An insulation having a gradually increasing
leakage current response can tolerate much
higher leakage current than an insulation
having abrupt current increases in the V-I
response curve, A test on an insulation
having a slowly increasing leakage current
response should be stopped when the total
insulation current becomes three to six times
larger than the capacitative charging
component of current.
43. Ramped-Voltage Test Results.-
insulation ramped voltage test results. Actual
record size is 25 x 38 cm (10 x 17 in).
of new epoxy mica insulation when ramp dc
tested. Above 5 kV, the curve is very linear,
indicating excellent insulation quality.
asphalt mica insulation. This curve is also
linear above 4 or 5 kV, indicating high-
leakage current suggesting the insulation of
one coil or portion of a coil is very
questionable. The test was terminated early
to avoid the possibility of damaging the
due to many minute discontinuities similar to
the one shown in
a very gradual increase in leakage current
and implies that a general deterioration of the
insulation has taken place. As the test voltage
increases, the slope of the leakage current
will continue to increase in a nonlinear
manner. Breakdown will occur as the
current asymptotically approaches the
vertical. Testing should be halted well before
this point is reached.
- The powerplant ground
mat is part of the high-voltage circuit. To mini-
mize induced alternating-current voltages in
this circuit, the test set grounds should be
connected to plant ground as close to the
stator core as possible. The discharge
resistor and grounding stick should be tied to
the ground mat in the same area where the
test set grounds are attached. Do not ground
these sticks at the test set ground terminals.
Due to the nature of the insulation absorption
current, the winding must remain grounded for
1.5 to 2 hours, or the voltage will recover by
itself to a fraction of the original dc test
voltage. An ac voltage should not be applied
until the winding has been thoroughly
discharged (grounded for 1.5 to 2 hours).
Failure to properly discharge the winding
constitutes a safety hazard and could also
result in failure of the insulation upon
application of an ac voltage.
45. Discharging the Winding.
highly capacitive and inductive properties of
stator windings make it essential that they be
discharged slowly. When the capacitively
stored energy in the winding is suddenly dis-
charged through the winding inductance, dan-
gerously high-voltage transients are
generated across the insulation. The voltage
should be reduced to a value below 5 kV with
a discharge resistor before applying the
15 (FIST 3-1 12/91)