Megger DELTA 4000, Reference Manual

Page: 33 / 48

ZM-AH02E DELTA 4000 33
3 TESTING POWER SYSTEM COMPONENTS
Air magnetic circuit-breakers
The tests and test modes on air-magnetic circuit-breakers
are conducted in the same manner as the vacuum circuit-
breakers. The test connections for tests 1 - 6 are conducted
in the GST mode with the opposite bushing in the same
phase guarded. Normally tests 1 - 6 are conducted with
the arc-chutes in place. If it is desirable to eliminate the
influence of the arc-chutes, raise or remove them and
repeat tests 1 - 6. Follow the same test procedure as for the
vacuum breakers and record the results. If nameplate or
factory readings are not available, compare the results or
prior tests on the same breaker and results of similar tests
on similar breakers. Tests 1 - 6 and the UST tests should be
analyzed on dielectric losses, not power factor. If bush-
ings are equipped with test taps follow the tests procedures
previously discussed in this guide.
Oil circuit reclosers
Testing of oil circuit reclosers is performed in the same
manner as oil circuit-breakers. The current and watts are re-
corded and corrected to 2.5kv or 10kv if necessary. Power
factor is recorded for the closed breaker tests, but not
corrected for temperature. Test results are evaluated in the
same manner as the oil circuit breakers and the tank-loss
index is computed..
Rotating machines
The main purpose of capacitance and dissipation factor
tests on rotating machines is to assess the extent of void
formation within the winding insulation and the resulting
damage to the insulation structure due to partial discharges
(ionization in voids). An overall measurement on a winding
will also give an indication of the inherent dissipation factor
of the winding insulation and will reveal potential problems
due to deterioration, contamination, or moisture penetra-
tion.
A power factor (dissipation factor) tip-up test is a widely
used maintenance test in evaluating the extent of insula-
tion deterioration caused by ionization. In this test, the
dissipation factor is measured at two different voltages, the
first low enough so that no ionization occurs (normally
25 percent of rated line-to-ground voltage), the second at
rated line to ground voltage or slightly above rated voltage.
The tip-up value is obtained by subtracting the value of the
dissipation factor measured at the lower test voltage from
that measured at the higher test voltage. When the dissipa-
tion factor increases significantly above a certain voltage, it
is evident that ionization is active and producing some loss.
An increase in dissipation factor above a certain voltage is a
guide to the rate at which ionization is occurring and gives
guidance as to how the ionization action may be expected
to accelerate. If voids are short-circuited when ionization
occurs, some increase of capacitance with voltage may also
result. Any forecast of remaining useful life must be based
upon knowledge of the resistance of the particular insula-
tion to ionization.
In general, the coils nearest the line terminals and operating
at the highest voltage to ground are most affected by ion-
ization. The reliable life remaining in a winding can often
be extended by obtaining dissipation factor versus voltage
curves on all coils, replacing only the worst, and regrouping
them so that the coils with the least increase of dissipation
factor, and preferably lower value of dissipation factor,
are nearest the line terminals. Considerable extension of
winding life can also be realized in many cases by measuring
dissipation factor versus voltage on groups of coils without
removal and rearranging the line and neutral connections
accordingly. This can be done several times in a lifetime so
that the coils are evenly deteriorated.
An overall measurement on a rotor or stator winding is
made on the insulation between the winding and ground. In
the case of three-phase stator windings, where the connec-
tion between the winding phases and neutral can be con-
veniently opened, additional measurements are also made
on the inter-winding or phase-to-phase insulation. When a
tip-up test is made on a complete phase winding, only the
average value is measured; an isolated section having an
abnormally high tip-up may be completely masked.
Table 3.10 shows the specific connections between the test
set and a typical generator three-phase stator winding as
well as the routine series of measurements performed on
the windings. It is assumed that the connection between the
winding phases and also neutral are opened. The phase-
to-ground insulation tests are made by the GST-GND test