High-Voltage Underground Cable Testing at LADWP
The work required one day for setup and for measurements. An engineer from the consulting company conducted the measurements, while LADWP personnel assisted with setting up the equipment in the substation.
The measurements were made using a model DTS-800 instrument for the entire length of the circuits, which were approximately 4.5 miles and 5 miles long each.
The DTS-800 testing instrument measures the temperatures by injecting 10-ns wide light pulses in the fibers. All measurements were single-ended and, thus, made by injecting the light pulses into one end of the fiber. In turn, the backscatter from impurities or “dopants” in the glass fibers is used for the temperature measurements. Using the multi-mode fibers for the measurements, a resolution of 1 m and an accuracy of ± 1°C were obtained.
Because of the length of the circuits double ended measurements were not made. For this case, measurements require connecting two ends of a fiber loop to the DTS-800. Due to the length of the circuits, fiber loops would exceed 10 miles when taking into account the lay of the fibers and thus exceed the measurement capability of the DTS-800 which was limited to 7 miles. Plus the additional fiber-optical splices along the loop would provide for additional optical losses.
The warm and cold spots were related to the location of manholes found through optical measurements. The manholes were identified by their fiber-optical splices and also by the ratio of the length of the fiber to the length of the cable, which was approximately 1.28. The fibers are applied in a helix pattern over bedding tapes placed around the extruded insulation shield.
The hottest spot — 29°C (84°F) — was between MH-9 and MH-10 (see diagram on page 48E), where the cable and ducts are installed in air in a compartment of a concrete bridge that crosses the Tujunga Wash Channel. The width of this warm spot is approximately 165 ft.
The coolest spot (between MH-15 and MH-16) was found where the cables are installed, directly under the Ventura Freeway overpass. This is due to lower ambient soil temperature at increased installation depths and shading from the sun.
The thermally limiting sections were found where the transmission cables cross distribution cables duct banks at the intersection of Burbank Blvd. and Lankershim Blvd. or the intersection of Burbank Blvd. and Satsuma Ave. At these crossings, the mutual heating between the distribution and transmission cables caused an increase in soil ambient temperature of about 4°C (39°F).
LADWP currently operates approximately 9.50-circuit miles of 230-kV and 2.2-circuit miles of 138-kV XLPE-insulated cables. However, there are two circuits planned at 230 kV that are due in service in 2009. Additionally, LADWP has an ongoing program to replace 70-circuit miles of 138-kV oil-filled cable with XLPE-insulated cable, including replacement of a circuit approximately 5 miles, which will be completed in June 2005. For these reasons, LADWP's test engineers will continue to perfect methods for commissioning and maintenance testing of high-voltage XLPE cable.
Vincent Curci has been an electrical engineer at the Los Angeles Department of Water and Power since 1983 and is currently in charge of underground transmission design. Curci, who holds a BSEE and two MSEE degrees, is a member of IEEE and also participates in the Insulated Conductors Committee of IEEE and EPRI Underground Transmission Task Force. Vincent.Curci@ladwp.com
Hassan Motallebi has been an electrical engineer at the Los Angeles Department of water and Power since 1993, working in the areas of control, automation, SCADA, and Underground Transmission. Motallebi who holds a BSEE and a MSEE degree, is a member of IEEE and participates in the Insulated Conductor Committee (ICC) of IEEE. Recently, he managed Toluca-Van Nuys Cable D project for LADWP, which he presented at ICC meeting held in Montreal, Canada, in May 2004. Hassan.Motallebi@ladwp.com
Kishan Kasondra is an electrical engineer associate at the Los Angeles Department of Water & Power, which he joined in May 2002. His responsibilities include designing, installing, inspecting, testing and maintenance of high-voltage underground transmission lines. Kishan is currently managing a cable project to replace a 4.5-mile long SCFF cable circuit with an XLPE cable system. He received a BSEE degree from Loyola Marymount University and is attending the University of Southern California to complete a MSEE degree specializing in power systems. He is a member of IEEE. Kishan.Kasondra@ladwp.com
The temperature measurements for the Toluca-Van Nuys Cable D were taken before the cable was energized and loaded, as a baseline, and afterwards, when carrying a predetermined load for a period of two weeks.
| Parameter | Value | Units |
|---|---|---|
| Concrete Duct bank dimensions | 36 × 36 | inches |
| Conduit type | Fiberglass (FRE) | — |
| Conduit inside diameter | 6.4 | inches |
| Spacing between cables | 17.5 | inches |
| Cable configuration | Triangular | — |
| Concrete thermal resistivity | 80 | °C-cm/W |
| Backfill thermal resistivity | 120 | °C-cm/W |
| Native soil thermal resistivity | 150 | °C-cm/W |
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