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Polymer matrix composites in high voltage transmission line applications

This paper introduces Polymer matrix composites in high voltage transmission line applications background and the polymeric materials are popularly used in high voltage insulators.

Polymer matrix composites (PMC) can be highly desirable materials for various types of electrical and mechanical applications because of their excellent specific properties. However, these properties can be seriously compromised if the composites are subjected to extreme environments. Many extreme environments can be envisioned, including high voltage (HV) transmission line applications.

Polymer matrix composites

Polymer (GRP) composites have been widely used in the designs of composite transmission line insulators (Fig. 2), transmission and distribution towers, and in substation applications. However, it was been shown by Kumosa et al. that the in-service conditions can be especially damaging to the structural integrity of transmission line insulators based on PMC if they are improperly designed.
Composite suspension insulators (also referred to as either non-ceramic, polymer or polymeric insulators) are used worldwide in overhead transmission line applications with line voltages in the range of 69 kV to 735 kV. The first composite insulator was developed in the US by General Electric in the 50s. Then, over the years, the technology has been developed predominantly in Europe and in the US into the second and third generation of insulators supporting, in some cases, the most critical transmission lines in many places of the world. Despite the fact that this technology has been dramatically improved, the insulators have been sporadically failing in service, dropping energized transmission lines and causing line outages at various utilities.
The design of composite suspension insulators is rather straightforward. The insulators rely on unidirectional GRP composite rods as the principle load-bearing component (Fig. 2). The rods, usually 15 mm in diameter, are manufactured by pultrusion and the constituents are either polyester, vinyl ester, or epoxy resins reinforced with either Eglass or Electric Corrosion Resistant (ECR)-glass (also called boron-free E-glass) fibers. The surface of the GRP rod is covered with a rubber housing material with multiple weathersheds. The purpose of the housing is to protect the GRP rods against outside environments (predominantly moisture, pollution and corona discharges).

The primary purpose of the weathersheds is to increase the leakage distance between the energized and ground ends of the insulators and to protect the GRP rod against the outside environment. Today, common housing materials are ethylene-propylene rubbers, different types of silicon rubbers and ethylene vinyl acetate-based elastomers. Other composite insulators such as substation or line post insulators are based on the same design. However, they usually rely on large GRP rods, up to 50 mm in diameter. There are two metal end fittings attached to the GRP rods at both ends of the insulators (Fig. 2). In modern composite insulators the fittings are usually attached to the rod by crimping .
In spite of many benefits, which the insulators can offer in comparison with their porcelain counterparts (high mechanical strength-toweight ratio, improved damage tolerance, flexibility, good impact resistance, and ease of installation), they can fail mechanically in service by rod fracture, and electrically by flashover.

PMCs are also beginning to be used in the next generation HV high temperature (HT) electric conductors. Present overhead electrical conductors are based on either the Aluminum Conductor Steel Reinforced (ACSR) or Aluminum Conductor Steel Supported (ACSS) design. These lines are subject to significant amounts of sag during HT operation. Thus, new designs of HV overhead conductors have been considered for potential applications [13], which could reduce the “sag problem,” and allow more power to be transmitted using the existing structures.
One of the new designs, known as the Aluminum Conducting Composite Core (ACCCTM) conductor, is based upon hybrid PMC core rods with two different reinforcing fibers (Fig. 3). The inner high strength core is based on continuous carbon fibers embedded in a high temperature epoxy. The carbon core is surrounded by a thin sheath of unidirectional dielectric ECR-glass fiber/high temperature epoxy composite. The glass/epoxy layer is incorporated into the design in order to prevent a direct electrical path between the conducting aluminum wires and the conductive carbon fibers, preventing a potential galvanic reaction.
To mitigate stress corrosion cracking of the glass fiber composite portion of the conductor [4], corrosion resistant ECR (boron free) glass fibers are used. The current expectation is that the PMC conductors, especially the ACCC design, should be able to transport, in theory, up to 3 times more electric current at much higher temperatures (up to 180°C) and significantly reduce sagging. These advantages could revolutionize power transmission world-wide.

In this review paper the most important accomplishments from the composite insulator research performed between 1993 and 2007 in our laboratory and its impact on the global transmission line systems and the global economy are described. Then, our current on-going HT PMC conductor research, its’ most important findings thus far, and its’ international importance is presented.

Nowadays, Polymer matrix composites in high voltage transmission line applications are widely in high voltage insulators. Silicone rubber is one of important insulation polymer materials for composite insulators.

 

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