Theory
13
Silicone liquid 0.001% 2.7
Varnished cambric, dry 1.0% 4.4
Water 100% 80
Ice 1.0%
@ 0°C
88
Note: Tests for moisture should not be made at freezing temperatures because of the 100 to 1 ratio difference
dissipation factor between water and ice.
5.7.4 Influence of Temperature
Most insulation measurements have to be interpreted based on the temperature of the specimen. The dielectric
losses of most insulation increase with temperature. In many cases, insulations have failed due to the cumulative
effect of temperature, e.g. a rise in temperature causes a rise in dielectric loss which causes a further rise in
temperature, etc.
It is important to determine the dissipation factor temperature characteristics of the insulation under test, at least in
a typical unit of each design of apparatus. Otherwise, all tests of the same spec should be made, as nearly as
practicable, at the same temperature. On transformers and similar apparatus, measurements during cooling (after
factory heat-run or after service load) can provide required temperature correction factors.
To compare the dissipation factor value of tests made on the same or similar type of equipment at different
temperatures, it is necessary to correct the value to reference temperature base, 20°C (68°F). The MIDAS micro
does that automatically, when the DUT is correctly defined. See also chapter 12.1 DUT tab in the description of
software setup.
The insulation material temperature for apparatus such as spare bushings, insulators, air or gas filled circuit
breaker and lightning arresters is normally assumed to be the same as the ambient temperature. For oil-filled
circuit breakers and transformers the insulation temperature is assumed to be the same as the oil temperature. The
(transformer mounted) bushing insulation temperature can be assumed to be the midpoint between the oil and
ambient temperatures.
The capacitance of dry insulation is not affected by temperature; however, in the case of wet insulation, there is a
tendency for the capacitance to increase with temperature.
Dissipation factor-temperature characteristics, as well as dissipation factor measurements at a given temperature,
may change with deterioration or damage of insulation. This suggests that any such change in temperature
characteristics may be helpful in assessing deteriorated conditions.
Be careful making measurements below the freezing point of water. A crack in an insulator, for example, is easily
detected if it contains a conducting film of water. When the water freezes, it becomes non-conducting, and the
defect may not be revealed by the measurement, because ice has a volumetric resistivity approximately 100 times
higher than that of water. Tests for the presence of moisture in solids intended to be dry should not be made at
freezing temperatures. Moisture in oil, or in oil-impregnated solids, has been found to be detectable in dissipation
factor measurements at temperatures far below freezing, with no discontinuity in the measurements at the freezing
point.
Insulating surfaces exposed to ambient weather conditions may also be affected by temperature. The surface
temperature of the insulation specimen should be above (never below) the ambient temperature to avoid the
effects of condensation on the exposed insulating surfaces.
5.7.5 Influence of Humidity
The exposed surface of bushings may, under adverse relative humidity conditions, acquire a deposit surface
moisture which can have a significant effect on surface losses and consequently on the results of a dissipation
factor test. This is particularly true if the porcelain surface of a bushing is at temperature below ambient
temperature (below dew point), because moisture will probably condense on the porcelain surface. Serious
measurement errors may result even at a relative humidity below 50% when moisture condenses on a porcelain
surface already contaminated with industrial chemical deposits.
It is important to note that an invisible thin surface film of moisture forms and dissipates rapidly on materials such
as glazed porcelain, which have negligible volume absorption. Equilibrium after a sudden wide change in relative
humidity is usually attained within a matter of minutes. This excludes thicker films which result from rain, fog, or
dew point condensation.
Surface leakage errors can be minimized if dissipation factor measurements are made under condition where the
weather is clear and sunny and where the relative humidity does not exceed 80%. In general, best results are
obtained if measurements are made during late morning through mid-afternoon. Consideration should be given to
the probability of moisture being deposited by rain or fog on equipment just prior to making any measurements.
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