- The power factor of an
insulation is a measure of its dielectric power
loss, and is not a measure of its dielectric
voltage strength. The power-factor method is
used primarily for testing the insulation
condition of high-voltage bushings, cables,
transformer windings, and transformer oil.
51. Theory of Power-Factor Mea-
.- Unless otherwise stated, the
terms "power factor" and dissipation factor"
are used interchangeably in this discussion.
Insulation power factor is the cosine of the
angle between the applied voltage and
current, and is obtained from measurement of
watts, volts, and amperes, usually with a
specialized power-factor test set. Dissipation
factor is the cotangent of the angle between
the applied voltage and current, and is usually
obtained from a direct reading of a dial on a
capacitance bridge. The values of power
factor and dissipation factor are within 1
percent of being equal between 0 and 8
percent power factor, which covers most test
values, but diverge as the values increase.
Thus, both have equal significance as a
measure of insulation value. It should be
remembered that power factor or dissipation
factor is a measure of insulation dielectric
power loss, and is not a direct measure of
dielectric strength. Conditions which cause
abnormal power loss usually also cause
reduction of dielectric strength. The power-
factor values are independent of insulation
area or thickness, and increase only with an
increase of contamination by moisture, other
foreign matter, or ionization, and therefore,
are easier to interpret than insulation
resistance values, which additionally depend
on insulation area and thickness.
52. Significance of Dielectric Losses.
An increase in dielectric loss may accelerate
insulation deterioration because of the
increased heating, but more commonly an
increase of dielectric loss is evidence of
other deterioration which also affects
dielectric strength. As in insulation resistance
tests, the change in periodic test readings is
more indicative of insulation deterioration
than is absolute magnitude of readings.
Insulation power factor increases directly with
temperature, and may be as much as 10
times as high at 80
C as at 20
Therefore, temperature corrections to a
base temperature must be made, usually to
C for a meaningful comparison between
values taken at different temperatures. See
table E for Temperature Correction Factors
of Power Transformers.
53. Generator Windings.
- The power-
factor-voltage characteristic (power- factor
tip-up) is used primarily as a quality-control
criterion in manufacturing. It is sometimes
used as an acceptance test on individual
coils. Power-factor tip-up has been used as
a maintenance test because a change in the
tip-up value over a period of time is an
indication of change of condition of the coil
insulation. The sensitivity of the power-factor
tip-up test decreases with the length of coil
included in the measurement. Therefore, one
coil, or coil side, is the preferred test unit,
although coils are sometimes tested in
groups of two or three to expedite testing.
Refer to IEEE Standard No. 286, July 1975,
"IEEE Recommended Practice for
Measurement of Power-Factor Tip-Up of
Rotating Machinery Stator Coil Insulation,"
for a description of the power-factor tip-up
54. Transformer windings.
- The power-
factor method is particularly recommended
for detecting moisture and other loss-
producing contaminants in transformer
windings. Experience has shown that the
power-factor test is more revealing than the
insulation-resistance test when there is a
high-loss dielectric in series (as in a
transformer winding surrounded by oil), and
is less influenced by surface leakage
components. Users have found many cases
where high-power-factor readings indicated
moisture in the windings, although the oil
dielectric tests were up to standard. The
power factor was brought back to normal by
a dry-out run. Time-saving techniques have
been developed whereby the losses in
transformer bushings or windings can be
segregated without disconnecting the
windings from the bushings.
17 (FIST 3-1 12/91)