Variable transmission facility ratings benefit the stability, capacity and reliability of the grid.
As electricity demand surges and renewable energy integration accelerates, the U.S. power grid faces mounting pressure to deliver more power, more efficiently. One promising solution lies not in building new infrastructure, but in optimizing the capacity of existing transmission lines through variable facility-rating methodologies—specifically, Static Line Ratings (SLRs), Ambient-Adjusted Ratings (AARs) and Dynamic Line Ratings (DLRs). Â
Understanding Transmission Line Ratings
Transmission line ratings determine how much electricity a line can safely carry. Traditionally, utilities have relied on SLRs—conservative, fixed values based on worst-case weather conditions. While safe, SLRs often underutilize available capacity, especially during favorable weather.
To address this, utilities are evaluating AARs and DLRs, which adjust line capacity based on real-time or near-real-time environmental conditions.
Relieving Grid Congestion
Variable Transmission Line Ratings (VTLRs) are gaining momentum due to several key market drivers. One of the primary motivations is the need to relieve grid congestion, especially in areas with increasing electricity demand and limited infrastructure. VTLRs also support the integration of renewable energy by enabling more flexible and efficient transmission, reducing the curtailment of wind and solar resources.
Additionally, as many transmissions systems age, VTLRs offer a cost-effective alternative to expensive infrastructure upgrades. Regulatory support is playing a significant role as well—most notably in the U.S., where FERC Order No. 881 mandates the use of AARs to improve grid reliability and efficiency. Similar regulatory momentum is seen in Europe, where grid operators are embracing dynamic line rating technologies to meet energy transition goals.
Comparing Methodologies
In our recent study with INL, we discuss how variable transmission facility ratings benefit the stability, capacity and reliability of the grid.
AARs provide a practical, lower-cost way to boost transmission capacity by about 15% over SLRs, though they deliver about 16% less capacity than DLRs. This makes them a useful intermediate solution for grid operators to ease congestion and integrate more renewables without the full complexity and cost of DLRs. As such, AARs can serve as a cost-effective bridge while utilities build the systems and expertise needed to fully implement real-time dynamic ratings.
Enhancing Grid Performance Through DLRs
We found that DLRs “have the potential to expand practical line capacity, improve line utilization, reduce transmission congestion and enhance market efficiency. In North America, ISOs and RTOs are counting heavily on mathematical optimization to dispatch generation resources and serve the net demand in their corresponding market.” (Idaho National Laboratory & POWER Engineers, member of WSP, 2024)
Consider these two recent case studies:
Pennsylvania Power & Light installed DLRs on three transmission lines and saw a $64 million reduction in congestion costs over one winter.
New York Power Authority reported up to 20% capacity increases, reduced wind curtailment, and improved system visibility using DLRs combined with machine learning.
These results underscore the potential of DLRs to optimize power flow, reduce congestion and enhance market efficiency, especially when integrated with other grid-enhancing technologies (GETs) that balance overloaded and underutilized lines.
GETs and DLRs offer innovative solutions to increase transmission capacity, improve grid flexibility and support renewable integration. Their effectiveness depends on how utilities and transmission operators implement them in practice, requiring coordinated planning, real-time data integration and clear operational protocols.
Moving Forward
As the grid evolves to meet 21st-century demands, variable line ratings offer a powerful, cost-effective way to unlock hidden capacity. By moving beyond static assumptions and embracing real-time data, utilities can enhance reliability, support renewable integration and deliver more power.
POWER partnered with Idaho National Laboratory (INL) to explore how electric utilities can modernize their infrastructure in response to the growing demand for energy and the need for expanded transmission capacity. Through a series of case studies, this collaboration examines how grid-enhancing technologies (GETs) can serve as effective interim solutions while new transmission lines are being planned and built.
This article is the second in a four-part series highlighting the role of GETs in strengthening the grid and supporting utilities in meeting future energy needs.
Explore all four case studies here: