New underground transmission and distribution cable circuits are expensive to install and difficult to site. Therefore, users are becoming more interested in accurately and confidently determining the maximum usable cable rating of existing lines for the full range of expected operating conditions. This is not easy. The cable environment can change significantly over the length of a feeder circuit because of differing thermal properties of the various soils encountered along the route. In certain locations, cables might be buried deeply (sometimes unknown to the design engineer), affecting the ability of the cable to dissipate heat. Heat generated from nearby distribution circuits also might raise the ambient earth temperature. Thermal conditions can even change with time because of moisture-dependent changes in soil thermal properties.

Historically, cable-rating calculations have included a substantial safety margin to account for uncertainty in circuit thermal conditions. In most cases, therefore, the “book” ratings are conservative and the line may be capable of higher power transfers by 5% to 10% or even more. Substantial savings can accrue if the user can characterize the full cable length and rate the underground circuit based upon actual conditions. Even better, if hot spots can be identified, the user can perhaps mitigate them and achieve even higher ratings.

Utilities are increasingly turning to distributed fiber-optic temperature sensing (DFOTS) to determine the temperature along the entire length of transmission and distribution feeders. Results of the temperature monitoring allow utilities to determine the maximum allowable feeder loading with a great level of confidence.

Distributed Fiber-Optic Temperature Monitoring System

As shown in Fig. 1, a pulsed laser beam is injected into a conventional multimode fiber that is installed in the cable outer shielding or alongside the cable. A small part of the signal is reflected back into the instrument from temperature-dependent molecular vibrations in the fiber. The magnitude of the vibration indicates temperature, and transit time indicates distance. DFOTS can determine temperatures with a 1°C (34°F) temperature resolution and a 1 m (3 ft) spatial resolution. These resolutions are ideal for underground cable systems. Distances up to about 8 km (5 miles) can be monitored. An instrument is available for monitoring temperature on single-mode fibers for much longer distances, but both temperature and spatial resolutions are lower.

Distribution Cable Monitoring

At least two utilities in the United States have conducted distribution-cable re-rating studies using DFOTS, for more than 24 circuits. In one case, the utility wanted to operate the cables above the traditional design conductor temperature of 90°C (194°F). The utility was concerned that some locations might operate even hotter, possibly drying out the soil, overheating the cables and creating premature failures. The other utility had multiple distribution feeders installed near a high-temperature water line and was concerned about overheating the cables. In both cases, fiber-optic cables were installed in spare ducts in the duct bank. Temperatures were monitored during high-load periods and ratings were determined as follows:

• The expected temperature at the fiber location was calculated based upon two weeks of load data for each of the feeders in the duct bank and upon the best estimate of the earth thermal conditions.

• Expected temperature was compared to measured temperature.

• Adjustments were made to the earth thermal parameters to have the calculated temperatures match the measured temperatures.

• The new thermal parameters were used for ampacity calculations.

This approach is accurate for the conditions at the time of the test. Knowledge of the annual ambient earth temperature, plus soil thermal analysis — including laboratory dryout curves — is necessary to translate results to other times of the year.

Transmission Cable Monitoring

Transmission circuits are an excellent target for DFOTS analysis, primarily because of the increased value of an additional ampere of current at transmission voltages.

The first system monitored in this country was a pair of heavily loaded 69-kV circuits on the Southern California Edison (SCE) system. The cables had reached their “book rating,” and new cable installations were scheduled to reinforce the area. SCE was able to install a fiber-optic cable in the same duct as one of the power cables. Procedures similar to those described above for distribution cables were used to demonstrate that an 8% to 10% ampacity increase was possible. Therefore, SCE deferred the installation of a new line by almost two years.

Similar tests were performed on other circuits at SCE and at many other utilities, in almost every case demonstrating that the cable rating could be safely increased. In a few instances, however, temperature monitoring showed significant hot spots at existing loadings that raised concerns about premature cable aging. The utilities are currently investigating ways to mitigate the hot spots.

ComEd recently installed 138-kV XLPE-insulated cables along with temperature sensing fiber-optic cables into duct that had been installed decades ago. Because of the unknown thermal conditions surrounding the existing duct, DFOTS was selected as a method to determine ratings of the new cables. ComEd is the first utility to use real-time DFOTS monitoring to rate critical circuits.

The success of DFO temperature monitoring led to its novel use as a “thermal acceptance test” for the cables. Cable suppliers size the cable based upon information supplied by the utility, and guarantee that the designed cable conductor temperature would not be exceed for rated loads. For the first time, DFO monitoring allows the utility to measure temperature along the line and to use the procedures described earlier to verify the cable sizing is indeed adequate.

Further Innovations

The success of this distributed-sensing technology has led to the following developments:

• Vendors are developing a more rugged, lower-cost sensing unit that is designed for continual monitoring in the field.

• Utilities and other users are routinely specifying installation of optical fibers in the outer shielding construction of transmission-voltage extruded-dielectric cables.

• Cable manufacturers, consultants and others are integrating DFO temperature monitoring with dynamic rating systems to insure that the increased cable rating is based upon actual hot spot temperatures.

Jay Williams has degrees from Brown University and New York University. He began work at ConEd in 1965 and was in charge of the Transmission Cable Group when he left to join Power Technologies Inc. (PTI) in 1973. He headed PTI's cable group until 1992, when he co-founded Power Delivery Consultants Inc. Williams specializes in underground-cable-system design and analysis. He is a registered professional engineer in New York.

Ahead of the Curve

Art Davoren, senior project manager with Nevada Power Co. (NPC) is finding that knowledge is power.

He states, “The use of DTS allows the cable system owner to verify the accuracy of the ampacity model used to determine conductor size and type. With known loads and conductor temperatures, an effective thermal resistivity can be calculated for the installed cable system. This effective thermal resistivity can be used to calculate what should be more accurate ratings for the installed cable system.

“By obtaining a cable temperature profile prior to energizing the cable, verification of soil ambient and detection/quantification of external heat sources which may have otherwise been overlooked are now easily accomplished. At NPC, we found that soil ambients under asphalt at typically installed cable depths run approximately 4°C (39°F) warmer than under other areas.”

Today, NPC specifies optical fibers in its 115-kV XLPE-insulated cables.

Figure 2 shows the DFOTS system in place at a NPC substation.