An advanced distance relay strategy for incipient fault detection and location in underground distribution systems

In recent years the use of underground cables in distribution networks has increased due to the reliability that it brings. However, despite of its advantages, this type of cables is still exposed to faults. Faults in underground cables occur as a series of faults of sub-cycle duration, also known as incipient faults. This type of fault has the characteristic of being a single-phase-to-ground type. Over time, incipient faults can evolve into a permanent fault, causing severe damage to the isolation of underground cables. In this context, detecting and locating incipient faults will provide an evidence of a defect in the cable that in the future will cause a major trouble, which help to improve the system's reliability indicators. Considering previously exposed, this paper presents a strategy for detecting and locating of incipient and permanent faults for Underground Distribution System (UDS) by using the principle of distance protection and the analysis of a time-domain model of the system. The strategy is based on the distance relay operation due to it can relate the estimated impedance by a protection device with the length to the fault; estimation of the impedance is made by using a novel estimator that takes the samples of the voltage and current signals to refine the value of the impedance seen in a period of time. Additionally, the strategy uses as arc fault model a static arc voltage which its formulation models the arc voltage as a square wave with a white noise in phase with the fault current. The strategy was validated in an UDS considering multiple scenarios of fault and nominal operating condition, also, it shows an outstanding behavior to detect both incipient and permanent faults for single-line to ground faults. The developed tests show an accuracy to identify among incipient faults, permanent faults and non-fault events with an average of 94.78% in all considered scenarios. Finally, the proposed protection strategy presents an average error of overestimation for the fault distance of 3.385%, which represents an average of 136.01 meters.