Effect of adjustable speed drives on the operation of low voltage ground fault indicators

Over the years, the petroleum and chemical industry has found increasing favor with 60 Hz low voltage (/spl les/600 VAC) power systems that utilize a high resistance grounded (HRG) neutral philosophy. Historically, the older generation of adjustable speed drives (ASDs) had little or no effect on the normal operation of ground fault indicators (GFI) used with the installed HRG systems. This paper focuses its investigation into nuisance GFI alarms that may occur when present generation ASDs are retrofitted into the existing plant. The paper first reviews possible neutral grounding systems, with emphasis on the types of HRG systems possible and GFI alarm philosophy. The paper then discusses how ASDs may generate zero sequence high frequency noise currents in the HRG neutral circuit, which may cause nuisance ground fault alarms and potentially mask a legitimate ground fault.