Development of methods for measuring partial discharges in the solid insulation with gaseous defects of power transformers

Abstract This article presents a research project aimed at developing a method for measuring the intensity of the electric field inducing partial discharges (PDs) in a gaseous defect within a solid insulator. Measurements performed with this method allow observation of PDs only within the defects, thus providing reliable information on the relationship between the temporal and dimensional characteristics of the PDs and the size and geometry of the defect. The advantages of a measurement cell filled with transformer oil compared to a “dry” cell, such as a parallel-plate capacitor, are demonstrated. To solve the problem related to measuring partial discharges in gaseous inclusions in order to determine the technical condition of the solid insulation, the intensity of the electric field in gaseous defects within a solid insulator was calculated using the finite element method with COMSOL Multiphysics 6.2 software. Partial discharge (PD) modeling in MultSim was employed, along with probabilistic and statistical methods for processing the results. The finite element method was used to calculate the electric field non-uniformity coefficient for samples containing gas defects of varying sizes. MultSim modeling allowed us to determine the phase angle between the current and voltage in the cell during partial discharge (PD) measurements. For a cell with a spherical electrode, each containing sectors of a sphere with a radius of R = 25 mm and a largest radius of x = 18 mm in a plane perpendicular to the sector axis, the expected partial discharge parameters were calculated as a function of the insulation defect size. Statistical parameters were determined to reproduce partial discharges (PDs) recorded over a given period and to calculate their duration. During laboratory measurements, a model consisting of eight samples of a paper dielectric of varying thickness: 0.15 mm, 0.2 mm, 0.25 mm, 0.3 mm, 0.35 mm, 0.4 mm, 0.45 mm, and 0.5 mm containing air inclusions simulating a 2 mm diameter defect located near the electrodes was used, but in this article, the results for a model of a single dielectric of thickness of 0.2 mm will be presented. A reference voltage of 12.5 kV was applied to the high-voltage electrode, and partial discharges (PD) were observed in an electric field of intensity 6.747 kV/mm. Based on the experimental results, electric charge diagrams and experimental setups simulating the change in electric fields causing partial discharges are presented. It was experimentally shown that partial discharges in a sample containing a single gas defect occur twice during the period of a sinusoidal voltage at the grid frequency, in the first and third quarters, with the same electric field amplitude inside the gas cavity of the defect.