High-voltage transmission line porcelain cross arm insulator

I. Overview

For insulation and supporting conductors in high-voltage overhead power lines with rated voltage of 35kV or below, frequency not exceeding 100Hz, and altitude not exceeding 1000m in three-phase power systems. Insulator installation location has an environmental temperature of -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 insulation from the ground, like common line insulators, they can also partially or fully replace iron or wooden crossarms. Originally developed in our country to save crossarm materials (metal, concrete, and wood) and 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 pole and simplify the pole structure.

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

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

The structure is assembled by solid ceramic pieces bonded with adhesive, equipped with mounting holes on the flange for screw installation. Medium-voltage and higher voltage porcelain crossarms commonly used feature metal attachments (flanges). To mitigate the impact force during line breaks, the flange also has stabilizing screw holes; when the load exceeds the strength of the stabilizing screws, the screws break, causing the porcelain crossarm to rotate around the mounting hole, increasing the wire sag and reducing tension, thus preventing insulator breakage or pole collapse. During normal operation, the stabilizing screws can overcome the tension difference between the two wires. The pure ceramic root lacks stabilizing screws; in this case, the friction at the mounting location overcomes the tension difference between the two wires, and the crossarm can also rotate around the mounting hole during a break. An elastic pad (usually oil felt paper) is placed between the flange and ceramic end face of the adhesive assembly structure to reduce thermal stress. The flange is typically cast from malleable cast iron or焊接 from steel plates, and all metal attachments are fully hot-dipped galvanized. The adhesive is prepared from cement not lower than 42.5 grade silicate cement and quartz sand.

The porcelain crossarm insulators come in two types: horizontal and vertical installations. The wire binding styles include direct binding and wire clip fixation. If the insulator is installed horizontally, the wire is tied with a fine metal wire to the side slot of the porcelain piece's head; in this case, the top slot on the insulator does not need to be formed. If the insulator is installed vertically, the wire is tied to the top slot of the porcelain piece. Therefore, if the porcelain crossarm insulator with the top phase is installed vertically during operation, the manufacturer must produce a portion of these insulators with a top slot. Another wire fixation style involves the porcelain piece's head having a connecting fitting, which is used to clamp the wire.

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

The electrical and mechanical properties of porcelain cross arm insulators are compared with those of ordinary line insulators at the same voltage level, characterized by:

(1) The insulating distance and creepage distance of porcelain crossarms are relatively large, with a higher full-wave impulse flashover voltage at 50% and higher dry and wet power frequency flashover voltages.

(2) The porcelain crossarm has a longer porcelain body with lower bending mechanical 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 crossarm distribution lines are:

(1) Utilizing a rotatable structure, when the line is broken, the unbalanced tension of the conductors causes the porcelain cross arm 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 ceramic body is susceptible to washing away by wind and rain, has good self-cleaning properties, and is not prone to puncturing; replacement and maintenance quantities are minimal.

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

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