Critical parameters for electrical tree formation in XLPE

Abstract

Published data for electrical tree inception field at a defect in XLPE vs. the Laplacian field at the tip of the defect are explained on the basis of a minimum distance which the space charged limited field must extend from the defect tip into the XLPE in order to damage enough dielectric in the field direction to facilitate PD inception and tree initiation,

Factors influencing electrical treeing in XLPE insulation

“…In one study, similar measurements were repeated using 27 samples of a given XLPE cable. The measured values of treeing inception stress were over a range of 253-328 kV rms /mm, with about onethird of the total samples showing stress of about 290 kV rms /mm [17]. Similar to the AC inception stress versus tip radius behavior, impulse tree inception stress also depends upon the defect tip radius.…”

Electrical aging and life models: the role of space charge

“…>100-200 kV/mm) the conductivity of insulating polymers can be enhanced by several orders of magnitude, due to the dependence of the activation energy on electric field. This favors local large current densities that may cause the formation and growth of local conductive paths, leading to breakdown once they reach a critical length [25][26][27][28][29]. In practical insulating systems, these phenomena are generally localized at interfaces between the bulk insulation and defects (such as protrusions and metallic inclusions), which are able to magnify the local electric field significantly with respect to the average design field, or associated with field amplification due to huge bipolar space charge accumulation, that can happen in the presence of strongly injecting electrodes and ionic contaminants [30][31][32].…”

“…Approaches following these premises have been developed by Zeller [25][26][27] and Boggs [28,29] having the common background of the so-called Field-Limited Space-charge Current (FLSC) theory. According to FLSC theory, there is a threshold field, E c , below which injected charges possess a low but finite mobility so that over a long period of time the equilibrium situation predicted by the Space Charge Limited Current (SCLC) model [11,12] will result.…”

A Novel Approach to the Positive DC Nonlinear Corona Design

“…Therefore, linear relations are only applicable as long as the electric field strength is smaller or equal to critical value (see Figure 1 with measured values of Laplacian and Poisson electric field [Boggs, 2005;Jiang et al, 1998). In cases when the electrode has mathematical peaks and/or sharp edges, the linear approaches cannot be applied.…”

“…The second assumption is rarely discussed in the literature. The area of approximately constant electric field is present around the electrode spike (Jiang et al, 1998). After increasing the charge, the field at the spike is no longer increasing, but it stays equal to the critical value of the field that corresponds to the charge when sparking begins (Kaptzov [1947] assumption), and the sparking area is increasingly spreading.…”