Partial discharges in free helium bubbles in transformer oil

Partial discharges (PD) in a floating bubble in transformer oil were studied experimentally and theoretically. In test cell, the PD inception was very rare event. Sometimes PD occurred after an exposition of more than 10 hours at the voltage that corresponds to the value three times larger than that required in accordance with the Paschen's law. The problem is in initiating electrons. Analysis showed that, at the conditions of PD occurrence in helium bubbles, the mechanism of breakdown corresponds to the streamer one. Registrations of electrical and optical signals of PD were performed to estimate both apparent charge and the number of radiated photons. As a result of estimation, number of electrons is approximately equal to the number of photons.

I. Introduction

Bubbles in oil filled high voltage equipment are frequently discussed problems. Among oil-filled apparatuses, the power transformer is one of the important and expensive units in power system. The main element in it is oil-barrier insulation with oil channel. Oil stream flowing in the channel has two functions: insulating and cooling of windings. The appearance of free bubbles in the oil channel and the PD activity in them may become the cause of the transformer outage. Some PD characteristics (PD inception voltage as a function of the bubble size, phase-resolved patterns etc.) were investigated in air and vapor bubbles [1]. The aim of this work is to analyze the physical picture of PD in a free helium bubble.