In this study, the author investigated the online deterioration diagnosis of hydro-generators. Simultaneous measurements were done under offline conditions by using an online PD monitor and an offline PD measuring system. When a stator had wave-winding structure, good correlation between the maximum electric discharges and PD values of the online PD monitor was seen. However, when a stator had lap-winding structure, a good correlation could not be seen clearly. PD values were small due to the weak capacitive coupling in a lap-winding structure. PD measurement of hydro-generators with wave-winding structure was done under online and offline conditions by using the PD monitor. Online PD values were slightly higher than offline PD values. The author believes that online PD monitoring is useful in diagnosis of hydro-generators when the stator has wave-winding structure.

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To investigate the temperature distribution on the surface of a silicone rubber exposed to partial arc discharges，experimental and numerical analyses on thermal characteristics were carried out. Tests were executed by placing the salt-waterlogged glass-filter paper on a silicone rubber sheet to simulate a polluted electrolyte layer. The results showed that the formation heights of arc channels from the surfaces of silicone rubbers and temperature distributions varied with the electrolyte conductivity. The temperature distribution on the specimen surface became high in the middle of the discharge gap while those became low at points of contact between an arc discharge and tips of glass-filter paper. The thermal diffusion from the points of contact between the arc discharge and the tips of glass-filter paper into electrolyte electrodes caused the temperature decrease at the points.

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