Abstract
As a key accessory of high-voltage (HV) insulated submarine cable, the factory joints of the cross-linked polyethylene (XLPE) represent an unpredictable uncertainty in cable-connecting fabrications by means of the extruded molding joint (EMJ) technique. The electrical breakdown pathways formed at the interfaces between recovery insulation and cable body under alternative current 500 kV voltages are specifically investigated by microstructure characterizations in combination with the electric field and fractal simulations. Dielectric-defected cracks in tens of micrometers in insulation interfaces are identified as the strings of voids, which dominate insulation damages. The abnormal arrangements of XLPE lamellae from scanning electron microscopy (SEM) imply that the structural micro-cracks will be formed under interface stresses. Electrical-tree inception is expedited to a faster propagation due to the poor dielectric property of interface region, manifesting as 30% lower of tree inception voltage. The longer free-paths for accelerating charge carriers in the cracks of interface region will stimulate partial discharging from needle electrodes. The carbonized discharging micro-channels arising in interface region illustrate that the partial discharging will be triggered by the electrical-trees growing preferentially along the defect cracks and could finally develop into insulation damages. The mechanism of forming cracks in the fusion processes between the molten XLPE of cable body and the molten cross-linkable PE of recovery insulation is elucidated, according to which the crack-caused degradation of insulation performance is expected to be alleviated.
Highlights
With the development of wind power generation in seashore and island power networks, great demand lies in the high-voltage submarine cable (HVSC) for electric power transmissions from wind power stations and island power systems to mainland power networks [1,2,3]
A large number of cracks are recognized being parallel to interface extension plane in interface region, as shown in Figure 3 of the morphology photos taken from transparent observations by the region, as shown in Figure 3 of the morphology photos taken from transparent observations by the optical microscope
30% lower of tree inception which can properties be attributed interface region in extruded molding joint (EMJ), showing about 30% lower of tree inception voltage which can be attributed to that the long free paths of charge carries in the cracks of interface region will trigger discharging from needle electrode
Summary
With the development of wind power generation in seashore and island power networks, great demand lies in the high-voltage submarine cable (HVSC) for electric power transmissions from wind power stations and island power systems to mainland power networks [1,2,3]. Cross-linked polyethylene (XLPE) has been successfully applied as the essential insulating material for 500 kV HVSC with a single cable length approaching to 18.5 km in China. It is comprehensively required for long submarine cable due to the specific operation environments. The factory joint fabricated through the extrusion molding technology (extrusion molding joint, EMJ) is a key factor for the reliability of HVSC system [7] Recovery insulation of this factory joint as a super-clean cross-linkable polyethylene compound material is extruded into joint mold with an almost identical diameter for cable connections to facilitate submarine cable laying. The dielectric failure mechanism is consistently confirmed by initiation and propagation characteristics of electrical-trees in interface region
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