Partial discharge is defined as the localized electrical discharge that only partially bridges the insulation between conductors and which can or can not occur adjacent to a conductor. Partial discharge is commonly a consequence of localized electrical stresses in the insulation or on the surface of the insulation accompanied by emission of sound, light, heat, and chemical reactions.
"Corona" is a form of partial discharge that occurs in gaseous media around conductors which are remote from solid or liquid insulation. However, partial discharge is not always "corona".
Partial discharges are commonly detected using electrical methods. Non-electrical methods of PD detection include acoustical, optical, and chemical methods and also, where practicable, the subsequent observation of the effects of any discharges on the test object. These methods are not suitable for the quantitative measurement of partial discharge quantities but are essentially used to detect and/or locate partial discharges.
- Acoustic detection
- Directionally selective microphones with high sensitivity above the audible frequency range are useful for locating corona discharges in air.
Acoustic transducers may also be used for locating discharges in gas-insulated switchgear or oil-immersed equipment such as transformers; they may be either inside or outside the enclosure.
- Visual or optical detection
- Visual observations can be carried out in a darkened room, after the eyes have become adapted to the dark and, if necessary, with the aid of binoculars of large aperture. Alternatively, a photographic record can be made, but fairly long exposure times are usually necessary.
- Chemical detection
- The presence of partial discharges in oil- or gas-insulated apparatus may be detected in some cases by the analysis of the decomposition products dissolved in the oil or in the gas. These products accumulate during prolonged operation, so chemical analysis may also be used to estimate the degradation which has been caused by partial discharges.
- IEC 60270 High-voltage test techniques - Partial discharge measurements
- IEC 60567 Guide for the sampling of gases and of oil from oil-filled electrical equipment and for the analysis of free and dissolved gases
- IEC 60599 Mineral oil-impregnated electrical equipment in service – Guide to the interpretation of dissolved and free gases analysis
- IEC 61181 Impregnated insulating materials – Application of dissolved gas analysis (DGA) to factory tests on electrical equipment
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