Role of the number of consecutive voltage zero-crossings on the water tree growth in polyethylene

With the progress in power electronics, ac voltages with various harmonic components are often applied to insulating materials of power apparatus. Water treeing is the most-observed degradation pattern in polyethylene-insulated power cables, and much work has been done. Nevertheless little is known about their dielectric properties under above-mentioned particular ac voltages. In this paper, the authors examine the influence of superposition of a low-frequency ac voltage (0.1 to 5 Hz) and a high-frequency 2-kHz ac voltage on the growth of water trees in polyethylene. It has become clear that the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage plays an important role in the length and shape of water trees.