Wildfires have swept across the United States since the 1800s. Notably, the Miramichi fire in the State of Maine in 1825 scorched an astonishing 893,000 acres. Over the years, massive wildfires have come and gone. However, in recent times, the incidence of wildfires has surged nonlinearly, reaching a point where they are nearly commonplace, particularly in the western states. While it is widely acknowledged that human actions account for 80-90% of wildfires—often due to irresponsible behavior like untended campfires, discarded cigarette butts, and automobile malfunctions—natural factors, such as lightning strikes, can also serve as ignition sources. Additionally, electric power lines pose another potential ignition risk.
In the ten-year period from 2013 to 2022, on average, a staggering 7.2 million acres per year were consumed by wildfires. The year 2023, however, shows only 2.7 million acres were burned. Wildfire damage is correlated with weather, fuel load, firefighting efforts, and other factors. So, while the 2023 wildfire damage numbers are still highly undesirable, can this be interpreted as turning the corner as a nation in our mitigation efforts?
Survival vs. Mitigation
There are various techniques to prepare the electric grid to better sustain, or survive a passing wildfire. These include but are not limited to clearing of fuel loads, pole changeouts to steel, Smart Grid monitoring combined with weather forecasting and predictive simulation to put first responders on alert or initiate a Public Service Power Shutoff (PSPS). The emphasis of this paper is solely mitigation. That is, to harden the grid so as to minimize the possibility of a wildfire ignition event caused by the powerlines interacting with the environment.
What is Aerial Covered Conductor
Aerial covered conductor uses standard aluminum and copper wires, similar to bare wire construction. On top of the bare wire is added a 3-layer insulation system. The insulation system starts with a thin semicon layer (to smooth out electric fields when in contact with a grounded object and minimize partial discharge) directly over the bare wire, an inner layer of low-density polyethylene (which gives a high BIL and is soft to enable stripping for taps and transitions), and an outer layer of High Density Polyethylene (providing UV inhibition, track resistance, abrasion resistance, and color). The system voltage class determines the total insulation thickness, as the design objective is to limit the available surface current should the phase conductor come into contact with a tree or other grounded object.
Aerial Covered Conductor Systems – Two Configurations to Choose From
It is helpful to first describe what is meant by covered conductor systems. Spacer Cable Systems consist of three heavily covered, but unshielded, phase conductors. The conductors are usually AAC when in a spacer configuration, since there is no tension on the phase conductors, but are usually ACSR or AAAC when installed in a self-supported or “Tree Wire” configuration.
In Spacer Cable construction, the phase conductors are attached to a messenger by spacers, installed every 30 ft. (10m.) along the messenger. The messenger is a high strength, alumoweld (AW) or alumoweld-aluminum (AWA) conductor which has several functions. The first is that the messenger is the mechanical strength member, holding the phase conductors up. The messenger can also be used as a system neutral, is a lightning shield, and provides a mechanical protection function by protecting the phase conductors from any items (leaves, branches, trees) which can fall onto the bundle from above. The spacers are made of High Density Polyethylene (HDPE), as are the pin or line post insulators used on the angles, to ensure dielectric compatibility with the phase conductors.
Tree Wire systems, on the other hand, look more like bare wire construction. They utilize the same 3-layer covered conductor design, but the phase conductors are usually either ACSR or AAAC (since it is self-supported and fully tensioned). Tree Wire systems are strung in an open wire configuration on crossarms with polyethylene insulators. The photo below on the left shows a Spacer Cable System, while the photo on the right shows a tree wire configuration.
How Bare Wire Systems Cause Wildfires
The most common causes of wildfire ignition from powerlines are as follows:
- A tree branch can come into contact with a powerline and become ignited.
- A tree branch can fall across two phase conductors, catch fire and fall to the ground, igniting dry brush or grass.
- A powerline can fall to the ground energized and not have enough current to trip a protective device. This “High Impedance Fault” can go unrecognized for long enough to ignite material on the ground.
- High winds can cause conductors to swing in the wind and come into contact with one another, a phenomenon known as “Conductor clashing” or “Conductor slap.” High energy plasma particles are emitted and can fall and ignite material on the ground.
- Mylar balloons used for parties and celebrations released into the air can come into contact with powerlines and release a massive fireball when they cause a flashover.
How Covered Conductor Systems Prevent Ignition
With covered conductor, if a tree branch comes into contact with a phase conductor, there is not enough current to cause ignition. If a high wind situation causes phase conductors to touch one another, there is not enough current to cause an issue and no flashover occurs. If a line is knocked to the ground, there is not enough current to ignite dry brush which may be present. And lastly, if a conductor falls to the ground, the polyethylene covering on the covered conductor will not cause a spark to be thrown. This is not like the equivalent bare wire scenario, where a metallic conductor can fall from a height, hit a rock on the ground, throw a spark, and cause ignition.
Several innovative protective relaying schemes have been employed to combat fallen wires causing ignition. Through frequency analysis, or sequence impedance analysis, the relay determines that the line has somehow faulted and is falling to the ground. It takes a little over a second to travel from pole height before hitting the ground, and the relay can de-energize the line before it hits the ground, ostensibly preventing ignition. Unfortunately, even if the line is de-energized when it hits the ground, it can still hit a rock, throw a spark, and cause ignition. Covered Conductor eliminates this concern.
Summary and Conclusions
At a recent conference on Wildfire Mitigation, a Relaying Engineer emphasized the importance of adopting a “layered approach” to address wildfire mitigation needs. This approach involves integrating innovative protective relaying, monitoring/forecasting, Smart Grid technologies, and system hardening. A fundamental component of this strategy is the utilization of Aerial Covered Conductor.
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