The effect of high continuous voltages on air, oil, and solid insulations

The dielectric strength of air, oil and solid insulations was determined for d-c. voltages up to 150 kv. The d-c. visual corona voltage is equal to the maximum a-c. corona voltage for wires varying in radius from 0.013 cm. to the largest sizes. The variation of d-c. and a-c. corona voltages with air density is the same over a large range. The laws already given for a-c. voltages apply equally well for d-c. voltages in terms of maximum values. The spark-over of gaps is the same on alternating current and direct current for equal maximum voltages when the gap is such that spark-over precedes corona. Thus, for the sphere gap the same laws apply for a-c. or d-c. voltages. This is true at various air densities. When corona precedes spark-over there is generally a difference in a-c. and d-c. spark-over voltages. For a non-symmetrical gap, spark-over at normal air density takes place at the lowest voltage when the electrode surrounded by the denser field is (+). At low air densities spark-over takes place when the electrode surrounded by the denser field is (−). Insulators spark-over at the lowest voltage when the cap, or electrode surrounded by the denser field, is (+). The (+) spark-over voltage generally corresponds closely to the maximum a-c. spark-over voltage. The d-c. spark-over voltages in oil generally correspond closely to the maximum a-c. spark-over voltages. In wet oil the d-c. spark-over voltage is lower than the a-c. The d-c. breakdown voltages of solid insulations, in good condition, are generally higher than the maximum a-c. voltages. This is especially so when the time of application is long and the insulation is thick. The d-c. breakdown voltage on insulations tested apparently increases directly with the thickness, while the a-c. breakdown voltage increases at a lesser rate. Laws are given. When the insulation is moist, the d-c. and maximum a-c. breakdown voltages are generally approximately the same. It appears that high- voltage direct current would be useful in certain high voltage cable testing, etc.