Time-Dependent Pressure Effect in Liquid Dielectrics

The electric strengths of n hexane and transformer oil have been measured at various pressures using linearly rising voltages having rates of rise between 50 kV/cm/¿s and 20 MV/ cm/¿s. Each measurement was made by applying a single pulse of sufficient magnitude to ensure breakdown in order to avoid the possible influence of prestressing with the normal multipulse technique. For both liquids the electric strengths are strongly dependent on the rate of rise of the applied field and on hydrostatic pressure, but the pressure dependence diminishes with increasing rate of rise of the applied field, and at rates of rise higher than about 4 MV/cm/¿s the pressure dependence practically disappears. The results indicate that a certain time may be required for bubbles to form and deform in an electrically stressed liquid and that the bubble mechanism may not be operative in the breakdown process for fast-rising voltages.