Measurement of the electric breakdown strength of transformer oil in the sub-nanosecond regime

The dielectric strength of highly purified insulating transformer oil has been measured in the sub-nanosecond regime under single pulse and repetitive burst conditions. Single pulse breakdown fields have been measured to be 11 MV/cm. Repetitive bursts to 1 kHz reduce the threshold field value by a factor of two, with lower breakdown fields recorded at a 1.2 kHz repetition rate. The high-pressure hydrogen source provides a 130 ps risetime and a 1 MV peak amplitude at repetition frequencies to 1.2 Hz. An experimental setup was used which permits the breakdown of the oil spark gap while protecting the high power source in case of total wave reflection, at the cost of excitation source fidelity. The breakdown electric fields are measured with self-integrating electric field sensors and an advanced diagnostic system which uses Fourier compensation to measure the fast risetime of the ultra-wideband pulse accurately. The anomalously high breakdown voltages measured with high power ultra-wideband sources compare favorably with Zheltov's prediction of the breakdown strength for sub-nanosecond pulse duration. The anomalously high field strengths permit the design of ultra wide band (UWB) high power microwave (HPM) sources with a reduced geometrical inductance which can result in significantly faster HPM UWB sources.