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String Efficiency of Suspension Insulator Disk Type


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What is Suspension Insulator?

The suspension insulator separates the line conductors and supports them electrically. It consists of the amount of ceramic ware insulator units connected with one another by metal links to create a versatile string. The conductor is attached at the last of the string.
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The suspension dielectric is mainly classified into 2 varieties.

  • Cap and Pin type
  • Hewlett or Interlink type.

The Main Fact of String Efficiency 

The quantitative relation of voltage across the full string to the merchandise of the number of discs and therefore the voltage across the disc nearest to the conductor is understood as string efficiency, i.e.,
Equation of String Efficiency for n number of Disc's:-
String efficiency = {Volatge across String}\{n*times Voltage across disc nearest to conductor}
where n = number of discs within the string.
String efficiency is a critical thought since it decides the potential distribution on the string. The larger the string Efficiency, a lot of uniforms is that the voltage distribution. So 100% string Efficiency is a perfect case that the voltage across every disc is precisely the same. though it's not possible to realize 100% string efficiency, nevertheless efforts ought to be created to boost it
as near to this worth as attainable.


Methods of up String Efficiency:-

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The maximum voltage seems across the dielectric nearest to the road conductor and reduces more and more because the is approached. If the insulation of the best-stressed dielectric (i.e., closest to the conductor) breaks down or flashover takes place, the breakdown of alternative units can come about in succession. This necessitates equalizing the potential across the different groups of the string, i.e., to boost the string Efficiency.
The various strategies for this purpose are :

By mistreatment longer cross-arms:-

The worth of string Efficiencydepends upon the value of K, i.e., magnitude relation of shunt capacitance to mutual capacitance. The lesser the worth of K, the more significant is that the string Efficiency and new uniform is that the voltage distribution. The value of K
can be reduced by reducing the shunt capacitance. To scale back shunt capacitance, the space of conductor from a tower should be exaggerated, i.e., longer cross-arms ought to be used. However, limitations of the value and strength of the tower don't permit the utilization of terribly long cross-arms. In observe, K = 0·1 is the limit that will be achieved by this methodology.

By grading the insulators:-

During this methodology, insulators totally different dimensions are thus chosen that each contains a different capacitance. The insulators are capacitance hierarchical, i.e., they're assembled within the string in such the way that the highest unit has the minimum capacitance, increasing more and more because the bottom group (i.e., nearest to the conductor) is reached. Since voltage is reciprocally proportional to capacitance, this methodology tends to equalize the potential distribution across the groups within the string. This methodology has the disadvantage that an outsized range of different-sized insulators is needed. However, quick results may be obtained by mistreatment commonplace insulators for many of the string and bigger units for that almost about the road conductor.

By employing a guard ring:-

 The potential across every unit during a string may be equalized by using a guard ring that could be a metal ring electrically connected to the conductor and close the lowest dielectric. The guard ring introduces capacitance between metal fittings and also the line conductor. The guard ring is contoured in such the way that shunt capacitance currents i1, i2, etc. are adequate to metal fitting line capacitance currents i′1, i′2, etc. The result's that very same charging current I flow through every unit of string. Consequently, there'll be uniform potential distribution across the groups.

Calculation of String Efficiency

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Potential Distribution Over A Suspension insulator String

The figure below shows a 3-disc string of suspension nonconductors. As every ceramic ware disc lies in between 2 metal links, it forms an electrical condenser. This capacitance is thought of as self-capacitance or mutual capacitance.
Moreover, air capacitance is additionally gifted between metal links and, therefore,, the earthed tower. This is often referred to as shunt capacitance. The figure below illustrates the equivalent circuit of a 3-disc suspension nonconductor (assuming that shunt capacitance is a few fractions of self-capacitance, i.e., shunt capacitance = k * self-capacitance).

From the above equivalent circuit, applying Kirchoff's current law to node A,
I2 = I1 + i1
V2ωC = V1ωC + V1ωkC
V2 = V1 + V1k
V2 = (1 + k)V1              ...... eq.(i)

applying Kirchoff's current law to node B,
I3 = I2 + i2
V3ωC = V2ωC + (V2 + V1)ωkC
V3 = V2 + (V1 + V2)k
V3 = kV1 + (1 + k) V2
V3 = kV1 + (1 + k)2 V1       ...... from eq.(i)
V3 = V1 [k + (1 + k)2]
V3 = V1 [k + 1 + 2k + k2]
V3 = V1 (1 + 3k + k2)              ...... eq.(ii)

Now, voltage between the conductor and the earther tower is,
V = V1 + V2 + V3
V = V1 + (1 + k)V1 + V1 (1 + 3k + k2)
V = V1 (3 + 4k + k2)               ...... eq.(iii)

From the above equations (i), (ii) & (iii), it is clear that the voltage across the top disc is minimum while the voltage across the disc nearest to the conductor is maximum, i.e., V3 = V1 (1 + 3k + k2). As we move towards the cross arm, the voltage across the disc goes on decreasing. Due to this non-uniform voltage distribution across the string, the unit nearest to the conductor is under maximum electrical stress and is likely to be punctured.

Why string potency for the DC system is 100 percent?

Higher the string potency, a lot of uniforms is that the voltage distribution. String potency becomes 100 percent if the voltage across every disc is precisely similar. However, this is often a perfect case and not possible in a sensible situation. However, for DC voltages, nonconductor capacitances square measure ineffective, and the voltage across every unit would be similar. This is often why string potency for the DC system is 100 percent.
Inequality in voltage distribution will increase with the rise within the range of discs in an exceeding string. Therefore, shorter strings square measure a lot of economical than longer string insulators.

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