The surface discharge along the interface of solid–liquid insulation is one of the main causes of the deterioration of the life of a power transformer. This experimental study investigates the effects of different solid and liquid insulation materials on the surface partial discharge (PD) characteristics. The liquid insulation materials utilized are mineral oil and synthetic ester, whereas the solid insulation materials are cellulose board and Nomex board. The surface PD characteristics in various combinations of solid and liquid insulation materials are investigated using PD inception voltages (PDIV), apparent charges (pC), and phase-resolved PD (PRPD) patterns, and the comparative analysis is presented. Moreover, topography and composition of samples during surface discharges phenomena have been discussed using high-resolution images and chemical composition obtained using the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDAX). The findings of this article will assist in understanding the behavior of surface PDs under various conditions.

Surface Discharge Characteristics of Different Solid–Liquid Insulation Materials in Power Transformers

Although oil-immersed power transformers generally use mineral oil as insulation and cooling fluid, this liquid does not meet the new technical requirements of dielectric fluids such as high biodegradability, non-toxicity and high safety. For these reasons, natural and synthetic esters as alternative to mineral oil have increased their utilization in some transformers installations. Despite the fact that there are several works that have demonstrated the suitability of these insulation fluids from the point of view of their stability, dielectric and thermal properties, there are very few works focused on the study of the effects of these liquids on impregnation process. The aim of this work is provide information about the behavior of different rigid insulation materials, not studied until now, during the impregnation process in a synthetic ester compared with a mineral oil.

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Impregnation processes of insulation rigid components of cellulose in synthetic ester and mineral oil

Pre-compressed pressboard was commonly used in power transformers along with mineral oil. Pressboard in combination with mineral oil is suitable composite insulation for oil filled transformers over several decades. Synthetic ester is a suitable alternative for mineral oil due to its biodegradable and fire resistant property. It is also compatible for transformer insulations as per recent research studies. Intent of this paper is to find out the suitability of synthetic ester as transformer insulation, with respect to partial discharge studies. Understanding partial discharge characteristics of synthetic ester with an influence of moisture, conducting particles and bubbles, the experimental study was carried out. Measurements of PDIV, PDEV, maximum apparent charges, and PRPD patterns were recorded in order to analyse the effect of various impurities over partial discharge activity. Oil moisture has more impact over board moisture.

G. B. Kumbhar

Papers

Investigation of partial discharges in synthetic ester-pressboard under AC stress