Composite insulator materials

Composite insulator materials include the core material, metal end fittings materials and housing materials. Each material is to provide different performance and properties.

1. Core: The internal core of a non-ceramic composite insulator consists of a fiberglass reinforced plastic (FRP) rod. It is the primary mechanical load-bearing component of the insulator. The strength of the rod depends on the types of materials used, the diameter, and the percentage composition of glass fiber and impregnating resin. Generally, almost all manufacturers use E-glass (electrical) fibers and epoxy resin. ECR-glass (electrical chemical resistant) fiber is generally no longer used in the manufacturing process, but acid-resistant fiber is becoming available upon request. Acid resistant fiber may protect against brittle fracture caused by stress corrosion of the rod when the rod is exposed to moisture and corona discharge. Polyester and vinylester resins have been used but are uncommon.

During fabrication, the individual glass fibers are equally tensioned to axially align each fiber parallel to the rod axis. The fibers are placed in a resin matrix such that each glass fiber is impregnated with the resin. The combination is then cured at a high temperature, allowing the individual fibers to bond to the resin matrix. This manufacturing technology is known as pultrusion. Generally, 60 to 70 percent of the FRP rod volume contains glass fibers.

2. Metal End Fittings: The strength of a composite insulator not only depends on rod materials and rod diameter, but also on the metal end fittings material type and the bonding procedures used to attach the metal end fittings to the FRP rod. Metal end fittings should be made from a good commercial grade of iron or steel and should be galvanized in accordance with ASTM A153, “Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware”. Metal end fittings may also be manufactured from a good grade of stainless steel. Aluminum is not recommended.

Metal end fittings should be designed to effectively transmit the mechanical loads to the insulator core (FRP rod). The attachment of the metal end fittings to the FRP rod is important and should be performed with a symmetrically controlled crimping method that compresses the metal radially onto the rod without damage to the rod fibers or resin matrix. Excessive compression may crush the rod and cause de-bonding between the rod and the end fittings, which may reduce the mechanical strength of the insulator. Insufficient compression may cause slippage between the rod and the end fittings. It is critical that the manufacturer provide the utility with documentation that details quality control procedures and crimping machine control mechanisms that prevent FRP rod damage and metal end fittings slippage.

Metal end fittings should be properly sealed to prevent moisture contact with the FRP rod. This seal should never be broken during testing or otherwise.

ANSI C29.11, “Composite Suspension Insulators for Overhead Transmission Lines - Tests” defines the following testing procedures for composite suspension insulators to confirm the integrity of the metal end fittings interface:
*Prototype Tests:

Power Frequency Voltage Test

Sudden Load Release Test

Thermal Mechanical Test

Water Penetration Test

Steep Front Impulse Test

*Sample Test:

Mechanical Load Test

Galvanizing Test

*Routine Tests: Tension – Proof Test

The tensile strength of metal end fittings attached to the FRP rod should equal or exceed the specified mechanical load (SML) rating defined for the composite suspension insulator. Similar testing procedures to confirm the integrity of the metal end fittings interface for composite line post insulators are specified in ANSI C29.17, “Insulators - Composite - Line Post Type”.

3. Housing: The housing is composed of a sheath and weathersheds, both are made of a polymeric compound. The sheath covers the fiberglass core to protect it from the external environment while the weathersheds provide the required leakage distance and electrical insulation strength.

It is important when considering housing materials to determine which type of material is best suited for the insulator application. Housing materials are generally made from EPR or SR. An alloy material of EPDM and silicone additive is also available. The weathersheds and sheath may be bonded together during the vulcanization process or molded into one piece by injection molding. The sheath is generally bonded to the rod, but a void-free silicone grease interface is also available. The housing may be either bonded to the metal end fittings or sealed to prevent moisture ingress to the FRP rod.

Experience has shown that the performance of different formulations varies for a given environment. Therefore, the utility should obtain written verification from the manufacturer that the insulator housing material considered will provide satisfactory performance in the particular environment to which the insulator is to be subjected by the purchaser. SR housing materials have exhibited the best short-term flashover performance and are generally recommended in areas with heavy contamination.

As one of composite insulator manufacturers in China, our Composite insulator materials include housing –silicone rubber, core-FRP and end fittings-galvanized steel.