Electrical and chemical diagnostics of transformer insulation

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The main function of a power system is to supply electrical energy to its customers with an acceptable degree of reliability and quality. Among many other things, the reliability of a power system depends on trouble free transformer operation. Now, in the electricity utilities around the world, a significant number of power transformers are operating beyond their design life. Most of these transformers are operating without evidence of distress. The same situation is evident in Australia. In PowaaerLink Queensland (PLQ), 25% of the power transformers were more than 25 years old in 1991. So priority attention should be directed to research into improved diagnostic techniques for determining the condition of the insulation in aged transformers.
The insulation system in a power transformer consists of cellulosic materials (paper, pressboard and transformerboard) and processed mineral oil. The cellulosic materials and oil insulation used in transformer degrade with time. The degradation depends on thermal, oxidative, hydrolytic, electrical and mechanical conditions which the transformer experienced during its lifetime.
The condition of the paper and pressboard insulation has been monitored by (a) bulk measurements (dissolved gas analysis (DGA) insulation resistance (IR), tanö and furans and (b) measurements on samples removed from the transformer (degree of polymerization (DP) tensile strength). At the interface between the paper and oil in the transformer, interfacial polarization may occur, resulting in an increase in the loss tangent and dielectric loss. A DC method was developed for measuring the interfacial polarization spectrum for the determination of insulation condition in aged transformers.
This paper makes contributions to the determination of the insulation condition of transformers by bulk measurements and measurements on samples removed from the transformer. It is based on a University of Queensland research project conducted with cooperation from the PLQ and the GEC-Alsthom.
Most of the currently used techniques have some drawbacks. Dissolved gas analysis requires a data bank based on experimental results from failed transformers for predicting the fault type. When transformer oil is rep or refurbished, the analysis of furans in the refurbished oil may not show any trace of degradation, although the cellulose may have degraded significantly. DP estimation is based on a single-point viscosity measurement. Molecular weight studies by single-point viscosity measurements are of limited value when dealing with a complex polymer blend, such as Kraft paper, particularly in cases where the molecular weight distribution of the paper changes significantly as the degradation proceeds. In these instances, a new technique, gel permeation chromatography (GPC), is likely to be more useful than the viscosity method, because it provides information about the change in molecular weight and molecular weight distribution. Investigation of the GPO technique has been included in this research to assess its effectiveness in determining the condition of insulation.
Conventional electrical properties (dissipation factor and breakdown strengths) of cellulosic materials are not significantly affected by ageing .so very little recent research has been directed to electrical diagnostic techniques, in this research project, thorough investigations were also undertaken of the conventional electrical properties, along with interfacial polarization parameters of the cellulosic insulation materials. The interfacial phenomena are strongly influenced by insulation degradation products, such as polar functionalities, water etc. The condition of the dielectric and its degradation due to ageing can be monitored by studying the rate and process of polarization and can be studied using a DC field. Furthermore, this is a non-destructive diagnostic test.
A retired power transformer (25 MVA, l1/132 kV) and several distribution transformers were used for the experimental work. The results from these transformers will be presented and an attempt will be made to correlate the electrical and chemical test results. The variation of the results through the different locations in a power transformer will be discussed with reference to their thermal stress distribution. Accelerated ageing experiments were conducted to predict the long term insulation behaviour and the results are presented in the accompanying paper.

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