Wireway porcelain insulators

High-voltage transmission line porcelain cross arm insulators

I. Overview

Used for overhead power transmission lines with insulation and supporting conductors in three-phase power systems with rated voltage of 35kV or less, frequency not exceeding 100Hz, and altitude not exceeding 1000m. Insulator installation site temperature ranges from -40℃ to +40℃.

II. Structural Features

Porcelain crossarm insulators are a conical or cylindrical porcelain insulator structure that serves both as a crossarm and an insulator. In addition to the functions of fixing conductors and providing ground insulation, like conventional line insulators, they can also partially or fully replace iron or wooden crossarms. Originally developed in China to save materials for crossarms (metals, cement, and wood) and to meet the needs of rural electrical line construction, these insulators can be installed horizontally or vertically. Compared to disk-shaped or long rod-shaped insulators, they can also reduce the height of the tower and simplify its structure. Since a porcelain crossarm line was put into operation in 1963, this type of line has seen rapid development over the decades. Currently, 110kV and below lines, especially 6～35kV lines, have been widely adopted and promoted.

In the initial development stage, there are two types: porcelain-style and glued-style.

A porcelain-type structure refers to a solid porcelain piece with an installation hole at the root. During use, the installation hole is threaded through with a screw and secured with a clamp. A gasket (such as asbestos cloth) should be placed between the porcelain and iron parts when clamping. The clamp should be tightened with a spring washer. However, due to its low strength and poor operational reliability, this structure has been phased out.

The structure is assembled with solid porcelain pieces bonded by adhesive, featuring mounting holes on the flange for screwing in with bolts. Porcelain cross arms used with medium voltage and higher voltage levels commonly include metal accessories (flanges). To mitigate the impact force when the line breaks, the flange also has stabilizing screw holes; when the load exceeds the strength of the stabilizing screws, they break, allowing the porcelain cross arm to rotate around the mounting hole, thereby increasing the wire sag and reducing tension, preventing the breakage of insulators or pole collapses. During normal operation, the stabilizing screws can overcome the tension difference between the two wires. Pure porcelain roots do not have stabilizing screws; instead, friction at the mounting site overcomes the tension difference between the wires, and can also rotate around the mounting hole during a break. An elastic pad (usually oil felt paper) is placed between the flanged structure and porcelain face to reduce thermal stress. The flange is typically cast from malleable cast iron or welded from steel plate, and all metal accessories are hot-dipped galvanized. The adhesive is made from a mixture of not less than 42.5-grade silicate cement and quartz sand.

The porcelain crossarm insulators are available in two types: horizontal and vertical installations. The methods of binding the conductors include direct binding and wire clip fixation. If the insulator is installed horizontally, the conductors are tied with fine metal wire to the side slot of the porcelain head; in this case, the top slot on the insulator does not need to be formed. If the insulator is installed vertically, the conductors are tied to the top slot of the porcelain. Therefore, if the porcelain crossarm insulators are installed vertically during operation, the manufacturer must produce a portion of the insulators with a top slot. Another method of securing the conductors is by using porcelain heads with connecting hardware, which are used to clamp the conductors.

Our porcelain crossarm insulators are actually a type of line column insulator that can be rotated during accidents.

The electrical and mechanical properties of ceramic crossarm insulators are compared with those of ordinary line insulators at the same voltage level, and their characteristics are:

(1) The insulation distance and creepage distance of ceramic cross arms are relatively large, with higher 50% full wave impulse flashover voltage and dry/wet power frequency flashover voltage.

(2) The porcelain crossarm has a longer porcelain body with lower bending strength. After considering the safety and reliability factor, the allowable heavy load is generally smaller than that of ordinary insulators at the same voltage level. Therefore, porcelain crossarms are not suitable for lines with larger conductor cross-sections and spans.

Compared to distribution lines installed using ordinary insulator brackets, the main advantages of porcelain crossarms for distribution lines are:

(1) Featuring a rotatable structure, when the line is broken, the unbalanced tension of the wire causes the porcelain crossarm to rotate, effectively mitigating the expansion of the broken wire incident.

(2) The insulation level and lightning resistance of the lines are high, with a low accident rate.

(3) The porcelain body is resistant to washing by rain and wind, has excellent self-cleaning properties, and is not prone to cracking. There is minimal need for replacement or maintenance.

(4) Easy to install and construct; can increase the distance of the conductor from the ground by approximately 0.3 to 2.2 meters at the same pole height.

(5) Save on raw materials such as steel and wood, and reduce the cost of single-pole towers by 10% to 50%.