AC Partial Dischatge Characteristic at Minute Air Gap under Non-uniform Electric Field

For a raise of the miniaturization and reliability of various electric power apparatuses, it is important to clarify the partial discharge characteristic in their insulation part. In many case, partial discharge may occur under the non-uniform field. So, it is necessary to understand the partial discharge characteristic under non-uniform electric filed. Partial discharge inception voltage (PDIV) was measured under AC voltage application using needle-plane electrode system. Partial discharge inception electric field (PDIE) was estimated using COMSOL that is a general purpose physical simulation software. When the air gap distance was below 50 gm and the tip radius was 10, 30 and 60 gm, PDIE at the needle tip was almost the same as the electric field estimated by the Paschen curve. In this case, it is considered that PDIV may be decided by the electric field at the needle tip and that the emission of initial electrons plays very important role in the partial discharge process. However, when the needle tip radius was 10 or 30 gm and the air gap distance became 50 gm, PDIE at the needle tip became higher than the electric field strength estimated by the Paschen curve. In this latter case, it is considered that even if initial electron was emitted from the needle tip, the electric field was not enough to develop the detectable electron avalanche. From our experiment, the electric field strength necessary to the detectable partial discharge development is estimated to be about 67 "A of the electric field strength necessary to the initial electron emission from the needle tip.