All windings not at test potential should be
grounded. Each winding should be measured
to all other windings and to ground, and also
all of the windings measured together to
ground. All possible winding combinations
should be tried. Refer to IEEE Standard No.
262, 1973, for more information concerning
this test technique.
- The power-factor test on ca-
bles is usually an insensitive indication of
deterioration except on very short lengths.
Cables often require more charging current
than the power-factor test set can supply.
However, "hot-collar" power-factor tests on
cable potheads are useful, sensitive
indications of moisture and voids or other
substandard conditions in the potheads.
Abnormally high values of current and power
indicate the presence of moisture and
abnormally low values indicate a void or the
absence of filling compound or oil. Hot-collar
tests do not require a high-capacity test set.
56. Test Equipment.
- Most of the power-
factor testing done in the United States
and Canada has been by means of
equipment supplied by the Doble Engineering
Co. The Doble test sets consist of a
completely shielded, high-voltage, 60-Hz
power supply which applies up to 10 kV to
the equipment being tested. Readings are
taken of volts, milliamperes, and watts, from
which power factor is determined. The
equipment is suitable for use near high-
voltage circuits without interference from
induced voltages. The capacitance-test
bridge, a much simpler and less expensive
tester, is available on some Bureau projects.
It applies about 80 volts to the equipment
being tested. Capacitance and dissipation
factor are read directly on dials when the
bridge is balanced. This equipment is not
sufficiently shielded against induced voltages
to be suitable for use near high-voltage
circuits, thus limiting its usefulness for field
maintenance work. Although shields of sheet
metal or screen can be used to enclose the
equipment bushings during tests, this method
is not desirable because of the danger in
handling these large metal parts near
The ionization test is
used primarily for detecting ionization
(corona discharge) and slot discharge in
generator windings. Both ionization and slot
discharge may cause deterioration of
insulation. Ionization generally occurs in
voids inside the insulation within the ground
shield section of the coil.
Some internal ionization is present in most
higher voltage stator insulation. If it is
intense enough, it produces destruction of
the binder and other organic components by
the chemical effects of the ozone and oxides
of nitrogen generated by the discharge (plus
moisture) and eventually by direct electronic
bombardment. Destruction is often
aggravated finally by mechanical vibration
of the conductors where the insulation has
been softened by the other effects.
Slot discharge is a capacitance discharge
and occurs across poor contact points
between the coil surface shielding and the
stator iron. Deterioration is from the
destructive effect of this relatively
concentrated discharge. Coils which fit
loosely in the slots may be subjected to this
Test apparatus for corona detection requires
much higher sensitivity than detection of slot
discharge. It is still of specialized category
and beyond the scope of this chapter.
(FIST 3-1 12/91)