Applying High-Resistance Neutral Grounding in Medium-Voltage Systems

Abstract

The application of high-resistance neutral grounding (HRNG) to medium-voltage (MV) distribution systems is one of the least understood and often misapplied methods of system neutral grounding. An HRNG grounded system is the only intentionally grounded neutral grounding method suitable for industrial systems that allows normal operation (no voltage dips, no power surges, no shutdowns, and minimal damage) for an indefinite time period after the inception of the most common of all faults, i.e., the single-line-to-ground fault. The complexity of applying an HRNG system is due to lesser understood factors, such as the relationship between system charging current, neutral grounding resistor let-through current, and point-of-fault ground-fault current; point-of-fault arcing voltage magnitudes; escalating arcing fault phenomena; and point-of-fault energy levels, all of which are not easily determined nor easily estimated. This paper addresses the application of HRNG neutral grounding systems on MV industrial ac power distribution systems. The seemingly perfect HRNG grounding system, with ground-fault-current magnitudes often limited to 10 A or less, has a limited window of application on MV systems, such that when misapplied may actually place the electrical system backbone components at risk, as well as trip the system offline due to escalating arcing faults resulting in phase-to-phase faults. HRNG applications in MV generator protection and MV arc flash mitigation are not within the scope of this paper.