Jarosław Gielniak

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.

Does the degree of cellulose polymerization affect the dielectric response

The purpose of this work is to evaluate the influence of the cellulose polymerization degree (DP) on the dielectric response. The research objects were new and aged pressboard samples of the polymerization degree 1435 and 406, respectively. The authors investigated the dielectric response in the time and frequency domains. The research showed that dielectric response characteristics of new and aged samples, of different DP and an identical water content are very close. Their differences are within the limits of statistical dispersion.

Dielectric responses of new and aged transformer pressboard in dry and wet states

This paper presents results of dielectric response measurements in frequency domain. obtained at different temperatures for oil-impregnated pressboard samples (new and aged) containing different amount of moisture. Description of the sample preparation and the ageing procedure is also included. The dielectric responses were used for defining master curves and activation energies at the different humidity levels, to be later used in the modelling of diagnostic measurements on transformers. Spread of the results from samples containing similar amount of moisture are also presented.

Fire safety and electrical properties of mineral oil/synthetic ester mixtures

The paper presents research results of mineral oil/synthetic ester mixtures comprising: flash point (open and closed cup), fire point (open cup), net calorific value, breakdown voltage, relative permittivity, dissipation factor, conductivity and dielectric losses. Mineral oil/synthetic ester mixtures for the following concentrations: 95/5, 90/10, 80/20, 60/40, 50/50, 40/60, 20/80, 10/90 and 5/95 were tested. In order to compare properties (especially electrical), prepared mixtures and base liquids were conditioned at uniform climatic conditions (temperature 25 °C, relative humidity 9%). This allowed to obtain mixtures with the same relative humidity. Obtained results were referenced to the properties of the base liquids. Furthermore, the results were analyzed in terms of fire safety and changes of electrical properties, in the context of the use of mixtures as insulation liquids in power transformers.