Electrical tree reconstruction method for oil-impregnated pressboards based on the inverse problem for the electrostatic field

There is still a lack of effective means of observation for electrical trees in opaque materials, which hampers experimental and theoretical research on the growth of such trees in the oil-impregnated pressboards of power transformers. This presents a great obstacle to the accurate diagnosis and prediction of partial discharge (PD) defects. This paper describes a method to monitor the growth process of electrical trees in pressboards based on solution of the inverse problem for the electrostatic field. The three-dimensional structure of the carbonized branches is reconstructed from the positions of PDs. Each PD is located by the charge remaining in the material. This charge is calculated from the electric field intensity or potential at some points in the field using the mirror image method and solution of the inverse problem for the electrostatic field. The electric field intensity is measured using an array of metal strips near the zero-potential border while PDs are occurring. High-voltage PD experiments are performed on oil-impregnated pressboard strips to verify the feasibility of the method. From the results of these experiments, the accuracy of PD site location is found to be as good as 0.35 mm in the χ direction (the horizontal coordinate), which is sufficient to describe the general structure of electrical trees in oil-impregnated pressboard. The movement of the PD positions indicates the growth process of tree branches. The calculated positions and lengths of the branches are consistent with observations of carbonized tracks after the pressboard has been torn open. The results obtained here will be of benefit to establishing a real-time monitoring method for three-dimensional reconstruction of electrical trees to reveal their internal growth mechanism in opaque materials such as oil-impregnated pressboards.