Lars E. Lundgaard

Dielectric response of mineral oil impregnated cellulose and the impact of aging

Dielectric response is a non-invasive diagnostic method with a potential to give information about the status of the solid insulation in a power transformer. To investigate what conditions the method can and cannot reveal, frequency domain spectroscopy has been applied to oil-impregnated paper and pressboard with and without moisture and/or acids added, and to paper at various stages of aging. Water and carboxylic acids are important aging indicators. It is found that moisture is clearly detectable. Also low molecular weight carboxylic acids will affect the dielectric response in a way similar to moisture, but high concentrations are needed for a contribution comparable to that of moisture. It is difficult to tell moisture and acids apart from the responses obtained. High molecular weight carboxylic acids make little or no contribution to the dielectric response of the solid insulation in a paper/pressboard - oil system. It was also observed that keeping paper or pressboard at elevated temperatures had a conditioning effect on the response

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.

Propagation of positive and negative streamers in oil with and without pressboard interfaces

Inception and propagation of streamers in a point-plane gap, with and without pressboard interface parallel to the field, have been studied. Current and light emission have been recorded. Positive streamers propagate at /spl sim/2 to 4 mm//spl mu/s at voltages to 2/spl times/ the minimum breakdown voltage. Above this voltage, a fast event (>100 mm//spl mu/s) may occur, preceded by streamers propagating with velocities in the 10 to 20 mm//spl mu/s range. The negative streamers have /spl sim/2/spl times/ the breakdown voltage as the positive ones. Even negative streamers may develop into a slower kind of fast event. Solid pressboard parallel to the field does not change the breakdown voltage, but makes inception of fast events easier. The breakdown process in oil shows several similarities to a gas breakdown.

Streamer propagation under impulse voltage in long point-plane oil gaps

A 67 mm point-plane gap immersed in transformer oil, at atmospheric pressure, stressed with a 1/180 /spl mu/s impulse, has been studied by recording gap current and light emission. Time-resolved images were obtained by use of an image converter camera. Minimum breakdown voltages were twice as high for negative as for positive points. All streamers at these and higher voltages were supersonic, with negative streamer velocities /spl les/50 km/s. The speed of the positive streamer gradually increased to 19 km/s with increasing voltage, where it saturated. At high stresses the propagating positive streamer has been observed to switch to a faster mode which propagates with speeds in the 65 to 200 km/s range. Positive streamers were bush shaped to 2/spl times/ the minimum breakdown voltage, the branching diminishing with increased voltage. Negative streamers were coarsely bush shaped or usually tree shaped. For both polarities, light emission consisted of a background light, with superposed pulses corresponding to 15 ns wide current pulses of up to 10 A. The pulses are caused by very bright, brief re-illuminations of single streamer channels. The tips of positive streamers are considerably more luminous than the channels, indicating that electron multiplication may take place at the tips. A model of the streamers as being plasma filled channels may explain the re-illuminations. >

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.

Oxidation of paper insulation in transformers

Oxidation ageing experiments on paper insulation and transformer oil have been performed using two different techniques. One is a traditional ageing study wherein degree of polymerization (DP) is measured after conditioning in sealed bottles containing paper submerged in oil, where the partial pressure of oxygen over the oil is varied. Paper is sampled at certain intervals during 8 months and DP-values are measured. The other study was carried out by means of a microcalorimeter; continuously measuring the heat flow from the paper-oil sample. We find that the degradation reaction rate is not linear with oxygen concentration and that the activation energy of oxidation is lower than what has earlier been found for hydrolysis. The microcalorimeter appears to mimic ageing characteristics for oxidation and can potentially be a useful tool for quick ageing investigations.

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.

Effects of electron-attaching and electron-releasing additives on streamers in liquid cyclohexane

The effects of electron-attaching and electron-releasing additives in cyclohexane on the initiation and propagation of positive and negative streamers have been studied quantitatively. Fast impulses (<20 ns, <40 kV) were applied on a 10 mm point-to-plane gap and studied by shadowgraphic imaging and a differential charge measurement technique. The properties of both positive and negative streamers depend on the specific electronic characteristic of the additives. Electron-attaching additives facilitate the propagation of negative streamers, whereas the most effective electron-releasing additives reduce initiation voltages and facilitate the propagation of positive streamers. Depending on the reactivity and concentration of the additives, streamer filaments become thinner and fewer while propagating faster and further. The Townsend-Meek theory for streamer inception in gases has been adapted to a solution and applied to analyze the voltage dependence of the positive streamer propagation. Results show a quantitative dependency on the ionization potential and concentration in agreement with experimental trends.

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.

Particles in GIS characterization from acoustic signatures

Acoustic techniques are accepted generally for diagnostics of gas-insulated systems (GIS). This paper addresses the problem of risk assessment of moving particles. The main part of the paper gives a general understanding of internal particle movement and excitation of external acoustic signals, leading to signature analysis and simple calculation of characteristics such as mass and length of the particle, and its elevation height in a GIS duct. This is supported by experimental studies of acoustic and electric signals from moving particles. It is shown how partial discharge (PD) at a particle reduces the jumping tendency, and how discharges with time will change when the shape of the particle changes. Experiments on sensor sensitivity and on the coefficient of restitution also have been carried out. The signature analysis and given formulas constitute a good tool for discriminating between harmless and harmful particles, as well as offering a tool for better understanding the particle movement.