Vegetation management is an evolving practice and one that will continue to challenge utility asset managers as they face an increased and evolving load and increasingly severe and unpredictable weather.
Vegetation is a major cause of overhead lines asset failure and could cause large wildfires like the one in California in 2018. Vegetation management is the most-costly expense for many North American utilities. According to the Electric Power Research Institute (EPRI), power outages and other power disturbances cost the U.S. economy nearly $120 billion every year. Much of this cost can be directly attributed to power outages triggered by overgrown vegetation.
How do most asset managers handle vegetation management?
Most vegetation managers have difficulty defining their preventive pruning plan. They are relatively unaware of what their vegetation portfolio represents – vegetation typology, location and health status. Therefore, it is likely they are not taking the right measures to efficiently mitigate the number of failures and their consequences, in relation to the thousands of miles of conductors. Utilities typically have a fixed budget for asset and vegetation management and deploly it based on a fixed timeline. Asset managers mitigate vegetation based on a schedule, not on what foliage is causing the highest risk.
Some utilities have started going beyond existing compliance requirements to address the wildfire risk conditions and run vegetation inspections, at least annually. To date, progress has been made for transmission lines, but vegetation management at the distribution level still leaves room for improvement. Risk-based, data-driven vegetation management results in more targeted tree trimming and removal activities, which improves the accuracy and cost of maintenance, safety, environmental impact and customer service.
How to determine linear asset failure risk due to vegetation
Asset failure risk is the combination of two dimensions: the probability and the consequences of failure.
The probability of a conductor failure depends mainly on the density of vegetation in close proximity to the line. Vegetation coming in contact with bare conductors is the main cause of failure. This causes the grounding of the bare conductor through the tree and its moisture. This mode of failure is the most frequent and can cause forest fires. Insulated conductors are not exposed to this mode of failure because the protective sheath around the conductors isolates them from the rest of the external aggressions, but they are exposed to falling trees and branches, that might cut the conductor.
In order to prevent vegetation impact, a corridor is put in place around the conductors and then the corridor must be cleared of vegetation on a regular basis. The width of the corridor depends on a number of parameters such as:
- The height and voltage of the line;
- The type of support of the conductors on the poles or tower (T-shaped or stepped lines along the pole); and
- The strength and direction of the prevailing winds.
To collect information about vegetation surrounding linear assets, utilities invest in employees to walk their power lines, take photos, and record damage. Others send helicopters to take images, some do lighter scans, and others use drones or satellites.
However, as the vegetation information is collected, it’s important that asset managers determine all potential consequences caused by vegetation, not just current damage to the line, pole or other assets. The number of connected customers, the voltage level of the conductors, the accessibility to the line (the slope, on/off road) are also important factors used to identify the severity of the failure. The higher the voltage, the greater the impact of failure.
Why use advanced analytics?
Switching from maintaining vegetation based on a schedule, to data-driven vegetation management with the help of risk modeling can enable asset managers to use their budget much more effectively and prevent interruptions and costly damages to assets. This means taking an inventory of vegetation, assessing its proximity to the lines, and evaluating the probability of outage where vegetation is likely to grow into the asset, or where trees are likely to fall or create short circuits. Understanding the probability and consequences of failure are critical for utilities to prioritize actions. With advanced analytics and technology, utilities can save up to 15% of operational costs for vegetation management annually.
Once utilities know the probability of failure of an asset based on surrounding vegetation, and the consequence of that failure, they can make informed decisions about what needs to be addressed in the short term and longer term. Utilities need to mitigate risks that could cause outages as well as potential catastrophic damage. The consequence of failure are assets with higher maintenance or replacements cost, safety and environmental impacts, loss of network performance and intangible consequences such as the utility’s reputation. There is an opportunity and urgency from regulators and the public for utilities to take better measures and find solutions that can help them better mitigate risks.
Making the switch
A data-driven solution, which takes vegetation risks into account and calculates the return on investment as well as potential consequences can help asset managers make informed decisions. There is no need to scan the whole grid, utilities can simply get started with a subset of their network to build a use case.
With just a few basic data points, companies like Aplines can help asset managers get started on risk-based asset and vegetation management quickly and easily. These data points are the location of the pole, the height of the conductor as well as the location of the surrounding vegetation. Satellite imagery is used to define the latter. With other remote sensing technologies, data such as vegetation height, species and growth rates can be added to refine the risk analysis. Identifying the current risk is important, but anticipating the future risk is even better to target the frequency and type of preventive actions.
Formatting and using remote sensing technology image analyses, the density and distance of vegetation around linear assets is identified. This information is then translated into a probability of failure for each span and transcribed on a map.
Efficient vegetation management is attainable
Vegetation management is an evolving practice and one that will continue to challenge utility asset managers as they face an increased and evolving load and increasingly severe and unpredictable weather. The accumulation of risk factors, such as historically dry weather, strong and dry wind, presence of dry vegetation are all important indicators in a first approach to the study of the probability of tragic events. However, it is difficult to anticipate the probability of feared events with precision if they have never occurred.
With a risk-based approach, preventive actions can be put in place to address such risks, but also to put in place contingency plans that are deployed when the catastrophic events occur, to minimize the impacts of failure. Shifting to a risk-based approach can save budget and prevent outages and can make efficient and effective vegetation management possible for utilities.
Author: Xavier Loin, Asset Management Consultant – published in power-grid.com, 23 November 2021