It was designed for 100°C (212°F) operation and built for 100°C operation, but can it safely operate at 100°C? This question of transmission facility functionality was posed by the North American Electric Reliability Corporation (NERC) to transmission planners, owners and operators last year. Following the discovery that some transmission facilities could not operate according to their specified rating, NERC issued a recommendation in October 2010 that utilities establish a plan to evaluate any discrepancies between design and field conditions of transmission facilities.
Utilities were asked to verify that the defined facility ratings methodology in place provided adequate tolerance for the variables of field conditions such as location, height, topographical constraints and maintenance of adequate conductor clearance. The NERC recommendation also required any ratings discrepancies be brought back into compliance with the utility facility ratings methodology within a year.
For Georgia Transmission Corp. (GTC), which builds and maintains the high-voltage power infrastructure on behalf of 39 of Georgia's electric membership cooperatives (EMCs), this meant submitting a plan to validate the ratings on more than 3,000 miles (4,828 km) of transmission lines crisscrossing the state.
Ratings figure heavily into long-range plans for facility additions and upgrades. Eliminating thermal and voltage constraints and meeting the need for load distribution are planned based on the specified capabilities of a line to transmit power as designed. In assessing how to meet future demand, planners rely on a given facility's ability to transmit at capacity. Those not performing to their specified rating may fault during peak load conditions, causing outages and threatening overall system stability and reliability.
Due to the age of many facilities in the electric grid, verification of ratings is especially necessary since construction took place prior to computer modeling techniques that could more closely account for varying field conditions, a fact GTC recognized well before the NERC recommendation was issued.
A Head Start
For some utilities, the NERC recommendation required starting from scratch to develop a step-by-step verification plan, which had to be submitted to NERC for consideration by the Dec. 15, 2010, deadline. But GTC was already in the midst of a systemwide assessment that would meet the criteria of the verification process.
Beginning in 2007, the utility determined the need to undertake a full system assessment to create a more comprehensive, complete and accurate engineering model of the grid facilities. This assessment would give GTC a baseline and measurable benchmark for system performance.
The utility prioritized facilities according to age; those built prior to 1997 were assigned to Group 1, the first set of facilities to be assessed. These older facilities, built according to paper-based engineering plans, were anticipated to have greater potential for discrepancies because of the limitations of integrating field conditions into a 2-D plan. Since 1997, the utility has used PLS-CADD, Power Line System's computer-aided 3-D design and modeling software, which allows for more precise design. Use of this technology means newer facilities were less likely to have discrepancies. Group 2, comprised of post-1997 facilities, represented just 25% of GTC's system.
In 2008, the utility partnered with Sanborn Map Co. to begin aerially surveying facilities designated for Group 1 with light detection and ranging technology (LiDAR). LiDAR uses laser technology to reflect objects and map the physical properties of a line and surrounding areas, providing the utility with, quite literally, a clear picture of the transmission system. Used for some time in military and environmental science applications, the utility industry adopted use of LiDAR technology within the last decade.
GTC recognized LiDAR's potential to create accurate engineering models defined by georeferenced data rather than relying strictly on paper-based engineering plans. In addition to modeling the exact placement of lines and poles, LiDAR also added the facility surroundings to the model including the topography and vegetation in or abutting the right-of-way, as well as actual conductor sag and ground-to-conductor clearance.
During the data collection process, lines were flown as efficiently as possible with parallel lines being flown from opposite directions for the most precise readings. GTC also used a third-party ground-based survey crew to validate the accuracy of the LiDAR data. Along with the processed data, other deliverables included oblique photographs taken approximately every 50 ft (15 m) and aerial imagery. Three terabytes of total data were delivered on external hard drives for archiving purposes.
By the end of 2008, 75% of GTC's transmission system — more than 2,000 miles (3,219 km) of line — had been flown and cataloged with LiDAR. In early 2009, GTC began feeding the data collected from the aerial survey into PLS-CADD to initiate the modeling process.
Responding to NERC
When the NERC recommendation was issued last year, GTC's modeling efforts were well under way. Plans requested in the NERC issuance included an evaluation to adequately account for field conditions, a process to review system facilities to verify the rating and remediation steps to be taken if discrepancies were identified. The NERC recommendation stated a one-year limit on addressing identified ratings issues.
NERC required that the assessment be made with technology that had the capability to deliver data with the precision to definitively note field conditions. In fact, the NERC recommendation indicated the impetus for the call to action was the result of a survey using LiDAR by a transmission owner after a ground-to-conductor clearance issue caused an outage. With its LiDAR survey in progress, GTC was able to assemble a plan that reported an assessment was under way and detailed both its time line to complete the process and its action plan for responding to discrepancies.
