Svein Magne Hellesø

Simulation of Water Diffusion in Polymeric Cables Using Finite Element Methods

The major aging mechanism in polymeric medium voltage cable insulation is water treeing. Water trees can grow from contaminations and protrusions in the cable insulation when the relative humidity (RH) in the cable insulation exceeds a value of about 70 %. Water trees can cause service breakdown of the cable when they reach a critical length. The service life of cables without any metallic water barrier can be prolonged by delaying the water ingress into the cable. The water ingress is dependent on the diffusion rate, water sorption, initial water content in the cable materials, and the temperature of the cable. A numerical model for simulating the water ingress into a cable has been made using a finite element method. For some of the cable materials the water solubility coefficient was concentration dependent, and details of modeling these coefficients are included. Simulations for several cable configurations have been performed to investigate the time it takes for the humidity in the cable insulation to reach the critical threshold for water tree growth. The results have been compared with a simplified method for determining the time to reach 70% RH. The results show that it is possible to include concentration-dependent solubility coefficients in the finite element method. The results from the simulations and the simplified method show good agreement. By taking account of the water absorbed by the semiconductors, the time to reach 70%RH is increased by about 10%.

Experimental and computational studies of water drops falling through model oil with surfactant and subjected to an electric field

The behaviour of a single sub-millimetre-size water drop falling through a viscous oil while subjected to an electric field is of fundamental importance to industrial applications such as crude oil electrocoalescers. Detailed studies, both experimental and computational, have been performed previously, but an often challenging issue has been the characterization of the fluids. As numerous authors have noted, it is very difficult to have a perfectly clean water-oil system even for very pure model oils, and the presence of trace chemicals may significantly alter the interface behaviour. In this work, we consider a well-characterized water-oil system where controlled amounts of a surface active agent (Span 80) have been added to the oil. This addition dominates any trace contaminants in the oil, such that the interface behaviour can also be well-characterized. We present the results of experiments and corresponding two-phase-flow simulations of a falling water drop covered in surfactant and subjected to a monopolar square voltage pulse. The results are compared and good agreement is found for surfactant concentrations below the critical micelle concentration.

Influence of surfactants on the electrohydrodynamic stretching of water drops in oil

Abstract In this paper we present experimental and numerical studies of the electrohydrodynamic stretching of a sub-millimetre-sized salt water drop, immersed in oil with added non-ionic surfactant, and subjected to a suddenly applied electric field of magnitude approaching 1  kV/mm. By varying the drop size, electric field strength and surfactant concentration we cover the whole range of electric capillary numbers (CaE) from 0 up to the limit of drop disintegration. The results are compared with the analytical result by Taylor (1964) which predicts the asymptotic deformation as a function of CaE. We find that the addition of surfactant damps the transient oscillations and that the drops may be stretched slightly beyond the stability limit found by Taylor. We proceed to study the damping of the oscillations, and show that increasing the surfactant concentration has a dual effect of first increasing the damping at low concentrations, and then increasing the asymptotic deformation at higher concentrations. We explain this by comparing the Marangoni forces and the interfacial tension as the drops deform. Finally, we have observed in the experiments a significant hysteresis effect when drops in oil with large concentration of surfactant are subjected to repeated deformations with increasing electric field strengths. This effect is not attributable to the flow nor the interfacial surfactant transport.

Water tree initiation and growth in XLPE cables under static and dynamic mechanical stress

Water treeing is the major aging mechanism in wet-designed cross-linked polyethylene (XLPE) cables causing a reduced lifetime. Several factors affect the initiation and growth of water trees. Without the presence of an electrical field, contaminants or moisture above a certain level, no water trees are observed. In addition, also e.g. mechanical load can influence water treeing. In this paper the effect of dynamic mechanical load on water treeing is studied. A novel test rig was designed for applying simultaneously dynamic mechanical and electrical stress on medium voltage cables in wet environment. The cables are dynamically bent to a given radius to induce a maximum strain of 1 % in the insulation. After ageing the cable insulation was examined for water trees. The water treeing is compared with a reference subjected to a static mechanical load. It is found that mainly small bow-tie trees grow in the insulation even after 0.8 million mechanical cycles. Significantly higher densities of the trees are found at the location with highest dynamic mechanical strain compared to the static reference cable.

Measurements and modeling of water diffusion in water blocking tapes for high voltage extruded cables

The main objective of this paper is to characterise and numerically model the sorption of water vapour in three commercially available water blocking tapes for medium and high voltage extruded cables. The main purpose of these tapes is to avoid longitudinal water penetration. However, the swelling tapes can also delay radial water ingress in the cable, thus delaying the formation of water trees and extending the life time of the cable. Sorption measurements have been perfomed in temperature and humidity controlled climate chambers using ultra-microbalances at 22-80%RH and 30degC. The results show that the tapes have high sorption capability and that the equlibrium water vapour content do not follow Henrys law. This is particularly clear at high water vapour pressures. The average diffusion coefficients increase by decades with increasing water vapour pressure indicating a strong dependence on water concentration.

Axial water ingress in watertight MV XLPE cable designs

When the outer sheath of a polymeric cable becomes severely damaged, humidity can enter the insulation system potentially increasing the relative humidity above a critical value (70% RH) making initiation and growth of water trees possible. The main purpose of this work is to determine how fast water vapour will diffuse axially in the cable. Sensitive relative humidity and temperature sensors were placed within the outer sheath at different axial positions. After drying (evacuation) a hole was cut at the cable end facilitating water ingress. Numerical calculations of axial water diffusion were performed using Comsol. The results so far show that the axial water vapour diffusion in the cable is slow and dependent of the air gap close to the swelling tape. After 130 days the humidity at 0.5 m had increased by about 40%, and the sensor at 1 m had increased by 20%. Numerical calculations of the water diffusion in the same section show a slower increase. The actual axial liquid water penetration is yet not determined. The numerical calculations show that this is an important factor, as the calculations are in more agreement with measurements when adjusting the position of the water front.