Water Tree Growth in Polyethylene under DC Voltage Stress

Water trees have been experimentally grown in high molecular weight, low density polyethylene when subjected to a constant dc stress. Two major types of trees have been observed; bushy trees (one-half of a bow tie) emanating outward from microvoids and larger water channels penciling inward from the perimeter or outward from the center conductor. The coaxial dielectric was subjected to an average stress of 22 kV/mm in 70°C sea water to yield a growth rate for the bushes of 0.04 im/hr. Careful microscopic observation has shown a correlation between the electrical stress, the size of the microvoids, and the length of the bush. The water channels grow inward from the perimeter when the center electrode is negative at the rate of 0.2 μm/hr. When the center electrode is positive however} the water trees initiate growth but reach saturation in a short time.