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High Resistance Grounding Analysis (HRG) using symmetrical components, 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.
High resistance and ungrounded systems are commonly used for critical processes where a high degree of reliability is required. Electrical safety and minimization of the probability of an arc flash incident is another benefit of the HRG and ungrounded systems. While both systems provide these benefits, there are other advantages in the use of HRG over an ungrounded system:
Elimination of potentially damaging transient over voltage conditions from an arcing ground fault on an ungrounded system.
Better detection of a ground fault on the system with High Resistance Grounding Analysis.
Additional options for sensing and locating an inadvertent ground on the system with the HRG system.
While use of the HRG system is becoming much more popular than the ungrounded system, there are some complexities that present unnecessary challenges to the practicing engineer in the understanding, design and application of HRG and ungrounded systems. The common issue that exists involves the stray capacitance of the power system which is an issue that affects both the HRG and ungrounded system. On small systems, the stray capacitance may be so small as to allow the design engineer to ignore its impact on the system. A few milli-Amps here or there can be ignored with little or no consequences.
On larger and more extensive systems, the stray capacitance from surge capacitors, cable capacitance and equipment capacitance can result in the flow of significant “charging current.” A significant amount of charging current may have different values to different designers. This author considers charging currents in excess of one Ampere to be significant and the calculation of that current is essential for designing an HRG system with the proper resistance such that IRO ≥ ICO where IRO is the design current for the HRG resistance and ICO is the zero-sequence current which flows during a ground fault.
In the opinion of the author, charging currents in excess of ten Amperes become problematic in that significant damage may occur at the point of fault due to the higher fault currents. With charging currents less than ten Ampere, there is minimal risk in allowing a ground fault to persist until reasonable efforts can be made to locate and isolate the ground fault. However, with currents of ten Amperes and more, consideration should be given to quickly isolate the ground fault due to potential arc damage to the equipment. These higher currents are more prevalent on systems greater than 7200 Volts.
|High Resistance Grounding Analysis Using Symmetrical Components|