High-voltage transmission line porcelain cross arm insulators
I. Overview
For overhead power transmission lines with insulation and support conductors used in three-phase power systems with rated voltage of 35kV and below, frequency not exceeding 100Hz, and altitude 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 a standard line insulator, 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 development stage, there are two types: porcelain-style and glued-style.
A porcelain sleeve structure is a solid porcelain piece with an installation hole at the base. During use, the installation hole is threaded through with a screw and secured with a clamp. A gasket (such as asbestos cloth) should be placed between the porcelain and metal parts during clamping, and a spring washer should be used to tighten the clamp. However, due to its low strength and poor operational reliability, this structure has been phased out.
The structure is assembled with solid ceramic pieces and bonded with adhesive, featuring mounting holes on the flange for screw installation. Most porcelain crossarms used in medium voltage and above voltage levels are equipped with metal accessories (flanges). To mitigate the impact force during line breakage, the flange also has stabilizing screw holes. When the load exceeds the strength of the stabilizing screws, the screws break, the porcelain crossarm rotates around the mounting hole, increasing the wire sag and reducing tension, thereby preventing insulator breakage or pole collapse. During normal operation, the stabilizing screws can overcome the tension difference between the two wires under normal conditions. The pure porcelain root has no stabilizing screws; instead, it relies on friction at the installation point to counteract the tension difference between the two wires, and 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 face of the adhesive assembly structure to reduce thermal stress. The flange is generally cast from malleable cast iron or welded from steel plate, 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.
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 wire is tied to the side groove of the porcelain head with a thin metal wire; in this case, the top groove does not need to be created. If the insulator is vertically installed, the wire is tied to the top groove of the porcelain. Therefore, if the vertical installation is used for the porcelain crossarm insulators during operation, the manufacturer needs to produce a portion of these insulators with a top groove. Another wire fixation style involves the porcelain head having a connecting fitting, which is used to clamp the wire.
Our porcelain cross arm insulators are actually a type of line pole insulator, which is a rotatable line pole insulator during accidents.
The electrical and mechanical properties of porcelain crossarm insulators are compared with those of ordinary line insulators of the same voltage grade, characterized by:
(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 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.
Ceramic crossarms for power distribution lines, when compared to those installed with standard insulator brackets, offer the following primary advantages:
(1) Featuring a rotatable structure, when the line is broken, the unbalanced tension of the conductors causes the porcelain crossarm to rotate, thereby 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) Porcelain body is easily washed by rain and wind, has 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 from the ground to the conductor 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%.
