The stator coil end of a rotating machine has the structure of electric field grading system (SG system) for the purpose of preventing partial discharge. Light emission of partial discharge at such SG on a model bar system under operating temperature is measured by a photomultiplier tube and a digital camera equipped with an image intensifier. It is found that light emission of PD on SG system decreases under operating temperature. Small PD which occurs at lower voltage under high temperature may prevent the occurrence of the subsequent PD.
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