Impedance Grounding for Variable-Frequency Electric Motor Drives

The grounding system for a variable-frequency electric motor drive (VFD) is typically designed to enhance the continuity of operation, limit ground fault currents, limit transient over-voltages during ground faults, improve safety, and reduce or eliminate common-mode voltages (CMVs). The majority of VFDs utilize power transformers to provide isolation between the VFD and its supply, and operate as the point-of-supply to the front-ac-dc power electronic converter (PEC). Grounding system designs for VFDs have been a subject of several standards, industrial codes, and recommended practices. This paper analyzes the performance of impedance grounding systems (during steady-state and fault conditions), when utilized in VFDs. Analyzed grounding systems are the low-resistance, and high-resistance grounding systems. The performance of analyzed grounding systems is evaluated based ground currents, ground potential, and CMVs. Several experimental tests are also conducted to draw conclusions and suggest recommendations for an adequate impedance grounding for industrial VFDs.