Low-Frequency Tests
There are two low-frequency tests:
1. Low-frequency wet-withstand voltage
test
2. Low-frequency dry-withstand voltage
test
Low-Frequency Wet-Withstand Voltage
Test — The low-frequency wet-withstand voltage test is applied
on bushings rated 242 kV and below while a waterfall at a particular
precipitation rate and conductivity is applied. The values of
precipitation rate, water resistivity, and the time of application
vary in different countries.
American standard practice is a
precipitation rate of 5 mm/min, a resistivity of 178 ohm-m, and a
test duration of 10 sec, whereas European practice is 3 mm/min, 100
ohm��m, and 60 sec, respectively.
If the bushing flashes over externally
during the test, it is allowed that the test be applied one
additional time. If this attempt also flashes over, then the test
fails and something must be done to modify the bushing design or test
setup so that the capability can be established.
Low-Frequency Dry-Withstand Voltage
Test — The low-frequency dry-withstand test was, until
recently, made for a 1-min duration without the aid of
partial-discharge measurements to detect incipient failures, but
standards currently specify a one-hour duration for the design test,
in addition to partial-discharge measurements.
The present test procedure is:
Partial discharge (either
radio-influence voltage or apparent charge) shall be measured at 1.5
times the maximum line-ground voltage. Maximum limits for partial
discharge vary for different bushing constructions and range from 10
to 100 ��V or pC.
A 1-min test at the dry-withstand
level, approximately 1.7 times the maximum line-ground voltage, is
applied. If an external flashover occurs, it is allowed to make
another attempt, but if this one also fails, the bushing fails the
test. No partial-discharge tests are required for this test.
Partial-discharge measurements are
repeated every 5 min during the one-hour test duration at 1.5 times
maximum line-ground voltage required for the design test. Routine
tests specify only a measurement of partial discharge at 1.5 times
maximum line-ground voltage, after which the test is considered
complete.
Bushing standards were changed in the
early 1990s to align with the transformer practice, which started to
use the one-hour test with partial-discharge measurements in the late
1970s. Experience with this new approach has been good in that
incipient failures were uncovered in the factory test laboratory,
rather than in service, and it was decided to add this procedure to
the bushing test procedure.
Also from a more practical standpoint,
bushings are applied to every transformer, and transformer
manufacturers require that these tests be applied to the bushings
prior to application so as to reduce the number of bushing failures
during the transformer tests.
No comments:
Post a Comment