The Risk of Losing Capacity with FERC 881

FERC's intention is to increase grid capacity

FERC Order 881 in the US obliges all transmission owners to implement Ambient Adjusted Ratings (AAR) by July 2025. Utilities must move away from using Static Line Ratings (SLR), in which grid capacity is based on assuming constant values, and must account for the grid cooling effect from air temperature.

The intention of the order is to reduce power prices for consumers by increasing grid capacity: Chairman Glick said: “If we are going to [keep] customer rates just and reasonable [...], we need to squeeze everything out of our existing grid,” (source ferc.gov)

In the winter when it’s cool outside, accounting with temperature does lead to an increase in capacity. The red line (AAR) is above the static ratings (SLR) most of the time in January, mainly because it’s colder outside. The grid can carry more power. The below image shows line ratings in one week in January in 2022, based on weather data from the Pioneer Airfield, Arizona.

Gain in capacity is not always guaranteed

Whether AAR leads to a gain or loss depends on what the assumptions were that the utility used in calculating ratings in the first place. Here we illustrate this based on a hypothetical example from Arizona.

In May, ambient ratings (AAR) are consistently below the static ratings (SLR) during the day, as outside air temperatures reach or exceed 100 degrees Fahrenheit.

Implementing AAR in this example would lead to a loss of capacity a whopping 40% of the time on an annual basis. It’s especially ironic that the loss occurs exactly when additional capacity would be needed - during the day when industry and air conditioning is turned on!

Utilities have been optimizing already for decades

The reason this happens is that many transmission owners have been pushing their grids to the maximum for decades already and might knowingly have been using optimistic assumptions for wind speed in calculating their static line ratings. 

In the example shown above, the static ratings were calculated using a wind speed of 4.4 ft/second perpendicular to the conductor (1.34 m/s). The implicit assumption by the grid operator has been that the excessive benefit from high wind speeds is compensated for by the margin from a conservative assumption for air temperature.

Now, however, FERC 881 requires the actual air temperature to be used, thus removing this buffer. Without this buffer utilities are now forced to revisit and reduce the wind speed assumption in their calculations. Instead of 4.4 ft/second they might use 2 ft/second (0.61 m/s) in AAR. This then leads to a loss of capacity when it’s hot outside.

An easy fix to this would be to take more risk and continue using a relatively high wind speed assumption. But this would run the risk of the wires actually overheating, sagging too close to the ground, causing a flashover and igniting a fire.

Dynamic Line Ratings help increase capacity while maintaining safety

A more customized solution is to implement Dynamic Line Ratings right away. Grid Raven’s wind forecast machine learning model, Boreas, covers the full network and helps utilities use accurate wind speeds in their calculations. The below graph shows the same week in May, with Dynamic Line Ratings (DLR) shown, in which actual wind speeds were taken into account (green line). Dynamic Line Ratings can help avoid safety problems while maximizing grid capacity.

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