Anette B. Johansson

A New Application Area for Fullerenes: Voltage Stabilizers for Power Cable Insulation

Fullerenes are shown to be efficient voltage-stabilizers for polyethylene, i.e., additives that increase the dielectric strength of the insulation material. Such compounds are highly sought-after because their use in power-cable insulation may considerably enhance the transmission efficiency of tomorrows power grids. On a molal basis, fullerenes are the most efficient voltage stabilizers reported to date.

Evaluation of the performance of several object types for electrical treeing experiments

A comparison of the efficiency between various test object types for studying the development of electrical trees in polymers is presented. Two types of newly developed wire-plane objects are investigated and compared with two versions of the traditionally used needle-needle object. The two needle-needle objects are prepared according to ASTM standard and act as references, whereas the alternative objects rely on the use of an ultra-thin tungsten wire (10 and 20 μm) for providing the highly divergent electric stress. In one of these object types the wire extends from a semiconducting tab embedded in the tested material whereas the embedded wire is extended and connected externally by means of a copper tape in the other type. The comparison is made using cross linked polyethylene (XLPE) for the testing purposes. The wire type objects provide some promising advantages, including parallel formation of several electrical trees and an exposure of a larger material volume. Further advantages include the simplification of both the manufacturing and the measuring procedures. Among the newly developed test objects, the one without the semiconducting tab is considered most advantageous as it allows for more accurate treeing initiation measurements at a lower voltage level. A disadvantage with the wire electrode is its tendency to form kinks during object preparation, which may lead to difficulties in correctly estimating the field strength locally as well put strain on the surrounding material. However the multitude of electric trees formed in each object allows for trees growing at such imperfections to be discarded in following data analyses. Although finding a statistical method that makes good use of all the data poses a challenge, a suggested approach is presented.

Electrical tree inhibition by voltage stabilizers

Electrical treeing is one of the main degradation mechanisms in polymer dielectrics at highly divergent electric fields and can, if initiated, cause dielectric breakdown as soon as an electrical tree channel has bridged the insulation. One way of increasing resistance to electrical treeing is the addition of certain organic additives such as polycyclic aromatic compounds or benzophenone-like structures. These additives act as voltage stabilizers that are believed to capture high energy electrons and dissipate their energy, preventing degradation of the polymer matrix by the impact of “hot electrons”. In this study, a benzil type compound is evaluated as voltage stabilizer in a superclean cross-linked polyethylene compound used for high voltage cable applications. The stabilizing effect of the additives is measured using test objects of wire-plane electrode geometry in ramped AC voltage experiments. For determining tree initiation conditions, optical detection is used and correlated to voltage measurements. In this way, the initiation voltage of each single tree is determined. Results show that the investigated voltage stabilizer raises the electrical tree inception level significantly. The stabilizing effect is dependent on voltage ramp rate and using a lower ramp rate results in a more pronounced effect of the stabilizer.

High electron affinity: a guiding criterion for voltage stabilizer design

Voltage stabilizers are an emerging class of additives that enhance the dielectric strength of an insulating polymer such as polyethylene. Several partially conflicting reports ascribe the stabilizing effect to either a high electron affinity or low ionization potential of the additive. Here, we report a clear correlation of the electron affinity and to a lesser extent the EHOMO–ELUMO difference of various voltage stabilizers with electrical tree initiation in cross-linked polyethylene. To facilitate a fair evaluation, the voltage-stabilizing efficiency of a set of 13 previously reported voltage stabilizers, which strongly differ in their chemical composition, is compared at equal stabilizer concentration and equivalent test methodology. These results are correlated with the electron affinity and EHOMO–ELUMO difference, as obtained from density functional theory (DFT) modeling, which agreed well with available literature values. Moreover, based on the here established strong correlation between dielectric strength and electron affinity, a new molecule with exceptionally high electron affinity is selected from the extended literature on organic photovoltaics. This malononitrile–benzothiadiazole–triarylamine based molecule with a high electron affinity of 3.4 eV gives rise to a 148% increase in tree initiation field compared to 40% obtained using anthracene, one of the most efficient previously reported voltage-stabilizers, under equivalent test conditions. Thus, we here propose to use the electron affinity as a guiding criterion for identifying novel high-efficiency voltage stabilizers, which opens up the vast library of organic semiconductors as potential candidates, as well as associated synthesis routines for the design of yet unexplored materials.

Effects of inclusions of oxidized particles in XLPE on treeing phenomena

One issue with the use of crosslinked polyethylene (XLPE) insulation in high voltage power cables is the presence of oxidized particles. Oxidation may occur during production of polyethylene, during the extrusion and curing process used to produce XLPE cables, or in subsequent processes/operation of the cables. Oxidized particles are here referred to as organic contaminants. In the present study artificial organic contaminants were introduced in XLPE samples made for electrical treeing measurements. Instead of using the common needle-needle or needle-plane configuration, a wire-plane configuration was used. Organic contaminants have an increasing conductivity and permittivity as function of increasing degree of oxidation. These properties most probably give rise to local electric field enhancements in the material. The morphology of organic contaminants also differs from virgin XLPE, which probably cause bad adhesion between the XLPE matrix and the contaminants. A combination of these factors was found to affect the electrical performance of the insulation. Several different parameters, such as dry and wet ageing, as well as AC voltage and DC voltage stress, were included in the study.