A considerable number of three-wire ungrounded or resonant-grounded systems are in North America and a prevalence of even more are on other continents. On ungrounded or resonant-grounded systems, when a single-phase-to-ground fault occurs on a feeder, the fault current is significantly less compared to a multigrounded or solidly grounded feeder for the same fault and it cannot be reliably detected by conventional overcurrent protection. Utilities today face an urgency to employ rapid fault detection and isolation methods on such systems due to an increase in wildfire hazards. Faults generating even a few amperes can cause wildfires or severe public safety hazards if the faults are not isolated promptly. This challenge warrants a reliable solution for single-phase-to-ground fault detection in such systems. Existing fault detection methods, especially overcurrent-based methods, perform poorly because of the small fault current magnitude in the system.
This paper presents a new single-phase-to-ground fault detection method that uses pure-fault current quantities, which eliminate the system load current effect. By using the identified unique relationship between pure-fault current characteristics of the faulted and unfaulted phases, the method detects single-phase-to-ground faults and reliably identifies the faulted feeder out of parallel feeders connected to the same bus. This method does not require any system parameters that are especially hard to obtain for distribution systems, allowing the method to be adapted to field reclosers. The associated number of settings used is minimal, and the values are easily obtained. This paper describes the method in detail and demonstrates its performance by using both actual fault events and staged system testing events.