High-voltage transmission line porcelain cross arm insulators

I. Overview

Used for overhead power transmission lines with insulation and support conductors in three-phase power systems with nominal voltage not exceeding 35kV, frequency not exceeding 100Hz, and altitude not exceeding 1000m. Insulator installation location has an environmental temperature range of -40℃ to +40℃.

II. Structural Features

Ceramic crossarm insulators are a conical or cylindrical porcelain insulator structure that serves both as a crossarm and an insulator. In addition to the common functions of fixing conductors and providing insulation from the ground like standard line insulators, they can also fully or partially replace iron or wooden crossarms. Initially developed in our country to save crossarm materials (metal, concrete, and wood) and to meet the needs of rural electrical line construction, these insulators can be installed horizontally or vertically, thus reducing the height of the pole and simplifying the pole structure compared to disk-shaped or long rod-shaped insulators. Since the operation of a porcelain crossarm line began in 1963, this type of line has developed rapidly over the past few decades. Currently, 110kV and below lines, especially 6～35kV lines, have been widely promoted and adopted.

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

A pure porcelain structure is a solid porcelain piece with an installation hole at the root. During use, the installation hole is passed through a screw and secured with a clamp. 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 clamp. However, this structure has been phased out due to its low strength and poor reliability.

The structure is assembled with solid ceramic pieces and bonded with adhesive, featuring mounting holes on the flanges for screw installation. Medium voltage and above voltage等级 porcelain crossarms commonly have metallic accessories (flanges). To mitigate the impact force during line breaks, the flanges also have stabilization screw holes. When the load exceeds the strength of the stabilization screws, the screws break, allowing the porcelain crossarm to rotate around the mounting hole, thus increasing the wire slack and reducing tension, preventing insulator breakage or pole collapse. During normal operation, the stabilization screws can overcome the tension difference between the two sides of the conductors. The pure ceramic base has no stabilization screws, relying on frictional force at the mounting location to counterbalance the tension difference on both sides of the conductors, and can also rotate around the mounting hole during breaks. An elastic pad (usually oil paper) is placed between the flange and ceramic end face of the adhesive assembly structure to reduce thermal stress. The flanges are generally cast from malleable cast iron or welded from steel plates, with all metallic accessories surface hot-dipped galvanized. The adhesive is made from cement not less than 42.5 grade silicate and quartz sand.

The porcelain crossarm insulators are available 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 wire is tied with a thin metal wire at 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 vertically installed, the wire is tied to 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 a top slot. Another wire fixation style involves the porcelain head being equipped with a connecting fitting 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 crossarm insulators are compared with those of ordinary line insulators at the same voltage level, featuring:

(1) The insulating distance and creepage distance of ceramic crossarms 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 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 mounted using ordinary insulator supports, the main advantages of porcelain cross arms for distribution lines are:

(1) Utilizing a rotatable structure, when the line is disconnected, the unbalanced tension of the conductors 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 susceptible to washing away by rain and wind, has good self-cleaning properties, and is not prone to puncturing; therefore, the amount of replacement and maintenance is minimal.

(4) Easy to install and construct; at the same pole height, it can increase the distance from the wire to the ground 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%.