Tettex MIDAS 2881, Operating Instructions Manual

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14 Theory
14
Rubber 4.0% 3.6
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.
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 does that automatically. See also chapter "Software : Menu Conditions
(Temperature correction)”.
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 far 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.
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.