(182g) On Polyethylene Cable Failure, Electric Fields, Water Clusters and Ions

Johansson, E., University College of Borås
Ahlström, P., University College of Borås
Bolton, K., University of Borås

Polyethylene high voltage cable insulation does frequently fail in the absence of voltage stabilizers (normally antioxidants). A main mechanism in cable degradation is water treeing, a phenomenom in which water forms tree-like structures inside the insulation. In a series of simulations the molecular mechanisms behind water treeing have been studied using an connectivity altering Gibbs ensemble Monte Carlo simulation method [1] to model the equilibrium of water molecules between a pure water phase and a polyethylene phase. Polyethylene has been modelled with united atom models, like TraPPE whereas water has been modelled using SPC and SPC/E models.

To model ionic impurities (from, e.g, the catalysts used in polyethylene production) in the polyethylene a NaCl ion pair was inserted into the simulation box. This led a dramatic increase in water solubility in the polymer and the formation of water clusters around the ions. If the distance between the ions was smaller than about 2 nm the the two clusters were united into one large cluster [2]. The effect of an additional electric field has also been investigated and the conclusion is that an applied field in the absence of ions has a minor effect but gives rise to a decreased solubility of water in the hydrocarbon phase at very high external fields (> 1 GV/m) ? partially because of an increased ordering of the water in the pure water box. If, on the other hand, an electric field is applied to a system containing an ion pair water solubilities increase when the electric field is aligned with that between the ions (except at the highest fields > 1 GV/m)

A mechanism for the degradation of high voltage cables is proposed based on the above results.

[1] .E.Johansson, K.Bolton, D.N.Theodorou, P.Ahlström, J.Chem.Phys.126, 224902 (2007)

[2] E.Johansson, K.Bolton, D.N.Theodorou, P.Ahlström, J. Chem. Phys. 127, 191101 (2007)