Tettex MIDAS 2881, Руководство по эксплуатации

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138 Applications Guide
should be selected. By choosing the normalized dissipation factor (to 20°C) as a measuring value the
automatically calculated value will be recorded.
The Liquid Insulation Test is made by
normal UST A mode.
Instead of the HV GND input the Input
A is used as low voltage to reach the
highest possible accuracy. But to be
able to switch HV ON the HV GND
Input has to be connected to Ground.
The test cell can be set in the bottom
part of the transportation case for this
measurement.
Test cell connections
14.4.2 Measuring Data Interpretation
It is suggested that the following guides serve for grading liquid insulation by dissipation factor tests:
Classification Dissipation factor @ 20 °C
Mineral Oil Synthetic Others
Good (new) < 0.05 % < 0.05 % < 0.05 %
Used - usually considered satisfactory for continued service < 0.3 % < 0.5 % < 0.3 %
Used - should be considered in doubtful condition, and at
least some type of investigation (dielectric breakdown tests)
should be made. > 0.5 % > 0.5 % > 0.5 %
Used - should be investigated, and either reconditioned or
replaced. Should be investigated to determine the cause of
the high power factor. > 1.0 % > 2.0 % > 1.0 %
Note: High dissipation factors indicates deterioration and/or contamination with moisture, carbon,
varnish, glyptal, sodium, asphalt compounds, deterioration products, gasket materials or other foreign
products.
Mineral Oil
Carbon or asphalt in oil can cause discoloration. Carbon in oil will not necessarily increase the power
factor of the oil unless moisture is also present.
Synthetic Insulation Liquid (e.g. Askarel ®
)
If the high dissipation factor is caused by water or other conducting matter, free chlorides or a high
neutralization number, the synthetic oil is probably an operating hazard. If the high dissipation factor is
not due to these causes, it is probably not an operating hazard, except that when the dissipation
factor is quite high it may result in excessive heating of the device in which it is used. Care should
also be taken that the high dissipation factor is not due to dissolved materials from gaskets or
insulation necessary for safe operation of the askarel filled device. High dissipation factor due to
askarel contamination may mask other defects in askarel-filled units.