PVC conduits were installed at the 10 o’clock and 2 o’clock positions immediately outside the cable pipe for distributed temperature monitoring fibers. Temperature monitoring and communications fibers were placed in larger conduits located away from the cable pipes.
Underground power transmission is an integral part of major metropolitan areas, and Washington, D.C. — where the confluence of political, social and, yes, electrical energy come together to direct and support the lives of people throughout the United States and the world — is no exception. With a history that started in the late 1800s, the Potomac Electric Power Co. (PEPCO), a PHI Service Co., and predecessor companies have been serving the district and areas of Maryland dating back to the Washington Traction and Electric Co., a street car company.
Over such an extended time, the power system has evolved with the ever-increasing demands to supply the energy needs of the district, and PEPCO is continuously challenged with meeting those needs. As part of the effort to enhance reliability and meet energy demands of the area, PEPCO — recognizing a migration in power generation outside of its region — constructed a 230-kV pipe-type cable system to connect major power substations in the district and neighboring Maryland along a 5.4-mile (8.7-km) route and incorporated 21st century smart technology into aspects of the line to optimize performance and control of the system.
The cable for this project was manufactured by The Okonite Cable Co. and consisted of a 3,000-kcmil segmental copper conductor with 0.5 inches (12.7 mm) of laminated paper polypropylene (LPP) insulation and two D-shaped stainless-steel skid wires. PEPCO contracted with W.A. Chester to install the 8.625-inch (220-mm) Schedule 40 (0.25-inch [6.4-mm]) cable pipe and to pull, splice and terminate all of the cable.
Taking the Cable’s Temperature
Old-school methods for measuring temperature typically involved a copper-constantine Type-T thermocouple junction with test leads run to a hand-hole for spot measurements with handheld meters or recording using battery-operated data loggers. These point sensors only measure the temperature at one location, and cable conditions can vary over short distances along the length of a circuit. As a result, hot spots — combinations of installation conditions and soil characteristics that limit ratings — may be missed, allowing the cable to operate above rated temperature.
Fortunately, there is now a state-of-the-art alternative method for temperature monitoring. An optical fiber is installed in close proximity to the cable system to measure the temperature. Ideally, the conductor temperature itself would be measured, as this is the temperature that limits most ratings, but it is impractical because of the energized line-to-ground voltage and, in a pipe-type cable, is further complicated by the pressurized dielectric liquid within the steel pipe. So, 2-inch (50-mm) conduits were installed immediately outside of the cable pipes at the 10 o’clock and 2 o’clock positions to contain the temperature measurement fibers.
PEPCO routinely also includes conduits in the center of the trench in which to put communications fibers. On the new circuit, these are used in a dual role to measure the temperature, as well.
As a test bed for fiber-optic-based temperature measurements, PEPCO wanted the ability to compare measurements right outside the cable pipe with temperatures from the positions of the communications conduits to gain experience for possible retrofit applications on pipe circuits that only had the communications conduits. The new lines have four active fibers, although only two were needed for temperature monitoring and to provide feedback for real-time ratings. Spare fibers also were included in each fiber-optic cable in the event a fiber became damaged during the installation or operation.
Use of an optical fiber for temperature monitoring of cable systems was considered in the mid-1990s based on a physics principal first identified in the 1920s — another instance of marrying old and new technologies. Incident laser light pulses sent into optical fibers produce backscatter. This phenomenon, known as the Raman effect, uses the temperature-dependent backscatter and signal processing to determine the temperature approximately once every meter (3.3 ft) along the fiber, giving a complete temperature profile and thousands of measured values over PEPCO’s 5.4-mile-long cable pipes. This distributed measurement avoids missing hot spots.
LIOS Technology supplied the distributed temperature sensing (DTS) system used by PEPCO. Many DTS-based temperature systems use optical time-domain reflectometry common to the communications industry, but LIOS’ system is based on an optical frequency domain method for faster measurement times and generally improved long-term reliability. Usually communications fiber is a low-loss single-mode type, but, for improved temperature measurement accuracy (1C° [1.8F°]) and spatial resolution (approximately 1 m), a 50-micron multimode optical fiber was selected.
As part of configuring the system, PEPCO’s cable system design consultant, Electrical Consulting Engineers, P.C. applied knowledge of the installation conditions along the cable route and details from as-built drawings to identify zones of interest for the purposes of measuring temperatures that would be most important for ratings. The benefit to identifying zones is that a few tens of temperature values must be transferred to the utility’s supervisory control and data acquisition (SCADA) system rather than the thousands of values from the detailed DTS measurements. If detailed studies are needed or PEPCO requires more information, the complete temperature measurement traces are still available on the system located in the substation.
Temperature traces and the corresponding zone temperatures are measured every 15 minutes, as this interval was deemed to be a reasonable balance for the processing time for each temperature measurement with the relatively long thermal time constant of the buried cable system.
In addition to the cable temperatures, loops of optical fiber were installed at three burial depths — 3 ft, 6 ft and 9 ft (about 1 m, 2 m and 3 m) — remote from the power cables and any other heat-producing sources to provide an ambient temperature input for rating calculations. The 3-ft to 9-ft range was representative of the range of depths for most of the installed cable along the route.