Thermal stability is a concept used to describe the ability of a moist soil
to maintain a relatively constant thermal resistivity when subjected to an
imposed heat load, thus preventing a power cable from exceeding its safe
operating temperature. Thermal instability (or "thermal runaway") occurs
when a soil is unable to sustain the heat from a cable and the soil
progressively dries resulting in a substantial increase in the soil thermal
resistivity and attendant increase in the cable operating temperature. If
moisture is not replenished or current reduced, the ultimate result may be
cable failure due to overheating.
The thermal stability can be
analyzed in the laboratory with a drying time test, which is basically an
extended time thermal resistivity test. The initial soil moisture content,
which should be the lowest expected in the field, is a crucial parameter
affecting the test results. A constant, realistic cable heat rate (W/cm) is
applied to a thermal probe, embedded in a large soil or backfill sample, and
continued until substantial drying has occurred near the probe. The onset of
thermal instability (initial drying time) is indicated by an upward swing of
the temperature vs. log time curve. It has been shown that the drying time
for a thermal probe can be scaled up to the drying time for a full size
cable, because the thermal stability limit is a constant:
It has also been shown that a
critical heat rate must be exceeded before thermal instability will be
instigated. The thermal stability should be measured at the driest expected
soil conditions and at normal and emergency cable heat loads. The initial
drying time for a cable indicates the length of time that the given constant
heat rate can be applied, before the heat rate must be reduced or soil
moisture replenished to prevent instability. Steady state drying times for a
cable system should be in the order of several weeks or months to ensure
stability over the dry season. For high emergency ampacities, the drying
time test may be used to indicate the allowable loading time before a moist
backfill would become unstable.
In a cable
installation, a good corrective thermal backfill must always be placed next
to the cable. The thermal dryout curve of a good backfill has a sharp knee
at a low critical moisture content; also the totally dry thermal resistivity
is quite low. For these backfills the thermal stability may be treated as a
binary concept. That is, if the lowest expected soil moisture is above the
critical moisture content, then the backfill is stable for normal heat rates
and the moist thermal resistivity may be used for the ampacity calculation.
If the moisture is below the critical moisture content, then the backfill is
unstable and the totally dry thermal resistivity must be used for the
design.
