Mechanism of partial discharges in free helium bubbles in transformer oil

Partial discharges (PD) in floating bubbles and in microspheres made from thin glass walls in transformer oil were studied experimentally and theoretically. In a test cell, the PD occurrence was a very rare event. Sometimes PD in bubbles occurred after exposure for more than 10 hours at a voltage that corresponds to a value three times the voltage in accordance with Paschen's law. In the case of a glass microsphere, PD takes place at a voltage according to Paschen's law after several hours but afterwards PD occurs more frequently. The reason for this is in the starting electrons which initiate the first avalanche. It is shown with 100 keV X-rays that PD occurs in all bubbles and its inception voltage corresponds to Paschen's law. Theoretical analysis shows that, at the conditions of PD occurrence in helium bubbles, breakdown fits the streamer mechanism. Registrations of electrical and optical signals of PD were performed to estimate apparent and true charges and the number of radiated photons. As a result of the evaluation, the number of electrons is approximately equal to the number of photons.