A New Ground Fault Detection Method In Three-Wire Distribution Systems To Reduce Fire Hazards

While most power distribution systems in the US are muti-grounded, four-wire systems, a considerable amount of single-grounded, three-wire, distribution systems do exist especially on the west coast. Single-grounded, three-wire systems refer to the distribution systems without dedicated ground wire where only the transformer neutral is grounded at substation. On such systems, when a single-phase-ground fault occurs on a feeder, the fault current is significantly less than if the same fault had occurred on a multi-grounded four-wire feeder. This is because of no low impedance return path for the fault current to flow back to the source at the substation other than through the line charging capacitance. Traditionally, such low fault currents during single phase to ground faults were sometimes seen as an advantage of these three-wire systems as it meant there was no urgency of tripping the line to disrupt service and the systems continued to supply power. However, today, utilities face the urgency to employ rapid fault detection and isolation methods due to an increase in the occurrences of wildfire hazards. Even a fault generating just a few amperes can cause a catastrophe. This challenge requires a reliable solution for single-phase to ground fault detection in single-grounded, three-wire systems. Existing fault detection methods using direct current measurement tend to perform poorly during load unbalance or for feeders with asymmetrical phase construction with different phase charging capacitance.

This paper proposes a new single-phase to ground fault detection method for three-wire system using only the pure-fault current quantities by removing the load current. Pure-fault quantities are quantities generated during a fault and are independent of the pre-fault system. By utilizing the identified unique relationship between pure-fault current characteristics of the faulted and un-faulted phases, a new method is devised to detect single-phase to ground faults with high fault impedance on three-wire systems and to reliably identify the faulted feeder out of parallel feeders connected to the same bus. This method does not require information about any system parameters that are especially hard to obtain for distribution systems. The number of settings associated with this logic is minimal and default settings are applicable to most systems. This paper describes the method in detail and, in the end, demonstrates the performance of this method using both simulated systems and field events.