Thermal bubble breakdown characteristics of LN2 at 0.1 MPa under a.c. and impulse electric fields
✍ Scribed by M. Hara; D.J. Kwak; M. Kubuki
- Publisher
- Elsevier Science
- Year
- 1989
- Tongue
- English
- Weight
- 789 KB
- Volume
- 29
- Category
- Article
- ISSN
- 0011-2275
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✦ Synopsis
A.c. and impulse dielectric strengths of cryogenic liquid under atmospheric pressure-boiling state were investigated experimentally, with reference to the behaviour of a thermally induced bubble, which might be generated at quenching condition of immersed-cooling superconducting devices, using a parallel plane electrode configuration. The experimental results with LN2 as the liquid show that the bubble shape under electric field stress depends significantly On the applied voltage waveform. That is, the thermal bubble under a.c. stress varies complicatedly in its shape according to the sinusoidal variation in voltage and vibrates violently, whereas the effect of impulse stress on the boiling phenomenon is negligible. With a.c. voltage, the breakdown voltage falls suddenly from liquid breakdown voltage near to one of the saturated gases at a threshold heater power of boiling onset.
In constrast to this, with impulse voltage the reduction of breakdown voltage with heater power is gradual and the time to breakdown depends on the existence of thermal bubbles. These breakdown characteristics are attributed to the bubble behaviour in the presence of electric field.