PD measurements
are an ideal method for evaluating switchgear apparatus with
non-self-restoring insulation. During a temporary overvoltage, during
a high-voltage test, or under transient voltage conditions during
operation, PDs may occur on insulation of this type, which includes
gas, liquid, and solid materials.
If these PDs
are sustained due to poor materials, design, and/or foreign
inclusions in the insulation, degradation and possible failure of the
insulation structure may occur.
Due to the
variability of performance of dielectric materials and system
designs, it is recommended that partial discharge tests be made as
design tests in conjunction with the other dielectric tests on new
switchgear equipment designs.
The partial
discharge test should be performed both before and after the impulse
and normal frequency dielectric tests. Once performance is
established, partial discharge tests on the switchgear equipment
design need only be performed following the normal frequency
withstand tests.
Due to the
possible influence that the impulse and normal frequency dielectric
tests may have on the outcome of the partial discharge test, the
partial discharge test may be performed a substantial time after
these dielectric tests.
In some cases,
the partial discharge test can be made as a part of the normal
frequency withstand test. The following procedure is recommended for
PD tests on switchgear equipment. The normal frequency voltage is
applied and raised to the dry normal frequency withstand voltage
level, as specified by standards, for no less than 10 s.
Partial
discharges may occur at this test voltage depending on the PD voltage
inception level of the equipment under test. The voltage is then
decreased to the partial discharge test voltage level as specified by
the equipment standard and held at that voltage for one minute.
If the measured
PD level exceeds the level allowed by the equipment standard at the
end of this time limit, the equipment is considered to have failed
this test. A PD level lower than that allowed by the equipment
standard signifies that the apparatus has passed.
Partial
discharge acceptance levels and related normal frequency test
voltages are not listed in this guide because the various switchgear
equipment standards list the specific test levels established for
that equipment.
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