Ceramic crossarm insulator

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 has an environmental temperature range of -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. Originally developed in our country to save materials for cross arms (metal, cement, and wood) and to meet the needs of rural electricity line construction, these insulators can be installed horizontally or vertically, thereby reducing the height of the towers and simplifying their structure compared to disk-shaped or long rod-shaped insulators. Since a porcelain cross arm line was put into operation in 1963, this type of line has developed rapidly over the past few decades. Currently, it is widely used in 110kV and below lines, especially in 6-35kV lines.

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

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

The structure is bonded with solid ceramic components and flanges using adhesive, featuring mounting holes for screwing through bolts. Most ceramic crossarms used with medium voltage and above voltage levels are equipped with metal accessories (flanges). To cushion the impact force generated when the line is broken, there are also stabilizing screw holes on the flange. When the load exceeds the strength of the stabilizing screws, the screws break, causing the ceramic crossarm to rotate around the mounting hole, thereby increasing the wire sag and reducing the wire tension, preventing insulator breakage or pole failure. During normal operation, the stabilizing screws can overcome the tension difference between the two wires under normal conditions. The pure ceramic root has no stabilizing screws; in this case, the friction force at the installation location is relied upon to counterbalance the tension difference between the two wires, and it can also rotate around the mounting hole during a break. An elastic pad (usually oil felt paper) is placed between the bonded flange and ceramic end face to reduce thermal stress. The flange is generally cast from malleable cast iron or can be焊接 from steel plates, with all metal accessories fully galvanized. The adhesive is prepared using not less than No. 42.5 silicate cement and quartz sand.

The porcelain cross arm insulators are available in two types: horizontal and vertical installations. The methods for securing the conductors include direct binding and wire clip fixation. If the insulators are installed horizontally, the conductors are tied with fine metal wire to the side slots of the porcelain head, and in this case, the top slot does not need to be made. If the insulators are installed vertically, the conductors are tied to the top slot of the porcelain. Therefore, if the insulators with vertical installation are used during operation, the manufacturer needs to produce a portion of these insulators with top slots. Another method for securing the conductors is by using porcelain heads with connecting fittings to 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, and their characteristics are:

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

(2) The porcelain crossarm has a longer porcelain body with lower bending strength. After considering the safety 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 suspended using ordinary insulator brackets, the main advantages of porcelain crossarm distribution lines are:

(1) Utilizing a rotatable structure, in the event of a wire break, the unbalanced tension in the conductors causes the porcelain crossarm to rotate, thereby effectively mitigating the expansion of the wire break incident.

(2) High insulation and lightning resistance levels with low accident rates.

(3) The porcelain body is easily washed by rain and wind, has excellent self-cleaning properties, and is not prone to cracking; replacement and maintenance requirements are minimal.

(4) Easy for construction and installation; at the same tower height, it can increase the wire-to-ground distance by approximately 0.3 to 2.2 meters.

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