SiC-based nonlinear resistive grading materials have been used for end turn stress grading of form-wound rotating electrical machines, such as high voltage motors and turbine generators, over a long period. Although these materials work well under power frequency, the electric field and local heating become problematic under PWM repetitive pulses for the stress grading systems of converter-fed rotating machines. In order to select the appropriate stress grading materials and to optimize the insulation design of end turn structure for PWM waveforms, accurate knowledge of the nonlinear conduction characteristics of stress grading materials as a function of temperature and electric field is required. In this paper, the authors analyze sophisticated sample configurations for accurate measurement of nonlinear conductivity characteristics of the materials based on transient nonlinear FEM computations.

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SUMMARYWater treeing occurs when an AC voltage is applied to an electrical insulation system such as those in power cables and power apparatus. From a practical viewpoint, research on water treeing has mostly been performed under high-frequency AC voltage application. However, power cables and power apparatus are also used under DC voltage application owing to progress in inverter technology. Inverter drive of rotating electrical machines has spread and expanded for global warming prevention and maintenance improvement. Underwater motors are used in water. In an underwater motor driven by the voltage of an inverter, it is necessary to consider water treeing of the motor insulation and cable insulation connected with a drive board. Thus far, a water electrode method that utilizes an AC voltage of high frequency has been used in the accelerated water treeing experimental method for a long time. On the other hand, an inverter with a surge voltage generates an equivalent AC frequency by the application of a multifrequency pulse voltage similar to a pulse width modulation (PWM) control wave. Therefore, there is a possibility of mistakes in realmachine reliability evaluation because the initiation and propagation mechanisms are different from those in a real machine, even if we can reproduce water treeing by increasing the basic frequency. This paper reports our investigation of a new water electrode method and of temperature effects on water tree initiation and propagation. We also compare experimental results between 500 Hz and 50 Hz using the new water electrode method from the viewpoint of temperature.