High-voltage transmission line porcelain cross arm insulator

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 cross arm insulators are a conical or cylindrical porcelain insulator structure that serves both as a cross arm and an insulator. In addition to the common functions of fixing conductors and providing ground insulation like regular line insulators, they can also fully or partially replace iron or wooden cross arms. This type of insulator was developed in our country to save cross arm materials (metal, concrete, and wood) and meet the needs of rural electricity line construction. It can be installed horizontally or vertically, thus, compared to disk-shaped or long rod-shaped insulators, it can also reduce the height of the pole and simplify the pole structure.

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

The porcelain-style structure refers to a solid porcelain piece with an installation hole at the root. During use, the installation hole is passed through with a screw and secured with a clamping plate. A gasket (such as asbestos cloth) should be placed between the porcelain and iron pieces during clamping, and a spring washer should be used to tighten the clamping plate. However, due to its low strength and poor reliability, this structure has been phased out.

The胶装式 structure is composed of solid porcelain components bonded with adhesive and equipped with mounting holes for screw installation. Medium-voltage and higher voltage porcelain crossarms commonly used in applications are fitted with metal accessories (flanges). To mitigate the impact force generated when the line breaks, the flanges also feature stabilizing screw holes. When the load exceeds the strength of the stabilizing screws, the screws shear off, allowing the porcelain crossarm to rotate around the mounting hole, thereby increasing the wire sag and reducing wire tension, preventing insulator breakage or pole collapse. During normal operation, the stabilizing screws can overcome the tension difference between the two sides of the conductors. The pure porcelain base does not have stabilizing screws; instead, it relies on friction at the mounting point to overcome the tension difference between the two conductors, and can also rotate around the mounting hole in the event of a break. An elastic pad (usually oil felt paper) is placed between the flange and porcelain end faces of the胶装式 structure to reduce thermal stress. The flanges are generally cast from malleable cast iron or焊接 using steel plates, with all metal accessories surface hot-dipped galvanized. The adhesive is prepared from not less 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 horizontally installed, the conductors are tied with thin metal wires at the side slots of the porcelain head, and in this case, the top slot does not need to be created. If the insulator is vertically installed, the conductors are tied on the top slot of the porcelain. Therefore, if the insulator with a vertical installation is used during operation, the manufacturer needs to produce a portion of the specifications with top slots. Another wire fixation style involves the porcelain head having a connecting fitting to securely clamp the conductors.

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 crossarm insulators are compared with those of ordinary line insulators of the same voltage grade, featuring:

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

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

Ceramic crossarm overhead distribution lines, compared to those mounted with ordinary insulators, offer the following primary advantages:

(1) The rotatable structure is adopted, allowing the porcelain cross arm to rotate when the wire is broken, effectively mitigating the expansion of the broken wire incident.

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

(3) The porcelain body is susceptible to washing away by rain and wind, exhibits good self-cleaning properties, and is not prone to cracking, resulting in minimal replacement and maintenance requirements.

(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%.