Krzysztof Walczak

Frequency response of oil impregnated pressboard and paper samples for estimating moisture in transformer insulation

Knowledge about moisture content in oil impregnated paper insulation is essential when estimating remaining lifetime of power transformers. Direct evaluation of moisture content is rarely possible due to inaccessibility of the internal insulation system in transformers. Therefore, various indirect estimation techniques are utilized. Frequency domain spectroscopy (FDS) measurements of transformer insulation belong to this group. To perform high quality interpretation of results of FDS measurements a good knowledge on dielectric responses in oil impregnated pressboard and paper is required, especially as it refers to their variation with water content and temperature. The aim of this paper is to provide an open access to the frequency domain spectra of oil impregnated paper and pressboard samples, which can then be used in modeling of the results of diagnostic measurements in power transformers.

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.

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.

Power Transformer Diagnostics Based on Acoustic Emission Method

Widely applied techniques for the fault location based on AE method are: (i) measurement of the time difference of arrival (TDOA) of the acoustic signals, (ii) measurement of the acoustic signal amplitude in different areas of a transformer tank (standard auscultatory technique, SAT), (iii) advanced auscultatory technique (AAT), (iv) estimation of the direction of arrival (DOA) of the acoustic signal based on the phased-array signal processing (Markalous et al., 2008; Tenbohlen et al., 2010; Qing et al., 2010).

Wyładowania niezupełne w układzie izolacyjnym celuloza-olej mineralny oraz celuloza-ester syntetyczny

The theme of the article are partial discharges on surface of pressboard immersed in mineral oil or synthetic ester. These are some of the most dangerous discharges. Partial discharges of surface type and sliding type were tested. It has been found that the partial discharges on the surface of the pressboard immersed in a synthetic ester have a slightly higher intensity than immersed in mineral oil. Partial discharges in electroinsulating system of cellulose-mineral oil and cellulose-synthetic ester. Słowa kluczowe: wyładowania niezupełne, układ izolacyjny celuloza-olej mineralny, układ izolacyjny celuloza-ester syntetyczny.

The bubble effect in bushings – investigations on models

This paper refers to the bubble effect in transformer bushings. The bubble effect consists in releasing water from moistened cellulose insulation after exceeding a critical temperature. The most dangerous result is a pressure increase that leads to an explosion and fire. Research was conducted using laboratory models of OIP (Oil- Impregnated Paper) and RBP (Resin-Bonded Paper) types of bushings immersed in mineral oil and synthetic ester. For all of the investigated systems we determined the dynamics of water adsorption upon contact of the insulating material with air and the temperature characteristics of bubble effect initiation depending on water content in cellulose. In the experiment we investigated the dynamics of changes of water content in dielectric liquid, water content in air over the surface of the dielectric liquid, and air pressure over the surface of the dielectric liquid. In the future, the technical possibilities will be analyzed of using the dynamic changes of the three quantities investigated here to detect the bubble effect in bushings.