By Paul Richardson, Engineering Director, NM Group
Every heatwave now tests the reliability of overhead power networks. As extreme heat becomes more frequent and severe across North America, the UK and Europe, network operators face a seasonal challenge that’s often underestimated: rising ambient temperature directly affects overhead line performance.
Unlike storm damage, which is sudden and visible, heat-driven effects build up quietly – within a single heatwave, and cumulatively across many summers – until clearance margins that once looked comfortable no longer are. High ambient temperature affects overhead power lines mainly through three mechanisms: thermal expansion, increased sag, and long-term creep of the conductor.
When a power line encounters an external condition, the effect is not always one-directional – many conditions carry both a negative and a positive impact. High wind speeds, for example, increase mechanical loading on the conductor system, affecting its structural capacity, but also reduce conductor temperature and improve still-air clearance performance. Conductor blackening – the gradual darkening of a bright silver conductor over its service life – increases the conductor’s absorptivity, meaning solar radiation has a greater impact on conductor temperature; conversely, blackening also increases emissivity, which increases the amount of heat the conductor loses through cooling.
In the case of increased ambient temperature, however, all impacts are negative, and all are cumulative.
Increased Conductor Temperature Means More Sag
When ambient temperature rises – particularly while the conductor is carrying current – the conductor heats up and expands. As conductor length increases, it sags further between towers:
Higher temperature → Longer conductor → Greater sag → Reduced ground clearance
This is typically the most immediate concern during periods of high ambient temperature.
Clearance Requirements Become More Critical
Because sag increases at high temperatures, utilities must ensure the line still maintains the minimum required clearance from:
- Ground
- Roads and railways
- Buildings
- Trees and vegetation
- Other power lines
To accommodate maximum expected sag, designers may:
- Use taller structures
- Increase span tension, within limits
- Select conductors with lower thermal expansion
- Design for higher operating temperatures
High temperature does not increase clearance itself – it requires more initial clearance during design, to ensure safe clearances remain under hot conditions.
Additional Creep
Creep is the permanent elongation of the conductor over time under mechanical stress. High temperatures accelerate creep, because the conductor material undergoes greater plastic deformation. The effects:
- Permanent increase in conductor length
- Permanent increase in sag
- Further reduction in clearance over years of operation
Creep is especially important for:
- Aluminum conductors
- High-temperature operation
- Long spans
- Conductors subjected to frequent thermal cycling
In the case of increased ambient temperature, all impacts are negative, and all are cumulative.
The Combined Effect
The total sag of a line can be expressed as:
Total Sag = Initial Sag + Thermal Sag + Creep Sag
Thermal sag is reversible: the conductor contracts again as it cools. Creep sag is permanent: the conductor remains longer even after it cools.
Thermal sag can be further differentiated into sag due to high ambient temperature and sag due to increased electrical load – both increase during periods of high ambient temperature. Creep sag, too, increases under periods of high ambient temperature, but behaves differently once the line cools: thermal sag will revert to a more typical condition, while creep sag will not. Creep sag is a permanent increase in conductor sag.
A Practical Example
Suppose a transmission line is installed in spring, at 59°F (15°C). At a summer temperature of 100°F (38°C), the conductor expands and sags more. Electrical load – likely increased by demand for air conditioning and cooling for data centers – also raises conductor temperature, and therefore thermal sag. This may well exceed the thermal limit originally considered for the line, such as 167°F (75°C).
After several years of operation at elevated temperatures, creep permanently lengthens the conductor. Creep is accounted for at the design phase of a project, but this is based on the ‘everyday tension case,’ typically taken as 60°F (15°C).
Where the temperature a line is exposed to is above the temperature accounted for in the creep design, the conductor will sag more than its as-designed condition. Once a conductor goes beyond its as-designed condition, not only are clearance and creep performance affected, but – in the case of greased conductors – grease may migrate from the conductor strands or core. The line then sags more than its original as-designed condition.
Therefore, high ambient temperature:
- Raises conductor temperature
- Increases temporary sag through thermal expansion
- Accelerates long-term creep, beyond that for which a line may have been designed
- Requires larger design clearances to maintain safe distances under worst-case conditions, greater than may have been accounted for in the design phase
What Can Be Done?
That’s a significant problem – and with everyday temperatures rising year on year, and demand for power increasing alongside them, the question for network operators is what can be done to mitigate it.
The good news is that a dual strategy of implementing a LiDAR survey program and adopting seasonal – or better still, ambient-adjusted – ratings will address all of these issues.
LiDAR Survey Programs
LiDAR survey removes the reliance on the as-designed condition. Creep can be measured, rather than assumed, and areas of concern for clearance can be quickly identified and addressed.
NM Group is a market leader in aerial LiDAR data capture, delivering survey-grade, high-density point-cloud data across some of the largest transmission and distribution networks in North America and Europe. Our Aerial LiDAR survey services give asset managers an accurate, current, and defensible record of conductor position and clearance – replacing assumption with evidence, network-wide.
Ambient Adjusted Ratings
Ambient-adjusted ratings ensure that the electrical load transmitted through the conductor will not exceed the conductor’s thermal limit – whether that limit relates to the conductor’s internal mechanical performance, or one identified through clearance studies and thermal rating assessment.
NM Group’s Caydence AAR solution turns real-world environmental data into safe, dynamic line ratings – unlocking additional network capacity where conditions allow, while protecting the network at exactly the moments heat and demand peak together.
Clearance can be maintained. Creep performance can be managed. Conductor reliability can be preserved. Increased safety, increased reliability, increased sustainability.
Contact NM Group today to discuss a LiDAR survey or Ambient Adjusted Rating program for your network.