High Resistance Grounding Analysis Using Symmetrical Components

Abstract

This paper provides a perspective on high resistance grounding (HRG) using symmetrical components. The paper notes that with the use of modern micro-processor-based relaying and zero-sequence current transformers, directional ground relays can sense the zero-sequence capacitive ground fault current on an HRG or ungrounded system. It further presents an analysis of ground fault currents based on distributed stray capacitance. This paper addresses the confusion within the industry by discussing the fact that both zero-sequence ground fault current and charging current flow during a ground fault on an HRG and ungrounded system. A discussion on the differences between charging current and ground fault current will establish that positive-sequence current flows into the capacitance from the system while zero sequence current flows from earth during a ground fault. The charging current is shown to be a positive-sequence current while the ground fault current is shown to be a zero-sequence current. This knowledge is important for the proper analysis and design of the protection system on an HRG or ungrounded system. Finally, the use of an HRG system with proper resistance is shown to be the best method in detecting and isolating a ground fault, especially if the ground fault needs to be automatically removed from the system.