S. Ingebrigtsen

Ageing of Mineral Oil Impregnated Cellulose by Acid Catalysis

Acid catalyzed ageing of oil impregnated kraft paper has been studied experimentally. Five different carboxylic acids (formic, acetic, laevulinic, stearic and naphtenic acid) have been added to mineral oil to arrive at a neutralization value of 0.4 mg KOH/g. Thereafter, kraft paper of two different humidities has been allowed to equilibrate with the oil. The results show that the lower the molecular weight of the carboxylic acids is the more is absorbed by the paper. Furthermore, a clear synergy for the paper ageing rates between moisture and the lower molecular weight acids is found, while for the higher molecular weight acids almost no effects were found. It is concluded that present techniques for measuring acidity of oils do not reflect these detailed conditions. It is suggested to introduce a new measuring technique for neutralization value where water rinsing is used to identify the content of low molecular weight, water soluble acids.

Aging of Kraft paper by acid catalyzed hydrolysis

Hydrolysis of transformer insulating Kraft paper is experimentally investigated. Paper moisture content was varied and five different acids representative of what may be found in an aging transformer - were added to the oil. The low molecular weight acids were to a large degree absorbed by the paper, contrary to the high molecular acids that remained in the oil. The three low molecular acids accelerated the aging of the paper, and a clear synergy between these acids and water content was found. The high molecular acids did not influence the paper aging significantly. The results support a theory explaining the hydrolysis as governed by acid catalysis. Examples from measurements on real transformers are shown. It is concluded that low molecular acids constitute an important aging factor for transformer windings and should be included in diagnostic schemes, and when estimating the pay-back from maintenance actions.

Solubility of carboxylic acids in paper (Kraft)-oil insulation systems

The solubility equilibriums of formic, acetic, levulinic, stearic and naphthenic acids in oil-paper (Kraft) insulation systems have been studied using titration techniques. Temperatures and moisture levels were varied. Low molecular weight carboxylic acids strongly favour the cellulose while the heavier stearic and naphthenic acids favour the oil. Paper moisture did not influence solubility in a systematic way. The presence of low molecular acids increases the water content of the oil. Oil and paper from service aged transformers, show that typically 10-15% of the oil acidity stems from low molecular weight, hydrophilic acids. The concentration of the same type of acids is a hundred-fold higher in the cellulose. Thus, most of the total acidity in the solid insulation consists of hydrophilic acids, while they constitute only a minor part of the acidity in the oil. The methods used here for extraction of low molecular weight acids are proposed as an addendum to international standards for determination of oil acidity.

Field dependence on the molecular ionization potential and excitation energies compared to conductivity models for insulation materials at high electric fields

The aim is to improve the understanding of high-field phenomena (such as preinception currents/conduction, streamer initiation and propagation) in insulating materials in terms of the molecular properties of the substances involved. In high electric fields, ionization is a likely process, and in all such processes, the ionization potential is an important parameter. A fundamental question is how these processes depend on the electric field, and therefore, based on the interaction between a negative point charge and a molecular cation as modeled by density functional theory, a field-dependent model for the ionization potential is developed. In addition, the first excitation energies as a function of the electric field are calculated using time-dependent density functional theory. It is demonstrated that empirical high-field conduction models for cyclohexane and n-tridecane can be explained in terms of the difference between the ionization potential and the first excitation energy. It is also suggested that...

Enhancements in the lightning impulse breakdown characteristics of natural ester dielectric liquids

In this letter, we experimentally demonstrate that the lightning impulse breakdown characteristics of natural ester liquids can be significantly enhanced by dispersing molecular additives possessing lower values of both the ionization potential and 1st excitation energies as compared to the base liquid. One such additive contributed to an increase in the breakdown and acceleration voltage of the base liquid by 32% and 90%, respectively. Apart from the expected influence of the low ionization potential of the additives, results also indicate a positive effect of lower value of 1st excitation energy.

