Effect of Floating Metallic Particles in Pre-Breakdown and Breakdown Characteristics of Oil Transformer under DC Voltage

Abstract

Contaminants in transformer oil insulation can float when meeting several conditions. Then, the presence of floating contaminants affects the electrical characteristics of oil insulation. Therefore, the pre-breakdown (corona) and breakdown characteristics due to metallic floating particles in transformer oil insulation would be investigated. This test used DC high voltage stress. A 56 Ω resistor was connected to the oscilloscope to detect the corona currents. The camera was used to capture the images of corona light emission. In addition, the electric field between the electrode and particles was simulated. The variables were the particle size, including its shape, and the distance between the particles and the grounded electrode. The experimental results show that the average value of corona inception and breakdown was inversely proportional to the size of floating particles. The peak value of corona current was directly proportional to the particle size. The lowest breakdown voltage was found when the particle was close to the electrode, but they were not in contact.

Motion behaviours and PD characteristic of free metallic particles in flowing transformer oil with paper‐covered electrodes

“…Figure 14 shows that the maximum electric field strength around the particle reaches ∼10 kV/mm, which generally exceeds the critical field strength for PDs in the oil [29, 32–34]. It can be seen that when the electrode was singly covered, the electric field intensity in the gap between the metallic particle and the electrode is smaller than that for the non‐covered condition and larger than that for the double‐covered condition.…”