Insulators Used in Overhead Transmission Lines
In overhead transmission lines, insulators support the conductor on metallic tower structures. At the same time, they electrically isolate the high-voltage conductor from the metallic body of the supporting towers.
In high-voltage overhead transmission lines, engineers typically install insulator strings at the tip of the cross arms of towers. According to the design, they position the strings either horizontally under tension or suspend them vertically to hang the conductor. In both configurations, the insulators effectively block electrical conductivity between the tower structure and the high-voltage conductor.
Insulator Materials
Manufacturers use two types of materials as insulating materials:
- Glazed Porcelain
- Toughened Glass
Nowadays, manufacturers commonly use either glazed porcelain or toughened glass to produce disc insulators for overhead transmission lines. Specifically, glazed porcelain consists of approximately 20% silica, 80% feldspar, and 50% special clay. Importantly, the porcelain body of the disc must be free from internal voids. Otherwise, any void present can reduce the dielectric strength and consequently lead to partial discharge within the insulator.
Porcelain has a dielectric strength of 3 to 6 kV per millimeter of thickness. Toughened glass has approximately double the dielectric strength compared to porcelain.
Nowadays, many manufacturers prefer toughened glass over glazed porcelain for producing disc insulators. Toughened glass offers several advantages. Defects such as air bubbles, voids, cracks, or opaque foreign materials are clearly visible within the glass, making inspection and defect detection easier.
However, toughened glass also has a disadvantage. It tends to hold condensed moisture more easily than polished glazed porcelain disc insulators.
Types of Insulators Used in Overhead Lines
There are mainly three types of insulators, engineers use in overhead lines.
- Pin-type Insulator
- Suspension-type Insulator
- Strain-type Insulator
Pin-type Insulator
A pin-type insulator typically consists of a single or multiple porcelain shells with the necessary number of rain sheds. In addition, the metal spindle of the insulator holds the porcelain body securely. To ensure strong bonding, manufacturers specially treat the interface between the spindle and the porcelain. Finally, the spindle is usually bolted vertically onto the cross arm.
The conductor rests on top of the pin-type insulator. The groove on the upper portion of the insulator holds the conductor securely. Soft, annealed aluminum is typically used to tie the conductor to the top of the pin-type insulator.
Multiple shells with rain sheds provide the required creepage distance. It is common practice to make the diameter of the uppermost rain shed larger. Because this design helps the upper shed act like an umbrella during rainfall, keeping the lower portion of the insulator dry and maintaining insulation.
Utilities usually avoid using pin-type design for voltages above 33 kV. Most of them prefer the pin-type variant for 11 kV overhead lines. Using pin-type insulators beyond 33 kV is not economical. For higher voltages, engineers commonly use disc insulators because they offer better performance and cost-effectiveness.