This paper reviews briefly the history of paper and pressboard insulation being used for electric insulation of oil immersed power transformers. An introduction to today's insulation technique is given. Important details of materials selection, as well as drying, clamping and oil impregnation procedures required to achieve short-circuit resistant windings are explained. Aging effects are described, as they gradually weaken the solid insulation mechanically and electrically. The value of post-mortem analysis of transformers before scrapping is shown with examples. A list of factors important for the manufacturing and operation of reliable transformers, yet often neglected, completes this article.

Power transformer insulation – history, technology and design

Presents a study of streamer inception in mineral transformer oil, in point-plane and rod-plane geometry under impulse voltage. The measurements performed with points and rods over a wide range of tip radii show a marked decrease of initiation fields when the electrode radius is increased. The initiation field is divided by 30 over the investigated range (from 1 /spl mu/m points up to 2 cm rods). This effect compares fairly well with the "surface effect" known for breakdown voltages under uniform field with large electrodes. Plotting these results together shows the decrease of streamer initiation fields over a very wide electrode surface range (12 decades), proportional to S/sup -0.17/ (S: electrode surface area, cm/sup 2/). These results suggest the influence of electrode surface defects on streamer initiation under impulse voltage.

Streamer initiation in mineral oil. Part I: electrode surface effect under impulse voltage

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.

Statistical Dielectric Degradation of Large-Volume Oil-Insulation

Breakdown of transformer oil is affected not only by electrostatic field configurations, but also by many other factors. The latters make it difficult to predict break- down voltage from electrostatic field, as made for air and S F6 gaps. Alternatively, many experimenters tried to express the breakdown stress of oil as a function of stressed oil volume. But, results are limited to their respective electrodes.

Breakdown of Transformer Oil

Mineral oil has been widely used in liquid insulation of power transformers. However, it is poorly biodegradable and could cause serious contamination to the environment if a spill occurs. With increasingly strict environmental rules and regulations, there is considerable interest from the Utilities to apply esters in power transformers as substitutions to mineral oil. In order to use esters in large power transformers, their dielectric properties should be thoroughly investigated. This PhD thesis covers the experimental studies on the dielectric properties of a type of synthetic ester (Midel 7131) and a type of natural ester (FR3) in both uniform and divergent electric fields, using a mineral oil (Gemini X) as the benchmark.The dielectric properties of transformer liquids in uniform fields were investigated using breakdown voltage tests. The breakdown voltages of esters should be at least similar to that of mineral oil to allow a replacement in transformers. To obtain a fair comparison, the AC breakdown voltages of well-processed transformer liquids were tested, and their distributions were statistically analyzed. Since the breakdowns of transformer liquids in uniform fields are caused by the weakest-links, conditions representative of in-service transformer liquids were also considered by testing the effects of extraneous factors, such as particles, water and electrode area.The divergent fields were produced by sharp needle electrodes with tip curvatures of a few micrometers. The dielectric properties of transformer liquids in such fields were studied using several methods. A traditional PD detector was used to study the partial discharge characteristics of insulating liquids, such as the inception voltages and the repetition rates. A high speed camera was utilized to identify the streamer generation, propagation and breakdown phenomena. An oscilloscope was used to investigate the current signals associated with these phenomena. The breakdown voltages of transformer liquids were also determined in the divergent field at various gaps. Furthermore, the fault gases in transformer liquids under partial discharge faults were determined and analyzed.The following findings and conclusions can be made from the research in this thesis:*	The AC dielectric strengths of esters in uniform fields are similar to that of mineral oil when they are in a well-processed condition. When practical liquid conditions are considered, the AC dielectric strengths of esters are higher than that of mineral oil.*	The partial discharge behaviours at overstressed voltages can be used to differentiate various transformer liquids. Esters are relatively inferior to mineral oil in terms of higher discharge amplitude, higher discharge repetition rate and more negative partial discharges.*	Mineral oil possesses a higher ability to suppress the propagation of negative streamers. Thus, the AC dielectric strength of mineral oil in the divergent field is relatively higher than those of esters.*	Esters generate the same types of fault gases due to electrical discharge as mineral oil, but in relatively larger amounts.

K. H. Weber

Papers

Extremal Area Effect for Large Area Electrodes for the Electric Breakdown of Transformer Oil