Abstract
The role of impurity particles on variable frequency pre-breakdown and breakdown behaviour of transformer oil has been investigated. Two types of particles were considered, those which were naturally present in the oil and those which were added, such as conducting copper and nonconducting selenium and silicon particles. Above a threshold stress and below a critical stress the particles formed bridges at frequencies between 20 Hz and 1000 Hz, when the gap length was below 1.5 mm. The stress required for bridge formation depended on gap and frequency. Bridges sometimes contributed to bubble formation and spurious sparks, which occasionally led to a fully developed breakdown. However, if the stress was increased above the critical value the gap was cleared of particles and breakdown did not occur over a range of stresses higher than that at which spurious sparks have been observed. The breakdown strength as a function of frequency was dependent on the type and size of particles present in the liquid and on the gap setting. It was also found to depend on the gap used for conditioning the samples. Added particles lowered the breakdown strength at all frequencies and for highly purified liquid samples the breakdown strength was found to be less dependent on frequency. A post-breakdown glow discharge was observed at gaps greater than 200 microns and frequencies higher than 200 Hz. In the light of the pre-breakdown experimental results a mathematical model for the bridge formation process has been put forward. In this model the relaxation effect of the ion atmosphere of the oscillating particles which are considered as dipoles and Bjerknes' hydrodynamic forces are regarded as mainly responsible for controlling the activity of particles in oil subjected to electric stress.