Megger DELTA 4000, Справочное руководство

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ZM-AH02E DELTA 4000 21
3 TESTING POWER SYSTEM COMPONENTS
the result of the low phase being wound around the center
leg of a three-legged core. The reluctance of the magnetic
circuit is less for the center leg of the core resulting in a
lower charging current.
Shunt reactors
When electrical energy is transmitted at extra high voltages,
special problems arise that require the installation of large
compensating devices to regulate the over-voltage condi-
tions and to guarantee stability. Among these devices are
shunt reactors. Shunt reactors are composed of a large coil
placed inside a tank and immersed in oil. They can be single
phase units or three phases in one tank. In both cases each
phase has its own neutral bushing.
Test connections
▪
For all tests, the line and neutral bushings for
corresponding phases must remain shorted.
Test procedure
▪
Record test results on the test form for Miscellaneous
Equipment Capacitance and Power Factor Tests.
▪
Test voltages are at 10kV. If 10kV exceeds the insulation
rating, test at or slightly below the insulation rating.
▪
For single phase units only one overall ground test is
performed in the GST mode.
Test results
Power factor and capacitance results should be recorded in
the same manner as for oil filled power transformers. Tem -
perature correction should be to the top oil temperature.
Compare test results to previous tests or tests on similar
units. Additional bushing tests should be performed if test
results are suspect.
Potential transformers
Potential transformers are installed on power systems for
the purpose of stepping down the voltage for the operation
of instruments such as Volt-meters, Watt-meters and relays
for various protective purposes. Typically the secondary
voltage of potential transformers is 120 V, so power factor
testing is performed on the primary winding. Potential
transformers are typically single phase with either single or
two bushing primaries. Single bushing primaries have one
end of the high voltage winding connected to ground. Sec-
ondary windings are normally three wire and dual identical
secondary windings are common.
Test connections
Ensure that the potential transformer is disconnected from
the primary source before testing begins.
1]
Remove any fusing on the secondary circuits to
prevent any type of back-feeding to the secondary.
2]
Ground one leg of each secondary winding for all
tests on two primary bushing transformers, for
dual secondary transformers it is typically X1 and
Y1.
3]
Ensure that the case of the potential transformer
is securely grounded to a system ground before
testing begins, this also includes testing of spare
transformers.
Test procedure
Ensure the test set is securely grounded.
1]
Record all tests results. Power factor tests should
be corrected to ambient temperature.
2]
Compare test results to prior tests on the same or
similar equipment.
Current transformers
Current transformers are used for stepping down primary
current for Ampere-meters, Watt-meters and for relaying.
Typical secondary current rating is 5 A. Current transform-
ers have ratings for high voltage and extra high voltage
application. The higher voltage classifications can be oiled
filled, dry type or porcelain construction. Tests on two
bushing primary currents transformers are performed
by shorting the primary winding, grounding all second-
ary windings and test in the GST mode. Some current
transformers in the high voltage classifications have test
taps similar to bushings. Tests can be performed on units
equipped with a test tap for the C
1
insulation and the C
2
tap insulation. Assure that the unit under test is grounded
before testing. Record all test results and correct the power
factor readings to the ambient temperature at the time of
the test.
Voltage regulators
Regulators are generally induction or step-by- step. The
induction regulator is a special type of transformer, built
like an induction motor with a coil-wound secondary, which
is used for varying the voltage delivered to a synchronous
converter or an ac feeder system. The step-by-step regulator
is a stationary transformer provided with a large number of
secondary taps and equipped with a switching mechanism
for joining any desired pair of these taps to the delivery
circuit. Voltage regulators may be single or three phase.
Single phase regulators consist of three bushings identified
as S (Source), L (Load) and SL (Neutral). The windings in
the regulator cannot be effectively separated, so one overall
power factor test is performed. All the bushings are shorted
together and tested in the GST-GND test mode.
Tests should be conducted with the tap changer moved to
some position off of neutral. Additional Hot Collar tests
may be conducted on bushings of suspect units. Excitation
tests may also be performed by energizing terminal L with
the high voltage lead and the low voltage lead on SL in the
UST position. Terminal S should be left floating. Power
factor results should be corrected to top oil temperature on
regulators just taken out of service. Ambient temperature
should be used for those that have been out of service for
any length of time. Power factor results should be compare
to previous tests on the same equipment or similar tests on
similar units.