rms ac puncture voltage for well-compacted
and impregnated mica insulation. IEEE
Standard No. 95 mentions the factor 1.7 as
appropriate for used insulation. Cases have
indicated that on winding insulation with some
deterioration, the application of ac
overpotential tests may cause further
deterioration, even though the insulation may
not puncture in the 1 -minute duration of the
and impregnated mica insulation. IEEE
Standard No. 95 mentions the factor 1.7 as
appropriate for used insulation. Cases have
indicated that on winding insulation with some
deterioration, the application of ac
overpotential tests may cause further
deterioration, even though the insulation may
not puncture in the 1 -minute duration of the
ac test. Consequently, on any but new
windings, if the dc test indicates weakness,
ac overpotential test should not be applied.
Deteriorated coils should be located by
testing groups of individual coils by the dc
method or by use of a clip-on ammeter.
Ionization and power-factor tip-up tests may
be useful supplements in important repair
work.
windings, if the dc test indicates weakness,
ac overpotential test should not be applied.
Deteriorated coils should be located by
testing groups of individual coils by the dc
method or by use of a clip-on ammeter.
Ionization and power-factor tip-up tests may
be useful supplements in important repair
work.
HIGH-VOLTAGE, DC TESTS - STEPPED-VOLTAGE METHOD
24. Description of Test.-
The stepped-
voltage technique of high-voltage, dc testing
of insulation consists of measuring the
insulation current at scheduled times for a
series of voltage steps up to an indicated
insulation weakness or to well above the
normal operating ac peak voltage (
of insulation consists of measuring the
insulation current at scheduled times for a
series of voltage steps up to an indicated
insulation weakness or to well above the
normal operating ac peak voltage (
x V
2
rms). The insulation is then discharged
through a microammeter and measurements
are taken of current versus time. The data
are interpreted to determine insulation
quality. For interpretation, it is convenient to
graphically plot the insulation current against
test voltage and to calculate and plot the
resistance versus test voltage.
through a microammeter and measurements
are taken of current versus time. The data
are interpreted to determine insulation
quality. For interpretation, it is convenient to
graphically plot the insulation current against
test voltage and to calculate and plot the
resistance versus test voltage.
25. Current Versus Voltage Curve.
-
As the data are recorded on the data sheet,
a graph should be plotted of the insulation
current (at the end of each voltage step)
versus the test voltage, see
a graph should be plotted of the insulation
current (at the end of each voltage step)
versus the test voltage, see
form of plotting has the advantage of
employing readings taken directly from the
test instrument. An incorrect reading or an
incorrectly plotted point is promptly apparent
and may be checked immediately. As long as
the insulation current is not excessive (see
employing readings taken directly from the
test instrument. An incorrect reading or an
incorrectly plotted point is promptly apparent
and may be checked immediately. As long as
the insulation current is not excessive (see
resistance values) and continues in a
straight line with increasing test voltages,
the test may be continued up to the
recommended maximum shown in table A.
straight line with increasing test voltages,
the test may be continued up to the
recommended maximum shown in table A.
26. Resistance Versus Voltage
Curve.
Curve.
- While the current versus voltage
curve is being plotted, the insulation
resistance (in megohms) can be calculated
at the end of each voltage step. A resistance
versus voltage curve should be plotted, as
shown on
resistance (in megohms) can be calculated
at the end of each voltage step. A resistance
versus voltage curve should be plotted, as
shown on
the first voltage step, the megohm scale for
the curve should be chosen so the first
plotted point will be above the zero axis
about equal to the distance from zero along
the horizontal scale to the maximum test
voltage. This will yield an approximately
"square" plot for test of any size specimen,
permitting comparison of curve shapes on a
more uniform basis.
the curve should be chosen so the first
plotted point will be above the zero axis
about equal to the distance from zero along
the horizontal scale to the maximum test
voltage. This will yield an approximately
"square" plot for test of any size specimen,
permitting comparison of curve shapes on a
more uniform basis.
27. Log-log Plots and Discharge
Curves.
Curves.
- In addition to the current versus
voltage and resistance versus voltage
curves, the following curves should be
plotted on three-by-three cycle, log-log paper
(K&E No. 359120): (1) current versus time
for the first voltage step, and (2) the
discharge current versus time after the last
voltage step, see
curves, the following curves should be
plotted on three-by-three cycle, log-log paper
(K&E No. 359120): (1) current versus time
for the first voltage step, and (2) the
discharge current versus time after the last
voltage step, see
permits determination of the absorption
exponent "n" and the proper template to be
used for determination of the true leakage
and absorption components of current from
the current versus time curve (1). Ordinarily,
the template analysis will be performed later,
see
exponent "n" and the proper template to be
used for determination of the true leakage
and absorption components of current from
the current versus time curve (1). Ordinarily,
the template analysis will be performed later,
see
28. Data Sheet and Voltage/Time
Schedules.
Schedules.
- Form PO&M-155 should be
used for recording data as shown by
example in
example in
(FIST 3-1 12/91)
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