Quantum chemistry simulations for multi-scale modelling of polymer ageing

The study focuses on degradation caused by partial discharges (PDs), that is, localized, lowenergy electrical discharges occurring within gaseous voids embedded in the material. The proposed approach is deterministic and grounded in fundamental physical principles, integrating simulations of both microscopic and macroscopic phenomena.

Polyethylene (PE) is a widely used dielectric in high-voltage cables, and its gradual deterioration under the influence of PD activity. Successive PDs generate a plasma within internal defects, interacting with the surrounding solid and leading to chemical transformations at the gas-solid interface (e.g. chemical reactions, surface erosion, increase of surface conductivity). In severe cases, PDs can promote the propagation of branched internal channels

(a phenomenon known as treeing) which represents one of the main causes of dielectric failure in AC regimes.

Density Functional Theory (DFT) calculations are employed to elucidate the chemical pathways involved in treeing formation. We estimate a series of microscopic parameters intended to be used in macroscopic models, enabling quantitative comparisons with experimental data. The study investigates the physico-chemical mechanisms involved in i) PDs’ triggering, ii) sustaining of the growth of tree-like structures and iii) their conductivity increase.

The triggering of each PD depends on the material’s work function, which varies with surface chemistry and electronic structure. Simulations indicate that Schottky emission becomes possible when excess electronic charge accumulates on the surface in the presence of specific chemical defects.

With molecular dynamics (MD), we explore the interactions between plasma ions and PE surface at different energies and plasma conditions. As the electric field intensity increases, high-energy ions induce sputtering, releasing carbon-based fragments into the gas phase. These fragments can subsequently redeposit onto the PE surface, forming new chemical phases with enhanced electrical conductivity.

The goal is to achieve predictive assessments of the remaining operational lifetime of electrical components such as high-voltage cables and dry transformers. With this contribution, we aim to advance toward a predictive understanding of the ageing mechanisms of polymeric insulation in electrical systems.