Megger DELTA 4000, Справочное руководство

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12 DELTA 4000 ZM-AH02E
2 INTERPRETATION OF MEASUREMENTS
Any sudden changes in ambient temperature will increase
the measurement error since the temperature of the appara-
tus will lag the ambient 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 char-
acteristics may be helpful in assessing deteriorated condi-
tions. As an example, bushings have typically a rather flat
temperature correction with only slightly elevated values
at high temperatures. Generally a bushing showing highly
increased dissipation factor at elevated temperature should
be considered “at risk”.
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. 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 tempera-
ture of the insulation specimen should be above and never
below the ambient temperature to avoid the effects of
condensation on the exposed insulating surfaces.
Significance 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; however, e.g.
dry oil-impregnated paper and polyethylene of good quality
exhibit decrease of dielectric losses when temperature is
raised moderately, e.g. from 20°C to 30°C. It is also known
that the effect of temperature depends on the aging status
of the insulation. In many cases, insulations have failed due
to the cumulative effect of temperature, i.e., a rise in tem-
perature causes a rise in dielectric loss which in turn causes
a further rise in temperature, etc (thermal runaway).
It is important to determine the dissipation factor-tempera-
ture characteristics of the insulation under test. Otherwise,
all tests of the same specimen should be made, as nearly as
practicable, at the same temperature.
To compare the dissipation factor value of tests made on
the same or similar type apparatus at different temperatures,
it is necessary to convert the value to a reference tempera-
ture base, usually 20
°
C (68
°
F). Examples of standard tables
of multipliers for use in converting dissipation factors at
test temperatures to dissipation factors at 20 ° C are found in
the Appendix A of this document.
In reality, temperature correction for a specific compo -
nent is always individual and pending age/condition.
DELTA 4000 has a unique and patented feature for estimat-
ing the individual temperature correction (ITC). By measur -
ing dissipation factor over frequency and using mathemati-
cal formulas and models of insulation characteristics, the
correct temperature correction can be determined from
5 to 50°C measurement temperature to 20°C reference
temperature. The input data for the calculation is dissipa-
tion factor measured from 1 to 500 Hz and the method
is principally based on Arrhenius’ law, describing how the
insulation properties are changing over temperature.
κ = κ
0
·exp(-W
a
/kT)
With activation energy W
a
and Boltzmann constant k
The test temperature for apparatus such as spare bushings,
insulators, air or gas filled circuit breakers, and lightning
arresters is normally assumed to be the same as the ambient
temperature. For oil-filled circuit breakers and transform -
ers the test temperature is assumed to be the same as the
top oil temperature or winding temperature. For installed
bushings where the lower end is immersed in oil the test
temperature lies somewhere between the oil and air tem-
perature.
In practice, the test temperature is assumed to be the same
as the ambient temperature for bushings installed in oil-
filled circuit breakers and also for oil-filled transformers
that have been out of service for approximately 12 hours.
In transformers removed from service just prior to test,
the temperature of the oil normally exceeds the ambient
temperature. The bushing test temperature for this case can
be assumed to be the midpoint between the oil and ambient
temperatures.