The discovery of these fouled insulators confirmed the recent faults were not caused by raptor streamers. Moreover, the faults also occurred in late winter and early spring during dense fog or misty precipitation. These local weather conditions appeared to act as a catalyst, which suggested insulation breakdown as the reason for the faults rather than air-gap compromise caused by raptor streamers. Examination of event oscillograph outputs reflected that raptor streamer faults occurred indiscriminately across the voltage waveform, whereas contaminated insulator faults occurred at or near peak voltage. The outputs also revealed a distinct voltage rise (~35%) for approximately two cycles due to neutral shift on the un-faulted phases of the same circuit, as well as a similar rise (~20%) on the nearest adjacent circuit phase, which, if similarly contaminated with droppings, helped to facilitate the simultaneous fault.
Second, tower crews replaced standard glass insulators with silicon-coated insulators on towers with the heaviest accumulation of droppings. Silicon-coated insulators historically have been used in marine environments to mitigate faults derived from salt accumulation and coastal fog. NorthWestern Energy believed coated insulators might also protect against contamination from bird droppings because of their physical properties: hydrophobic, higher-flashover voltage ratings and longer current leakage paths.
Identifying The Source
During the non-breeding season, ravens gather in groups to roost at night on trees, cliffs and anthropogenic structures such as towers, buildings and bridges. The single-night number of ravens at an individual roost can be impressive, approaching or exceeding 2000 birds. Raven roosts are typically seasonal, forming in the fall and disappearing in late spring as individuals disperse to breeding territories. This pattern of tower use coincided with the occurrence of NorthWestern Energy’s recent troublesome faults: most in the winter, fewest in the summer.
A Third Strategy
Crews installed and positioned perch deterrents to protect approximately half of each insulator string — a length of 126.5 inches (3213 mm) — from accumulating droppings. These arrangements effectively broke the leakage path of current responsible for faults and represented the utility’s working 500-kV hot-line gap settings of 50 inches to 55 inches (1270 mm to 1397 mm), adjusted for worksite elevation. By design, deterrents limited but did not completely exclude ravens from roosting on specific towers. Somewhat counterintuitively, NorthWestern Energy’s goal was to keep ravens on the towers they were using to discourage them from spreading to new towers, which would have created additional fault risks and mitigation efforts.
The three-pronged approach of washing insulators, installing perch deterrents and replacing glass with silicon-coated insulators has proven highly successful. The number of sustained faults declined from an average of 13 per year from 2016 to 2018 to an average of six faults per year from 2019 to 2021, and then dropped to only one fault in 2022. Although wind-dispersed raven droppings eventually contaminated the entire length of insulator strings on towers with perch deterrents during the winter, the rate of accumulation greatly decreased, reducing the time and effort needed for washing. Seasonal rains and declining roost sizes kept insulators free of contamination from spring to fall.
Why Ravens Now?
Data from the National Audubon Society’s Christmas Bird Count show the winter raven population in central Montana is exhibiting exponential growth. This trend was statistically associated with the increasing number of faults annually from October to April before NorthWestern Energy took corrective actions. Therefore, continuing challenges to the operation of the 500-kV transmission lines should be expected. Existing roosts have the potential to become larger and spread onto previously unused towers as the wintering population increases. Ravens commute daily 15.5 miles to 34 miles (25 km to 55 km) one way to nocturnal roosts in other regions of the U.S. and Europe, so roosts along the transmission lines probably attract ravens from a wide geographic area in central Montana. Moreover, seasonal movements of ravens also play a role in roost formation, where individuals can migrate over 300 miles (483) from their summer breeding ranges to food-rich areas in the winter.