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.