Overview & Events

Project description, task information, and event opportunities.

Transmission rating engineers are faced with unique challenges. There are increased demands for powerflow, reliability, and safe operation. There are complex tradeoffs between capacity improvement options. Different approaches will not only carry different costs and require different times to complete, but there may also be considerations for line designs, system operations, or environmental impact. For example, an ACSS conductor upgrade may cost less than a HTLS conductor but it will be more likely to exceed NESC required clearances to ground during emergency load events. This EPRI project seeks to explore and identify the tradeoffs for different methods to increasing power flow, determining ratings, and managing risk. Comparative data from laboratory testing and utility demonstration projects are combined to give engineers information to guide decisions.

This project addresses multiple industry needs including the following:

• Documenting lessons learned from utility uprating projects
• Assessing the performance of commercial rating systems
• Quantifying the cost, system risks, and time to complete of uprating alternatives
• Testing utility conductors to guide rating assumptions
• Exploring novel uprating technologies such as conductor coatings
• Investigating industry standards and improving calculation algorithms
• Providing reference and training material
• Developing engineering software tools

Objective

This project intends to capture and document knowledge regarding selection, design, and application of transmission ratings and uprating methods. This material includes reference guides, update videos, technical reports, software tools, and industry insight whitepapers. The overall objective is to allow increased utilization of transmission assets to maximize power flow while managing risks. This optimization allows more efficient, reliable, and resilient operation of the grid.

Research Insight Example

In transmission line clearance profiles developed from LiDAR data sets there are errors present caused by the measurement technique as well as from the methods used to establish the conductor temperature at the time of the flight. In general, errors caused from conductor temperature estimates result in final sags having errors of several feet. Whereas errors caused from poor measurement methods or poor LiDAR scanner design most often result in missing underbuilt lines, fences, or line crossings. Further information is available in a 2022 Technical Update on transmission line clearances.

Planned 2024 Research

This project conducts desktop, laboratory, and full-scale testing of ratings technologies, utility methods, and transmission assets. The overall research objective is to provide information needed to develop a technical basis for ratings decisions. The 2024 project includes the following tasks:

Risk Mitigation and Uprating Clearance Limited Lines: As transmission lines carry increasing amounts of power they operated at higher temperatures. When conductors heat up, they expand increasing their sag and reducing the physical and electrical clearances to other objects. To maintain safe clearances, some lines must operate at reduced ampacity, which decreases system performance and reliability. 2024 research will explore the range of upgrade strategies available to increase transmission capacity by removing power flow limits that caused by clearance issues. The intent is to develop guidance that will help “right size” the solution for their specific line designs and ampacity needs; which should shorten upgrade durations and reduce costs.

Guidance for Ratings Practices Aligned with FERC 881: In the US utilities are required to update their ratings practices based on a recent FERC order. This order changes not only real-time ratings (ambient adjusted ratings), but also short-term forecasts and seasonal ratings. EPRI has developed several reports on these ratings topics since 2019 (prior to the release of the order). In 2024 we plan to merge all of the guidance documents into one comprehensive document with updates based on key learnings from one-on-one projects where EPRI has supported utilities in developing new ratings.

Understanding Climate and Extreme Weather Challenges for Line Ratings: Maintaining accurate transmission ratings heavily depends on understanding weather conditions and how the system reacts to changes in weather and climate patterns. Lines are often designed and built based on historical data. However, the conditions they are exposed to in the future can differ greatly from past weather. In 2024 we plan to publish the first in a series of reports to help utilities shift towards using climate data for ratings decisions. The initial report will expand on work completed in 2023 that explored potential improvements to emergency rating practices to improve power flow during extreme weather events.

Updating AAR Calculation Tools: The goal of this task is to update ratings calculation tools with new layouts and features based on user feedback received from utility beta testers in 2023. The updated calculators are intended to streamline the development of table-based ambient adjusted ratings and expand on the list of calculators available in Transmission Ratings Workstation (TRW). Updates are also being performed on the conductor database and for software security.

Considerations for Realtime Ratings: The utility industry is seeing increased adoption of ambient adjusted and dynamic line ratings. While this naturally effects real time operation, it will also require a new philosophy for line design, conductor selection, and identification of circuit upgrades. In 2024 two activities will be performed to support these efforts. EPRI will continue to host information sharing sessions for utilities to communicate lessons learned and emerging issues among peers. In addition, sections on real time ratings within the EPRI Increased Power Flow Guidebook will continue a multi-year update.

