Scale effect of the impluse breakdown in transformer oil

As an important dielectric liquid, transformer oil is widely used in power systems due to its higher breakdown strength than compressed gases. The key factor affecting the safe operation of power apparatus is decided by the steep impulse electric strength. In this paper, an impulse voltage generator with adjustable pulse width was set up to generate 0.12/50μs impulse voltage. Breakdown characteristics in transformer oil under pulsed voltages are studied over a range of electrode sizes and gap distances with a hemisphere electrode configuration of various geometric configurations. It is verified that both area and volume effects simultaneously lead to the degradation of the breakdown strength in transformer oil. The impulse breakdown strength is statistically analyzed by using Weibull distribution. By comparing two results of Weibull fitting function, it reveals that the 3-parameter Weibull distribution has a higher goodness-of-fit than the 2-parameter for impulse breakdown in transformer oil. The 3-parameter Weibull shape parameter m for the quasi-uniform field are estimated to be 2.4 to 2.58 and 2.79 to 4.11 under different electrode area and gap distance respectively. These effects are corresponding to the variation of breakdown initiation and propagation process when the dimension of electrode configuration changes. The relationships between the shape parameter m and electrode diameter as well as gap distance are in the forms of m(D)=0.004D+2.384(d=0.5mm) and m(d)=5.27-1.25d
^-0.423
(D=30mm) and the field utilization factor η as a function of shape parameter m has been calculated in which case the relationship between electrode physical dimension and stressed area is established, which can be provided for insulation design directly.