String Efficiency & Improving Methods

String Efficiency:

The total voltage applied across the string of suspension type insulators is not equally distributed across all discs.In this distribution of voltage, disc nearest to the conductor will be at higher potential than the other discs. This unequal potential distribution is undesirable and is usually expressed in terms of string efficiency

What Is String Efficiency?

"The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency."

i.e.,String efficiency =Voltage across the string/(n ×Voltage across disc nearest to conductor)

Where n = number of discs in the string.

String efficiency is an important factor is transmission line designing.since it decides the potential distribution along the string.to get uniform distribution string efficiency should be high.Thus 100% string efficiency is an ideal case for which the volatge across each disc will be exactly the same.that gives easy calculations to no.of discs to be added.but it is impossible to achieve 100% string efficiency,yet efforts should be made to improve it as close to this value as possible.

Mathematical expression for String Efficiency:-

Above Fig. shows the equivalent circuit for a 3-disc string. Let us suppose that self capacitance of each disc is C. Let us further assume that shunt capacitance C1 is some fraction K of selfcapacitance i.e., C1 = KC. Starting from the cross arm or tower, the voltage across each unit is V1,V2 and V3 respectively as shown. 
Applying Kirchhoff’s current law to node A, we get,

I2 = I1 + i1
or V2ω C* = V1ω C + V1ω C1
or V2ω C = V1ω C + V1ω K C
∴ V2 = V1 (1 + K) ...(i)

Applying Kirchhoff’s current law to node B, we get
I3 = I2 + i2
or V3 ω C = V2ω C + (V1 + V2) ω C1 
or V3 ω C = V2ω C + (V1 + V2) ω K C
or V3 = V2 + (V1 + V2)K
= KV1 + V2 (1 + K)
= KV1 + V1 (1 + K)2   since [ V2 = V1 (1 + K)]
= V1 [K + (1 + K)²]
∴ V3 = V1[1 + 3K + K²] ...(ii)
Voltage between conductor and earth (i.e., tower) is
V = V1 + V2 + V3
= V1 + V1(1 + K) + V1 (1 + 3K + K²)
= V1 (3 + 4K + K²)
∴ V = V1(1 + K) (3 + K) ...(iii)
From expressions (i), (ii) and (iii), we get,
 V1/1=V2/(1+K)=V3/(1 + 3K + K²)=V/(1+K)(3=K)

∴Voltage across top unit, V1 = V/(1 + K)(3 + K)
Voltage across second unit from top, V2 = V1 (1 + K)

Voltage across third unit from top, V3 = V1 (1 + 3K + K²)
%age String efficiency =Voltage across the string*100/(n ×Voltage across disc nearest to conductor)
=V*100/3V3

Methods of Improving String Efficiency:

By using longer cross-arms:-

From above observation we can say The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance.For the low the value of K, the string efficiency will be high and we get uniform voltage distribution.value of K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used.

By grading the insulators:-

In this method, different dimensions of  insulators are chosen which have different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required.

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 insulator. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units.

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Types of Insulators In Overhead Transmission Lines

Types of Insulators In Overhead Transmission Lines

Transmission line insulators separate, contain and suspend transmission line conductors.they protects short circuit path between earth and transmission line.There are several types of insulators In overhead transmission lines.but the most know are pin type, suspension type, strain insulator,shackle insulator and Stay Insulator.

Types of electrical insulators in transmission lines:

1. Pin type insulators:

The name "pin" came from constructional design of it.Pin type insulators are used for the transmission of lower voltages.transmission range of pin type is up to 33 kV (kilovolts).Pin type insulators are secured with steel or lead bolts onto transmission poles. These are typically used for straight-running transmission lines.There is a groove on the upper end of the insulator for housing the conductor. The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.we can use this type of insulator beyond 33kv,but that becomes bulky in size & costly. 

2.Suspension type insulators :

Suspension type insulators are used to transmit above 33kv.because above 33kv using of pin type is uneconomical.In suspension type insulator numbers of insulators are connected in series to form a string and the conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.Each insulator of a suspension string is called disc.

Advantages of Suspension Type insulators :

(i)Suspension type insulators are very economical voltages beyond 33 kV.

(ii) Each unit or disc of suspension type insulator is designed for low voltage,usually 11 kV.As per requirement of operating voltage we can add no.of discs in series. 

(iii) If any one disc is damaged, the whole string does not become useless because the damaged disc can be replaced by the sound one.

(iv) The suspension arrangement provides greater flexibility to the line. The connection at the cross arm is such that insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum.

(v) The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning.

3.Strain Type insulators:

Strain type insulators are simply horizontally suspended suspension insulators.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.The discs of strain insulators are used in the vertical plane. When the tension in lines is exceedingly high, at long river spans, two or more strings are used in parallel.voltage rating almost equal to suspension type.

4.Shackle Type Insulators:

Shackle type insulators, similar to strain type insulators, are used on sharp curves, end poles and in section poles.These insulators are single, round porcelain parts that are mounted horizontally or vertically.The shackle insulator or spool insulator is usually used in low voltage distribution system.They can be directly fixed to the pole with a bolt or to the cross arm.

5.Stay insulators :

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.

This Is Complete Information On Types of Electrical Insulators ; Insulators In Overhead Transmission Lines.

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