Type of Insulators Used in Transmission lines
There are 5 types of insulators used in transmission lines as overhead insulation:
- Pin Insulator
- Suspension Insulator
- Strain Insulator
- Stay Insulator
- Shackle Insulator
Pin, Suspension, and Strain insulators are used in medium to high voltage systems. While Stay and Shackle Insulators are mainly used in low voltage applications.
Pin insulators are the earliest developed overhead insulator, but are still commonly used in power networks up to 33 kV system. Pin type insulator can be one part, two parts or three parts type, depending upon application voltage.
In a 11 kV system we generally use one part type insulator where whole pin insulator is one piece of properly shaped porcelain or glass.
As the leakage path of insulator is through its surface, it is desirable to increase the vertical length of the insulator surface area for lengthening leakage path. We provide one, two or more rain sheds or petticoats on the insulator body to obtain long leakage path.
In addition to that rain shed or petticoats on an insulator serve another purpose. We design these rain sheds or petticoats in such a way that while raining the outer surface of the rain shed becomes wet but the inner surface remains dry and non-conductive. So there will be discontinuations of conducting path through the damp pin insulator surface.
In higher voltage systems – like 33KV and 66KV – manufacturing of one part porcelain pin insulator becomes more difficult. The higher the voltage, the thicker the insulator must be to provide sufficient insulation. A very thick single piece porcelain insulator is not practical to manufacture.
In this case, we use multiple part pin insulator, where some properly designed porcelain shells are fixed together by Portland cement to form one complete insulator unit. We generally use two parts pin insulators for 33KV, and three parts pin insulator for 66KV systems.
Designing Consideration of Electrical Insulator
The live conductor attached to the top of the pin insulator which is at the live potential. We fix the bottom of the insulator to supporting structure of earth potential. The insulator has to withstand the potential stresses between conductor and earth. The shortest distance between conductor and earth, surrounding the insulator body, along which electrical discharge may take place through the air, is known as flashover distance.
- When the insulator is wet, its outer surface becomes almost conducting. Hence the flashover distance of insulator is decreased. The design of an electrical insulator should be such that the decrease of flashover distance is minimum when the insulator is wet. That is why the uppermost petticoat of a pin insulator has umbrella type designed so that it can protect, the rest lower part of the insulator from the rain. The upper surface of the topmost petticoat is inclined as less as possible to maintain maximum flashover voltage during raining.
- The rain sheds are made in such a way that they should not disturb the voltage distribution. They are so designed that their subsurface at a right angle to the electromagnetic lines of force.
Post insulators are similar to Pin insulators, but post insulators are more suitable for higher voltage applications.
Post insulators have a higher number of petticoats and a greated height compared to pin insulators. We can mount this type of insulator on supporting structure horizontally as well as vertically. The insulator is made of one piece of porcelain and it has clamp arrangement are in both top and bottom end for fixing.
The main differences between pin insulator and post insulator are:
|It is generally used up to 33KV system
|It is suitable for lower voltage and also for higher voltage
|It is single stag
|It can be single stag as well as multiple stags
|Conductor is fixed on the top of the insulator by binding
|Conductor is fixed on the top of the insulator with help of connector clamp
|Two insulators cannot be fixed together for higher voltage application
|Two or more insulators can be fixed together one above other for higher voltage application
|Metallic fixing arrangement provided only on bottom end of the insulator
|Metallic fixing arrangement provided on both top and bottom ends of the insulator
In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size, weight of the insulator become more. Handling and replacing bigger size single unit insulator are quite difficult task. For overcoming these difficulties, suspension insulator was developed.
In suspension insulator numbers of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator because of their disc like shape.
Advantages of Suspension Insulator
- Each suspension disc is designed for normal voltage rating 11KV (Higher voltage rating 15KV), so by using different numbers of discs, a suspension string can be made suitable for any voltage level.
- If any one of the disc insulators in a suspension string is damaged, it can be replaced much easily.
- Mechanical stresses on the suspension insulator is less since the line hanged on a flexible suspension string.
- As the current carrying conductors are suspended from supporting structure by suspension string, the height of the conductor position is always less than the total height of the supporting structure. Therefore, the conductors may be safe from lightening.
Disadvantages of Suspension Insulator
- Suspension insulator string costlier than pin and post type insulator.
- Suspension string requires more height of supporting structure than that for pin or post insulator to maintain same ground clearance of current conductor.
- The amplitude of free swing of conductors is larger in suspension insulator system, hence, more spacing between conductors should be provided.
When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties.
|Rated System Voltage
|Number of disc insulator used in strain type tension insulator string
|Number of disc insulator used in suspension insulator string
For low voltage lines, the stays are to be insulated from ground at a height. The insulator used in the stay wire is called as the stay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the guy-wire will not fall to the ground.
The shackle insulator (also known as a spool insulator) is usually used in low voltage distribution network. It can be used in both the horizontal or vertical positions. The use of such insulator has decreased recently after increasing the using of underground cable for distribution purpose.
The tapered hole of the spool insulator distributes the load more evenly and minimizes the possibility of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.