Types of Insulators-Pin, Suspension,Strain type

Types of insulator used in power system pdf

Types of Insulators are used in an overhead line to provide insulation to power conductor from ground and tower or supporting structure. Generally, insulators are connected to cross an arm of supporting structure and power conductor pass through the clamp of the insulator.

In this article, we will learn about different types of insulators used in power system transmission line, like Pin typeSuspension typeStrain type insulator, and operating voltages, with diagram and uses advantages 

Insulators are made up of either porcelain or glass, the material used for porcelain are silica 20% feldspar 30% and clay 50%. The porcelain which is used for making insulators must be free from defects and impurities, it should be checked because the presence of pores or Air Bubbles will lower down dielectric strength of insulators.

The dielectric strength of porcelain should be 15KV to 17KV for every one-tenth inch of thickness. porcelain is used for the manufacturing insulators because it is mechanically strong and less affected by temperature and has a minimum leakage problem. 

Sometimes glass is also used for insulation purposes because it has higher dielectric strength which makes it possible to use of single-piece construction. And glass being transparent, because it is very easy to find out the defect and pores into the glass, which leads to a reduction in the dielectric strength of insulators.

The major drawback of Glass type insulator is moisture gets condenses very easily on its surface which limits it's used up-to 33 KV operating voltage.

Types of insulator use in transmission line as follows 
  • Pin type insulator
  • Suspension type insulator
  • Strain type insulator

Pin type insulator

Pin-type-insulator-diagram
Pin type insulator

Pin type insulator consists of single or multiple shells that mounted on the spindle to be fixed to the cross arm of the tower. Multiple shells are provided to obtain sufficient length of leakage path so that flash overvoltage between conductor and pin type insulator is increased. 

The design of shells is made such that if the uppermost shell gets wet due to raining, the lower shells are dry and provide sufficient leakage resistance.
In pin type overhead line insulator, power conductor passed through the groove at the top of the insulator.

Pin type insulator is used for transmission and distribution of electric power at voltage up to 33kv  beyond 33kv operating voltage pin type insulator become too bulky and uneconomical.

The cost of insulator beyond 33kv increase is given by

Cost directly proportional to V^X

X>2

Advantages of Pin Insulator

  • Pin type insulator has high mechanical strength.
  • It has a good creepage distance.
  • Pin type insulator used on the high voltage distribution line.
  • Construction is simple and requires less maintenance.
  • Pin type insulator can be used vertically as well as horizontally.

Disadvantages of Pin Insulator

  • Pin type insulator used up to 33kv operating voltage
  • Beyond 33kv cost increase and become uneconomical.
  • Replacement of insulator is expensive
  • Pin type insulator only used in the distribution line.

    Suspension type insulator

    suspension type Insulators diagram
    suspension type Insulators

    Because of limitations of pin type insulators for higher voltage, suspension type Insulators are used. Suspension type insulator consist one or more insulator unit connected together to form a string and hanged to the cross arm of supporting structure or tower and carry power conductor at its lowest extremity. This type of composite unit also called as string type insulator.

    The conductor is suspended at the bottom end of the string while the other end of the string is attached to the cross arm of the supporting structure.
    In suspension type insulator each disc of porcelain is designed to operate at 11KV, as operating voltage increase number of disc connected in series increases. e.g 132 KV transmission the number of insulator disc required is 12.

    Advantages of suspension type insulator

    • Suspension type insulator being economical as compared to pin type insulator for high voltage overhead transmission line
    • Each insulator disc is designed to operate for 11KV. hence for any operating voltage a string of insulator can be used. for example for 66kv 6 number of discs are required.
    • In case of failure of one of unit in string only that unit/disc need to replace rather than whole string.

    Strain type insulators


    Strain-type-insulator-diagram
    Strain type insulators

    When there is a dead end of  transmission line or there is corner or sharp curve on line, the transmission line is subjected to greater tension. In order to relieve the line of excessive tension, strain type insulators are used. For low voltage lines (< 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strain insulator consists of an assembly of suspension insulators as shown in Figure.The discs of strain insulators are used in the horizontal plane. When the tension in lines is exceedingly high, as at long river spans, two or more strings are used in parallel.



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