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Ground Fault Relays for Variable Frequency Drives (VFDs)

60 cycle GFRs have limitations when the circuitry involves VFDs (Variable Frequency Drives). A variable frequency drive converts the incoming AC internally into DC, which will then be modulated again into a variable cycle AC leading to the load. Internal VFD - DC grounds can not be detected with conventional GFR technology. The common “passive” CT needs alternating currents to detect a ground fault, therefore DC currents will go unnoticed. Some drives may be equipped with their own internal scheme to detect ground faults which will eventually trip in the high Ampere range. Early warning or personnel protection cannot be guaranteed in this case.

Figure 1: The grounded system with variable frequency drive and BENDER RCMA technology

Other possible issues include:

  • EMI filtering sometimes incorporated into drives can provide a leakage path to ground and add to the overall system leakage
  • Multiple kHz carrier frequencies used can cross the gap between insulation and ground
  • Harmonic content

The solution: Protect by means of an active current transformer with built-in filtering technology. RCMA devices employ a double-coil system which enables them to accurately measure AC, DC, and mixed AC/DC currents from 0 to 2000 Hz.