Partial-Discharge Characteristics of Free Spherical Conducting Particles Under AC Condition in Transformer Oils

Abstract

One of the major problems in the liquid insulation is the particle contamination, which can seriously degrade the withstand voltage of the insulation. The movement of conducting particles under operation voltage can generate incipient discharges that can lead to the complete failure of the insulation system during operation. The partial-discharge characteristics of the free spherical conducting particle in the quasiuniform electric field were studied in this paper in order to better explain the partial-discharge (PD) measurement signals for particle movement and implications for insulation diagnostics in power transformers. The phase-resolved partial-discharge (PRPD) pattern was measured and the PD inception voltage, PD charges, and PD frequency were studied under different applied voltages. The influence of oil aging degree to particle PD characteristics was also studied in this paper. It is observed that the PD pattern is different in a different particle motion stage and the number of particles has a significant influence on PD frequency and a slight influence on PD pattern. The particle is inactive in severely aged transformer oil. The PD charges and PD frequency (pulses number per half ac cycle) reduce with the aging degree of the oil sample.Index Terms-Partial discharge (PD), particle measurement, transformer oil.

Copper Particle Effect on the Breakdown Strength of Insulating Oil at Combined AC and DC Voltage

“…The CIGRE report [6] suggested that particle identification and counting were found to be necessary procedures of condition monitoring of converter transformers. In view of the research progress of the particle effect on the breakdown of insulating oil, focus was made on the particle types [7][8][9], voltage types [10,11] particle movement [12,13] under electric field, and partial discharge [10,13] in insulating oil. Reference [7,8] observed that conducting particles affect the dielectric strength much more than insulating particles, and larger particles reduce the breakdown voltage even more significantly.…”

“…The AC breakdown strength of insulating oil contaminated with different copper particle concentrations was obtained. Reference [13] also investigated the motion of single steel particle in insulating oil at AC voltage and obtained the relationship between the PDs on particle and the motion of particle in insulating oil. Most publications investigated particle effect on the breakdown characteristics of insulating oil at individual AC voltage and individual DC voltage.…”

Investigation of partial discharge between moving charged metal particles and electrodes in insulating oil under flow state and AC condition

“…The first term on the right side of equation (1) is the oil flow traction force F D expressed as equation (2) and the direction of the horizontal of oil flow. The particle movement in the horizontal direction of the oil flow is determined by F D along the horizontal direction [16].…”

“…When charged metal particles move to the weak parts of the insulation, they may cause electric field distortion that leads to partial discharge (PD). In addition, when charged metal particles collide with electrodes, charge transfer occurs between charged metal particles and the electrodes, thereby generating PD [2][3][4][5][6][7].…”

“…Sarathi et al [3,8,9] observed that the levitation voltage of free particles in engineering insulating oil decreases with an increase in total harmonic distortion of the supply voltage regardless of the series of harmonics, and the rise time of the PD current pulse of free particles is a few nanoseconds (ns). Li and Hu et al [2] studied the PD characteristics of the free spherical metal particle in the quasiuniform electric field in engineering insulating oil. They concluded that the PD pattern is diverse in a different particle movement stage, and the number of particles has a significant effect on the PD frequency and a minor effect on the PD pattern.…”