A New Water Electrode Method for Short‐Time Testing of Water Treeing and Consideration of Temperature Effects of Water Tree Initiation and Propagation

Abstract

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.