Abstract
Partial discharge (PD) measurement in a needle to plane gap is suggested by IEC used for ranking dielectric liquids. This paper investigates PD behavior in eight liquids using a 20 mm ac stressed needle to plane gap. Parameters varied were tip radius, frequency and voltage. Both discharge phase distributions and pulse sequences were studied. PD occurrence was highly correlated to the frequency of the applied voltage. A higher frequency of 600 Hz gave a higher PD rate than the lower frequencies, like 60 Hz. The tip radius does also affect the PD pattern. The influences from tip radius and frequency indicate a strong correlation between space charges and PD initiation. It was observed that large positive PDs almost always was preceded and followed by a smaller negative PD in the neighboring half cycles, indicating an influence from charges of opposite polarity created in the previous half cycle. Exposing the gap to X-rays gave a PD burst for both polarities. This indicates that electrons are needed for initiation of PDs and that lack of electrons is the reason for low occurrence rate of positive PDs.