Electrical Tree Inception Research Articles

Dependence of electrical tree inception and growth on mechanical properties

The mechanical properties of an unsaturated polyester resin have been varied by the addition of a plasticising polyester resin. Specifically, the tensile strength, modulus of elasticity and fracture toughness have been determined over a range of plasticisation. The initiation and growth of electrical trees in a needle-plane geometry in this range of materials have been investigated. Two conflicting criteria determine the initiation and growth: the presence of internal mechanical strain, and the intrinsic mechanical properties of the material. The initial internal mechanical strain, resulting from the manufacturing process, greatly enhances tree inception and propagation. This effect is dominant and masks the intrinsic dependence of tree initiation and growth on mechanical properties. In strain-free samples, however, the reduction in inception time and the increase in growth rate resulting from the plasticisation of the material, and the ensuing changes in mechanical properties, are clearly shown.

The statistics of electrical tree inception

A theoretical model is proposed to explain the statistics of electrical tree inception based on the geometry of the initiating electrode and the field variation around its surface. Numerical calculations show that the model is sufficient to describe qualitatively the observed features of both the time to inception distribution and the distribution of inception voltages (fields) obtained in ramp experiments. The implications of the model for the analysis of such experiments are discussed. >

Threshold voltage of luminescence and electrical tree inception in low-density polyethylene

The characteristics of luminescence observed during the initiation of an electrical tree have been studied for polyethylene subjected to high electrical stresses. Use of a light detection system which is at least three orders-of-magnitude more sensitive than conventional partial discharge detectors shows that luminescence occurs during tree initiation and this luminescence is not due to partial discharges. Applying a constant high voltage to the polymer or interrupting the voltage several times does not change the behavior of luminescence. The light starts at about the same voltage and its intensity returns to the same value as before the interruption. This behavior is contrary to that observed for partial discharges. The luminescence inception voltage is the threshold voltage at which the polymer starts to degrade. Thus, it is important that the insulation of electrical equipment be protected from transient overvoltages which exceed the threshold level.