The electric utility industry has been afflicted by punishing wildfires for decades, and the problem has seemed to peak in recent years. The toll of destruction has included millions of acres of land burned per year, lives lost, habitats destroyed, and natural resources damaged, such as reservoirs used to store precious drinking water. A fundamental goal of utility planners is to fortify the grid infrastructure so that the possibility of electric power lines being the cause of a wildfire ignition is reduced to the lowest risk factor possible. This paper will look at various construction techniques and mitigation strategies for each.
The first line of defense is Vegetation Management (VM). The power line Right-of-Way (ROW) must be cleared regularly of any vegetation that may either interact with a power line, or, conversely, be abundant fuel for a wildfire should an event occur. This includes but is not limited to tree trimming, removing danger trees (trees which may touch the power line if it falls over), and generally ensuring that there is a safe distance between the power line and any vegetation. ROWs are typically 50-500 feet for high voltage, but much less for distribution voltages, often as narrow as 30-50 feet. On the low end, this means that the edge of the ROW will only be 15 feet from the centerline of the power line. It is difficult for utilities to properly clean ROWs this narrow, and permission for robust ROW cleaning from empowered jurisdictions is often granted only after a serious wildfire event at a given location. This means that the utility must keep a sustained and intense focus on ROW vegetation to ensure that it does not inadvertently touch power lines, and the narrower the ROW the more difficult this task is.
Statistics vary, but it is estimated that power lines account for only three percent of the wildfire ignition scenarios, but fully 19 percent of all the acreage burned in California from 2016 to 2020. What percentage of the property damage is attributed to powerline wildfire ignition is not known with accuracy but is significant, since wildfires are often in remote areas where access is difficult and response time slower. The data shows that United States wildfire causes damage equivalent to 2-4% of U. S. GDP, which translates to between $394 billion and $894 billion dollars annually. Reducing these events and associated costs is of paramount importance to the electric utility industry and to the nation as a whole.
Bare Wire Lines
Assuming bare wire power lines are responsible for the vast majority of wildfire ignitions when power lines are involved, it is imperative to devise mitigation measures. Bare wires can cause ignition by contacting trees in windy conditions, conductor clashing, falling from the pole and igniting dry brush on the ground, or falling to the ground, hitting a rock, throwing a spark, and initiating ignition that way.
The risk of conductor clashing on bare wire lines can be reduced by using relatively low technology interphase spacers, such as the one shown in the figure below.
Interphase spacers come with either metallic clamps on either end of a fiberglass rod, or with standard pin type insulators, and are installed at the mid-span between the phase conductors. The fiberglass rod ensures the phase conductors won’t blow into each other during high wind conditions, and the insulators augment the BIL and Voltage Withstand.
Another bare wire construction mitigation strategy is to use Flame Retardant (FR) insulators. Insulators with FR additives will have a higher ignition temperature. This may or may not help that much if the wildfire is stagnant, but, if the fire is moving rapidly in windy conditions, the increase in ignition temperature is assumed to provide a margin of safety. Most important, however, is the anti-drip properties of FR insulators. In a wildfire scenario, if for any reason the insulator becomes ignited, we don’t want flaming drips of polyethylene to fall onto material on the ground and start a fire there. The anti-drip properties of the FR insulators then remove the concern for bringing fire at top of pole (which will extinguish as soon as WF passes) to ignite something on the crossarm or beneath the pole.
Put the Lines Underground (UG)
An almost universal cry across the electric utility industry is that the power lines should be put UG. The benefits include improved aesthetics (out of sight, out of mind), the lines are not subject to vegetation contact, and there is really no risk of throwing a spark. While undergrounding is assumed to be a panacea for the threat of wildfire ignition (and it just may well be), there are some caveats to adopting this technology as the predominant mitigation strategy.
The cost of materials for UG cable is 3x - 4x the cost of aerial covered conductor, and fully 5x – 10x the cost of bare wire construction. To this is added the cost of UG transformers, cabinets, and switches. There is potential harm to tree roots and endangered flora. Another concern is the lack of flexibility with undergrounding. How long will the medium voltage network remain unchanged? What is the future loading, and how will future changes be handled? Changes are much more easily managed with aerial construction. So, while undergrounding is a technically viable and aesthetically attractive option for wildfire mitigation, it imposes a cost structure which may not be economically justified systemwide.
Aerial Covered Conductor
Aerial covered conductor is like bare wire construction, in that it uses aluminum or copper conductors. The 3-layer insulation system on top of the bare wire includes a semiconducting layer (to smooth out electric fields when in contact with a grounded object), 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, with thicker insulation as the voltage class increases.
Aerial Covered Conductor comes in two standard configurations. The first is Spacer Cable, where phase conductors attach to an overhead messenger wire using High Density Polyethylene (HDPE) spacers every 30 feet. The messenger wire serves multiple purposes: it provides mechanical strength, acts as a system neutral, serves as a lightning shield, and protects the phase conductors from falling debris. Both the spacers and insulators are made of HDPE to ensure dielectric compatibility.
Tree Wire systems resemble bare wire construction but incorporate the exact same 3-layer covered conductor design used in Spacer Cable Systems. Typically, the phase conductors in Tree Wire systems are either ACSR or AAAC, as they are self-supported and fully tensioned.
These systems are strung in an open wire configuration on crossarms, using polyethylene insulators. The photo below on the left shows a Spacer Cable System, while the photo on the right shows a tree wire configuration.
Wildfire Risk Aversion with Covered Conductor
The most common modes of bare wire power lines causing wildfire ignition are when power lines come into contact with the surrounding environment. Possibilities include conductor clashing, live conductors falling to the ground and igniting dry brush, de-energized conductors falling to the ground, hitting a rock and throwing a spark onto dry brush, or a tree branch lying across two phases, igniting, and falling to the ground in flames. Animals and birds can also get into the lines, and either get between the phases or between the phase and neutral, become engulfed with flames and drop to the ground. The last cause is mylar balloons being irresponsibly released into the air which sometimes end up on pole tops and create flashovers with massive fireballs.
With covered conductors, incidents like tree branches touching phase conductors, phase conductors touching each other in high winds, or lines knocked to the ground do not result in enough current to cause ignition. Additionally, the polyethylene covering on the covered conductors prevents sparks when they fall to the ground. This contrasts with bare wire scenarios, where metallic conductors can cause sparks upon impact. Extensive testing of mylar balloons and covered conductor has verified that whether lodged phase to phase or between the phase and ground of a covered conductor system, there is no flashover.
Summary and Conclusions
Power lines, while essential for modern society, pose an inherent risk when it comes to wildfire ignition. Fortunately, there are mitigation strategies available for different types of power line construction options: bare wire, underground, and covered conductor. Among these, covered conductor systems stand out as the most practical and cost-effective tool for combating this phenomenon.
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