A Model Response
LiDAR was already giving the utility a clear picture of its system and the field conditions. In some cases, the LiDAR survey revealed changes in terrain that required updates to the facility. In others, additions and changes by other utilities, such as raising or lowering crossing distribution, altered the rating of the line. The survey also provided improved accuracy on sag modeling through the use of finite element analysis and calculating actual ground-to-conductor clearance.
Upon identifying field conditions that might indicate a discrepancy with the potential to impact safety or reliability, GTC immediately dispatched a dedicated field response team to verify conditions visually. This team, comprised of all the necessary functional areas — project management, design, environmental, construction inspection, land, procurement and maintenance — developed the most effective remediation plan to achieve the facility rating. If necessary, GTC downgraded the facility and issued a derate notification letter to the system operator.
No findings to date posed an imminent threat, but GTC acted quickly to complete required upgrades and repairs to bring the line back to its as-engineered rating. With the stability and reliability of the system in mind, GTC made the necessary repairs to bring facilities back up to the intended rating within weeks — in some cases within days — of an identified issue.
Through the experienced gained over the first two years of the program, GTC has learned what works best for it. Whether prioritizing lines to verify within a given year or reacting to discrepancies found on the system, processes to address the NERC recommendation were created, modified and refined to protect reliability, minimize the manpower needed and reduce costs.
Lines should not be modeled all at once. Determining the order in which lines are modeled requires consideration of many things, including time of year and existing planned outages on the lines for project or maintenance activities. Since all utilities are verifying their facilities per the recommendation, inter-utility connections also need to be coordinated.
By developing a dedicated field response team that consists of the critical functional areas of the organization, facility derates are kept to a minimum time frame, reducing system reliability exposure. The dedicated field team considered issues such as access, materials, and road and railway permits needed during an initial field meeting, which is treated as a kickoff to developing the least-cost option for resolving any potential discrepancies. When that least-cost option involves a collocated utility, such as a local distribution owner, coordination takes place as soon as the discrepancy is discovered to reduce the total time needed to resolve it.
To prepare for the potential increase in facility modifications needed to resolve discrepancies, GTC implemented a temporary increase to material safety stock based on the most cost-effective engineering solutions. This ensured the material for any necessary upgrades would be available when needed.
Derating a facility involves a high level of urgency to restore its rating that is felt across the entire organization. With the emphasis on returning the system to normal, there is the potential for unnecessary shortcuts. Stressing safe working practices to all involved must be at the forefront of any LiDAR program.
The modeling completed in 2009 provided a system snapshot of the field conditions in each of GTC's voltage classes. From there, GTC evaluated the priorities for additional rating verification procedures and any necessary upgrades and repairs. GTC accounted for outage constraints and priority project areas in defining the order of importance. High-capacity bulk lines of 230 kV and 500 kV are designated top priority for completion by the end of 2011. All 115-kV lines have been identified as second-tier priority for completion by the end of 2012.
The remaining 25% of the system — those facilities constructed after 1997 — will be flown with the LiDAR technology next year to complete the system model and validate ratings. Any necessary ratings adjustments will be scheduled for completion by the Dec. 31, 2013, NERC deadline.
From an operational standpoint, GTC will have a way to evaluate system performance in the long term based on the completed model. Through the LiDAR survey, all system facilities will be modeled with PLS-CADD, providing a complete picture of the transmission grid and the actual field conditions in which it operates. The model will not only help GTC monitor potential issues in existing facilities, it also will help the utility to refine future facility design and modifications to meet the challenges and variables presented. This increased operational efficiency will allow the utility to better serve its EMC members and bulk customers.
GTC's initiative has led to these early lessons and, over time, some refined processes. As the industry embarks on the path to compliance with the NERC recommendation, many more lessons will be learned and should be shared among the industry, giving all utilities the tools to complete the verification process safely and with the least impact to reliability. By 2013, the NERC recommendation will have led to recommended practices to keep transmission lines performing as designed.
Michael Fourman (firstname.lastname@example.org) holds an electrical engineering degree from Sinclair College and a management degree from DeVry University. He began his career in the electric utility industry at Dayton Power and Light in 1988. He has been with Georgia Transmission Corp. since 1998, where he has worked in line design and line maintenance. He is currently the manager of transmission line design.
Power Line Systems www.powline.com
Sanborn Map www.sanborn.com