Excitation energies and ionization potentials at high electric fields for molecules relevant for electrically insulating liquids

The electric-field dependence of the molecular ionization potential and excitation energies is investigated by density-functional theory calculations. It is demonstrated that the ionization potential has a strong field dependence and decreases with increasing field. The excitation energies depend weakly on the field and the number of available excited states decreases with increasing field since the ionization potential has a stronger field dependence. Above a specific field, different for each molecule, a two-state model is obtained consisting of the electronic ground state and the ionized state. Implications for streamer propagation and electrically insulating materials are discussed.

Calculation of ionization potentials and electron affinities for molecules relevant for streamer initiation and propagation

We investigate different quantum chemical methods to compute ionization potentials and electron affinities for various molecules of interest to streamer propagation experiments in liquids. Solvation effects have been studied for the ionization potential using a polarizable continuum model. The ionization potentials can be reasonably well predicted by the methods used, but electron affinities are more problematic. We discuss possible reasons for these problems. Our primary interest in exploiting these calculations is to aid the understanding of discharges in liquids, and to help predict the utility of various additives for insulating liquids.

Streamers in long point-plane gaps in cyclohexane with and without additives under step voltage

This is a study of the effect of the additives dimethylaniline (DMA) and trichloroethylene (TCE) on the pre-breakdown phenomena known as streamers, in a simple base liquid, cyclohexane. This has been done under a step-like impulse voltage in a long point-plane gap. The objective was mainly to find out which aspects of the chemistry of mineral transformer oils cause their good streamer-related dielectric properties, in particular the high voltage for the transition to a very fast mode of positive streamers, as pure cyclohexane does not have these good properties. It is found that DMA added indeed causes positive streamers to become very similar to positive streamers in mineral oil and has no effect on negative streamers. TCE causes the negative streamers to become more mineral-oil-like, and has only a little effect on positive streamers.

Effects of N,N-dimethylaniline and trichloroethene on prebreakdown phenomena in liquid and solid N-tridecane

Prebreakdown phenomena in 0.1 M N,N-dimethylaniline (DMA) and 0.1 M trichloroethene (TCE) in n-tridecane have been investigated experimentally in liquid and solid phases in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Results have been compared with results obtained in neat n-tridecane and in cyclohexane with the same additives. Adding TCE to n-tridecane increases the p re-inception current and decreases the inception voltage for negative and positive polarity. The increase in pre-inception current is probably caused by trap assisted conduction, and may be responsible for the decrease in inception voltage. TCE had no effect on propagation of streamers in n-tridecane. The main effect of DMA in n-tridecane is to enhance the propagation of streamers and electrical trees with positive polarity. This is in line with what has been reported for cyclohexane, and is explained by the low ionization potential of DMA. Charge injection for negative streamers is reduced by the addition of DMA in n-tridecane, while the emitted light is increased. This indicates that some of the energy otherwise used for propagation is emitted as light. A low electronic excitation energy for DMA at 3.8 eV supports this hypothesis. The additives have the same effect on prebreakdown phenomena in solid and liquid phases with positive polarity, and thus the same mechanism is suggested to be responsible for electrical treeing and streamer inception and propagation. The scatter is generally found to increase when going from liquid to solid phase, which is explained by the inhomogeneity in the solid phase. The transition to solid phase with negative polarity typically results in an even larger scatter than for positive polarity. The effect of additives seems to be secondary to the morphology at the point electrode with negative polarity.

Streamer injection and growth under impulse voltage: a comparison of cyclohexane, Midel 7131 and Nytro 10X

Streamer growth and charge injection vs. applied impulse voltage has been studied in cyclohexane, Midel 7131 (ester based transformer liquid) and Nytro 10X (mineral transformer oil). Experiments were conducted in point-plane geometry under 50 ns/740 /spl mu/s impulse voltage. The tip electrodes were electrochemically etched to 2.0 /spl mu/m and 2.3 /spl mu/m with electrode gaps of respectively 19.5 mm and 17 mm. Shadowgraphic imaging and a sensitive differential charge measurement technique were used to detect the streamer shape and charge. The experimental setup used can detect charge injections as low as 0.1 pC, and has an image resolution of 4.5 /spl mu/m. When comparing Midel 7131 to Nytro 10X similar streamer shape and length under positive and negative polarities at the same voltage were found, but Midel 7131 had a higher initiation voltage and less charge was injected into this liquid compared to Nytro 10X at the same voltage.