considerations, describes the selection of a retrofit- and new-installation solution, and offers a suggestion for the optimal ZSCT location.