Megger DELTA 4000, Справочное руководство

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ZM-AH02E DELTA 4000 13
2 INTERPRETATION OF MEASUREMENTS
Significance of humidity
The exposed surface of bushings may, under adverse rela-
tive humidity conditions, acquire a deposit of surface mois-
ture 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 bush-
ing is at a 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 percent 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 humid-
ity is usually attained within a matter of minutes. This,
however, 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 conditions where the
weather is clear and sunny and where the relative humid-
ity does not exceed 80 percent. 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.
Surface leakage
Any leakage over the insulation surfaces of the specimen
will be added to the losses in the volume insulation and may
give a false impression as to the condition of the specimen.
Even a bushing with a voltage rating much greater than the
test voltage may be contaminated enough to cause a signifi -
cant error. Surfaces of potheads, bushings, and insulators
should be clean and dry when making a measurement.
It should be noted that a straight line plot of surface resis-
tivity against relative humidity for an uncontaminated por-
celain bushing surface results in a decrease of one decade
in resistivity for a nominal 15 percent increase in relative
humidity and vice versa.
On bushings provided with a power factor or capacitance
tap, the effect of leakage current over the surface of a
porcelain bushing may be eliminated from the measurement
by testing the bushing by the ungrounded specimen test
(UST).
When testing bushings without a test tap under high
humidity conditions, numerous companies have reported
that the effects of surface leakage can be substantially
minimized by cleaning and drying the porcelain surface and
applying a very thin coat of Dow Corning #4 insulating
grease (or equal) to the entire porcelain surface. When mak -
ing a hot collar test, the grease is generally only applied to
the porcelain surface on which the hot collar band is to be
located and to that of one petticoat above and one below
the hot collar band.
When testing potheads, bushings (without test tap), and
insulators under unfavorable weather conditions, the dis-
sipation factor reading may, at times, appear to be unstable
and may vary slightly over a very short period of time. The
variation is caused by such factors as the amount of surface
exposure to sun or shade, variations in wind velocity, and
gradual changes in ambient temperature and relative humid-
ity. Similar bushings may have appreciably different dissipa-
tion factor values for the case where one bushing is located
in the sun while the other is in the shade. A test made on
the same bushing may have a different dissipation factor
value between a morning and an afternoon reading. Due
consideration must be given to variations in readings when
tests are made under unfavorable weather conditions.