Haefely 2830, Руководство пользователя, Страница: 26 / 124

18 Theory
5.7 Evaluation of Test Results
Significance of Capacitance and Dissipation Factor and DC Resistivity
A large percentage of electrical apparatus failures are due to a deteriorated condition of the
insulation. Many of these failures can be anticipated by regular application of simple tests and with
timely maintenance indicated by the tests. An insulation system or apparatus should not be
condemned until it has been completely isolated, cleaned, or serviced. The correct interpretation
of capacitance and dissipation factor tests generally requires a knowledge of he apparatus
construction and the characteristics of the types of insulating material in used.
Changes in the normal capacitance of insulation indicate such abnormal conditions as the
presence of a moisture layer, short circuits, or open circuits in the capacitance network. Dissipation
factor measurements indicate the following conditions in the insulation of a wide range of electrical
apparatus:
•
Chemical deterioration due to time and temperature, including certain eases of acute
deterioration caused by local overheating.
•
Contamination by water, carbon deposits, bad oil, dirt and other chemicals.
•
Severe leakage through cracks and over surfaces.
•
Ionization.
The interpretation of measurements is usually based on experience, recommendations of the
manufacturer of the insulating materials , and by observing these differences:
•
Between measurements on the same material after successive intervals of time.
An increase of dissipation factor above a typical value may indicate conditions such as those
showed above: If the dissipation factor varies significantly with voltage down to some voltage
below which it is substantially constant, then ionization is indicated. If this extinction voltage is
below the operating level, then ionization progress in operation with consequent deterioration.
Some increase of capacitance (increase in charging current) may also be observed above the
extinction voltage because of the short-circuiting of numerous voids by the ionization process.
An increase of dissipation factor accompanied by a marked increase of the capacitance usually
indicates excessive moisture in the insulation. Increase of dissipation factor alone may be caused
by thermal deterioration or by contamination other than water.