The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. In this article, the second part of a three-article series, face and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be used to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.@en

Nanocomposites have been known in the field of electrical engineering for more than a decade, albeit there are still uncertainties of how the filler properties influence the properties of the composite material. Filler size, shape, aspect ratio and the surface modification, which consequently leads to even dispersion or clusters of particles, are all suspected to influence the dielectric behaviour of the insulation. But it is disputed what influences the properties to what extent. The focus of this paper is the dependence of the short term breakdown (BD) strength of polymer based composites filled with boron nitride (BN) on the filler size. Base polymer is commercially available bisphenol A epoxy with anhydrite hardener. As filler material we use boron nitride powder with different average particle sizes. These are ranging from 70 to 5000 nm. The particles were not surface modified in order to see solely the influence of the filler size on the BD strength. The short term BD strength was measured for negative DC voltages with Rogowski shaped electrodes. Dielectric spectroscopy was done complementarily to see how the filler size influences the relative permittivity of the composites.@en

This paper deals with the analysis of the effect of temperature gradient applied to polymeric cables on space charge accumulation inside insulation bulk. In particular, experimental measurements on HVDC cable models showed that space charge due to temperature gradient is generally negligible with respect to excess charge injected from the electrodes at electrical fields larger than the threshold for space charge accumulation. The effect of temperature gradient can be revealed only at fields close to the threshold, i.e. 5 to 10 kV/mm, leading, however, to a small extent of charge accumulation, of the order of tens of mC/m3.@en

This paper presents a discussion of the concept of threshold for trapped space charge accumulation in solid dielectrics submitted to a DC field. The starting point is the fact that it is often possible to define a critical field separating an ohmic type of conduction from a nonlinear regime in the current-voltage characteristic of solid dielectrics. In the space charge limited conduction theory, this critical field corresponds to the onset of space charge accumulation. However, other conduction processes, such as hopping conduction for example, can also explain nonlinearity in the current-voltage characteristic, which does not involve space charge. It is proposed to check for the existence of a critical field for space charge accumulation using complementary techniques, i.e., space charge detection and electroluminescence techniques. Polyethylene, polyester and polycarbonate were investigated as being representative of three different families of polymers. It is shown that similar values of thresholds are found for a given material using the three above-mentioned techniques, lending support to the physical explanation of a threshold for trapped space charge accumulation.@en

The European Commission's Energy Green Paper 2006 identifies the need to develop a flexible single European grid as one of the most important priorities of the European Union energy policy for the next few years. This fits a background of increased renewables generation and an increasing mix of generation and transmission technologies. In this paper we will present a scheme that addresses an approach of increasing the trading capacity, controllability, stability and reliability of such a future power transmission network. Further, the scheme seeks to achieve this with reduced life cycle costs and enhanced environmental benefits.@en