D. Linhjell

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

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.

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.

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.

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.

Streamer propagation in long point-plane oil gaps

Streamer propagation in a 67-mm point-plane gap has been studied by means of current and light registrations and streak and framing mode fast photography of the light emission. A strong bush-shape of positive streamers decreases gradually with increasing overvoltage. Negative streamers tend to resemble branched trees. In both, single channels flash up more or less regularly with associated current pulses. Negative streamers, and positive streamers at low overvoltages, grow fairly steadily across the gap at high supersonic speeds. Positive streamers at higher overvoltages bridge the gap from a mid-gap position in a fast event. Positive streamers seem to have their electron multiplication in the tips.

Dielectric response of oil-impregnated paper insulation: variation with humidity and ageing level [power transformer applications]

The dielectric response of oil-impregnated Kraft paper with 3 initial humidity levels (0,17%, 1,75% and 3,5%) has been studied in the frequency range 0.1 mHz -1 kHz, before ageing at DP (degree of polymerisation) 1200 and after ageing to DP 900, 600 and 2-300, primarily to investigate if the method can be used for non-destructive characterization of the state of cellulose insulation in electrical power devices, e.g. transformers. Also, because ageing produces acids, known acids have been added to unaged paper to investigate the influence on the dielectric response. Results indicate that the effect of ageing is small compared to the effect of moisture, but the loss peak of the humid samples become more prominent. Light acids (formic, acetic), which are absorbed in the paper, have an effect on the dielectric response, similar to water.

Dielectric response of oil-impregnated cellulose from 0.1 mHz to 3 MHz

Pressboard and Kraft paper with and without moisture, with and without added acids have been tested with frequency domain spectroscopy up to 1 kHz. Kraft paper with three moisture levels has also been tested at various ageing levels. Moisture and low-molecular carboxylic acids cause shifts in the permittivity vs. frequency curves. In addition, in paper but not pressboard, moisture is associated with an extra relaxation in the low-frequency part. High-molecular carboxylic acids have little effect on the permittivity. The same is the case for reduction in chain length of the cellulose polymer. The method should be useful as a non-invasive field test method for destructive agents in transformers, but not for the ageing (chain scission) itself. The measurements up to 3 MHz are so far inconclusive, but have yielded a few results that blend in well with the measurements below 1 kHz.