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Theory
19
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.
The DC resistivity is very sensitive to changes of temperature, its dependence of the inverse of
the temperature, expressed in Kelvin, (1/K) is generally exponential. It is therefore necessary to
carry out measurements under sufficiently precise temperature condition.
Influence of Humidity
The major electrical effect of humidity on an insulating material is to increase greatly the magnitude
of its interfacial polarization, thus increasing both its permittivity and tan

as well as the dc
conductance. These effects of humidity are caused by absorption of water into the volume of the
material and by the formation of an ionized water film on its surface.
Influence of electrode Surface
The surfaces of the oil test cell or the solid test cell should be clean and with out any scratch or
other damaged when making measurement. Scratches and other mechanical damages increase
the tan

and in the worst case a break down can occur.
5.8 Other Test Methods
As of today there exists no other test method that can replace the currently used C & tan

test.
Nevertheless, several measurement methods exist which compliment dissipation factor
measurement and assist in localization of poor quality of liquid or solid insulating materials.
Oil Analysis Measurements provide useful information about the insulating oil in transformers
and oil-paper insulation systems.
Solid Analysis Measurements provide useful information about the insulating of solid material
like paper, rubber, ceramic etc.