Hiroshi Nieda

Studies of elastic waves caused by corona discharges in oil

In order to diagnose the deterioration of insulation and evaluate its location in oil-filled pole transformers, measurements of elastic waves brought by corona discharges in oil were performed, based on an acoustic emission (AE) technique. The detected signals in these measurements were analyzed by means of a fast Fourier transform (FFT) and their properties were discussed theoretically. It was found that the AE signals due to the Lamb waves of symmetric mode caused in a thin steel tank could be recognized easily by the FFT analysis of initial stages of the detected AE signals. This suggests that the accuracy on location of the AE sources in the transformer would be improved. Also, it was shown that a low pass filter was effective in improving the location accuracy.

Characteristics of elastic waves caused by corona discharges in an oil-immersed pole transformer

Measurements of elastic waves brought by a pulse of simulated corona discharge in an oil immersed pole transformer and a model tank filled with oil were performed in order to identify the location of insulation deterioration, using an acoustic emission (AE) technique. The detected AE signals were analyzed by means of a fast Fourier transform (FFT), and properties of elastic waves influenced by the existence of insulated windings and magnetic core for power transformation in the transformer were investigated. The result shows that almost all the initial AE signals detected on the-wall of the transformer tank were Lamb waves caused by the oscillation waves passing through the insulating paper and oil with repeating transmission, reflection and refraction. Also, it was possible to recognize whether the measured elastic wave was influenced by windings and their insulating papers or not, by FFT analysis. This suggests that the location accuracy of the corona discharge in the transformers would be improved.

Gas Temperatures in Impulse Discharge Channels at Low-Pressure Air

Two types of glow-to-arc transition processes, following constricted and transitional glows, in a nonuniform field gap between a point anode and a protrusion on a plane cathode in low-pressure air under impulse voltage are investigated, observing the spatiotemporal variation of emission intensity of the discharges by means of an image converter camera. At the same time, the distribution of gas temperature along the gap axis is measured spectroscopically with high resolution in time. The results show that the gas temperature is raised to a considerably high, though not extreme level. Along the positive column, the gas temperature rise is higher in the constricted region, especially at two parts near the respective tips of the point anode and the cathode protrusion. The transition mechanisms from the glow to the arc are discussed.