As the transmission infrastructure ages — and as permitting and constructing new facilities become increasingly difficult — pressure to increase the use of existing facilities is mounting. Finding available capacity within current systems has emerged as an issue of prime importance as we are required to move more and more power over existing rights-of-way (R/Ws).

The maximum line loading is subject to its “rating.” While any one of several variables can govern the rating of a circuit, perhaps the most common limiting factor is the National Electrical Safety Code (NESC)-imposed minimum clearance.

As utilities load their transmission lines to higher levels, the conductors heat up, elongate and sag. The real limiting factor, and hence the basis for a line's rating, is often the ability of the utility to maintain a safe clearance between energized conductors and the ground, trees, vehicles and other objects directly below the line, as set forth in regulations such as the NESC.

Both Public Service Company of New Mexico (PNM, Albuquerque, New Mexico, U.S.) and Tennessee Valley Authority (TVA, Knoxville, Tennessee, U.S.) have installed innovative sag- monitoring devices to tap into the additional capacity that exists in most overhead transmission circuits.

PNM and TVA were the first utilities to adopt the use of the “Sagometer,” which is a transmission line sag-monitoring device jointly developed by EPRI (Palo Alto, California, U.S.), EDM International Inc. (Fort Collins, Colorado, U.S.) and the Southwest Research Institute (San Antonio, Texas, U.S.). The California Energy Commission also committed resources to further system development through its Public Interest Energy Research program.

Normally, when utilities calculate static ratings they make conservative assumptions about ambient conditions (low wind and high summertime temperatures) coinciding with transmission lines heating up due to high electrical loads. They also typically include a buffer in their design clearance calculations.

Collectively, these factors manifest themselves in ratings that are much lower than the true power capacity of the line. As a result, lines often have significant “hidden” capacity available for use if unknowns associated with how the conductors are responding in real-world conditions could be lessened or eliminated. Some studies show most transmission lines can carry 5% to 20% more power than they do under present limitations. The Sagometer's direct measurement of conductor ground clearance and sag under actual operating conditions eliminates the unknowns and enables utilization of the hidden capacity.

PNM Enhances Ratings

While PNM's transmission system had sufficient capacity for many years, times have changed. In anticipation of the impacts of looming pressures to increase circuit and system capacity, PNM investigated what it could do to cost-effectively increase capacity while maintaining reliability and minimizing the need for new lines. PNM's multifaceted approach has enabled the utility to meet system demands and defer major capital expenditures.

The Sagometer is the most recent addition to the utility's thermal rating “tool box” that includes several approaches to enhancing ratings such as:

• Improving line surveys using conventional survey techniques and advanced airborne methods to determine as-built terrain, conductor attachment heights, spans and clearances.

• Improving line models using tools, such as PLS-CADD, to determine if and where there are phase-to-ground clearances that are limiting thermal circuit ratings.

• Making line/structure modifications including, inserting structures or replacing existing structures with taller structures; installing pole-top extenders and raising crossarms; and raising complete structures using Laminated Wood Systems' PhaseRaiser.

• Improving meteorological data used for thermal rating calculations. This has involved probabilistic modeling of conductor temperature performance on a seasonal and time-of-day bases using historic data supplemented with data gathered by several weather stations. PNM's weather stations are equipped with anemometers sensitive to low-speed winds.

PNM found that long-used assumptions for conductor-rating calculations are conservative in most instances. In many cases, PNM justified increasing the assumed wind speed from the long-used 2 ft/sec value. This alone has a significant impact on circuit ampacity as controlled by conductor temperature. PNM increased the ratings of several 115-kV circuits by 15% at 100°C (212°F).

In 1998, PNM became interested in ongoing EPRI-sponsored research to develop a sensor that used imaging technology to accurately monitor ground clearance of conductors. PNM thought that if the research was successful, the information provided by the sensors — coupled with other easily measured parameters and the PLS-CADD line-and design software from Power Line Systems (Madison Wisconsin, U.S.), already in use at PNM — could answer some important questions about the capacity of its system, particularly the 115-kV circuits looping the city of Albuquerque.

Based on the potential benefit, PNM entered into a contract to deploy the first prototype sensors in a real-world setting. PNM recognized the risk using technology yet to be proven in the field, but its supportive management team encourages innovation and recognizes that if the technology was a success, the rewards would far outweigh the risks. PNM's transmission engineering department deployed the first field prototypes on its 115-kV system in May 1999.

These first-generation prototypes have provided accurate measurements and have been very reliable. Prior to PNM's deployment, the only existing Sagometers were the research prototypes developed and tested at EDM's laboratory in Fort Collins and EPRI's high-voltage test facility in Lenox, Massachusetts. After gaining experience with the first prototypes, PNM planned to install more Sagometers. While the first-generation prototypes were somewhat bulky, the second-generation Sagometer design produced significantly smaller, lighter units that consumed less power.

PNM initially used the Sagometers to make measurements supporting calculations to optimize static ratings for normal and contingency operations, and to develop a better understanding of line behavior. In addition, PNM moved a couple of its Sagometers around on selected lines to take a first look at defining how much monitoring technology was needed to adequately model an entire line or groups of lines using PLS-CADD.

Assessing Conductor Life

The application that is currently the highest priority for several of PNM's Sagometers is monitoring the cumulative duration of high-temperature operations of several key circuits. This provides a means to track/estimate thermal degradation in conductor strength on portions of its 115-kV system that may operate at high temperatures during contingencies. PNM has established a threshold strength loss as the trigger for conductor replacement. However, until PNM installed the Sagometers, it had no reliable method for tracking high-temperature operations and estimating strength loss.

