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
Sample data from DTS measurements showing that the hot spot along the cable route
exists under a parking lot.