Fault free operation of power transformers is of great importance to the electricity industry. The operational reliability of a transformer is substantially influenced by the characteristics of the insulation material used. This is particularly important for aged transformers, especially so for those which are older than their design life. Monitoring the general condition of transformer insulation is a major concern to the electricity utilities and several attempts have been made to predict the insulation condition and the remaining life time of a transformer. The ageing process of insulation paper is usually monitored by several properties, including tensile strength and degree of polymerization, dissolved gas analysis, furan content in the oil. The first two methods are the destructive tests and require paper samples which have to be collected from the in-service transformer. The other two methods are indirect techniques for monitoring the degradation of cellulose in oil.
It is also observed that very limited research has been conducted in diagnosing insulation condition by electrical properties. However, very recently an attempt has been made to utilize the interfacial polarization spectra (IPS) parameters for the diagnosis of insulation condition in a transformer. The validity of this test has not yet been confirmed by other established diagnostic techniques.
With the support of the electricity supply and manufacturing industry, the Departments of Electrical Engineering and Chemistry at the University of Queensland began a research project in 1990 aimed at assessing the condition of aged transformer insulation using conventional and new electrical and chemical diagnostic techniques. This thesis describes the electrical aspects of the project and their correlation with chemical aspects.
To prove the usefulness of electrical properties for the assessment of insulation condition in transformers, several electrical test methods were used in this research project. These include measurements of: dissipation factor and 50 Hz capacitance, breakdown strengths (power frequency and lightning impulse), partial discharge inception stress and interfacial polarization spectra parameters.
To correlate these electrical properties with the molecular weight of the cellulose insulation, a gel permeation chromatography (GPC) technique was developed and used by our partners in the Chemistry Department. The GPC technique has been used to measure the molecular weight distribution of cellulose paper and pressboard. High performance liquid chromatography (HPLC) analysis of the oil for furan contents and tensile strength measurement of cellulose were also developed and used for correlation purposes.
The insulation of two aged power transformers and several distribution transformers was investigated by the developed techniques. IPS measurements were found to be very sensitive to ageing. For the 25 year old transformer, significant differences in the condition of the insulation were observed at different locations of the winding. IPS parameters showed a good correlation with the peak molecular weight and tensile strength. The second power transformer (which failed after 40,000 hours of operation) showed no significant differences in the condition of insulation among different locations. The ageing condition of the distribution transformers analysed by the IPS measurement technique showed consistency with the dissipation factor results and with the ages of the transformers. For the distribution transformers, the IPS parameters correlated well with the peak molecular weights.
New insulation was subjected to accelerated ageing at high temperatures to provide samples with controlled levels of ageing for comparison with the insulation samples taken from old transformers. The ageing was carried out at temperatures in the range 129 to 166°C for periods up to 28 days. The conventional electrical properties were found to be unchanged except for lightning impulse strength and 50 Hz capacitance. IPS parameters and peak molecular weights were found to vary significantly during the ageing of samples. Correlation has been obtained between IPS parameters and peak molecular weights. The IPS parameters also correlated well with the tensile strength of cellulose. A correlation between IPS parameters and the peak molecular weights validated the usefulness and importance of the IPS measurement method for diagnosing insulation condition in aged transformers.