In the absence of the Sagometer, PNM's approach was subjective and believed to be too conservative because of assumptions regarding ambient conditions during periods of high current. By instituting this innovative monitoring approach, PNM believes collected data may justify adding 15 to 25 years to the currently projected end-of-life for the conductors. And, even then, PNM may choose to cut samples to check for annealing before beginning wholesale replacement.

Deferring Line Rebuilds

The Sagometer data enables PNM to defer rebuilding its urban 115-kV lines to support contingency operations within safety-code-clearance limitations. This is key because local government regulations require line re-permitting if the utility rebuilds the lines using larger conductors or taller structures.

To avoid the potential future complications associated with re-permitting, PNM may choose to reconductor the lines with the same conductor even when the projected end-of-life of the existing conductors is reached, thus avoiding the need to re-permit. Using the same conductor enables re-use of the existing structures.

PNM funded most of its early work with the Sagometers through EPRI's Tailored Collaboration (TC) program. EPRI's TC program enables EPRI member utilities to leverage their investment in research and development for projects targeted at their specific needs.

Increasing Power Flow at TVA

TVA initially installed two Sagometers on 161-kV lines from a large fossil plant in its western service area. On hot days coinciding with peak summer loads, TVA did not have enough 161-kV transmission capacity at its 1369-MW Shawnee Fossil Plant to meet demand. In lieu of spending millions of dollars to build new lines, TVA installed Sagometers. Now operators monitor clearances in real time and expect this will allow 5% to 10% more power to be safely transmitted over the existing lines. The Sagometers create 24-hour plots of line clearance, which enables dispatchers to see how the lines move under various operating conditions and get a more accurate forecast of how the lines respond.

Ease of Use and Installation

A major benefit to the Sagometer system for PNM and TVA is the ease of installation and the ability to install the system on an energized line. Utilities find that for use in short-term studies, the Sagometers easily relocate with a single crew. The current Sagometer design enables a single crew to install up to three systems in a day. In addition, one crew can relocate two Sagometers in a day (remove them from one site and install them at another).

Future Uses of the Sagometer

Utilities eventually may use information from Sagometers to support a shift in their operations toward dynamic or quasi-dynamic ratings. Such a shift may be several years away, in that they will first have to make other system changes to gain the full benefits of dynamic rating. For instance, with enhanced static ratings, the limiting factor for many utilities will shift from the conductor to the substation equipment, which the utility will need to upgrade to realize further gains.

Accurately knowing conductor sag provides valuable knowledge that can help utilities with long-range planning for bulk-power transfers. With deregulation, utilities often must commit months in advance to wheeling large blocks of power on specific dates. However, because no one can accurately predict future weather trends, an agreement regarding the amount of power than can be shipped is constrained, once again, by worst-case assumptions.

Although real-time monitoring of transmission line sag does not answer questions about the future, utilities can use the Sagometer to develop a database of line behavior in all kinds of weather conditions. Then, instead of having to rely on worst-case assumptions, utilities can use real-world data to redefine the relationship between power flows, ambient temperatures and clearances on a given line, and make more realistic plans for bulk-power transfers.

Use of the Sagometer technology is helping PNM and TVA to meet capacity requirements during a time when rules related to bulk-power transfer are constantly changing and the lines are loaded in ways for which they were not designed. It is also enabling these utilities to defer making system changes until the future direction of how the system will be operated in the longer term becomes clearer.

Application of this technology is consistent with utilities' goals of maximizing the use of current assets. No one knows what the future will hold, but in the meantime, prudent application of technology to bolster capacity is a cost-effective solution.

Blake Forbes, a principal engineer in PNM's System Engineering group, has 23 years experience in the design, construction and project management of various transmission projects. He has the BSCE and MSCE degrees from the University of South Carolina, and is a registered professional engineer in New Mexico.

Dale Bradshaw is a senior manager in TVA's Energy Research and Technology Applications Group, where he manages the Power Delivery Technology Group. He has been with TVA for 25 years and received the BSEP and MSME degrees from the University of Oklahoma, and the MBA from the University of Tennessee at Chattanooga.

Fisher Campbell is a project manager at TVA, where he works in the Energy Research and Technology Applications Group demonstrating new transmission and substation technologies. He received a BSEE degree from Vanderbilt University and is a member of IEEE and PES.

About the Sagometer

The Sagometer uses machine vision to measure conductor ground clearance or sag. A video camera, coupled with sophisticated image capture and analysis technology, monitors the movement of a passive target mounted on a conductor. A special low-power illuminator mounted in the same housing as the camera enables the camera to see the target at night. The target contains a pattern that the machine vision can distinguish from the surroundings.

At user-defined intervals, typically 10 minutes, the system captures and processes images and provides accurate measurements of ground clearance and horizontal movement of the conductor. Measurements can be transmitted on an ongoing basis or archived on an on-board data logger for periodic downloading. Sagometers have been deployed via cellular phones, spread spectrum radios, radio links to RTUs and wireless Internet systems. The device can be powered by AC or solar panels.

(Note: Almost all Sagometer systems have been installed on energized lines).

The Sagometer normally transmits numerical data on ground clearance or sag, horizontal movement, ambient temperature, and other parameters. Also, depending on the communications technology employed, operators can use the Sagometer to view video images of the monitored line segments. This can be especially helpful during extreme weather events including icing events.

The technology was developed by a team of engineers and scientists from EDM International Inc., Southwest Research Institute and EPRIsolutions.