Mohamad Ghaffarian Niasar

Effect of partial discharges on thermal breakdown of oil impregnated paper

In this paper the effect of partial discharges on the thermal breakdown voltage of oil-impregnated paper is investigated. A disc shaped cavity in a stack of paper sheets formed a PD source. The paper sample was exposed to partial discharges for 18 hours. Dielectric spectroscopy measurements were performed on the samples before and after exposure to PD activity. The dielectric spectroscopy results showed a big change in the real and imaginary part of the permittivity. These results were used to calculate activation energy and heat losses inside the insulation. In order to investigate the possibility of thermal breakdown in the system, the oil-impregnated paper system was simulated by using the FEM software Comsol Multiphysics. The heat produced by PD pulses and dielectric losses was considered in the simulation. The results show that for the oil-impregnated paper insulation, the PD activity can reduce the thermal breakdown voltage by a factor up to 4 times.

Partial Discharge analysis in a metal-dielectric air gap on machine insulation at arbitrary testing voltage

Partial Discharge (PD) tests were performed in a metal-dielectric air gap between a spherical electrode and the surface of machine insulation, which consists of mica, epoxy resin and glass-fiber. The PD activities were investigated at three types of applied voltages: triangular, square and trapezoidal waveform. In the trapezoidal waveform, the duration of the linearly rising part of the applied voltage was varied. The effect of the time derivative of the applied voltage dU/dt on the PD activities was studied. The PD activities were studied at a power frequency (50 Hz) and at a lower frequency (10 Hz) of the applied voltage. The results show that the time derivative of the applied voltage has a significant effect on the PD behavior. In each waveform, most of the PD pulses occur during the linearly rising period, but never occur during the linearly falling period. At lower frequency, compared with the positive pulses, the number of negative ones decreases dramatically with the increasing rising time, and PD activities even disappear at the triangular waveform.

Impact of thermal and electrical aging on breakdown strength of oil-impregnated paper

In this paper the impact of thermal and electrical aging on AC and impulse breakdown strength of oil-impregnated paper was investigated. Sheets of paper were thermally aged at 140 °C for 30 days. A comparison between AC and impulse breakdown strength of unaged paper layers, 10, 20 and 30 days thermally aged paper was performed. In order to investigate the effect of electrical aging, first the paper layers were exposed to partial discharges for 6 hours at accelerated electric stress. The partial discharge was created inside paper samples by introducing a disc shaped cavity on the center of the paper samples. The effect of partial discharges was then investigated on the impulse breakdown strength of oil-impregnated paper. The study shows that thermal aging has very small effect on AC breakdown strength. However, impulse breakdown strength of oil-impregnated paper may reduce up to 40% due to exposure to PD.