Partial discharge detection and analysis in polymeric HVDC insulations

Abstract

The growing use of polymeric insulation in modern high-voltage direct current (HVDC) cable insulation systems has underscored the importance of reliable insulation diagnostics. Defects in insulation systems develop through the manufacturing process and operational stresses and are prone to partial discharge (PD). Sustained PD activity causes thermal stress and chemical modifications in the insulation system, thereby advancing the degradation of the insulation. PD behavior under DC voltage is relatively complex due to conductivity-dependent electric field evolution and accumulation of space charges. These challenges highlight the requirement of PD behavior analysis in HVDC polymeric insulation for the enhancement of developing insulation systems under the NEWGEN project. This thesis aims to provide a detailed study on the PD behavior in polymeric insulation systems and to experimentally detect and analyze the PD activity in different defect geometries in crosslinked polyethylene (XLPE) and polypropylene (PP) based flat insulation samples. XLPE and PP blends are used in HVDC cable insulation due to their high dielectric strength and very low conductivity. The comprehensive literature review presented in the thesis addresses polymeric insulations, electric field formation in HV cables, partial discharge physics, PD-induced insulation aging, and PD detection and analysis techniques. The empirical work was focused on detecting and analyzing the partial discharges under varying defect geometries, applied electric field, insulation material, and applied voltage types. Phase-resolved partial discharge (PRPD) and time-resolved partial discharge (TRPD) patterns, trends in PD magnitude frequency, and PD pulse shapes are evaluated. During experimental work, five defect geometries in PP insulation samples and five defect types in DC-XLPE insulation samples are stressed under AC and DC voltages. PRPD and TRPD analysis suggested that all samples with void-type defects followed a similar PD trend. Under AC excitation, PD activity was concentrated near the sinusoidal voltage maxima and exhibited symmetrical PRPD clusters across the voltage axis. Under DC excitation, the Laplacian field in the void and enormous free electron availability result in high PD activity initially, followed by longer pauses with high magnitude bursts of partial discharges. Space charges and increased material bulk conductivity evolve the electric field to a Poissonian distribution. As a result, void experiences low residual voltage, slower field recovery, and longer pauses between discharges. The experimental work pointed out signs of sustained PD activity with a higher repetition rate for voids with comparatively bigger diameters. Bigger void geometry supports a high electric field and a high number of initiating electrons for frequent PD inception. Evaluating the effect of applied electric stress on PD behavior, it is observed that PD magnitude is rather independent of the applied electric field. However, the discharge magnitude increases with longer pauses between PD activity. This behavior is supported by literature, as discharge magnitude is a function of time-lag and overvoltage. The descending trend in PD rate was observed in both insulation materials. According to literature, space charge accumulation reduces the local field intensity; moreover, formation of oxidation by-products and increased surface conductivity lowers the rate of electric field recovery and PD inception naturally. DC-XLPE shows more sustained partial discharges due to the slow charge decay rate associated with lower conductivity relative to PP blends. The results demonstrate that PD detection and analysis methods used in this study provide adequate resolution to reveal the effects of defect geometry, material properties, and voltage type on PD behavior of polymeric HVDC insulations in a laboratory environment.