Piotr Przybyłek

The influence of cellulose insulation aging degree on its water sorption properties and bubble evolution

The paper concerns the issue of water in a oil-paper insulating system of a transformer. The author described the influence of Kraft paper ageing on its water sorption and desorption properties. The research was done for new and substantially aged paper. The paper ageing degree was described by means of the cellulose polymerization degree. Volume absorption and line sorption of new and aged paper were investigated. The research was done taking into consideration moisture distribution in the transformers insulating system and possibilities to generate a particular kind of water desorption called the bubble effect. The article discusses this effect, which is dangerous for the transformer. The author refers to factors which affect the initiation temperature of the bubble effect. Moreover, the article presents research results of the bubble effect initiation temperature for new and aged paper as a function of water content in paper insulation. Formulae describing the initiation temperature of the bubble effect are given for some assumed conditions.

Moisture in cellulose insulation of power transformers - statistics

The article presents the results of extensive investigations aimed at determining the moisture content in a large population of power transformers (161 units). The project was carried out during the period of 2003-2011 in Poland, Germany, and Sweden. All results were obtained by means of the Frequency Response Spectroscopy (FDS) method. Among the transformers tested were grid transformers, generator step-up (GSU) transformers, and distribution transformers. According to the classification rules established by IEEE, 30% among the investigated transformers could be classified as “wet” (2-4% moisture content). The observed moisture variation with age was found to depend on transformer design and operating conditions. It is suggested that, based on the presented statistical results and knowledge concerning transformer type and age, one may discriminate between probably dry transformers and transformers that require actual moisture analysis and therefore prioritize further condition assessment.

The influence of water content and ageing degree of paper insulation on its mechanical strength

The article concerns the issue of mechanical strength of paper insulation in dependence of its ageing degree and water content. The authors focused on tearing resistance of Kraft paper. The tearing resistance of paper was determined using Elmendorf method. The ageing level of the paper samples was described by means of cellulose polymerization degree. The water content in paper samples was determined using Karl Fischer titration method. The investigations were conducted for various combinations of water content in the range from 7.24% to 0.12% and polymerization degree in the range from 1360 to 272. The results proved significant influence of water content in paper insulation on its mechanical strength. The drying of cellulose insulation causes significant weakness of mechanical strength of paper. This conclusion is particularly important in aspect of drying of aged transformer.

The influence of temperature and aging of cellulose on water distribution in oil-paper insulation

This paper deals with the problem of moisture in the transformer insulation system. The author proposed that apart from the temperature, the uneven aging of cellulose insulation may have a substantial effect on moisture distribution. The main goal of the research was to check the influence of the degree of aging of winding paper on moisture distribution by taking into account temperature distribution. New paper and paper at three different stages of aging were tested. The obtained results confirmed the hypothesis - it was proven that the uneven aging of insulation may enhance moisture distribution in the insulation system.

Water saturation limit of insulating liquids and hygroscopicity of cellulose in aspect of moisture determination in oil-paper insulation

The article concerns the presence of water in insulating systems consisting of cellulose materials impregnated with different electro-insulting liquids. This is a particularly important issue regarding power transformers with their strategic importance for an electrical power system. Water is a serious threat and can be the reason of a dangerous breakdown. Thus there is a need to have insulating system diagnostics in terms of water content. The article discusses the issue of water solubility in different electro-insulating liquids. The author determined the water saturation limits for these liquids. Moreover, the author determined water sorption isotherms for cellulose materials of different aging degree. On the basis of the obtained results, equilibrium curves were constructed for some selected kinds of insulation. The research results presented here allow to improve the reliability of determining moisture in cellulose insulation of various polymerization degree on the basis of water content measurement in an electro-insulating liquid.

Thermal properties of a mixture of mineral oil and synthetic ester in terms of its application in the transformer

The article presents an analysis of thermal properties of a mixture of mineral oil and synthetic ester used in transformers. The oil/ester rate was as follows: 100%/0%, 95%/5%, 80%/20%, 50%/50%, 20%/80%, 5%/95%, and 0%/100% The measured thermal properties were: thermal conductivity, specific heat, viscosity, density, and the thermal expansion factor. On the basis of measurement results, the authors calculated the heat transfer factor α. This factor determines the ability of a liquid and it has the key influence in transformer cooling. The measurements were done for a wide temperature range from 25 to 80°C.

Can the bubble effect occur in an oil impregnated paper bushing

This paper elucidates the question whether bubble effect can possibly take place in the insulation of OIP (Oil Impregnated Paper) type of bushings. The investigations were performed on a bushing insulation model and indicated to a possibility that the effect may occur. Presence of metallic screens in the insulation does not limit the development of gas bubbling; they only force the motion of water vapor bubbles along the model axis. The temperature dependence of bubble initiation on the water content in paper part of the bushing insulation is similar to the presented earlier in literature characteristics for bubbling in main insulation of power transformers. The possibility of bubble effect occurrence may explain the relatively frequent cases of transformer bushing failures.

A new method for indirect measurement of water content in fibrous electro-insulating materials using near-infrared spectroscopy

The article presents a new method for determining moisture of solid materials using near-infrared spectroscopy. In this method, the absorbance of a near-infrared wave passing through methanol is measured; water is extracted beforehand from the material in methanol. The mass of water extracted from the sample is determined on the basis of dependence of absorbance on the water concentration in methanol. The determined water mass to dry mass ratio of the sample allows to calculate the percentage moisture of the investigated material. This new method of water content measurement was prepared in order to determine moisture in fibrous electro-insulating materials that are applied in, e.g. power transformers. The investigations described here allow to determine the time necessary to extract water from cellulose and aramid materials. The author also compared the research results of water content obtained by means of this new method with results obtained by using the normalized Karl Fischer titration method and the weight method. It was proven, based on this comparison, that the new method can be used to measure water content in fibrous materials that are either impregnated or unimpregnated with mineral oil. According to the author, the method of measuring water content presented here can be widely applied in different branches of the industry, such as in food processing and pharmaceutical manufacture.

A comparison of bubble evolution temperature in aramid and cellulose paper

The paper deals with the problem of the bubble effect in transformer winding insulation. The bubble effect should be understood as the release of water vapor from insulation. The main goal of the research was to compare the initiation temperature of bubble evolution in aramid and cellulose paper depending on the water content. Cellulose and aramid paper samples of different water content were prepared for the research study. The obtained results confirmed that aramid paper is less hygroscopic than cellulose paper. It was also proven that the initiation temperature of bubble evolution is lower for aramid paper than for cellulose paper with the same water content. However, the initiation temperature of bubble evolution is very similar for aramid and cellulose paper conditioned at the same relative humidity.

Thermal properties of mineral oil admixed with C 60 and TiO 2 nanoparticles

Work on modification of insulating liquids with different nanoparticles to improve their electric properties has been done for many years. However, possible changes of thermal properties of such liquids have not been in focus. The article presents an analysis of nanoparticle influence on thermal properties of mineral oil used in transformers. The investigated nanoparticles were fullerenes C60 and barium oxide TiO2. The measured thermal properties were thermal conductivity, specific heat, viscosity, density, and the thermal expansion. On the basis of measurement results of the mentioned thermal properties, the authors calculated the heat transfer factor α. This factor determines the ability of a liquid to carry away heat and it has the key influence in transformer cooling. The measurements were done for a wide temperature range from 25 to 80°C.