needed to remove dirt, oil, and other deposits
that may reduce the flashover value.
that may reduce the flashover value.
2. Oil level.- Check the oil level on
bushings equipped with sight gauges or
other types of oil-level indicators and
add oil as necessary. Low-oil levels with
no sign of an external leak may Indicate
a leak within the bushings which may
require replacement of the bushing as
field corrections would be difficult to
accomplish. The oil level of bushings
without oil-level indicators is not
normally checked unless there is
evidence of leakage.
bushings equipped with sight gauges or
other types of oil-level indicators and
add oil as necessary. Low-oil levels with
no sign of an external leak may Indicate
a leak within the bushings which may
require replacement of the bushing as
field corrections would be difficult to
accomplish. The oil level of bushings
without oil-level indicators is not
normally checked unless there is
evidence of leakage.
C. Maintenance Tests.- Common mainte-
nance tests are power factor, RIV (radio-in-
fluence-voltage), dc insulation resistance, and
testing oil or compound for moisture.
Descriptions of these various tests follow:
nance tests are power factor, RIV (radio-in-
fluence-voltage), dc insulation resistance, and
testing oil or compound for moisture.
Descriptions of these various tests follow:
1. Power-factor Doble Tests.- The
power-factor test is the most effective
known field test procedure for the
early detection of bushing
contamination and deterioration. This
test also provides measurement of ac
test current which is directly
proportional to bushing capacitance.
power-factor test is the most effective
known field test procedure for the
early detection of bushing
contamination and deterioration. This
test also provides measurement of ac
test current which is directly
proportional to bushing capacitance.
Bushings may be tested by one or
more of four methods depending
upon the type of bushing and the
power-factor test set available. For
more complete detailed instructions
on the method of test and test
procedure, please see the
appropriate power-factor test set
instruction book. The four test
methods are as follows:
more of four methods depending
upon the type of bushing and the
power-factor test set available. For
more complete detailed instructions
on the method of test and test
procedure, please see the
appropriate power-factor test set
instruction book. The four test
methods are as follows:
a. The GST (grounded specimen
test).- This test measures the
insulating qualities of the insulation
between the current carrying or
center conductor and the mounting
flange of a bushing. The application
of such a test is necessarily limited to
bushings out of the apparatus such
as spare bushings, or bushings which
test).- This test measures the
insulating qualities of the insulation
between the current carrying or
center conductor and the mounting
flange of a bushing. The application
of such a test is necessarily limited to
bushings out of the apparatus such
as spare bushings, or bushings which
have been isolated from connected
windings and interrupters. The test is
performed by energizing the bushing
conductor and grounding the flange.
windings and interrupters. The test is
performed by energizing the bushing
conductor and grounding the flange.
Large variations in temperature have
a significant effect on power-factor
readings on certain types of
bushings. For comparative purposes,
readings should be taken at the same
temperature, or corrections should be
applied before comparing readings
taken at different temperatures.
a significant effect on power-factor
readings on certain types of
bushings. For comparative purposes,
readings should be taken at the same
temperature, or corrections should be
applied before comparing readings
taken at different temperatures.
b. The hot-guard test.- This test mea-
sures the insulation between the cur-
rent-carrying or center conductor and
the mounting flange of a bushing.
The test was designed specifically for
"draw-lead" type bushings but is
applicable to any bushing in
apparatus which can be isolated from
connecting windings and bus, but not
sufficiently to withstand test potential.
Both the bushing and the draw-lead,
winding, and bus are energized at the
same test potential, but only the
current and losses of the bushing are
measured.
sures the insulation between the cur-
rent-carrying or center conductor and
the mounting flange of a bushing.
The test was designed specifically for
"draw-lead" type bushings but is
applicable to any bushing in
apparatus which can be isolated from
connecting windings and bus, but not
sufficiently to withstand test potential.
Both the bushing and the draw-lead,
winding, and bus are energized at the
same test potential, but only the
current and losses of the bushing are
measured.
c. The UST (ungrounded-specimen
test).- This test measures the insula-
tion between the current-carrying or
center conductor and the capacitance
tap, power-factor tap, and/or un-
grounded flange of a bushing. This
test may be applied to any bushing in
or out of apparatus which is either
equipped with capacitance or power-
factor taps or the flange of which can
be isolated from the grounded tank in
which the bushing is installed. The
insulation resistance between the
taps or insulated flanges and ground
should be 0.5 meg-ohm or better.
While in this case, anything that is
attached to the bushing (such as
contact assemblies or transformer
windings) would also be energized;
test).- This test measures the insula-
tion between the current-carrying or
center conductor and the capacitance
tap, power-factor tap, and/or un-
grounded flange of a bushing. This
test may be applied to any bushing in
or out of apparatus which is either
equipped with capacitance or power-
factor taps or the flange of which can
be isolated from the grounded tank in
which the bushing is installed. The
insulation resistance between the
taps or insulated flanges and ground
should be 0.5 meg-ohm or better.
While in this case, anything that is
attached to the bushing (such as
contact assemblies or transformer
windings) would also be energized;
only the insulation of the bushing between
(FIST 3-2 11/91)
4