Optimizing Existing Transmission Capacity: Dynamic Line Ratings (DLR)

All transmission and generation equipment have limits on their useful capacity and these are expressed through what we call “facility ratings”.   Facility ratings include things like the maximum voltage, current, real power, and reactive power of various types of power system equipment.  The ratings of equipment are affected by things like the ambient temperature, wind (which cools equipment), solar heating, and distance from other equipment or objects.  For instance, a transmission line rating takes into account the maximum temperature the metal can continually operate at and the amount of sag that results when the metal expands when heated to ensure it doesn’t come into contact with other lines, structures, or equipment underneath it.

FAC-008

NERC reliability standard FAC-008-5 governs the way transmission and generation equipment are rated “To ensure that Facility Ratings used in the reliable planning and operation of the Bulk Electric System (BES) are determined based on technically sound principles.”  The equivalent standard in Alberta is Alberta Reliability Standard FAC-008-AB-3 and it specifies the factors that must be considered to establish facility ratings.

Two Different Approaches

There are two approaches to rating power system equipment – dynamic and static.  A dynamic rating approach takes into account changing ambient temperature, wind speed, and solar heating to optimize the rating of the equipment for the changing operating conditions it actually experiences.  A static rating approach provides a constant, or “static”, rating based on assumed values for ambient temperature, wind speed, and solar heating for a specified period of time. 

By nature, the static approach is typically far more conservative than the dynamic approach, because the rating will be based on the worst operating conditions expected, even though the actual operating conditions will be far less onerous the vast majority of the time.  However, there are situations where the dynamic rating would be more conservative than the static rating, such as an unseasonably warm winter day for transmission line with a winter rating, example.   

While FAC-008 allows for both approaches, the static approach is most common.  It is more conservative than the dynamic approach and it is much easier to implement and operationalize because it does not require the collection and management of large volumes of data on an ongoing basis by the system operator, and simply uses constant assumptions.  However, it also means that there are many times when the full capacity of the equipment cannot be used because the actual ambient conditions are less onerous than the assumed ambient conditions that the static rating is based on. 

Alberta currently uses static equipment ratings for two seasons, summer and winter, defined in FAC-008-AB-3 as follows:

• Summer: May 1^st to October 31^st

• Winter: November 1^st to April 30^th

What this means is that for most of the time, equipment on the power system is operating with limits based on assumed ambient conditions that are quite different than the ambient conditions the equipment is actually experiencing.  This leaves capacity unused and could mean that generation, say wind or solar, is being transmission constrained simply because of the way the transmission lines are rated, not because there isn’t transmission capacity available at the actual ambient conditions at the time.  Using static assumptions also means that new transmission may be built before all available existing transmission capacity is utilized, which is both operationally and economically inefficient. With many capacity constrained areas on the Alberta power system, the broader use of dynamic line ratings (DLR) could make a significant difference in reducing the volume of congestion we’re currently experiencing and defer the need to build new transmission capacity.

FERC Investigating DLR

Recently, FERC announced an Advance Notice of Proposed Rulemaking (ANOPR) to consider potential requirements for transmission providers to use dynamic line ratings. In its announcement, FERC states that “The proposed framework would also require transmission line ratings to reflect forecasts of wind speed and of wind direction for certain transmission lines in windy and congested areas. In order to identify candidate transmission lines for a DLR requirement related to wind, the draft ANOPR proposes metrics to measure congestion both in RTO/ISO regions and in non-RTO/ISO regions.” and the intent is to “make more efficient and cost-effective use of the power grid by requiring the application of dynamic line ratings (DLR), which use up-to-date forecasts of weather conditions to improve the accuracy and transparency of transmission line ratings.”

FERC’s recognition of the value of DLR will hopefully result in broad adoption of DLR over time to maximize the value of existing North American transmission infrastructure.

The Path Forward

While the dynamic approach is harder to operationalize, I believe it can provide significant optimization value and more fully utilize the billions of dollars we have already invested in the Alberta transmission system. The AESO is already piloting DLR for some transmission lines and this is something that should play an increasing role as power system planning evolves in Alberta. 

In conclusion, instead of writing my own explanation of the potential benefits of DLR in Alberta, I’m going to provide an excerpt from the Alberta Market Surveillance Administrator (MSA) Q4 2023 Quarterly Report that does a much better job than I can :-).

   

Excerpt from the Q4 2023 Market Surveillance Administrator Quarterly Report, February 12, 2024:

2.2 Dynamic line rating

The Alberta electricity transmission line rating is the maximum limit that power can flow through the line, which can be varied by season, as appropriate. The static line ratings are fixed at a certain threshold and referred to as the line capacity. Two seasonal ratings are applied to adjust the line rating, one in the winter months (November 1 to April 30) and one in the summer months (May 1 to October 31). The seasonal shifts from four sampled regions varied greatly from 0% to an increase of 36% (or 0 MW to 142 MW) in 2022, depending on the line.

