This article describes different types of power line insulators used in overhead transmission lines. In an electrical power system, the overhead power lines carry electric power to consumers through the conductors supported on poles or towers.
In a transmission line, a line insulator is a crucial component used to prevent leakage from line conductors to the pole support and, hence, to the ground. In overhead transmission lines, various types of power line insulators are used based on different line configurations.
Table of contents
- What is an Overhead Line Insulator?
- Properties of Overhead Power Line insulators
- Materials used for Overhead Power Line Insulators
- Functions of Overhead Line Insulators
- Types of Insulators Used in Overhead Power Lines
- Types of Overhead Power Line Insulators based on Material
- Types of Overhead Line Insulators based on Construction
What is an Overhead Line Insulator?
An overhead power line insulator is used to prevent the leakage of electric current from line conductors to the conducting surface of the pole or tower support or the ground.
Hence, an overhead power line insulator electrically separates the line conductor and the pole support.
The line insulators are crucial components of an overhead transmission and distribution system.
Properties of Overhead Power Line insulators
An ideal overhead line insulator should have the following properties:
- Very high electrical resistance to prevent leakage of electric current from line conductors to the pole or tower.
- High mechanical strength to handle the load of the conductor and environmental impacts like wind load.
- High electrical permittivity and dielectric strength.
- High ratio of puncture strength to flashover
The high ratio of puncture strength to flashover is a crucial parameter for an insulator. Let’s understand these technologies and their importance.
- Flash-over: The flashover occurs in an insulator when an arc forms between the live conductor and the insulator pin. The arc is resistive; as a result, an electric discharge occurs from the live conductor to the ground through the surrounding air. The insulator continues to act as an insulator during flashover. However, the arc heats the insulator’s surface, and the excessive heat may cause permanent insulator failure.
- Puncture: When the current flows from the conductor to the insulator’s body, it causes insulator puncture. In the case of a puncture, the electric current finds its path from the live conductor to the insulator body. As a result, high current flows between the phase and the earth. Insulators are designed to sustain the nominal, transient, and lightning voltages. However, if the voltage increases above its designed value, it can puncture the insulator. Thus, in the case of a puncture, the insulator gets permanently damaged.
- Safety factor of insulator: The safety factor of the insulator is the ratio of puncture strength to flash over voltage. It is desired to have a high value of puncture strength to avoid the insulator’s failure. The high insulator’s safety causes flashover before puncture; thus, the insulator does not damage.
Materials used for Overhead Power Line Insulators
The insulating materials have the following properties to manufacture an overhead line insulator.
Some of the most commonly used materials to design overline insulators are given below:
- Porcelain
- Glass
- Steatite
- Polymer
- Ceramic
- PVC
Functions of Overhead Line Insulators
The following two are the primary functions of the overhead line insulators in an overhead transmission and distribution system:
- Provide insulation between line conductors and conducting parts of the system.
- Provide mechanical support to the line conductors.
Types of Insulators Used in Overhead Power Lines
Based on the insulating material used, the following three types of insulators are most widely used in overhead lines:
- Porcelain insulators
- Glass insulators
- Polymer insulators
Based on the constructional design, the following four types of insulators are commonly used in overhead power lines:
- Pin insulator
- Suspension insulator
- Strain insulator
- Shackle insulator
Let us discuss these types of insulators used in overhead lines in detail.
Types of Overhead Power Line Insulators based on Material
(1). Porcelain Insulator:
Porcelain insulators are made of clay, alumina, quartz, and feldspar.
Porcelain insulators are widely used in overhead transmission lines where very high mechanical strength is desired.
Electrically, porcelain insulators provide a fair dielectric strength of 4 kV/mm to 10 kV/mm.
The great thing about porcelain insulators is that they are highly resistant to corrosion and, hence, can be used in adverse environmental conditions.
Advantages of Porcelain Insulators:
Some of the key advantages of porcelain insulators are listed below:
- Porcelain insulators have high dielectric strength and provide good electrical insulation.
- They have longer lifespans.
- They can be used in very high or very low-temperature conditions.
- Porcelain insulators provide high corrosion strength and, hence, they are suitable for use in outdoor applications.
- Porcelain insulators are made up of natural materials, which are environment-friendly.
(2). Glass Insulator:
Glass insulators are another overhead power line insulator that provides electrical insulation between the conductor and pole/tower.
This is called a glass insulator because it is made of toughened glass to provide excellent electrical insulation and mechanical strength.
Glass insulators can provide a significantly high dielectric strength of up to 14 kV/mm.
The fault detection in the case of glass insulators is relatively easy due to their transparent design.
However, glass insulators have a shorter lifespan than porcelain insulators.
Advantages of Glass Insulators:
Some key benefits of glass insulators are given below:
- Glass insulators are lightweight.
- They can be used for very high voltage ratings.
- Glass insulators provide ease in fault identification due to their transparent design.
- They are relatively less expensive than porcelain insulators.
- Glass insulators also offer better insulation due to high dielectric strength.
(3). Polymer Insulator:
As the name implies, a polymer insulator is a type of overhead power line insulator made of polymer materials.
In a polymer insulator, the insulation part is made of fiberglass, silicon rubber, EPM, CE, Teflon, etc.
Polymer insulators provide a dielectric strength of about 4 kV/mm.
Advantages of Polymer Insulators:
The following are some key benefits of polymer insulators:
- Polymer insulators provide a reasonably high dielectric strength and electrical insulation.
- They are lightweight.
- In polymer insulators, there is no issue of brittle failure.
