Electricity Asset Management Plan 2019-2029
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Vector Limited://
4.1.8 OUR VEGETATION MANAGEMENT STRATEGY Effective vegetation management, with changing environmental factors such as climate change, is essential to ensure a safe and reliable network. According to a 2017 NIWA study, extreme rainfall, severe droughts and wildfires could hit Auckland in years to come. A report by Ernst and Young (EY) commissioned by Vector, to look at how the Auckland area network could be impacted by climate change, concluded that occurrences of high wind speeds are likely to increase significantly resulting in trees and tree branches bringing down power lines more often. We have experienced this first hand in recent times. The challenge for network companies – under the Electricity (Hazards from Trees) Regulations 2003, is that only vegetation in a limited area can be trimmed. Essentially, this is where it is almost directly against power lines – the ‘growth limit zone’. This hinders the ability of EDBs to adequately protect the network in adverse weather conditions, when trees from outside the growth limit zone damage power lines. The regulations are highly prescriptive as they focus on set distances between trees and lines. For the vast majority of trees, these distances are grossly inadequate. For example, in some cases no action can be taken until a tree branch is as close as half a metre from a power line. This is a very small physical gap and insufficient to prevent trees swaying in high winds and clashing with lines. Some trees are also very fast growing and might require two trims in a season, which is both costly and inefficient (as mentioned above, growth is exacerbated by climate change). Fast-growing trees also tend to be less resilient to high winds and therefore pose a greater risk to the network. The regulations take account of only two parties – the power-line company and the tree ‘owner’. There can be significant issues identifying the tree owner, who can be different to the landowner or occupier. For example, in the case of forestry, the tree owner might be a Post Office box in Geneva. While tree owners may be difficult to find and communicate with, trees that pose a risk to the network continue to grow. The two parties must follow a complex process involving the measurements of tree distances within various zones, issuance of formal ‘cut and trim’ notices for every tree, with punitive action procedures to be followed. While the failure to obey a ‘cut and trim’ notice could result in a theoretical $10,000 fine, there is no record of a fine ever being imposed. Even after a tree is cut the problems can persist. While a newly pruned tree might be physically separated by up to 1.5m from a line, the tree might tower many metres directly above a line, meaning branches can fall across conductors, shorting them or bringing them down. A reliability and safety-focused vegetation programme, completed during the 2016 and 2017 financial years, surveyed around 20 per cent of the overhead network to risk-assess vegetation that could affect the performance of the associated distribution feeders. Of the sites identified, two-thirds (67 percent) involved trees outside of the growth limit zone – a mixture of trees likely to shed debris onto or fall across the lines, causing an outage or risk to public safety. There is an urgent need to move to a modern, principles-based framework that would allow EDBs to carry out, and act on, risk assessments for trees near power lines and oblige tree-owners to take more responsibility for their trees. The risk assessment could include factors such as customer numbers that might be affected by an outage, tree species, age and condition of tree, overhanging branches and fall distance, and issues of public safety, risk of fire etc. Service-level performance analysis has identified that outages caused by vegetation contribute to 17% of SAIDI and 12% of SAIFI (reliability indicators). This, in turn, adversely impacts the service level for ‘customer interruption breaches’. While ‘asset safety incidents’ haven’t occurred in relation to vegetation, having physical contact between network assets and vegetation poses a significant risk to safety, as well as having the potential to cause fires. Effective vegetation management practice is essential to ensure the safe and reliable operation of the network, and to ensure Vector maintains its regulatory compliance in terms of reliability indices, SoSS and the experience of the customer. In the meantime, we are focused on addressing the vegetation issue through a proactive programme to identify trees at high risk of causing an outage and affecting our SAIDI, SAIFI or Customer Interruption service levels, as well as the public’s safety. This is documented in our Vegetation Management investment programme in Section 5.
Lastly, following a successful trial in 2016, we are embarking on a programme of Light Detection and Ranging (LiDAR) surveys, in 2019, with two of the main deliverables being a series of measurements relating to vegetation:
• Clearances between vegetation and conductors (to monitor growth limit zone encroachment) • Height of trees when compared to the distance from conductors (to monitor fall hazards)
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