Power lines do not fail due to internal freezing. Instead, the danger to electrical infrastructure comes from atmospheric icing, which is the accumulation of ice on the conductor’s exterior surface. This buildup is a significant meteorological hazard that threatens the reliability of the power grid. Understanding how water transforms into a destructive layer of ice on these wires is key to mitigating system failures. The resulting ice load dramatically increases the stress on the conductors and supporting towers, leading to widespread outages during severe winter weather events.
How Ice Accumulates on Conductors
Ice accumulation on overhead lines primarily involves supercooling. Supercooled water droplets are liquid water that remains fluid despite being below the standard freezing point of \(0^\circ\text{C}\). This unstable state is common in weather patterns like freezing rain or supercooled fog and requires a surface for nucleation, which triggers freezing.
When these supercooled droplets contact a power line, they instantly freeze upon impact. The conductor’s temperature, typically at or below the ambient air temperature, facilitates this rapid phase change. The rate of ice accumulation is influenced by air temperature, wind speed, and the amount of liquid water present.
Freezing rain forms when melted snowflakes pass through a sub-freezing layer of air near the ground, causing the droplets to become supercooled. This sets the stage for substantial ice formation on any object they strike. The continuous impingement of these droplets leads to a steady outward growth of the ice layer, changing the wire’s physical properties.
Different Forms of Conductor Icing
Atmospheric icing is categorized into glaze ice and rime ice. Glaze ice, often called clear ice, is the densest and most damaging type, forming from freezing rain or drizzle. This ice is clear, has a smooth surface, and can achieve a high density, often ranging from 900 to 920 kilograms per cubic meter (\(\text{kg/m}^3\)).
The high density of glaze ice means a thin layer imposes an enormous weight load on the conductor. Glaze ice accretion occurs when droplets freeze slowly, allowing air bubbles to escape and resulting in a solid, homogeneous structure. This heavy ice adheres tightly to the wire, maximizing strain on the support structure.
In contrast, rime ice forms when smaller supercooled cloud or fog droplets freeze rapidly upon impact, often at colder temperatures below \(-5^\circ\text{C}\). This rapid freezing traps air, giving rime ice an opaque, white, or feathery appearance and a much lower density, typically between 150 and 700 \(\text{kg/m}^3\). Rime ice can be further classified as soft or hard.
Structural Stress and System Failure
The accumulation of ice, particularly the dense glaze variety, leads to a massive increase in the static weight load on the transmission tower and conductor. A quarter-inch of ice can significantly increase the conductor’s diameter, potentially causing a quarter-mile span of wire to accumulate thousands of pounds of additional weight. This load increase can cause the conductor to stretch beyond its elastic limit or lead to the collapse of supporting towers.
The ice layer also significantly increases the conductor’s surface area exposed to wind, raising the wind loading. This combination of increased weight and greater wind resistance places enormous mechanical stress on the system. A common failure mode is conductor galloping, a large-amplitude, low-frequency oscillation of the power line.
Conductor galloping is triggered when ice accretion forms asymmetrically on the wire, creating a non-circular profile that acts like an airfoil, similar to an airplane wing. A steady wind blowing across this iced shape generates aerodynamic lift, causing the line to oscillate dramatically in large, vertical waves. These oscillations can exceed a meter in amplitude, causing adjacent conductors to come into contact, resulting in a flashover or short circuit. The sustained motion of galloping accelerates the mechanical fatigue of the line and its hardware, ultimately leading to conductor breakage and widespread power outages.