Power generated at the generating stations is transmitted and distributed over long distance by using overhead line conductors. Overhead line conductors are supported on the poles and towers in such a way that the current from the conductor doesn’t flow to the earth through metallic tower structure. In simple words, line conductors must be insulated from their supports (poles, tower). If line conductors are not insulated from their poles, current will flow to the poles. In that case it will lead to electric shock or this might cause death when a living being touch the pole. Hence it is necessary to provide insulation.
The necessary insulation is provided by securing the line conductors to supports with the help of insulators. Insulators are attached in between the line conductors and tower which insulate the overhead line conductors from the metallic tower. In this article, we will discuss the most common types of overhead insulators. Types of insulators are :
Pin insulators are generally used in distribution and short transmission lines for voltages up to 33KV. Above 33KV( high voltages), suspension and strain type insulators are used. Shackle and stay insulators on the other hand are used in low voltage systems. Let discuss these types of insulators in detail.
Pin Type Insulator :
This type of insulators are generally used in power distribution and short transmission lines for voltages up to 33kV. Pin insulators are used when line is straight. Construction of pin type insulator is simple and they also requires less maintenance as compared to the other insulators. Pin insulator is designed to be mounted on a galvanized steel pin which in turn fixed with the cross arm of the tower. There is a groove on the top end of insulator for keeping the line conductor. Conductor is tied with the groove by wrapping with the same ( copper or aluminum) wire as that of conductor. For low voltages, one piece of pin insulator is used. In case of high voltage system, two or more pieces of pin insulators are used which are cemented together.
These insulators can be connected in horizontal as well as vertical position. In order to read the full detail of pin Insulators, kindly read the following article :
Suspension insulators are generally used for voltages higher than 33KV. These insulators consists of number of porcelain discs which are connected in series with the help of metallic links to form a suspension string. Each insulator of suspension string is called as disc insulator because of the shape looks like. Line conductor is suspended from the bottom most insulator while the top end of insulator is secured to the cross arm of the tower.
Each disc is designed for 11KV voltage. For high voltages, a large number of discs are provided on the string in order to increase the insulation strength of suspension insulators. There are two types of suspension insulators which are design and connected with different connections.
Cap and Pin Type Insulator
Hewlett or Interlinking Type Suspension Insulator
Advantages of Suspension Insulators:
Current carrying line conductors are suspended from the bottom insulator of suspension string which means that they are placed below the cross arms of the metallic tower. This arrangement makes the conductor to be protected from lightening.
Suspension type insulators are cheaper and economical than pin insulators for voltages beyond 33 kV.
If any disc is damaged in a suspension string, it can be replaced easily. There is no need to change the whole string of suspension insulators.
Each suspension disc is designed for 11 KV. We can increase the voltage handling capacity of these insulators by connecting a number of discs in series. In this way, system can be designed for any operating voltage.
Suspension string is free to swing in any direction due to which it can easily take the position where mechanical stresses are minimum. Moreover, this arrangement also provides great flexibility to the transmission line.
Disadvantages of Suspension Insulators:
Since insulator string is free to swing in any direction in case of suspension insulators due to which the amplitude of conductor swing is large. Hence more spacing between the conductors is required which is provided by making the arms of the tower longer.
These insulators becomes costly if they are used for voltages below 33KV.
Suspension insulators requires a large supporting structure (tower) to give a support to the conductor as compared to pin insulator.
Shackle insulator is also known as spool insulator. This type of insulator is generally used in low voltage distribution lines. Shackle insulators are used in both horizontal and vertical position. They can be directly connected to the pole by using bolt or cross-arm. Line conductor is fixed in the groove with soft copper or aluminum binding wire. Tapered hole in shackle insulator distributes the load more evenly due to which possibility of breakage reduces during heavily loaded condition. The below figure shows a shackle insulator fixed to the pole.
Wet and dry flash-over voltages for shackle insulator is about 10 kV and 25 kV respectively while puncture voltage↗ is approximately 35 kV. It is to be noted that the use of shackle insulators decreases with the use of underground cables in distribution system.
When there is a dead end or there is a sharp corner or curve in the transmission line, the line experiences excessive tension. In order to sustain a conductor load in tension, strain insulators are used. Strain insulator is shown in the below figure.
For low-voltage lines (up to 11 KV ), shackle insulators are used as strain insulators. In case of high voltage transmission lines, strain insulator consists of an assembly of suspension insulators are used. Discs of strain insulators are used in vertical plane while suspension string is arranged in horizontal manner. Strain insulator must have good mechanical strength and necessary dielectric properties so that it can withstand the tensile load of conductor. If tension in the line is high as in case of long river spans, two or more suspension strings can be used in parallel to sustain greater tension.(as shown in the below fig.)
Stay Insulator :
Stay wire (also known as guy wire) are generally used with the poles at the end of streets, river or lakes in low voltage lines. If stay or guy wire is not installed with the pole (when there is turn or span in the electrical line), the poles may bend or fall down since there is tension on the pole at such positions. Hence stay wire support the poles by providing mechanical strength.
Stay insulator also known as guy insulator is made up of porcelain piece. It is rectangular in shape and it is installed in the middle of stay wire. If electric fault occurs and current flows in the stay wire, the lower part of the guy wire remains insulated due to stay insulator. In this way, living beings on the ground remains safe during electric faults.