GUIDED BY: DR. NILESH CHOTHANI PREPARED BY: SEJAL SHAH(130010109054) “TYPES OF INSULATOR & STRING EFFICIENCY” ELECTRICAL POWER SYSTEM(2150908) GUIDED BY: DR. NILESH CHOTHANI PREPARED BY: SEJAL SHAH(130010109054)

OVERHAED LINE INSULATORS The insulators for overhead lines provide insulation to the power conductor from ground. The insulators are connected to the cross arm of the tower and the power conductor passes through the clamp of the insulator. The insulators have high mechanical strength, high electrical strength, high insulation resistance. They are free from impurities and moisture and can withstand the flashover phenomenon.

TYPES OF INSULATORS 1) PIN TYPE INSULATORS Pin type insulator consists of a single or a multiple units. They are secured to the cross arm of the pole. Pin type insulator can be used only up to 33kv. Beyond operating voltage of 33kv, the pin type insulator become too bulky and hence uneconomical.

2) SUSPENSION TYPE INSULATORS: Suspension insulator consists of porcelain disc units mounted one above each other. Each disc is provided with a meta cap at the top and a metal pin under. A string of any number of units can be built according to the line operating voltage. The conductor is suspended below the point of support by means of insulator string. The number of discs in a string depends on the line voltage and the atmospheric conditions.

The usual number of discs are: Voltage (kv) : 66 132 220 400 Number of discs: 4-5 9-10 15-16 22-23 Suspension type insulators are cheaper than pin type insulator. If any one string is damaged, it can be replaced by a sound one. Each unit is designed for a low voltage. Depending upon the working voltage, number of units can be connected.

These are special mechanical strong suspension insulators. 3) STRAIN INSULATORS: These are special mechanical strong suspension insulators. They are used to take the tension of the conductors at the line terminals, at angle towers and at road crossings. The string are placed in horizontal plane Two or three strings of insulators in parallel can ne used when the tension in conductor is very high.

4) SHACKLE INSULATORS In early days, the shackle insulators were used as strain insulators. But now a days, they are used for low voltage distribution lines. Such insulators can either be used in horizontal or vertical position. They can directly be fixed to the pole with a bolt or to the cross arm.

STRING EFFICIENCY Cs: capacitance of each insulator mCs: capacitance to ground V1,V2,V3: the voltage across each unit V= V1+V2+V3 At point A: I2= I1+ i1 wC.V2= wC.V1+ wmC.V1 V2= (1+m).V1 At point B: I3= I2+ i2 wC.V3= wmC.(V1+V2) + wC.V1 V3= (1+3m+m2).V1

For m<1 V3>V2>V1 Insulator efficiency: %n= (V/n.Vmax) * 100 String Efficiency = voltage across the string n* (voltage across disc nearest to conductor) V: Voltage across the insulator string n: number of insulator units Vmax: Voltage across the insulator unit near to conductor.

METHODS TO IMPROVE STRING EFFICIENCY 1) By using longer cross arm: the value of string efficiency depends upon m (Ce/Cs). The lesser the value of m, greater the string efficiency. The value of m can be decreased by reducing Cs. In order to reduce Cs, longer cross arm should be used 2) By guarding the insulators: Insulators of different dimension having different capacitance are chosen. They are assembled in a way that the top unit has minimum capacitance and the unit nearest to conductor has maximum capacitance. This method tends to equalize the potential distribution across the units of each string.

3) By using a guard ring: The potential across each unit in a string can be equalized by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom of insulator. The guard ring introduces capacitance between metal fittings and the line conductor, resulting in same charging current I flowing through each unit of string.

CONCLUSION Thus, a brief idea about the types of insulator used in overhead transmission line has been obtained. Each type of insulator is having its own area of application For the equal potential distribution across the strings of insulator, the string efficiency and the methods of improving the string efficiency has been concluded.