SALT RIVER PROJECT (SRP) EXPERIENCED AN OUTAGE IN JUNE 2003 when a high-voltage line sagged into a 12-kV circuit, despite the fact that the line was supposedly operating at 60% of its rating. A ground survey of the line, which identified other crossings with the same problem, led to an investigation that would improve the reliability of the urban 230-kV and 500-kV systems. To determine the extent of the problem, and at the same time investigate an opportunity to uprate its system, SRP (Tempe, Arizona, U.S.) contracted with Itron Inc. (Spokane, Washington, U.S.) to conduct an analysis of its line ratings. Itron was asked to subcontract with Opten Ltd. (Moscow, Russia) to operate survey equipment in SRP's helicopter. Data collection started in December 2003. Within just four flight days, 290 miles (467 km) of urban single- and double-circuit line had been surveyed.

Taking economic advantage of double-circuit line similarities and frequency of crossings per mile on low-priority lines, the analysis focused on 190 miles (206 km) of line data. Ground clearances and more than 560 line crossings were identified as ones to study.


Initial data consisted of feature-coded LiDAR points, orthogonally rectified (ground-relational) digital imagery, MPEG-formatted video and weather conditions. SRP provided additional information that consisted of original line design criteria, insulator details, and plan and profile drawings. This design information was used to model the system, as well as to check survey results relative to conductor attachment points and conductor tensions.

The LiDAR data were processed by Opten into ASCII output files, including ground features, structure attachment points and conductor “cloud” points, showing the location of each conductor. Conductor tension was determined by using the best-fit calculated catenary values for each ruling span. Conductor temperature was calculated using IEEE Standard 738-1993 with input variables, consisting of line current and weather conditions, taken from existing SRP databases. The catenary curves were compared directly to the actual conductor survey shots. Additional verification of the aerial survey was performed by comparing ground survey measurements at several attachment points. The LiDAR survey points fell well within the survey tolerances. Conductor temperature was verified by attaching thermometers on the conductor at three points along the survey. The measured temperature values differed from the calculated values by less than 5°F (3°C).


Itron loaded all data into the TL-Pro line design software, currently a Pondera Engineers' (Spokane) product, and made a re-rating analysis. The data were superimposed on existing system maps provided by SRP's GIS. The analysis consisted of modeling each line at the time of the survey (the base case) as well as incremental temperature increases. The base-case conductor tension, temperature and weather information were used to model conductor behavior at the elevated temperatures. Each temperature corresponded to a line rating, and each circuit had a maximum allowable operating temperature based on SRP's clearance criteria. This maximum allowable conductor temperature was used to prioritize any needed corrections. The data were compared to existing line-rating criteria and used to judge the feasibility of increasing line ratings.


The report data included:

  • General line information

  • Crossing and ground violation tables showing violation distance by temperature and span

  • Profile exhibits of spans with violations

  • Summary of all crossings analyzed

  • Separate line-by-line vegetation clearance reports

Additionally, system models as well as the raw and processed electronic data prepared by Opten included:

  • LiDAR point files

  • Orthogonally rectified digital imagery

  • Time-stamped video

  • Base conductor temperature and catenary values by span

  • TL-Pro model of the entire system analyzed in state plane coordinates

  • Electronic .dxf files of all profiles showing conductor points and LiDAR survey shots.

Additionally, potential vegetation problem areas were provided to SRP's vegetation management group for further analysis.


The study showed that 95% of all crossings examined complied with SRP's standards and 91% could accommodate an increased line rating based on a 212°F (100°C) ACSR conductor temperature. With minimal effort, the identified violations could be corrected to meet the increased temperature-rating criterion. The few ground-clearance issues identified were resolved with the responsible party and/or handled through easement adherence policies. Elsewhere, the metropolitan system had sufficient ground clearance to justify a possible systemwide increase in line rating. Corrective action was prioritized and designed by SRP Engineering to meet the increased temperature rating. SRP Engineering continues to investigate alternative uses for valuable survey data, including improved accuracy for asset allocation, ROW management and other transmission system reliability improvements.


The author would like to thank a few of those who assisted in this project and with this article: Darold Orgill, formerly of Itron, Konstantin Mekhanoshin, of Opten, and Dan Stevanovic and Drew Vallorano, both of SRP.

Daryl Chipman is a senior engineer in SRP's Line Maintenance Engineering department. He graduated with a BSEE degree from Montana State University and has been with SRP for six years. Chipman is a licensed engineer intern in the state of Montana.


ALTM: Airborne laser terrain mapper units include a LiDAR system, inertial measurement unit (IMU), differential GPS, video camera and on-board computer. Coupled with a high-resolution digital camera, it provides all the equipment necessary to create a 3-D survey from an aircraft.

LiDAR: Light detection and ranging is a pulsed high-power laser rangefinder to scan and define the 3-D location of objects that reflect light in relation to the known location of the helicopter. Integrated with an IMU and differential GPS, the location of the helicopter is precisely known and, therefore, describes all 3-D data collected.

IMU: Inertial measurement unit is cruise-missile technology vocabulary to define aircraft pitch, yaw and roll.

Differential GPS: Differential GPS uses airborne and ground-based GPS receivers located at points with known coordinates.

Feature coding: Feature coding is a post-process that identifies the 3-D object, such as a tower, conductor, tree, building or road.

Orthogonally rectified digital imagery: Orthogonically rectified digital imagery is high-resolution downward photos that were processed to eliminate the distortions caused by camera lens, variations in terrain elevation and perspective geometry to produce a photo with metric qualities of a map.