TCI-China composite insulator, porcelain insulator, power fitting, cable and accessory

Moisture Absorption Properties of Unidirectional Glass/Polymer Composites Used in Composite (Non-Ceramic) Insulators

L. Kumosa, B. Benedikt, D. Armentrout and M. Kumosa

Center for Advanced Materials and Structures, Department of Engineering, University of Denver 2390 S. York St., Denver, CO 80208

Abstract:

Moisture absorption tests were performed on unidirectional glass/polymer composite materials used in high voltage composite (non-ceramic)insulators. The composites were based on E-glass, high seed count ECR-glass and low seed count ECR-glass fibers with modified polyester, epoxy and vinyl ester resins. The composites were exposed to a moist environment with a relative humidity of 80% at 50°C, and their resistance to moisture absorption in terms of the rate of moisture absorption, maximum moisture content and apparent diffusivities was determined. Moisture absorption in unidirectional composite specimens was also studied numerically in this work using finite element techniques. In particular, the effects of specimen geometry (the edge effect) and specimen orthotropy on moisture absorption by the composites were evaluated. Significant differences in the moisture absorption properties of the composites were found indicating that the effect of moisture on the mechanical and electrical properties of the insulators will depend on the type of composite used. It was shown that the vinyl ester based materials had the most favorable moisture diffusion properties for use in high voltage composite insulator applications. The modified polyester based composites exhibited the worst moisture absorption performance. The epoxy-based materials had adequate rates of absorption; however these materials did not reach equilibrium and kept slowly taking on more moisture in a non-Fickian manner.

1. Introduction

Unidirectional glass reinforced polymer (GRP) composites have become a popular alternative to porcelain in the manufacture of high voltage insulators (see Figure 1) [1-23]. Their mechanical and electrical properties are of great importance, and both these factors will greatly depend on the presence of moisture in the composites. Under certain circumstances the GRP rods of composite insulators can be exposed to moisture ingress. This can happen if the end-fittings of the insulators are not protected against moisture [5, 8-10, 18, 19]. Water can also penetrate the rubber-housing layer if the thickness of the rubber is insufficient, especially if the rubber is damaged by erosion (cracks or abrasion) [8, 19]. The GRP rods of the insulators can also be damaged by gunshots [11] which seem to be a commonly occurring in-service phenomenon. Water inside can become nitric acid [10, 13] causing brittle fracture [1-10, 12-20]. However, if the acid concentration is not high, brittle fracture will not occur [10]. Instead, the moisture will propagate along the GRP rod leading to electrical failures [8, 10].

The brittle fracture process, which is actually stress corrosion cracking (SCC) of the rod composites based on E-glass fibers [5, 7-10, 12, 17, 19, 20, 24-30], can occur in the insulators and the initiation of SCC is strongly controlled by the water/acid diffusion through a polymer matrix [27-30]. It has been shown that brittle fracture of composite insulators can be prevented by the proper design of the composite rod materials [18, 20, 23, 27-30]. However, to prevent SCC of the composites used in the insulators and thus brittle fracture, the water/acid absorption properties of the composites must be well understood.

It has been shown that the electrical properties of the GRP rods depend very strongly on the amount of moisture absorbed [20,31,32]. In order to evaluate the effect of absorbed moisture on the electrical properties of composites used in composite insulators, the moisture absorption properties of the composites need to be first determined. Then,

absorbed moisture vs. leakage current relations can be established [32].

There are several experimental and numerical procedures which can be used to evaluate the resistance of composite materials to moisture absorption [33-48]. In this study, the moisture absorption properties of the composites commonly used in composite (nonceramic) insulators were investigated following well established experimental and numerical techniques. The composites were then compared for their resistance to

moisture and a recommendation was provided on the most suitable glass/polymer composite system for composite insulator applications.

Note: (Since this thesis is too long and we can't list them all in our web page. If you need the whole paper, please contact us in E-mail and we will send you the whole one.)