Effects of temperature on water tree initiation and propagation at equivalent AC frequency

Water 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 the application of a high-frequency ac voltage. However, in recent years, power cables and power apparatus have also been used under dc voltage application owing to progress in inverter technology. The use of rotating electrical machines with an inverter has become widespread owing to their contribution to the prevention of global warming, high effectiveness and the reduced maintenance required. In an underwater motor driven by the voltage of an inverter, it is necessary to consider the water treeing of the motor insulation and the cable insulation connected to the drive board. Up to now, we have performed accelerated deterioration tests at the inverter carrier frequency of 500Hz. However, in the case of an inverter waveform, an accelerated deterioration test cannot be performed because the inverter waveform becomes markedly different from that of an actual machine when the basic frequency is increased. In this paper, we report the development of a new water electrode test method and the investigation of the effects of temperature on water tree initiation and propagation utilizing an equivalent ac frequency of 50Hz. We found that a water tree was initiated within two weeks even at the basic frequency of 50Hz. Furthermore, we compared the test results obtained at 500Hz and 50Hz at different temperatures.