High-voltage transmission line porcelain cross arm insulator
I. Overview
For insulated and supporting conductors in high-voltage overhead power lines for three-phase power systems with rated voltages of 35kV and below, frequencies not exceeding 100Hz, and altitudes not exceeding 1000m. Insulators are installed in environments with temperatures ranging from -40℃ to +40℃.
II. Structural Features
The porcelain crossarm insulator is 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 standard line insulators, it can fully or partially replace iron or wooden crossarms. This insulator was developed in our country to save crossarm materials (metal, cement, and wood) and meet the needs of rural electrical 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 tower and simplify the tower structure. Since a porcelain crossarm line was put into operation 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 adopted.
In the initial stage of development, there are two types: porcelain-style and glue-bound.
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 tightened with a clamping plate. A washer (such as asbestos cloth) should be placed between the porcelain and iron pieces during tightening, and a spring washer should be added to the clamping plate for secure tightening. However, due to its low strength and poor reliability, this structure has been phased out.
The structure is composed of solid ceramic pieces bonded with adhesive, featuring mounting holes on the flange for screw installation. Medium-voltage and higher voltage porcelain crossarms commonly include metal accessories (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, they shear off, allowing the porcelain crossarm to rotate around the mounting hole, thereby increasing the wire sag and reducing tension, 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 has no stabilizing screws; instead, it relies on friction at the mounting location to counteract the tension difference between the two wires, and it can also rotate around the mounting hole during a break. An elastic gasket (usually oil felt paper) is placed between the flange and ceramic end faces to reduce thermal stress. The flange is typically cast from malleable cast iron or焊接 from steel plates, with all metal accessories surface hot-dipped galvanized. The adhesive is made from a mixture of not less than 42.5-grade silicate cement and quartz sand.
Ceramic cross arm insulators come in two types: horizontal and vertical installations. The wire binding styles include direct binding and wire clamp fixation. If the insulator is horizontally installed, the conductors are tied with thin metal wire to the side slots of the ceramic head; in this case, the top slot does not need to be created. If the insulator is vertically installed, the conductors are tied to the top slot of the ceramic head. 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 type of wire fixation is by using a ceramic head with connecting hardware, which is used to clamp the conductors.
Our porcelain cross arm insulators are actually a type of line column insulator, which 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 cross arms are relatively large, with higher 50% full wave impulse flashover voltage and dry/wet AC frequency flashover voltage.
(2) The ceramic crossarm has a longer ceramic 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, ceramic crossarms are not suitable for lines with larger conductor cross-sections and spans.
Ceramic cross arm distribution lines, when compared to distribution lines suspended with ordinary insulator brackets, have the following main advantages:
(1) Utilizes a rotatable structure; in the event of a wire break, the unbalanced tension of the conductors causes the porcelain crossarm to rotate, 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 susceptible to washing away by rain and wind, exhibits good self-cleaning properties, and is not prone to cracking; as a result, 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 conductor 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 a single-pole tower by 10% to 50%.
