The Influence of Dynamic Mechanical Strain on the Electrical Treeing Characteristics in XLPE Insulation

Dynamic power cables used in floating offshore renewable energy systems are subjected to continuous mechanical strain from hydrodynamic forces, making insulation reliability a critical design consideration. Current qualification standards predominantly rely on sequential testing of mechanical and electrical properties, which may overlook degradation mechanisms arising from their interaction under real-world conditions. This study presents a novel in-situ approach that combines electrical, mechanical and optical monitoring and testing of cross-linked polyethylene (XLPE) cable specimens, under strain conditions representative of dynamic offshore environments. In contrast to previous studies that examined only static strain, the present work investigates cyclic tensile loading to capture the coupled effects of time-varying mechanical strain and electrical energization. Five samples per condition were subjected to cyclic tensile strain amplitudes up to 2.8% at frequencies of 0.05, 0.1, and 0.2 Hz, under a 50 Hz AC voltage of 7.5 kVrms. Results demonstrate that dynamic strain accelerates tree growth and narrows final electrical tree geometries, height-width ratios doubled under dynamic conditions. Static tensile strain significantly shortens the initiation time and time-to-failure from an average of 306 down to 155 minutes. When comparing samples that were tested sequentially, similar breakdown characteristics were observed with subtle differences in tree morphology and propagation characteristics. These findings confirm that sequential testing does not fully replicate the degradation behavior observed under simultaneous loading. This work underscores the need for rigorous combined electro-mechanical testing when evaluating new cable materials and designs intended for dynamic applications in floating offshore wind.