Analysis of Radial Conductor Temperature Datasets: Due to the fundamental properties of heat transfer transmission conductors are hotter at the core than the outer surface. The increased core heat can lead to lines clearances being lower than projected in software tools. In 2024 we plan to perform a review of available data, guides, and industry literature. This will be used to develop plans to address knowledge gaps identified in the present data state-of-the-science.

Testing of Conductor Emissivity: Overhead line ratings depend on a factor known as emissivity. This factor is used to determine how well conductors can self-cool via IR emission. This is particularly important for high temperature conductors or when wind speeds are low. If a utility assumes a value that is too low, they may underestimate the capacity of their lines. If they use a value that is too high, then ampacity may be overestimated and lead to risks. This research task will test samples provided by utilities using an EPRI developed test apparatus; allowing utilities to replace assumed values with accurate data. Utilities are encouraged to provide samples from their new stock or lines removed from service as applying these results can improve the accuracy of their line ratings, LiDAR models, and IR inspections.

Testing of Conductor Absorptivity: Overhead line ratings depend on a factor known as absorptivity. This factor is used to determine how hot conductors will become when exposed to sunlight. If a utility assumes a value that is too high they may underestimate the capacity of their lines. If they use a value that is too low capacity may be overestimated and lead to risks. In 2024 EPRI plans to improve the accuracy of the algorithms uses for absorptivity analysis and to test a large number of samples with a known emissivity to inform the relationship between emissivity and absorptivity.

Applying the Research & Utilizing Value

The results from this research will provide distribution utility personnel with information and tools to improve specification and selection decisions for new overhead assets, increase system reliability and resiliency, enhance inspection and assessment effectiveness, lower inspection, and maintenance costs, and increase awareness of emerging technologies.

Ways to Engage

Utility subject matter experts in line design, transmission planning, standards/compliance, transmission ratings, and operations can utilize the results from this project.

Members are encouraged to participate in several ways:

• Attend task-force meetings and update webcasts
• Provide conductor samples for evaluation
• Share information on your utility’s standards or practices and uprating experiences
• Collaborate in one-on-one training and research summaries (contact Justin Bell)
• Attend testing at EPRI laboratories
• Provide guidance on utility needs and emerging issues
• Perform comparative assessments using TRW software tools
• Participate in customized projects to address uprating needs, such as assessing commercial line rating sensors
• This project provides an opportunity to meet and network with ratings experts from other utilities.

Supplemental Projects

Given that ratings practices and needs are highly utility specific, often projects in this area are bespoke and crafted to solve a specific challenged. Utilities are encouraged to reach out regarding their specific challenges if they are not addressed by the projects listed below.

Risk Management for Increased Transmission Capacity

Transmission lines are constantly being pushed to and beyond their limits. Much of the grid is nearing the planned retirement ages and utilities seek to extend the useful life of their lines while ensuring reliable transfer of power. When lines are pushed to their limits line clearances can be encroached on. It is important for utilities to maintain a safe distance between lines and objects such as buildings, trees, and underbuilt lines. This is particularly the case for older lines where LiDAR information indicates there is little to no clearance buffer. EPRI has developed a one-on-one project to help utilities understand the balance between capacity and risk for their lines. Methods to optimize ratings while maintain safety and reliability criteria have been developed by EPRI and can be applied to congested spans or larger parts of the grid. This is of particular importance in North America where the adoption of AAR will significantly increase risks to transmission assets and can result in violation of line clearances without operators being aware. The main outcomes of this project are improved ratings methods and assumptions, utilities will also have a strong technical basis that considers system risk for decision moving forward.

Methods to Improve Weather Modeling for Transmission Ratings

Weather data is a key input into both AAR and DLR, it is also needed by utilities to drive decisions about seasonal ratings and other topics such as vibration, wind loads, etc. Extensive research has conclusively shown that most weather models and weather stations are not suitable for ratings purposes. They can create significant errors in ratings causing utilities to absorb unnecessary costs from congestion, curtailments, and upgrades. They can also result in asset damage from thermal effects and violation of line clearances. The cost of deploying weather stations or commercial DLR technologies can be extensive, particularly when considering long term maintenance, calibrations, and replacements. This project seeks to apply weather modeling in combination with local weather stations to extend the coverage area of local measurements. This project also explores the accuracy of existing weather models and forecast tools as they continually evolve.

Engagement Opportunities