Electric breakdown of transformer oil

                                                                                                                      Electric breakdown of transformer oil

    A research programme has been commenced which is designed to determine the mechanisms of electric breakdown in insulating liquids under various conditions. The paper describes experimental equipment and techniques, and some results which have been obtained on transformer oil.Special equipment includes a transparent pressure chamber with an internal glass test-vessel, which enables the liquid under test to be subjected to hydrostatic pressures up to 500 lb/in2 and voltages up to 100 kV peak. A protective circuit hasbeen developed, incorporating triggered spark-gap, which limits the duration of breakdown currents to a few microseconds.A wide range of breakdown strengths has been obtained, varying from 5 000 kVpeak/cm with 1-microsec impulses, to below 100 kV(r.m.s.)/cm for a fibre contaminated oil under a.c. test conditions. The removal of fibres raises the a.c. strength of otherwise untreated oil to 400 kV(r.m.s.)/cm.It is concluded that a breakdown mechanism initiated by cavitation is operative in certain circumstances.

Breakdown of transformer oil is affected not only by electrostatic field configurations, but also by many other factors. The latters make it difficult to predict break- down voltage from electrostatic field, as made for air and S F6 gaps. Alternatively, many experimenters tried to express the breakdown stress of oil as a function of stressed oil volume. But, results are limited to their respective electrodes.