Megger DELTA 4000, Справочное руководство

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14 DELTA 4000 ZM-AH02E
2 INTERPRETATION OF MEASUREMENTS
Negative dissipation factor
Creep currents inside an insulation system or more com-
monly on surfaces; create change of potential distribution
that may give increased or decreased dissipation factor, and
in some cases also negative dissipation factor. This condi-
tion is most likely to arise when making UST and GST
measurements on specimens who have a capacitance value
of a few hundred picofarad or less. Equipment such as
bushings, circuit breakers, and low loss surge arresters fall
into this category.
The error is usually accentuated if tests are made under
unfavorable weather conditions, especially a high relative
humidity which increases surface leakage.
There appears to be no clear-cut way of knowing whether
an error is significant or what remedies should be taken to
overcome an error. A frequency sweep may give additional
information. The best advice is to avoid making measure-
ments on equipment in locations where negative dissipation
factors are known to present a problem when unfavorable
weather conditions exist, especially high relative humidity.
Make sure the surface of porcelain bushings are clean and
dry to minimize the effects of surface leakage. Make sure all
items such as wooden ladders or nylon ropes are removed
from the equipment to be tested and are brought out of
any electrostatic interference fields that could influence a
measurement.
Connected bus work, cables etc
A complete disconnected component is preferred when
performing dissipation factor measurements. All connected
bus work, cables, disconnect switches etc may add signifi -
cant capacitance and losses in GST measurements where
they are in parallel with the desired insulation measure-
ment. For this reason, many test engineers will ask that the
equipment under test be totally isolated from connected
apparatus.
UST data is principally possible to measure without fully
disconnecting the test object. The capacitance from the
connected parts results only in a current to ground that is
not measured in UST test mode.
Electrostatic interference
When tests are conducted in energized substations, the
readings may be influenced by electrostatic interference
currents resulting from the capacitive coupling between
energized lines and bus work to the test specimen. Other
sources for interference may be corona discharges (espe -
cially at high humidity) and is some cases DC fluctuations
in the grounding system. Trouble from magnetic fields
encountered in high-voltage substations is very unlikely.
To counter the effects of severe electrostatic interference
on the measurement, it may be necessary to disconnect the
specimen from disconnect switches and bus work. Experi-
ence in making measurements will establish the particular
equipment locations where it is necessary to break the
connections. The related disconnect switches, leads and
bus work, if not energized, should be solidly grounded to
minimize electrostatic coupling to the test set.
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. Unfa-
vorable 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
greater the difficulty is to perform accurate measurements.
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. Specifically, it has been found that
some difficulty may be expected when measuring capaci -
tance by the GST test method in high interference switch-
yards when the capacitance value is less than 100 pF. This
difficulty 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 percent, the wind velocity is low, and the surface
temperature of exposed insulation is above the ambient
temperature.
Tests made by the UST method are less susceptible to in-
terference pickup than are tests made by the GST method.
In the UST test method, the capacitive coupled pickup
current in the high-voltage circuit flows directly to ground
after having passed through the high-voltage winding of
the power supply transformer. In the GST test method the
same pickup current, after passing through the high-voltage
transformer winding, must pass through one of the bridge
transformer-ratio measuring arms before reaching ground.