Dielectric Withstand of a Mixture of Fluoroketone C5F10O and Air When Cooling Below its Dew Point

We investigate the temperature dependence of the dielectric withstand of a specific gas mixture consisting of a C5-fluoroketone (C5-FK) and air below its dew point. Both lightning impulse (LI) and ac dielectric withstand are investigated. Using standard testing techniques, it is found that the ac withstand stays constant until the dew point of the mixture and then decreases following the predicted withstand for the concentration of fluoroketone at the given temperature. On the contrary, when measuring the LI withstand using a common dielectric testing approach, one does not find a decrease but instead, in certain cases, an increase of the dielectric performance. We attribute this behavior to the charging droplets of the condensate fluoroketone after each dielectric breakdown event. Flipping the polarization of the LI after every breakdown event reestablishes the behavior seen with the ac stress application. Overall, these results confirm that the dielectric behavior of this alternative insulation gas for electrical devices is well understood and that the dielectric withstand at
$- 40\,\,^{\circ }\text{C}$
of a mixture specified for medium-voltage applications down to
$- 25\,\,^{\circ }\text{C}$
still shows 80% of the dielectric withstand measured at the dew point.