Phenomenological investigation of breakdown in oil dielectrics

The physics of low density channel formation due to electrical breakdown in trans-former oil is investigated using high speed electrical and optical diagnostics. The experiments done in pulsed and self-breakdown mode utilize a coaxial system with a point/plane axial discharge in transformer oil. Construction of the discharge chamber, oil filtration system and implementation of a matched 50 impedance throughout the system are presented. Preliminary experiments conducted in Univolt 61 and Envirotemp FR3 are compared for base line results for self and pulsed breakdown. Further experiments were conducted in an effort to alter the direction or structure of the low density channels with an applied magnetic field and at lowered hydrostatic pressure. While no change was seen in the low density channels with an applied magnetic field, expansion of the low density channels was recorded in shadowgraphy images. Expansion was seen in a small percentage of anode initiated low density channels but was predominant in the channels initiated from the cathode indicating a channel consisting of a gas or vapor for this polarity. Initial results support previous research that has indicated that current injection leading to the formation of a single bubble is likely the building block for these low density channels for cathode initi-ated self breakdown. Further experiments were undertaken to compare single bubble dynamics to channel formation. The experiments conclude that low density channels are formed due to the generation of gas or vapor during cathode-initiated breakdown.

A correlation between similar characteristics such as lifetime, separation, charge in-jection and pressure dependence were found. Systematic variation of exposure times and time delays between simultaneous luminosity and shadowgraphy images has also been performed to spatially and temporally resolve the luminosity emitted during prebreakdown events against the low density channels seen in the shadowgraphy im-ages at atmospheric pressure and lowered hydrostatic pressure. Composite imaging of cathode initiated low density channels that have become separated from the cathode, suggest a discharge from the bubble back to the anode occurs and was verified with shadowgraphy images at atmospheric pressure. Based on the interaction between separated bubbles and the cathode and the confirmation that low density channels adhere to the same mechanisms that control bubble dynamics, a model for cathode-initiated self breakdown at lowered hydrostatic pressure is presented. A partial discharge at low hydrostatic pressure forms a single bubble. The bubble oats into the gap and becomes polarized. The bubble then arcs back to the cathode shortening the gap dis-tance between the anode and the bubble causing breakdown to occur on a nanosecond timescale.