5.7.6 Influence of 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 voltage rating much greater
than the test voltage may be contaminated enough to cause a significant error. Surfaces of potheads, bushings,
and insulators should be clean and dry when making measurement.
It should be noted that a straight line plot of surface resistivity against relative humidity for an uncontaminated
porcelain bushing surface results in a decrease of one decade in resistivity for a nominal 15% increase in relative
humidity.
5.7.7 Electrostatic Interference
When tests are conducted in energized sub stations, the readings may be influenced by electrostatic interference
currents resulting from the capacitance coupling between energized lines and bus work to the test specimen.
The measurement difficulty, which is encountered when testing in the presence of interference, depends not only
upon the severity of the interference field but also on the capacitance and dissipation factor of the specimen.
Unfavourable weather conditions such as high relative humidity, fog, overcast sky, and high wind velocity will
increase the severity and variability of the interference field. The lower the specimen capacitance (and its
dissipation factor), the bigger the difficulty to make an accurate measurement. It is also possible that a negative
dissipation factor reading may be obtained so it is necessary to observe the polarity sign for each reading.
The MIDAS micro 2883 interference suppression feature minimizes the influences but however, the influences may
be minimized considerably by:
• Using the maximum voltage of the test set if possible.
• Disconnecting and grounding as much bus work as possible from the specimen terminals.
• Making measurements on a day when the weather is sunny and clear, the relative humidity is
less than 80%, the wind velocity is low, and the surface temperature of exposed insulation is above the
ambient temperature.
5.7.8 Negative Dissipation Factor
It is believed that a complex tree network of capacitances and resistances, which exist within a piece of equipment,
cause the negative dissipation factor phenomenon. Error currents may flow into the measuring circuit in instances
where phantom multiple terminals or a guard terminal appear in the measurement system. It is also believed that a
negative dissipation factor may be produced by currents flowing into a tee network as a result of space coupling
from electrostatic interference field.
If the dissipation factor of the measured capacitance would be lower than the one of the built-in standard capacitor
the displayed factor would be negative. But that’s only a theoretical case. If negative dissipation factors are seen in
daily work one should carefully recheck the test setup and all connections.
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