Three Key Factors for Preventing Power Line Wildfires
Updated: Apr 20, 2021
When drought conditions occur, wildfires can be caused by power line faults. While current mitigation techniques are somewhat effective, they are temporary and cannot prevent a fault from sparking a wildfire during unanticipated conditions.
They can also lead to negative repercussions for energy consumers. For example, electric utilities prevent wildfires by de-energizing power lines in “at risk” areas when high winds are forecasted. This temporary solution leaves hundreds of thousands of consumers without power for hours or days. On top of that, if a sudden windstorm occurred unexpectedly, this method would fail to prevent a faulted power line from sparking a wildfire.
Prescient’s innovative wildfire risk assessment and analysis technique, outlined in our post Calculating the Risk of Wildfires Caused by Sparking Power Lines, will allow electric utilities to calculate the risk of wildfires being caused by their power lines. Once the risk is identified and understood, Prescient will recommend strategic enhancements for the installation and maintenance of electric power lines and equipment in wildfire risk areas.
Our recommended enhancements are defined by three key factors: rapidly isolating power line faults, minimizing energy deposition on nearby dry vegetation, and creating zones free of combustible materials. Implementation of these enhancements will assure that the risk of wildfires is minimized for any power company. Most excitingly, these enhancements can be implemented within the existing grid as an update, not an overhaul.
Let’s take a deeper look at these three enhancements. We’re going to get a little technical, so feel free to skip ahead when you get the gist of each.
Enhancement 1: Rapid Fault Isolation
By rapidly isolating power line faults, the risk of igniting nearby vegetation is significantly reduced. Prescient proposes that when the risk of wildfires is high, electric utilities replace the traditional practice of coordinating Fault Interrupting Devices (FIDs), in which the FID that is closest to a fault is the only FID to open. Instead, when the risk of wildfires is high and a fault occurs, any FID should open in 100 milliseconds.
Figure 1 illustrates a typical power distribution line with four FIDs installed on the line. During low wildfire risk conditions, when a fault occurs beyond FID 2, FID 1 does not open. Similarly, when a fault occurs beyond FID 5, FID 3 does not open.
Prescient proposes that when the risk of wildfires is extreme, FID 1 be allowed to open at the same time as FID 2 through FID 5, whenever and wherever a fault occurs.
This change eliminates the need for proactive power outages (selective blackouts). By implementing this change, the risk of a fault igniting a wildfire will be eliminated.
Enhancement 2: Minimal Energy Deposition
Traditional practice is for electric utilities to use time overcurrent relays to detect faults on distribution lines, and to install fuses that open to isolate faulty transformers. The settings for these devices are based on transformer withstand and conductor annealing considerations.
When electric utilities consider energy deposition, the usual concern is the threshold of 2 calories per square centimeter on personal protective equipment (PPEs). Today, minimal energy deposition on nearby vegetation is not a design consideration. Consider a 600 amp, 12.47 KV distribution line equipped with time overcurrent ground relays set to actuate at 120 amps. The possibility of a wildfire caused by a faulted power line is problematic because the actuation time of a 120 amp ground relay is too slow.
To illustrate this concept, Figure 2 plots the following: the withstand time of a 50 KVA, 7.2 KV single phase transformer; the clearing time of a 10 amp fuse; the operating time of a 120 amp time overcurrent ground relay, set on the #1 time dial; and the time to deposit 2 calories per square centimeter on vegetation that is 12 inches from an arcing fault.
The 50 KVA, 7.2 KV transformer withstand time is included because its withstand is comparable to energy deposition of 2 calories per square centimeter at 12 inches. The significant point is that the protective relay does not actuate before 2 calories per square centimeter are deposited on vegetation. Consider 250 amps of fault current. Two calories per square centimeter are deposited in 1 second, and the time overcurrent ground relay actuate time is 8 seconds. Similar results occur at other fault current values.
Fortunately, dry vegetation is not expected to ignite at this energy level. But, if the distance is 6 inches, rather than 12 inches, energy deposition would be 8 calories per square centimeter and the time to deposit 2 calories per square centimeter would be 250 milliseconds.
This is further complicated by the fact that energy release is a function of voltage. The amount of energy released increases as voltage increases.
A key point is that the energy deposition curve is a function of both distance from an arc to dry vegetation and power system voltage. Prescient has calculated energy deposition during arcing faults for voltages from 4.16 KV through 765 KV as a function of fault clearing times and distance from a fault to dry vegetation, among other factors. The calculations, while tedious and voluminous, are straight forward and insightful.
Enhancement 3: Zones Free of Combustible Materials
Currently, danger tree and hazard tree assessments are routinely developed to determine the possibility of trees and tree branches accidentally contacting power lines. This strategy is effective for these risks. However, this approach fails to account for other wildfire risk factors.
Prescient recommends establishing zones free of combustible material so that the distance between faulted power lines and dry vegetation can be maximized. To determine which areas should be designated combustible-material-free zones, Prescient recommends assessing each power line row of way (ROW) with a variety of risk factors in mind, including the potential impact of an arc across an insulator, a pole with a broken crossarm, or a pole that broke at the ground line. Each risk factor presents different variables, including the distance between an arc and dry vegetation and the amount of energy deposited on nearby vegetation.
Figure 2 illustrates an arc across an insulator with vegetation located 12 inches from the arc. If dry vegetation is located 24 inches from an arc, energy deposited on vegetation is reduced and the likelihood of ignition is reduced. When a pole breaks at the ground line, the distance between an arc and dry vegetation can be less than 12 inches, and the likelihood of ignition is increased. Establishing zones free of combustible materials reduces unknowns in energy deposition calculations.
These zones do not need to be free of all vegetation. In combustible-material-free zones, clippings should be removed when fields are mowed, slash piles should be removed when logging occurs, and the ROW should be cleared of debris on a scheduled basis.
Let’s Mitigate Wildfire Risk Together
By implementing rapid fault isolation techniques, minimizing energy deposition on vegetation, and creating zones free of combustible materials, the wildfire risk due to faulted electric power lines can be significantly diminished. These changes should be implemented in selected areas, for specific power lines, which Prescient can identify for you using our wildfire risk assessment and analysis service.
Contact us to learn more about Prescient’s recommended actions to reduce wildfire risk due to faulted power lines. Together, we can play a part in preventing wildfires.