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

The petroleum and chemical industry has found increasing favor with 60 Hz low-voltage (⩽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 (GFIs) used with the installed HRG systems. This paper focuses on 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. GFI noise current magnitude is defined for both present and older ASD technologies. The effect this transient zero-sequence noise current magnitude has on GFI operation is described. Mitigation methods used at the drive to reduce ASD noise current magnitude to acceptable nonalarm levels is investigated. Filter solutions located at the HRG/GFI meter that reduce nuisance alarms are also investigated. The pros and cons of at the drive or at the meter filter solutions are supported with laboratory and field test data. Application guidelines are given to help avoid nuisance problems with a plant ground-fault protection scheme, which needs to successfully operate in the presence of multiple ASDs