- Polymer insulators are less expensive.
- Polymer insulators require no cleaning during the entire lifespan.
Let us now discuss the types of power line insulators based on their construction and design.
Types of Overhead Line Insulators based on Construction
(1). Pin Type Insulator:
The pin-type insulator is the simplest type of overhead line insulator. It is mainly used in low-voltage distribution lines up to 11 kV.
It is called a type insulator because its construction has a single insulting part made up of porcelain or glass and mounted on a pin with threads on the free end to fix at the cross-arm of the pole or tower.
The pin insulator has a groove or slot cut on its top to house the line conductor. The conductor passes through this groove and is firmly fixed by binding it with a wire of the same material as the conductor.
A pin insulator is generally made from porcelain. They are used in the open air. Therefore, adequate insulation is required during rain. To mitigate this problem, these insulators are designed with petticoats. They are used up to 33 kV power system. Beyond 33 kV, the insulator becomes too bulky.
Characteristics of Pin Insulator:
The following are some key characteristics of the pin insulators:
- Pin insulators have a simple construction.
- They provide significantly high insulation between the line conductor and conducting surfaces.
- Pin insulators can be used in both vertical and horizontal directions.
- They have reasonable creepage distance and mechanical strength.
- Pin insulators require less and easier maintenance.
(2). Post Insulator
The construction of post insulators is similar to pin insulators. However, post insulators are bulkier and used for higher voltage applications.
The Post insulators have a more significant number of petticoats than the pin insulators, and thus, they have more creepage distance. They can be mounted horizontally or vertically on the support structure.
(3). Stay Insulator
A stay insulator, also known as a stay type or egg insulator, is a low-voltage insulator. It is connected with a stay wire to fasten and counterweight the dead-end pole.
(4). Suspension Insulator:
Suspension type insulator is a power line insulator mainly used in high-voltage overhead lines. This type of insulator consists of one or more insulator disks connected in series. Generally, each insulator disc has a working voltage of 11 kV. The number of discs required for a 132 kV system is 7; if you consider mechanical safety factor 1, the discs will be 8. The discs will be 9 if you consider a mechanical safety factor of 2.
The number of discs of suspension insulators for rated voltage systems is tabulated below.
| Rated System Voltage (kV) | Number of discs in suspension insulator string |
| 33 | 3 |
| 66 | 4 |
| 132 | 8 |
| 220 | 14 |
These insulators hang to the cross-arm of the tower, and the line conductor is installed at the lower end of the insulator string.
Characteristics of Suspension Insulator:
The following are the main characteristics of suspension insulators.
- Suspension insulators provide higher dielectric strength and insulation between the tower and the conductor.
- In high-voltage lines, suspension insulators are the more economical choice.
- Suspension insulators can be designed for any voltage level using a certain number of discs in the string.
- Suspension-type power line insulators have less mechanical tension due to flexible suspension string.
- The initial cost of suspension insulators is comparatively high.
- Suspension insulators require a higher supporting structure to keep significant ground clearance.
- The suspension-type insulators are mounted below the earthed cross-arms of the towers. Therefore, they are not apt to fail with lightning strikes.
(5). Strain Insulator:
The strain insulator is an overhead power line insulator used to provide mechanical strain or tension and withstand the pull of a suspended line conductor.
The transmission line experiences a great tensile load at the dead end and sharp curves. In this condition, the strain insulators are used to sustain the mechanical tensile load of the conductors.
The strain assembly consisting of a string of suspension insulators is used for high-voltage transmission lines. In this case, suspension strings are arranged horizontally, while insulator discs are placed vertically. Multiple suspension strings can be placed parallel to withstand higher tensions. Shackle insulators are strain insulators for low-voltage lines, less than 11 kV.
This type of insulator is mainly used between two-line conductors to separate them electrically while providing a mechanical connection between them.
The strain insulator is made up of a piece of porcelain, glass, etc.
The following table shows the rated voltage and the corresponding number of strain-type insulator discs.
| Rated System Voltage | Number of discs in strain type insulator string |
| 33KV | 3 |
| 66KV | 5 |
| 132KV | 9 |
| 220KV | 15 |
Characteristics of Strain Insulator:
The main characteristics of strain insulators include the following:
- Strain insulators are mainly designed to be used at dead ends of an overhead transmission line.
- Strain insulators are used to provide mechanical support to overhead line conductors.
- Strain insulators reduce the mechanical tension on the transmission line, which can damage the line conductors.
(6). Shackle Insulator:
The shackle insulator is a type of overhead power line insulator used in low-voltage overhead lines. It is also called a spool insulator.
The most significant advantage of this type of insulator is that it can be used in horizontal and vertical positions.
A typical shackle insulator has a round porcelain insulator with a tapered hole in the middle and grooves on the outer surface. The tapered hole of the shackle insulator uniformly distributes the load and minimizes the possibility of breakage. The conductor is secured in the shackle insulator groove using soft binding wire.
This type of insulator is used in areas where the overhead lines change their angle.
Characteristics of Shackle Insulator:
The following are the main characteristics of the shackle insulators:
- Shackle insulators are designed to be used where the overhead line changes direction.
- Shackle insulators can be used in both horizontal and vertical positions.
- Shackle insulators are suitable for use in low voltage levels.
Conclusion
This is all about different types of overhead line insulators used in power transmission lines. The line insulator is one of the most critical components of an overhead line. It provides insulation as well as mechanical support to the power line system. In this article, I have explained all the major types of overhead power line insulators and their key characteristics.