HIGH-VOLTAGE, DC TESTS - CABLES
46. Cable Testing.-
When the cables can
be isolated from other equipment, they may
be tested to much higher limits than rotating
machine insulation. Accordingly, a table of
cable test voltages allowable for
maintenance tests is included (see table D).
The recommended limits for maintenance
tests of cables are much higher than for
rotating machine insulation because: (1)
cable insulation is less subject to cracking,
ionization, and erosion than rotating machine
insulation; (2) a higher voltage is likely to be
necessary to show up the defects; and (3) a
higher voltage is allowable in manner of a
proof test as there is less investment at
stake and the cost in time and expense of
repairing a cable failure from test is
comparatively small. The deterioration of
sheathed cables in service is most likely to
occur at the potheads or the joints due to
loss of impregnating oil or pothead
compound, entrance of air, moisture, and co-
rona action in the air voids.
be tested to much higher limits than rotating
machine insulation. Accordingly, a table of
cable test voltages allowable for
maintenance tests is included (see table D).
The recommended limits for maintenance
tests of cables are much higher than for
rotating machine insulation because: (1)
cable insulation is less subject to cracking,
ionization, and erosion than rotating machine
insulation; (2) a higher voltage is likely to be
necessary to show up the defects; and (3) a
higher voltage is allowable in manner of a
proof test as there is less investment at
stake and the cost in time and expense of
repairing a cable failure from test is
comparatively small. The deterioration of
sheathed cables in service is most likely to
occur at the potheads or the joints due to
loss of impregnating oil or pothead
compound, entrance of air, moisture, and co-
rona action in the air voids.
WARNING
High voltage cables should be tested with
negative voltage dc applied to the test
specimen. Never apply positive voltage
dc. Experiences by other utilities indicate
that positive voltage dc greatly accelerates
deterioration.
negative voltage dc applied to the test
specimen. Never apply positive voltage
dc. Experiences by other utilities indicate
that positive voltage dc greatly accelerates
deterioration.
47. Advantages of DC High-Potential
Test.
Test.
- AC high-potential tests on long high-
voltage cables would be impractical because
of the high-charging kilovolt amperes
required and nonuniform voltage distribution
over the length of the cable. The DC, high-
potential test method has the following
advantages over the ac method: (1) direct-
potential testing causes no deterioration of
the insulation up to within a few percent of
the breakdown point; (2) the small leakage
of the high-charging kilovolt amperes
required and nonuniform voltage distribution
over the length of the cable. The DC, high-
potential test method has the following
advantages over the ac method: (1) direct-
potential testing causes no deterioration of
the insulation up to within a few percent of
the breakdown point; (2) the small leakage
current drawn can be readily measured and
tendencies to increase and decrease noted;
(3) much less damage is done if a
breakdown occurs because the capacity of
the equipment is small; and (4) the test
equipment is relatively small and light
because it needs to be only large enough to
supply the insulation leakage current. A
special precaution to be observed with the
dc method is the long time required to drain
off the charge after a test is made.
tendencies to increase and decrease noted;
(3) much less damage is done if a
breakdown occurs because the capacity of
the equipment is small; and (4) the test
equipment is relatively small and light
because it needs to be only large enough to
supply the insulation leakage current. A
special precaution to be observed with the
dc method is the long time required to drain
off the charge after a test is made.
48. Test Technique.- In maintenance test-
lng of cables, it is practical to combine the
more searching ability of the step-voltage
method with test to reasonable proof level
(see
lng of cables, it is practical to combine the
more searching ability of the step-voltage
method with test to reasonable proof level
(see
absorption is not so prominent as to require
a voltage/time schedule as extensive as for
generator insulation. Five to seven equal
voltage steps up to the desired test level,
allowing 2 minutes at each step, are usually
sufficient. If the insulation is type with much
absorption, a generator insulation test
schedule may be used. The ramped-voltage
test technique may be used for cable testing
if the test set has a high enough voltage
rating to reach a reasonable proof level for
the cable.
a voltage/time schedule as extensive as for
generator insulation. Five to seven equal
voltage steps up to the desired test level,
allowing 2 minutes at each step, are usually
sufficient. If the insulation is type with much
absorption, a generator insulation test
schedule may be used. The ramped-voltage
test technique may be used for cable testing
if the test set has a high enough voltage
rating to reach a reasonable proof level for
the cable.
49. Interpretation of Test Curve.
Warning of approaching breakdown on test
is usually given by upward bend of the V-I
curve. However, with cables when weakness
is indicated, it is less important that the test
be terminated than with generators for the
reasons previously pointed out.
is usually given by upward bend of the V-I
curve. However, with cables when weakness
is indicated, it is less important that the test
be terminated than with generators for the
reasons previously pointed out.
Upward bend of the V-I curve is most often
an indication of trouble in the pothead such
as voids, moisture, or imperfect makeup.
See
an indication of trouble in the pothead such
as voids, moisture, or imperfect makeup.
See
If resealing of potheads is required, it is
advised that another recheck be made a
year later. Otherwise, routine tests of cables
need to be conducted only at intervals of
about 5 years.
advised that another recheck be made a
year later. Otherwise, routine tests of cables
need to be conducted only at intervals of
about 5 years.
(FIST 3-1 12/91)
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