Dielectric analysis of high-voltage power transformers
Dielectric tests have been used for decades for diagnosis of power transformers and bushings. In addition to the traditional tests at line frequency, modern types of dielectric tests using different frequencies and voltages have evolved. The article considers the scope of this principle, explaining the most common test procedures and describing what information can be derived from the measured values.
Keywords: dielectric response, power factor, dissipation factor, power transformer, bushing
Power transformers are the most expensive assets used in power transmission and distribution. Their owners expect a long working life without unplanned outages. In order to fulfil this task, the transformer has to be in healthy condition which must be verified by diagnostic means. If the results are not optimal, actions can then be taken to prevent failures. One group of diagnostic methods are dielectric measurements. Their intention is to identify the status of the insulation of the power transformer and its bushings. The integrity of their insulations is one basic requirement for their safe operation.
During the last decades, a variety of different types of dielectric measurements have been developed. The common basis of all those dielectric diagnostic measurement techniques is that they are non-invasive and identify capacitance and losses of the insulation they are applied on. Their difference is the investigated frequency range, the applied voltage and the assessment technique used to interpret the results. Collectively, these different techniques provide a variety of information about the measured insulation.
2 Dielectric properties of insulation systems
The dielectric properties of paper and pressboard are influenced by their temperature and water content. The influence of water is high, especially for the tan(δ) at lower frequencies where water increases both polarization and conductivity (1)  . Dielectric properties at line frequency and above are only slightly influenced; a significant influence is visible only where there is a higher water content.