Statistical Dielectric Degradation of Large-Volume Oil-Insulation

In an attempt to clarify the persisting controversy over the effect of the electrode area versus that of the stressed oil volume in large-oil-volume- breakdown, a study was carried out under well defined conditions of the oil quality, particularly with regard to the degree of particle contamination. The results indicate that both the electrode area and the stressed oil volume can affect the dielectric strength of transformer oil, with the stressed-oil-volume effect being most pronounced under particle contamination conditions. Test results with technically clean transformer oil as currently accepted for use in power apparatus indicate that the degree of particle contamination in these oils is sufficient to produce an observable stressed-oil-volume effect. Finally, it is demonstrated that the observed phenomenon can be interpreted in terms of an apparent effect of either the electrode area or the stressed oil volume. This has led to the development of a semi-empirical method of quantitatively assessing the breakdown phenomenon in large oil volumes with reasonable accuracy.