Research paperAC breakdown channel of PPLP multi-layer insulation for HTS cable
Introduction
The high temperature superconducting (HTS) cable was considered as an alternative for transmitting electric power with a significant reduction in conductor loss at medium applied voltage [1], [2]. PPLP has been used as tape insulation for oil-filled power cable because of its lower dielectric loss and higher dielectric strength in comparison with Kraft paper [3]. Recently, PPLP has been investigated for using as tape insulation for HTS power cable because it is easily impregnated with liquid nitrogen (LN2) and has excellent dielectric characteristics [4], [5], [6]. It is well known that breakdown in the multi-layer insulation of oil power cable is originated from partial discharges in the butt-gaps and creepage discharges at interfaces between layers. The partial discharges in the butt-gaps in LN2 was mentioned in previous studies [2], [7]. However, the occurrence of creepage discharges in the multi-layer insulation in LN2 has been rarely reported. Moreover, the influence of applied electric field on the shape of breakdown channel in multi-layer insulation and the degradation of insulation layers has not been fully studied.
In this study, the AC breakdown channel in the PPLP multi-layer insulation was investigated, and the deterioration of samples after breakdown was examined with different values of applied electric field. Besides, the electric field distribution of the electrode system was simulated with the finite element method (FEM) by using the Maxwell program, and suggested breakdown mechanism for multi-layer insulation was introduced. The Maxwell program solves two-dimensional electromagnetic problems in a number of steps such as modelling, material definition, boundary conditions and sources, solution criteria, meshing, solution and post-processing.
Section snippets
Materials
The insulating material used in this experiment is PPLP imported from Finland. The PPLP has thickness of 0.119 (mm) and consists of a PP layer (0.069 mm) laminated by two layers of Kraft paper of 0.025 mm (Fig. 1a). Density of PPLP is 0.89 g/cm3. Because of the highly hygroscopic nature of Kraft layers, vacuum drying of sample was performed at a temperature of 100 (oC) for 24 (h) prior to testing [8]. The water content of PPLP was about 5.8 wt% and 0.5 wt% for before and after drying,
Breakdown strength of LN2 and PPLP
Fig. 3 shows that the breakdown strength (EBD) of both LN2 and PPLP significantly decreases with increasing the electrode gap (d). However, it is obtained that data are better fit to the model of LN2 than that of PPLP (R2 = 1,0 for LN2 compared to R2 = 0,91 for PPLP). This can be explained by the nonuniform quality and thickness of PPLP layers as wells as the presence of random impurities at the interfaces between layers. The EBD of PPLP is about 2 times higher than that of LN2. This result is
Conclusions
The AC breakdown channels of PPLP multi-layer insulation was investigated with varying applied electric field stress for samples with and without butt-gap. The experimental results show that partial discharges in the butt-gaps and creepage discharges at interfaces between layers play a decision role in the formation of the shape and position of breakdown path. The occurrence of creepage discharges are more frequently with higher length for samples without butt-gap at a lower applied electric
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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