Applications
The Biddle dc dielectric test
sets are used to make proof tests and insulation tests on:
- Electrical power cables.
- Motors.
- Switchgear.
- Insulators.
- Transformers.
- Capacitors.
Both types of tests
are performed by applying controlled high voltages to the unit
under test at or above insulation
system operating level. Measuring the leakage current helps
determine the unit under test’s ability to withstand
overvoltages such as lightning strikes and switching surges.
The three models described cover a range of output voltages
that meet the most commonly specified ratings in 5-kV to 69-kV
class cable. All are suitable for testing power cable,
switchgear and rotating machinery in accordance with IEEE,
IPCEA, NEMA and ANSI guidelines.
Proof
Test
Proof testing is used for
acceptance testing of newly installed cable and maintenance
testing of aged and/or repaired cable. For the proof test, the
unit under test will either withstand the test voltage or it
will “break down,” providing the user with a go/no-go” answer.
This is essentially the same as the proof test performed by
Megger impulse generator systems, within voltage limitations.
Insulation Resistance Tests
To make appropriate tests on
healthy insulation, the test instrument must have microampere
sensitivity. Insulation resistance can be measured in at least
three different ways:
The insulation resistance test is often referred to as a “spot
check,” and is performed by applying a predetermined voltage
to the unit under test, holding it until the apparent leakage
current becomes stable and recording the readings with
adjustments for temperature. This test is especially
applicable to low-capacitance units under test.
Time-varying tests
such as the polarization index test (PI test) are independent
of temperature effects and save time. To perform this test, a
predetermined test voltage is applied to the unit under test
and readings are taken at 1 minute and 10 minutes. The
resulting ratio is analyzed to determine insulation quality.
This type of test is especially appropriate for
high-capacitance samples.
The step-voltage test
is independent of temperature effects and saves time. To
perform this test, the output voltage is increased in even
steps at regular intervals over a fixed period of time. As
long as the resistance of the unit under test increases with
time, it has high-quality insulation. This type of test is
only useful for high-capacitance samples.
Fault Location
When modified for use with Biddle accessory impulse
generators, the dc dielectric test sets provide an economical
means of locating faults. |
