Megger DELTA 4000, Справочное руководство

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10 DELTA 4000 ZM-AH02E
2 INTERPRETATION OF MEASUREMENTS
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 in severe
cases possible increase of 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.
Unless bushing and pothead surfaces, terminal boards, etc.,
are clean and dry, measured quantities may not necessar-
ily apply to the volume of the insulation under test. Any
leakage over terminal surfaces may add to the losses of the
insulation itself and may, if excessive, give a false indication
of its condition.
2
I
nterpretation of
measurements
Significance of capacitance and
dissipation factor
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 con-
demned until it has been completely isolated, cleaned, or
serviced and measurements compensated for temperature.
The correct interpretation of capacitance and dissipation
factor tests generally requires knowledge of the apparatus
construction and the characteristics of the particular types
of insulation used.
Changes in the normal capacitance of an insulation material
indicate such abnormal conditions as the presence of a
moisture layer, short circuits, or open circuits in the capaci-
tance 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 cases of acute deterioration caused by
localized 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
equipment being tested, and by observing these differences:
▪
Between measurements on the same unit after successive
intervals of time.
▪
Between measurements on duplicate units or a similar
part of one unit, tested under the same conditions around
the same time, e.g., several identical transformers or one
winding of a three-phase transformer tested separately.
▪
Between measurements made at different test voltages
on one part of a unit; an increase in slope (tip-up) of a
dissipation factor versus voltage curve at a given voltage is
an indication of ionization commencing at that voltage.
An increase of dissipation factor above a typical value may
indicate conditions such as those given in the previous para-
graph, any of which may be general or localized in charac-
ter. If the dissipation factor varies significantly with voltage
down to some voltage below which it is substantially con-
stant, then ionization is indicated. If this extinction voltage
is below the operating level, then ionization may progress