Cable Insulation Testing for Mechanistic Degradation FEA Modeling

摘要

The potential for degradation of the polymer jacket and insulation of medium and low voltage power cables represents a concern for lifetime extension of nuclear power plants. A growing concern especially for medium voltage cables is failures that have been observed in submerged environments. According to a NEI survey, over 50 circuit failures have been reported in 21 nuclear power plants (NPPs), perhaps the result of aqueous impact . Given that the number of cables in submerged environments for a period of greater than 30 years is growing and the majority of reported failures range from 20 to 40 years , mechanistic understanding of the degradation process is critical to confidently predict functional properties and safety margins for dielectric breakdown over timeframes of 80 years. Degradation of cable jackets and insulation with respect to exposure in wet and/or submerged environments is attributed to its possible interaction with pore formation within the polymer. It is hypothesized that solvent escape paths are generated during the polymer extrusion process in cable manufacturing. As solvents and low molecular weight polymer escape at extrusion temperatures and pressures, pores form in the polymer matrix. A combination of oxidation and polymer chains reorientation due to aqueous pressure drives the pore surface to become more hydrophilic and allow greater penetration by water molecules (see Figure 1).. The presence of pores is supported by molecular dynamics simulations and positron annihilation and equivalent circuit models of polymer coatings have shown a cyclic dependence on immersion when the thickness and pore contributions are taken into account. While commercial cable jackets and insulation are designed to greatly minimize porosity, thus delaying the initiation of barrier compromise that leads to electrical breakdowns, long-term performance on the order of years would suggest that degradation is enhanced by the presence of electric field. This degradation should be considered as a coupled problem since the higher electric field produced by pores is a driver for ion transport to these pores and thus advance of the pore to the conductor, water treeing.