Distributed Temperature Sensing (DTS) Measurements

For additional information on field testing, please contact John Cooper.

Underground cable circuits generally have varied conditions along the route - changing soil thermal resistivity, varying burial depths, proximity to other power cables, etc. - which cannot be completely assessed when a cable circuit is installed. These varying conditions mean that the cable will operate at different temperatures, some of which may be above the maximum allowable operating temperature for the cable.
Because the cable temperature is very important for long-term reliability, it is important to assess the thermal bottleneck -- hot spot -- along the cable route. Thermocouples are impractical for wide application on long cable circuits, although they continue to be used on a limited basis for selected locations. However, distributed temperature sensing (DTS) using an optical fiber is a state-of-the-art technology for measuring temperatures along an entire circuit.

How does it work?

DTS works using the physics principle that results when light is scattered by molecules, resulting in a frequency change in the light waves. The "backscattering" phenomenon was discovered in 1928 by Raman and is known as the Raman Effect. During DTS measurements, a light pulse is sent down an optical fiber. The Raman Effect is a function of temperature such that the backscattering gives an indication of the molecular temperature at the location of the reflections. By monitoring the time it takes for the reflections to reach the transmitter, it is possible to resolve the temperature along the entire optical fiber with a spatial resolution of approximately 1 meter (3.3 feet).
Attenuation along multi-mode optical fibers limits the measurement length to approximately 8km, but this is a practical working length for many cable circuits.

In the field...

To perform the temperature measurements, generally w 50/125 fibers are required so that a "double-ended" measurement can be performed (single-ended measurements are also possible, but require some special callibration steps). The optical fibers are attached by the utility (preferably by fusion splice -- see the figure below) to special "pig tails" that connect directly to the equipment.
The following figure shows testing being done on 115kV cables that have an optical fiber embedded under the cable jacket. Testing is also possible on cables using separate fibers.
Sample data from DTS measurements showing that the hot spot along the cable route exists under a parking lot.