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Ablation of the buffer layer in high voltage cross-linked polyethylene (XLPE) cables has been the attractive type of cable failure. The ablation of buffer layer commonly takes place under the pressure and moisture. Therefore,it is important to investigate the effect mechanism of pressure and moisture on the ablation of buffer layer. In this study,the simulated experiments of dry and wet water-blocking buffer tape under different pressures are carried out,whereby dynamic variations of current density,voltage and alternating current (AC) volume resistivity in the processes of ablation are obtained. The micromorphology of buffer tape specimens is obtained through the scanning electronic spectroscopy. It is shown that the AC volume resistivity of dry buffer tape samples gradually increases with time in the process of ablation,accompanied with the transient surge of current and the plunge of voltage. While the AC volume resistivity of wet buffer tape samples gradually decreases with time. Moreover,a decrease of AC volume resistivity occurs to the buffer tape samples after the water absorption. With the increase of pressure from 1.09 kPa to 5.45 kPa,a decrease of volume resistivity occurs to both the dry and wet buffer tape samples. Combined with the results of stimulated ablation experiments and the micromorphology of buffer tape,the effect mechanisms of the pressure and moisture on the conductive property of buffer tape are explained.

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• The buffer layer is not treated as an ideal homogeneous material as previously researched. • The failure mechanism is investigated from a perspective of material inhomogeneous microstructures. • Small fiber protrusions are found on the buffer layer surface. • The geometric models for the fiber protrusions are proposed quantitatively. • Significant electric field distortion is observed at the fiber protrusion regions. In recent years, growing attention has been paid to the buffer layer ablation failure in high voltage cables. The buffer layer was always regarded as a homogeneous material in previous studies, while in this paper, the effects of its structural inhomogeneity on failure is found unneglectable. The buffer layer is recognized to be a porous material comprising fibers and air pores through microscopic characterization. Its porosity is measured to be 85% by testing mercury intrusion porosimetry. Especially, some small fiber protrusions are observed on the surface of buffer layer, which are categorized and named as tip protrusions and arched protrusions. Subsequently, the influence of these fiber protrusions is investigated by quantitatively proposing their geometric models. The electric field distortion is observed at the fiber regions after finite element simulation. Under the extreme circumstances when the tip protrusions are vertical and close to the central area of air gap, the maximum electric field at tip site can reach to 28.5 kV/mm, which is 9.5 times of the breakdown threshold of air. And the maximum electric field of arched protrusions region can reach to 5.94 kV/mm, which is more moderate but still higher than the threshold. Therefore, it is demonstrated that the breakdown of air gap inner cables can be induced by the inhomogeneous fiber protrusions. [ABSTRACT FROM AUTHOR]

Aims We aimed to determine how many electrical cardioversions (ECs) should be applied to treat repetitive persistent recurrences of atrial fibrillation (AF) following ablation of persistent AF within the early post-procedural period. Methods and results A total of 40 patients with >1 episode of recurrent AF in the form of persistent atrial arrhythmias within 3 months following the ablation were recruited from 108 patients who underwent ablation for persistent or long-standing persistent AF. Electrical cardioversions were applied up to six times, if necessary, to restore sinus rhythm at clinical visits at 2-week intervals until 3 months after the ablation. Fourteen (35%) ablation failures defined as recurrences of AF identified from the 3rd month after the ablation procedure were finally diagnosed during the follow-up period (14 ± 4 month). The patients with an ablation failure more frequently required ECs than those without (3.7 ± 0.3 vs. 1.2 ± 0.2 times; P < 0.0001). A receiver-operating characteristic curve identified a number of ECs of ≥3 as the optimal cut-off value for predicting an ablation failure (area under the curve 0.91; sensitivity, 86%, and specificity, 96%; P = 0.0007). In the multivariate logistic regression analysis, a number of ECs of ≥3 was the only independent predictor of an ablation failure (odds ratio, 11.32; 95% confidence interval, 3.83–58.22; P = 0.0019). Conclusion It was difficult to maintain sinus rhythm in patients with persistent AF who required several ECs for recurrences of AF within the early post-ablation period. [ABSTRACT FROM PUBLISHER]