Water trees

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R. Vogelsang, R. Brütsch, T. Farr, K. Fröhlich: „Electrical tree propagation along barrier-interfaces in epoxy resin“

Electrical tree propagation along barrier-interfaces in epoxy resin R. Vogelsang1 (Student member IEEE), R. Brütsch2, T. Farr1 and K. Fröhlich1 (Fellow IEEE) 1 High Voltage Laboratory at the Swiss Federal Institute of Technology Zürich, Switzerland 2 Von Roll Isola AG Breitenbach, Switzerland

Abstract: The propagation of electrical trees along interfaces in epoxy resin is described. The electrical trees grew in needle-plane samples without and with internal barriers up to the final breakdown. The barrier materials glass, mica and PTFE had been chosen because they have significantly different bond mechanisms to the epoxy resin as well as being common materials in various composite high voltage insulation systems. The mechanical strength of the systems barrier material – epoxy resin was determined for the materials tested. The results show that the propagation of the tree along the barrier is dependent on the type of chemical bond and the shear strength between the epoxy resin and the barrier material. The higher the bond strength between the barrier and the epoxy the higher the resistance to electrical tree propagation. Furthermore it appears that the wettability of the barrier materials plays a minor role in tree propagation along them. The results give a better understanding of the failure mechanism of certain composite insulating materials exposed to high voltage.

Introduction Composite insulation systems are widely used in high voltage equipment. Typical composites are mica-epoxy systems of high voltage rotating machines or glass fibre reinforced materials. During their service the composites may be exposed to partial discharges and electrical treeing for long periods. The electrical treeing, a process in which fine erosion channels propagate through the material, is often referred to as the most important degradation mechanism in solid polymeric insulation [1–3]. Much work has been done over the years to investigate tree inception and tree growth in homogeneous materials. A comprehensive collection is that of Dissado [1]. In samples with barriers, Varlow et al. [4] and Cooper [5] have pointed out the importance of mechanical factors on the propagation of electrical treeing. Sweeney et al. [6] and Farr et al. [7] have mainly concentrated on the simulation of treeing in samples without and with barriers. To the authors it seemed to be interesting to investigate the electrical tree propagation at the interface by concentrating on the chemical bonds between the barrier and epoxy resin and their strength.

This approach is expanded by the investigations of Auckland et al. [8] who found that adhesion mechanisms may play an important role in tree growth at barriers. In these investigations, the electrical tree could travel along as well as through the barriers. In contrast, the present authors ensured that the electrical tree had to propagate along the barriers and could not penetrate them. The electrical breakdown values are compared to the externally measurable values of the shear strength at the interface and the wettability of the barrier material.

Bond mechanisms between the different barrier materials and epoxy resin When two materials are brought very closely together the binding forces of the atoms are responsible for the interfacial strength [9]. Between the barrier materials and the epoxy resin different types of bonds can occur. Each type of bond has different binding energies, which determine the strength of the interface between the barrier material and the epoxy resin. The strength of the interface is composed of the strengths of the different bond types. To investigate the influence of interface properties on tree growth, barrier materials with a significant difference in the bond strength to the epoxy resin have been chosen. Their bonds can be classified as follows: Glass – epoxy resin: The bonds between the glass and the epoxy are most likely determined by strong covalent or hydrogen bonds between the OH-groups of the glass and the amine-groups of the epoxy resin. In addition, van der Waals forces between the glass and the epoxy resin can also occur but are generally considered to contribute less to the adhesive strength than covalent or hydrogen bonds. Mica – epoxy resin: Since there are far fewer OHgroups at the surface of the mica compared to glass, there are also fewer covalent or hydrogen bonds between them and the amine-groups of the epoxy resin. In addition van der Waals forces occur between the mica and the epoxy resin. PTFE – epoxy resin: The bonds between the polytetrafluorethylene (PTFE) and the epoxy resin are assumed to be determined by van der Waals forces only.

Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2002, Cancun, Mexico

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