1. Porcelain / Glass Disc-Type Suspension Insulators

1.1 External shape (shed profile)

For insulators used on AC lines, porcelain and glass disc-type suspension insulators can be classified by shed profile into the standard type (also called ordinary type), the bell-shaped type, and the outer-shed type. The outer-shed type can be further subdivided into double-shed, triple-shed, and aerodynamic shed profiles.

The standard (ordinary) type usually has shallow ribs/grooves under the shed; these ribs increase the creepage distance, thereby raising the flashover voltage. The bell-shaped insulator (bell profile) has deeper skirt depth at the outer rim and often has a central groove whose depth exceeds that of the outer skirt. Outer-shed type insulators have no ribs under the skirt.

For porcelain and glass disc suspension insulators used on DC lines, the shapes are categorized into bell-shaped (which corresponds to the standard type) and outer-shed type (including double- and triple-shed). In other words, for DC service the “standard” designation essentially refers to the bell-shaped profile — a difference from AC practice. This is because DC insulators require deeper grooves to achieve higher external insulation flashover voltages.

Different shed profiles produce significant differences in pollution accumulation characteristics. Based on operating experience in Zhejiang province for AC straight-line string insulators, outer-shed type insulators exhibit the lightest pollution accumulation; ordinary (standard) type accumulates roughly twice the pollution level of outer-shed type; and bell-shaped insulators accumulate about 1.5 times the pollution of ordinary type. These multipliers are averages derived from large sample sets — actual differences can vary considerably depending on tower location. There is currently insufficient data to quantify how shed profile affects pollution accumulation on DC insulators.

The model/designation rules for porcelain and glass disc-type suspension insulators follow the standard JB/T 9683-2012.

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1.2 Internal structure

The internal structure of an AC porcelain insulator is shown in Figure .

(1 – steel cap; 2 – porcelain body; 3 – steel foot; 4 – locking pin; 5 – elastic pad; 6 – cement-bonding)

The steel cap and steel foot provide mechanical connections between insulators and between insulators and hardware fittings. The locking pin fixes the relative position between the steel foot and steel cap, preventing the insulator from dislodging.

The porcelain body carries the insulator’s electrical insulation function. The portion of the porcelain located inside the metal cap is called the porcelain head; this region has the highest internal electric field intensity and its material and design are critical to the internal insulation performance.

The cement-bonding (cement mortar adhesive) connects the porcelain body and the metal cap and is a key element for the insulator’s mechanical strength and internal insulation reliability.

For AC disc-type glass suspension insulators, the structure is similar to the porcelain insulator shown in Figure, except that the porcelain element is replaced by a glass element.

For DC disc-type suspension insulators, zinc rings are added around the steel foot and steel cap to serve as sacrificial electrodes; these zinc rings protect the steel fittings from electrochemical corrosion. (Earlier DC insulators without zinc rings experienced fitting corrosion during service.)

2. Composite (Polymer) Insulators

2.1 External shape (shed profile)

Composite insulators are easy to mold and are available in many shed-profile geometries. Parameters that describe polymer/silicone rubber shed geometry include shed skirt angle, skirt overhang ratio (protrusion ratio), and the arrangement of large vs. small sheds within one shed module (for example: equal-diameter sheds, large–small combinations, or more complex patterns such as large–small–small–medium–small–small).

Composite insulators have no ribs under the sheds. Different arrangements of large and small sheds have relatively little effect on contamination performance, but they significantly affect anti-icing (de-icing) behavior: if large sheds are too close together the insulator is more susceptible to ice bridging across sheds, which reduces the ice flashover voltage. (See discussion of composite insulator anti-icing performance and shed spacing below.)

2.2 Internal structure

(1) Overall structure

A composite insulator consists of a fiberglass core rod (core), a shed-protective housing (silicone rubber shed housing), and end fittings. The core rod is made of glass-fiber-reinforced plastic (GRP, or fiberglass-reinforced epoxy) and provides the mechanical strength. The housing is bonded to the core rod surface with an adhesive and protects the core from environmental exposure. End fittings are installed at both ends to connect the composite insulator to tower hardware.
(Keyword reference: rod-type composite suspension insulator structure, composite insulator silicone rubber shed profile.)

(2) End fittings and sealing designs

Composite insulators are classified by end-fitting design into wedge-type, bonded-type, and compression-type fittings. Presently, most in-service composite insulators use compression-type end fittings; wedge-type is now rare.

1) Wedge-type fittings and their sealing

Wedge-type fittings are divided into internal-wedge and external-wedge designs; schematic diagrams are shown in Figure.

The internal-wedge design (see right side of Figure (a)) is made by sawing a slot in the core rod, inserting the slotted core into the fitting’s bore, and driving a tapered wedge into the rod slot. The wedge compresses the rod surface against the inner wall of the metal fitting, generating frictional force so the insulator can carry mechanical tensile loads.

The external-wedge design (see left side of Figure (a)) places the core rod into the fitting bore and inserts multiple conical wedges between the rod outer surface and the fitting inner wall. The wedges press radially to create frictional engagement among the fitting, wedges, and core rod so that the insulator can carry tensile load.

Wedge-type sealing structures are shown in Figure 7.

In Figure the seal between core rod and fitting relies on a sealing adhesive (commonly a room-temperature-vulcanizing silicone rubber); the two metal fittings are usually connected by threads. Figure 7(b) adds an O-ring between the core rod and fitting and achieves sealing by the combination of the compressed O-ring and sealing adhesive.

These designs contain multiple sealed interfaces: between core rod and fitting 1 (sleeve), and between fitting 1 (sleeve) and fitting 2 (cap). Multiple interfaces increase the probability of seal failure. Room-temperature-vulcanizing silicone rubber used for sealant is prone to aging and cracking, allowing moisture ingress.

2) Compression-type fittings and their sealing

Compression-type fittings are manufactured by making metal end fittings with a cylindrical bore, sliding the fitting over the core rod end, and using automated crimping/pressing equipment to apply uniform radial pressure to the fitting’s outer circumference. Plastic (metal) deformation of the fitting causes it to form a tight, interference-fit sleeve onto the core rod, enabling the insulator to carry mechanical tensile loads.

Compression-type end fittings used in Zhejiang are shown in Figure.

Figure shows a groove machined in the fitting bore; a sealing ring is pressed between the housing and the groove, and external sealing is performed with room-temperature-vulcanizing silicone rubber. Figure adds a sealing ring on the fitting end into which a sealing O-ring is pressed. Figure adopts an integral injection-molding process in which high-temperature-vulcanized silicone rubber is directly molded/extruded onto the core rod and the fitting; grooves on the fitting surface enhance sealing reliability.

( a ) Room-temperature-vulcanizing silicone rubber + O-ring sealing
( b ) Sealing ring + O-ring sealing

Wedge-type fittings are structurally complex, have multiple sealing interfaces, and driving wedges into the core rod can damage the rod, which undermines long-term mechanical performance. Historically there have been several cases where wedge-type composite insulators experienced seal failure, end-water ingress, and subsequent brittle fracture; therefore wedge-type fittings are no longer produced and only remain in some older in-service insulators.

Compression-type fittings are simple, provide good sealing, and the uniform pressing force during crimping avoids damage to the core rod. Over more than a decade of operation, compression-type composite insulators have shown virtually no problems with end-seal failure.