Recovery voltage in transformer oil-paper insulation diagnosis

Kuang, Ye Chow (2004) Recovery voltage in transformer oil-paper insulation diagnosis. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Using recovery voltage to assess the state of electrical insulation has been a very popular method among engineers in the power industry. As a result of its popularity, there is growing interest among engineers to quantify and to have more thorough understanding of the underlying physical processes. The conventional method of measuring the recovery voltage does not record all of the information.

While this is understandable for practical purposes it is not helpful for understanding the recovery voltage as a physical process. Therefore a new experimental rig has been built to study the recovery voltage in oil-paper systems. This new experiment records the entire voltage profile for detail analysis. The system also controls the level of moisture and temperature of the insulation material to study the influence of each factor on the recovery voltage of oil-paper insulation. This simplified version of an insulation system in the experiment will help to understand the recovery voltage as an electrical response of insulation material avoiding complication of a real transformer. Various standard and non-standard measurements have been performed on the dry paper and oil-paper samples. The results are discussed and a model is proposed based on these results. The measurement results obtained in a series of experiments exposed two very important shortcomings of the commonly accepted model. Firstly it fails to explain many variations in the observed recovery voltage and secondly the parameters in the circuit representation lack direct physical interpretation.

A new model is proposed based on indirect evidences inferred from the results of the experiments and published properties of oil-paper insulation using other measurement techniques. The new model is built by assuming that the bulk dipole polarisation plays less significant role in the formation of recovery voltage. Instead, the interfacial charge transfers and charge diffusion on the surface of paper are used to explain the formation of recovery voltage in the oil-paper insulation system. The new model fits the experimental results well using small number of parameters with direct physical interpretations. This qualitative leap in the understanding of recovery voltage will help engineers to interpret the results of field measurements more accurately.

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