Underground AC Circuits in North America: Inventory Attributes and Sustained Outages

This paper presents a first comprehensive statistical study of transmission inventory and outage data for underground AC circuits on the North American scale. The analysis is based on the data collected and submitted by member utilities to the North American Electric Reliability Corporation’s (NERC’s) Transmission Availability Data System (TADS) during the years 2013 to 2018. The statistical approach considers the random nature of automatic outages and connects the outage frequency on an underground circuit and the outage duration with the circuit inventory attributes both numerical and categorical (voltage class, mileage, number of terminals, terrain etc.)A comparative analysis of reliability statistics for underground and overhead transmission circuits is also presented. It shows that automatic outages on the underground circuits are significantly rarer but much longer than on the overhead circuits. The greater durations result in a higher unavailability of the underground transmission lines compared with the overhead ones—on average, an underground AC circuit is unavailable 29 hours a year due to sustained automatic outages versus 6 hours for an overhead ac circuit.