Assessment of Water Tree Formation in XLPE Insulation Variants to Validate Design of 132 kV Wet-Type Dynamic Cables

To address these operational challenges wet design cables are increasingly deployed, where the polymer insulation is exposed to continuous moisture ingress throughout service, introducing increased long-term reliability concerns. The combination of moisture, high electric field, insulation defects and a prolonged duration can promote water treeing: a degradation mechanism in which diffuse water-filled channels form and progressively weaken dielectric strength. Over time, water trees can transition into electrical trees, ultimately leading to catastrophic cable failure. Tree Retardant XLPE (TR-XLPE) offers improved resistance to this degradation, but uncertainties remain regarding water tree mechanisms, long-term dielectric performance and the effectiveness of mitigation in 132 kV wet-design dynamic cable.

This study investigates water tree initiation and propagation under accelerated ageing conditions representative of 132 kV wet design cables. Small scale laboratory experiments were conducted on XLPE half-ring samples with water needle defects that simulate controlled sites of water ingress and local electric field intensification. This allowed controlled ageing of the insulation material from initiation points characteristic of vented water trees, tree formations of particular concern in wet-design cable due to constant water access and accelerated growth towards the conductor. Three XLPE material performances were evaluated. All samples were pre-soaked in deionised water and then aged for 100 Hours at 8 kV/mm AC (50 Hz) by a water needle defect filled with 3 % NaCl solution. After ageing, aged samples were analysed using optical microscopy to quantify water tree length, density, geometry and associated material morphology.

Results showed material-dependent water tree behaviour. XLPE 1 with high crystallinity and purity, exhibited small, dense circular water trees localised near the needle tip. XLPE 2 displayed an alternate behaviour with diffuse water trees encasing the needle defect and a visible material deformation. XLPE 3, characterised by a higher void content and more open microstructure, developed the longest and most diffuse water trees with evidence of transition to electrical trees. Crystallisation deposits in XLPE 1and 2 samples highlighted ionic migration during ageing while their absence in XLPE 3 indicated moisture retention within the polymer.

Electrical tree transition occurred in XLPE 3, suggesting that void-rich microstructures under high electric stress create localised field enhancements that can accelerate treeing degradation.

The study demonstrates that elevated electric stress in 132 kV wet design cables may accelerate water tree growth and compromise insulation reliability, with material morphology playing a key role in water tree propagation and electrical tree susceptibility. Future work will explore additional ageing factors such as mechanical strain and thermal stress to quantify water tree evolution. This testing regime provides initial insight into the effectiveness of XLPE insulation ageing when subject to the elevated ageing conditions required for 132 kV dynamic cable.