Review of Ground Fault Protection Methods for Grounded, Ungrounded, and Compensated Distribution Systems

This paper reviews ground fault protection and detection methods for distribution systems. First, we review and compare medium-voltage distribution-system grounding methods. Next, we describe directional elements suitable to provide ground fault protection in solidlyand lowimpedance grounded distribution systems. We then analyze the behavior of ungrounded systems under ground fault conditions and introduce a new ground directional element for these systems. Then we examine the behavior of compensated systems during ground faults and describe traditional fault detection methods. We conclude by introducing new ground fault detection methods for compensated systems. INTRODUCTION Ground fault current magnitudes depend on the system grounding method. Solidlyand lowimpedance grounded systems may have high levels of ground fault currents. These high levels typically require line tripping to remove the fault from the system. Ground overcurrent and directional overcurrent relays are the typical ground fault protection solution for such systems. However, high-impedance ground fault detection is difficult in multigrounded four-wire systems, in which the relay measures the ground fault current combined with the unbalance current generated by line phasing and configuration and load unbalance. Ungrounded systems have no intentional ground. For a single-line-to-ground fault on these systems, the only path for ground current to flow is through the distributed line-to-ground capacitance of the surrounding system and of the two remaining unfaulted phases of the faulted circuit. In resonant-grounded or compensated distribution networks the system is grounded through a variable impedance reactor connected to the power transformer secondary neutral or the neutral of a grounding bank. This reactor compensates the system phase-to-ground capacitance such that the zero-sequence network becomes a very high impedance path. The reactor, known as the Petersen coil, permits adjustment of the inductance value to preserve the tuning condition of the system for different network topologies. Resonant grounding provides self-extinction of the fault arc in overhead lines for about 80 percent of temporary ground faults [1]. Considering that about 80 percent of ground faults are temporary, we conclude that more than 60 percent of overhead line ground faults clear without breaker tripping. High-impedance grounded systems are grounded through a high-impedance resistor or reactor with an impedance equal to or slightly less than the total system capacitive reactance to ground. The neutral resistor is of such a high value that ground faults on such systems have very similar characteristics to those of resonant-grounded systems.