As the transmission system is built with a zero-congestion policy, there is relatively low congestion compared to generation and congestion generally only occurs in large volumes during line outages or high wind events. However, in regions of increasing congestion, such as areas with multiple wind assets, the returns to using non-wire solutions are high. Even small changes in line ratings in discrete areas or specific lines can create a valuable opportunity for increased flows.

Dynamic line rating (DLR) is a non-wires method to vary an overhead transmission line’s thermal rating limit in response to environmental and weather conditions. Conditions, such as lower temperatures or increased wind speeds, can have cooling effects on the line, reducing sag and increasing the potential safe operating rating. As a result, wind generation and line rating are naturally correlated. DLR is a supporting relief to congested areas, offering additional capacity to lines where the area would have been otherwise constrained, adding to the efficient operation of the electricity grid and market.

There are multiple parameters and conditions that are considered when calculating the real time DLR. The weather factors can include wind speed, direction and angle, solar radiation, ambient temperature, humidity, and more. The physical considerations include line current, sag and tension, material, and construction of the physical line and insulation. Although there are different tools and methods, the more comprehensive the evaluation, the less risky and the more accurate the result.

DLR pilots have led to promising results, ranging from a 20% to 177% increase in line ratings and are currently implemented in Germany, Belgium, the United Kingdom, New York, PJM, and Texas. A 2017 study within a PJM region on a 22-mile, 345 kV line with 3 DLR monitored sections, was estimated to cost $500,000 USD. However, the study also notes that the net congestion savings was over $4 million USD over one year. There are many considerations when implementing DLR including safety, information quality and speed, increased physical monitoring of equipment, and data integration. Using a cost benefit approach could help determine the optimal implementation plan.

2.2.1 Alberta

DLR was previously tested in Alberta from June to December of 2015, when AltaLink conducted a DLR study on four segments of a 138 kV line. The site was of interest as the majority of the power in the test area was from wind generation. The tested lines include both east-west and north-south components in an L-shape, which was chosen to assess the impact of wind direction on line cooling.

The results included a mean increase in over 75% of the study period (June to December 2015). The increase in mean transmission line rating ranged from 22% to 72%, depending on the line. Of note, during certain instances the dynamic rating was lower than the original static rating. The report theorizes that this was due to the wind reaching the generators before the lines, where the increased flow from generation was heating the line before the wind was able to cool it. Although no major issues were reported in this study, AltaLink stated in 2022 that the province’s harsh climate has previously caused different DLR devices to stop working, go out of calibration, or be blown from the line.

Since this time, the AESO has begun to explore using DLR for real time operations in the South and Central regions (AltaLink and ATCO TFO regions, respectively). During the 2023 grid reliability session, the new pilots were discussed, and the in-service date for DLR was updated to 2025. The proposal includes the 240 kV double circuit lines 924L and 927L as well as the 138kV line 7L128. There have been previous mentions of DLR in AESO’s 2021 technology forward publication and in the 2022 year in review where the AESO states that DLR would be implemented where feasible in 2023.

2.2.2 Theoretical DLR Example

Line 610L is a 138 kV line located in the Vauxhall region and runs between Fincastle and Taber. The seasonal shift in thermal line rating is approximately 6%. On November 15, 2022, the AESO filed a development NID which will include a new line and the removal of the current 610L. The line is frequently overloaded in real time and, consequently, multiple solar and wind farms are constrained down. The proximity to wind generation assets, length and congestion makes 610L a candidate for DLR.

The data from the 610L is outlined below as a simplified example of the potential increase in capacity from wind generation. A previous study by the Idaho National Laboratory estimated that a 12 km/h wind speed can increase the dynamic line rating by 35% in a conservative scenario. Using this metric, the dashed line represents the dynamic line rating, where the line rating is increased by 35% when wind speeds met or exceeded 12 km/h. The yellow line represents the wind speed in km/h, the orange is the current seasonal line ratings employed by the AESO, and the blue line shows actual flow in MVA.

The overall modelling trends demonstrate the overlapping pattern of wind speed and increased flow. The actual real flow on 610L is frequently above the seasonal line rating, as shown where the solid blue line exceeds the orange. When capacity is modeled with the increase in line rating from increased wind speeds, the line would not have been overloaded (in most of situations that occurred). An example is from November 14 to 28, 2022, where actual flow exceeded the line rating, but was below the DLR theoretical limit. 610L is a demonstration of how DLR could help increase the line rating and flow, reduce congestion, and contribute to efficient use of the existing transmission system.

The MSA recommends that the AESO continue to study and implement DLR in Alberta in targeted areas, such a high wind or frequently constrained areas. More tangible deadlines with near term implementation could facilitate optimal planning and use of the transmission system. Understanding DLR’s role in operations is an opportunity to incorporate this method into the future grid.