The Number of High-Voltage Underground Cables is Expected to Increase Rapidly in the future as a result of the population-density increase in urban areas and environmental restrictions. As with many utilities around the world, Israel Electric Corp. (IEC; Haifa, Israel) is actively seeking ways to enhance the reliability and quality of supply. IEC's transmission system includes some 160 substations with operating voltages of 110 kV, 161 kV and 400 kV. Currently, IEC has several 161-kV cross-linked polyethylene (XLPE)-insulated underground cables in commission.

Temperature monitoring of underground cables is the most important feature for real-time rating of the cables, and several utilities around the world are testing or implementing continuous temperature monitoring techniques for underground cables. Continuous cable temperature monitoring offers the ability to detect hot-spot locations, and most importantly, such monitoring is the key enabler for maximum use of the cable.

In line with the utility's strategy to improve the security of its supply, IEC decided to launch a pilot project for the continuous temperature monitoring of a section of its 161-kV underground cable system.

PILOT PROJECT CIRCUIT

The cable section selected for the pilot project is comprised of two high-voltage XLPE-insulated underground cable circuits. The complete route includes an underground cable section some 2 km (1.2 miles) in length.

The underground cable section is characterized by several cable-laying methods, including direct burial, road crossings in pipes, a railway crossing in pipes and a 250-m (820-ft) length in a tunnel. This cable circuit was laid in four sections connected by joint bays.

Temperature monitoring relies on optoelectronic equipment that launches laser light into a fiber-optic cable. A fraction of the light that is reflected back to the source is temperature-dependent (Raman bands), so temperature measurements are determined by analyzing the Raman backscatter signals. The position of each measurement point is determined by the time taken for the light source to travel to and from the source to the scattering point. If the fiber-optic cable is located in close proximity of the power cable, the measured temperature is determined from the heat generated in the power cable.

IEC purchased Model DTS 800-S15, a direct continuous temperature monitoring system manufactured by York Ltd. (Southampton, Hampshire, U.K.).

INFRASTRUCTURE FOR MONITORING

The infrastructure for temperature monitoring is composed of plastic micro ducts strapped to the surface of the power cable. Two single-mode and two multi-mode fiber-optic cables were blown into the ducts and the cable trench was backfilled.

Special attention was given to the underground cable joints, since the joints could be the weakest points of the cable due to overheating. The plastic micro duct was wrapped around each joint several times in order to lengthen the fiber optics that measured the joint temperature (to increase the spatial resolution). The infrastructure for temperature monitoring was installed alongside one of the three single-phase cables of each circuit and around 18 joints (2 circuits × 3 joint bays × 3 phases = 18 joints).

MONITORING RESULTS

The temperature monitoring of this pilot project was conducted in August 2004. Figure 1 shows the temperature profile of the power cable as measured by the DTS. The locations of the joints of the power cable are marked on this profile. As shown in the figure, the temperature range is 27°C to 37°C (80.6°F to 98.6°F) and no abnormal temperatures were recorded in the joints. Figure 2 shows the three temperature graphs that were derived from the results of a 24-hour span of measurements in the cable tunnel.

The purpose of these measurements was to compare the DTS monitoring results with those of conventional measuring techniques using thermocouples. The results show good correlation. The temperature-monitoring results were analyzed in comparison with the load current that was also measured at the same time. Figure 3 shows the graphs of hourly average temperatures during a five-day period.

As can be seen in Fig. 3, the temperature of the cable in the tunnel is lower than that of the cable laid directly in the ground. During the measuring period, there was a circuit outage of 10 hours; however, this interruption did not cause a significant reduction in the cable temperature. While the temperature of the power cable in the tunnel shows a good correlation with the current changes, the correlation between the temperature and the current changes in the ground is less obvious (apparently due to the large thermal inertia of the ground mass).

ANALYSIS OF RESULTS

The temperature of the power cable was calculated by the cable ampacity program (CAP) software, which is based on international standard IEC 60287. The calculation was performed based on the two power-cable circuits directly buried, with the three phases of each circuit laid horizontally and the trench filled with a thermal backfill. Other cable circuit parameters are listed in the following table.

The cable load was set at 500 A, the average current in the cable during the measuring period. The CAP software predicted that the temperature of the cable's outer protective sheath was 33.5°C to 34.9°C (92.3°F to 94.8°F) and that the temperature of the cable conductor was 35.5°C to 36°C (95.9°F to 96.8°F). These results correspond well with the measured results shown in Figs. 1 and 3.

RESULTS COMPARISON

The CAP software is capable of calculating the temperature difference between the outer protective sheath and the conductor. This difference depends on many parameters such as cable type, loading, ground thermal resistance, surrounding temperature, width and depth of cable-laying trench and so forth. Taking into account these parameters, the temperature difference between the insulation outer layer and the conductor can reach up to 18 centigrade degrees (32.4 Fahrenheit degrees) for loading of 1300 A (the conductor temperature is 90°C [194°F] and the temperature of the insulation's outer layer is 72°C [161.6°F]). Hence, by measuring the temperature of the insulation's outer layer with the aid of fiber optics, the conductor temperature can be calculated. The measurements and calculations are most critical when the loading of the power cable is relatively high.

The main benefit of monitoring the temperature of high-voltage underground cables using fiber optics is the acquisition of the real-time thermal behavior of the cable, determined under actual cable loading conditions. Moreover, in cases when hot spots are detected, the decision on the remedial action required is based on real-time data.

This pilot project proved that the DTS temperature-measuring method is valid, with the calculated results supporting the measured results. The calculation software enables the determination of the conductor temperature based on the measured temperature of the outer layer of the insulation. It should be appreciated that the temperature difference between the conductor and the outer protective sheath is a variable influenced by many parameters that must be considered when this difference is calculated.

As a result of this pilot project, the requirement for cable thermal measurements is now included in IEC's specification for all future 161-kV cable systems.

Cable parameters.
Copper conductor cross-section	1200 mm2 (1.86 inches2)
XLPE insulation thickness	22 mm (0.87 inches)
Lead sheath thickness	4.5 mm (0.177 inches)
Cable external sheath diameter	117 mm (4.6 inches)
Trench depth and width	2.5 m by 2.3 m (8.2 ft by 7.5 ft)
Max ground temperature	30°C (86°F)
Thermal resistivity of ground	75 cm°C/W
Thermal resistivity of backfill	160 cm°C/W
Cable daily load factor	0.70
Earthing system	cross bonding

Dr. Daniel Kottick was awarded his bachelor's, master's and Ph.D. degrees in electrical engineering in 1980, 1982 and 1986, respectively, by the Ben-Gurion University, Israel, and his MBA degree in 1998 from the Technion Israel Institute of Technology. He joined Israel Electric Corp. in 1986. Currently, Kottick is a senior expert at the R&D Laboratory and his main fields of interest include quality of supply, CTM of cables, electric vehicles and dynamic security of supply. He is a senior member of the IEEE. kotek@iec.co.il

Irena Kviatkovsky received her master's degree in 1982 from the Leningrad Polytechnic Institute, Russia, and is currently project manager of Israel Electric Co.'s R&D laboratory. Kviatkovsky has more than 20 years experience in high-voltage cable tests and loading calculations. irenakv@iec.co.il