Pulse electrical strengths of hot-pressed MgO containing 0.3 wt% LiF were measured. Strengths as high as 5.9×108 V/in. were obtained on samples 99.8% of theoretical density with 1-to 3-μm grain size. An increase of either porosity or grain size resulted in a monotonic decrease in strength. Single-crystal strengths of 4.0×106 V/in. were obtained by polishing samples chemically. A simple recessed-electrode configuration eliminated corona effects.

Effect of Microstructure on Pulse Electrical Strength of MgO

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

A detailed examination of the aging processes, that may take place in solid-dielectric extruded-type high voltage transmission cables under dry conditions, is presented. Particular emphasis is placed on the aging process as affected by the separate and simultaneous action of four main aging factors, namely the electrical, mechanical and thermal stresses, and the physical and chemical environment. A number of pertinent aging models are considered and their validity and applicability to accelerated aging tests on solid-type transmission cables are discussed. >

Multiple stress aging of solid-dielectric extruded dry-cured insulation systems for power transmission cables

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.

Partial Discharge Behaviours and Breakdown Mechanisms of Ester Transformer Liquids under ac Stress

In a previous paper (1), the necessary experimental and theoretical basis was established for a discussion of the effect of electrode area on the 60-cycle breakdown of transformer oil. A study of 1600 observations established the extremal nature (2) of such breakdown distributions and yielded an equation relating area and voltage which agreed closely with test results. This equation is: V A1 − V A2 = s v /σ N 1n A 1 /A 2 (1) where V is the modal strength and s v is the standard deviation of N measured breakdown values; σ N is a function of the number of breakdowns N only.

Electrode Area Effect for the Impulse Breakdown of Transformer Oil

The 60-cycle breakdown strength of good quality transformer oil has been shown to depend upon electrode size for a physical area ratio of 20 to 1 and for an additional ratio of 400 to 1 by means of a “group-of-minima” inference (1) The extremal nature of the breakdown distribution found to exist by these tests was also found to apply to impulse breakdown (2)

http://ieeexplore.ieee.org/iel7/7540441/7540442/07541061.pdf

Extremal area effect for large-area electrodes for the electric breakdown of transformer oil

The dielectric strength of liquids has been found to depend on electrode area. A study of 1600 observations of transformer oil breakdown, using four pairs of uniform-field (Rogowski) gaps, has established the extremal(1) nature of such breakdown distributions, and yielded an area-effect equation which agrees closely with test results.

K. H. Weber

Papers

Electrode Area Effect for the Impulse Breakdown of Transformer Oil

Extremal area effect for large-area electrodes for the electric breakdown of transformer oil

Area effect - and its extremal basis - for the electric breakdown of insulating oil