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Study of dielectric strength of high voltage cables

Messaritakis Georgios

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Summary

Reliable dielectric performance of high voltage (HV) cables is critical for power system integrity, yet the outer oversheath can present unexpected weaknesses during lightning transient events. This thesis undertakes a thorough investigation of dielectric strength in two extruded HV cable constructions—a 150 kV AL/XLPE/SWAS/PE design and a 110 kV N2XS(F)2Y variant—focusing on the cables’ oversheath behavior under impulse voltage stress. International test protocols (IEC and ASTM) that guide specimen preparation, measurement, and testing were used.Initial evaluation revealed that factory-applied semiconductive jacket layers often exceed the thin screening film assumed in standards, creating thicknesses on the order of millimeters and resulting in small surface resistances in the kiloohm range. Such oversheath characteristics led to surface conduction and creepage discharge when applying lightning impulses, visibly carbonizing the oversheath and producing distorted waveforms with abnormally rapid tail decay and erratic discharge paths. Mechanical removal of these semiconductive layers—achieved by rasping, after abrasion proved insufficient—restored insulation surfaces to GΩ-range resistivity, eliminating leakage paths and enabling correct 1.2/50 μs waveform generation during oil-immersed impulse tests. Clean electrode installation and controlled immersion protocols ensured reproducible conditions.For the 150 kV cable, only after complete removal of the multi-millimeter semiconductive oversheath did the impulse waveforms conform to IEC tolerances and the insulation exhibit clean puncture breakdown without lateral tracking. Similar findings are true for the 110 kV cable, where surface-resistance verification and targeted layer removal preceded successful impulse withstand tests.Experimental container design also evolved through prototypes to address mechanical failures under high-stress impulses, culminating in a nested, oil-filled assembly that maintained dielectric clearance and structural integrity during repetitive testing. The Marx generator was analyzed, modified to a single-stage impulse voltage generator for lower voltage impulses. The produced waveforms were validated via simulations and measurements.This work studies the dielectric strength, providing detailed procedures for oversheath assessment, mechanical removal techniques, electrode assembly, oil-immersion setup, and impulse voltage application. Finally, this study can contribute empirical dielectric-strength limits under transient stresses, supporting improved testing protocols and cable design and production evaluations.

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