Insulators are used in distribution and transmission systems to separate the line conductor from the supporting poles or tower . If transmission and distribution lines are not insulated from their supports (poles, tower), the current will flow to the poles in that case. If any of the living being touch the pole during that period, it will get the shock or this might cause death. Hence it is necessary to use insulators in transmission and distribution systems. There are four common types of overhead insulators which are used to insulate the high voltage line conductors from the tower. These are termed as:
Pin Type Insulators
Suspension Type Insulators
Strain Type Insulators
Shackle Type Insulators
Pin, suspension and strain type insulators are generally used in medium to high voltage lines while shackle insulators are applicable in low voltage distribution systems. In this article, we will discuss about the pin insulators in detail.
Pin Type Insulator :
Pin type insulators are generally used in electrical power distribution for voltages up to 33kV. It consists of one, two or three piece unit insulators depending upon their use in power networks. In a 11 KV power system (lower voltage), we use single piece of pin insulator. For high voltage network system, two or more units of insulators are used which are cemented together.
Construction of Pin Type Insulator:
Construction of pin insulator is simple. These insulators are made with porcelain. Pin type insulator is mounted on a galvanized steel pin which in turn fixed with the cross arm of the pole. Top end of the insulator has a groove for keeping the line conductor. Conductor is tied with the groove by wrapping with the same wire as that of conductor.
Since insulators prevent the flow of leakage current from the line conductor to earth by providing the necessary insulation. Moreover, we also know that electric current easily flow across the surface of insulator. Hence we can say that the leakage path for the current depends on the vertical surface of insulating material. For that reason, it is desirable to have the large vertical surface area of an insulator. Once the length of leakage path increases, leakage current will be less. In order to have a long surface area, several (two or three) petticoats or sheds are used. The units (pieces) of insulators are called petticoats or rain sheds. By providing two or three petticoats or rain sheds, the length of leakage path can be increased.
One more advantage of rain sheds or petticoats is that they are designed in such a way that when the outer surface of petticoats becomes wet during the rain, the inner side of pin insulator remains dry and non conductive. Hence the resistance of leakage path is still maintained by the rain sheds.
Causes of Insulator Failure:
The main purpose of using an insulator is to insulate the line conductor from the metal tower. Hence insulator must be designed in such a way so that it can withstand mechanical and electrical stresses. The electrical breakdown of insulator occurs mainly due to Flashover or Puncture. If there is moisture or dirt deposits in the insulating medium, small leakage current will start to flow. If leakage current will flow for a long period, excessive heat will be generated which will damage the insulating material permanently. This is called puncture of insulator. If insulator gets punctured, this simply means that insulator is electrically break down and damage permanently. In that case, the current will pass through the insulator to the pin or tower and makes the metal tower conducting. In order to avoid punctures, insulating material (porcelain) is provided with sufficient thickness.
Flashover is caused by the arc which occurs between the line conductor and insulator pin. Instant flash over of insulator generally occurs during lightning strokes and switching surges. Flashover may also occur during an abnormal increase of voltage( voltage transients) since it may exceeds the voltage handling capacity of insulator during that period. Insulators withstand the flashover without permanently damaged, however their insulation property will be affected. Moreover, insulators are designed in such a way that flashover takes place before the puncture of insulator. In other words, puncture voltage is always more than the flash over voltage. The below figure shows the flashover of an insulator.
It is defined as the ratio of puncture strength to the flashover voltage. It is desirable to have high safety factor value so that flash over takes before the insulator gets punctured.
In case of pin insulators, the value of safety factor is about 10.
Advantages of Pin Insulator :
The main advantage of pin type insulator is that it is cheap.
Construction of pin insulator is simple and also requires less maintenance.
Pin insulator requires a shorter supporting structure (tower) to give a support to the conductor as compared to the other insulators.
In case of carrying less load, light conductors are used for low voltages. In that case, one pin insulator is enough for performing the work instead of several units which is very economical.
Mechanical strength of pin insulator is high.
Multi piece units of pin insulators are used in high voltages. One advantage of using several units of insulator is that defect in one petticoat or rain shed doesn’t affect the mechanical strength of insulator and it continues to functions with full capacity.
Pin insulators has a good creepage distance. During flash over, creepage distance keeps the flashover not to intervene with the conducting system.
It can be connected in horizontal as well as vertical position.
Disadvantages of Pin Insulator:
Insulator pin damages the thread of insulator.
Cost of pin type insulator increases rapidly when it is used for high voltages.
Pin insulators can only be used up to 33KV. Beyond operating this voltage, it becomes bulky and uneconomical.
Applications of Pin Insulator:
In some cases, one pin insulator can perform the work instead of using two suspension insulators.
This type of insulators are used on intermediate poles on a straight run.