Factors influencing electrical treeing in XLPE insulation

Abstract

SUMMARYThis paper reviews the factors that influence the electrical stress required for initiation of electrical trees in crosslinked polyethylene (XLPE) insulation. Mathematical model relating the average tree inception stress with the defect tip radius of curvature has been provided for AC and impulse voltages. The resulting equation for AC treeing inception stress is verified by experimental measurements. Moreover, the effects of voltage magnitude, defect tip radius, and time of voltage application on partial discharge (PD) properties are studied, and the conditions for the initiation of bush-and branch-type trees are identified. For each type of tree, the variation of PD level with tree length is investigated experimentally. The results show that a minimum stress is required below which treeing cannot initiate in XLPE insulation. Moreover, during the early phase of tree propagation, tree length follows a self-limiting growth model and in this phase, tree length and the maximum PD levels are correlated.

A System Based on Artificial Neural Networks for Automatic Classification of Hydro-generator Stator Windings Partial Discharges

“…Insulation defects represent localized regions usually with lesser permittivity than the surrounding medium. Due to the smaller permittivity, electric field is more intense within imperfections, and it may cause localized breakdown of the medium even at operating voltage, resulting in PDs [4]. These imperfections are caused mainly by manufacturing deficiencies, load cycling and excessive winding vibrations [2].…”

“…Partial discharges manifest physically in the form of sound, light, electromagnetic radiation and current pulses [4], and therefore can be detected in several ways. The electrical and electromagnetic signals, on which most measurement methods are based, are usually of high frequency and low amplitude.…”

“…Substantial PD levels are an important symptom of stator insulation aging in rotating machines, including hydro-generators [4]. If not properly treated, PDs may progressively evolve to the point of disruptive failure in insulation [5], [2], causing great losses.…”

Influence of empty space and internal stresses on dielectric strength in two-phase polymer

“…The mechanism of treeing is very similar to electric breakdown [5]. In fact, treeing breakdown starts in the regions with the highest and most divergent electrical stresses [5,8,13]. The practical importance of treeing has been emphasized in many cases, particularly for polyethylene cables [5,13].…”

“…Several works focused on the study of the electric breakdown phenomenon of insulating materials and its corresponding driving force have been reported [1,3,[5][6][7][8][9][10][11][12][13][14]. Electric breakdown occurs when the current flowing through the sample increases, leading to its failure and consequently, the applied voltage cannot be stable.…”