Safely operating power lines during hot summer days

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Georg Rute, CEO

Image Blog

Safety is the number one concern for all transmission service providers. Unfortunately accidents do happen despite the best efforts to prevent them. One of the factors that contributes to accidents is the fact that transmission lines may dangerously overheat on hot, sunny, windless days.

Power lines may overheat when there's no wind

Every transmission line has a maximum operating temperature that it can tolerate. But line temperature is not measured in 99% of high-voltage transmission lines. Instead, grid operators rely on calculations to determine how much power a line can handle.

These calculations are done according to international standards. The ability of a power line to carry energy depends on the type of conductor as well as the weather conditions. Since weather is variable, the standard approach is to select certain agreed-upon conservative assumptions. The key assumption in these calculations is that wind blows at 0.61 m/s onto the wires.

Under most circumstances, a wind speed of 0.61 m/s is very conservative. But power lines cover entire countries and some parts of a line may be in forested areas or inside valleys. It is those parts of a power line which are sheltered from the wind which are most at risk of overheating.

Importantly, if the actual wind is in fact 0 m/s, then the capacity of a line may be one third less than the international standard specifies. So there is a risk that a power line is overloaded by one third.

Weather conditions determine the amount of power that can be transmitted over a power line. The greatest effect comes from wind. Dynamic Line Ratings account for the wind cooling effect, so that on cold and windy hours a power line can carry multiple times more power than when it’s hot and still. Historically grid operators have relied on Static Ratings, which means that changes in weather are not accounted for. Instead, assumptions are made about weather conditions, for example that wind blows at a constant 0.61 m/s. However, these standard assumptions are too optimistic roughly 3% of the time, which can lead to dangerous overheating of the power line.

As a power line heats up the wires expand and drop closer to the ground. During normal operation the wires may move up and down by several meters. Normally this is not a problem, since each power line is designed to be high enough from the ground in the first place, with proper safety margins.

However, if there has been an error during design, or if the power line is old and has deformed over time, then these safety margins might already be breached. These safety concerns are exacerbated by the calculation method that is used for determining line capacity.

A 132,000 volt power line can electrocute an object from a few meters away even if it's not in physical contact directly. If there happens to be something underneath the line when it overheats and sags too close to the ground then there is a risk of a flashover. For example if a tree is too close to the line then it might spark a wildfire.

FERC Order 881 may lead to power lines overheating

The assumption that there is a 0.61 m/s wind blowing onto the conductor and cooling it down is overly optimistic at least 3% of the time (see for example WATT). To illustrate this let’s look at Estonia. It’s a cold and windy place, almost as far up north as Alaska. Yet even here the overheating of power lines is a problem. Air temperature goes above 25°C / 77°F only about 2.5% of the time, but when it does it’s usually sunny. Half the country is forested, so power lines are often sheltered by trees. The assumption that wind blows at 0.61 m/s is overly optimistic in these cases and flashovers do happen, especially under older power lines inside forests.

In the US grid operators are mandated by FERC to implement so-called Ambient Adjusted Ratings (AAR) by July, 2025. This means that grid operators must use ambient conditions, such as actual air temperature, when calculating the ratings of their power lines. However, AAR solutions still typically make the assumption that wind is a constant 0.61 m/s. Therefore the risk of overheating on hot summer days remains.

Accurate wind predictions are needed to ensure safety

Grid operators need accurate wind information in order to remove the risk of lines overheating during windless hours. But it is not feasible to install physical sensors under every section of every power line in the country. Grid Raven is solving this problem by employing machine learning for modeling wind with meter-scale accuracy. This covers every section of all power lines and provides an accurate view of the network to grid operators.

Until Grid Raven’s technology has been implemented everywhere, when you’re walking in a forest on a hot, sunny day and you see a high-voltage line that is suspiciously close to the ground, then it might in fact be so. Don’t raise your hands while walking underneath it!

Grid Raven's AAR solution takes wind into account. Read more about it here:

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