String Efficiency of Suspension Insulators & Methods of Improving Voltage Distribution

In the previous article, we have discussed different types of insulators used in an overhead transmission line. We talked about advantages and disadvantages of each type of insulators. In this article, we will be talking about voltage distribution and string efficiency of Suspension insulator.


Voltage Distribution Over A Suspension Insulator String

Fig.1 Suspension Insulator String


Fig.2 Voltage distribution across units of a string
Fig. 1 shows the Suspension Insulator String. The insulator disc between two metallic pins forms a capacitor C which is known as Self Capacitance. The air insulating medium between metal pin and tower forms another capacitance kC known as Capacitance to earth. It is shown in fig. 2. The relation between Capacitance to earth and self-capacitance is given by


In a string, line conductor is held by the lowest insulator disc. When the line conductor voltage is applied across insulator string, it is not equally distributed across the individual discs. Actually, disc nearer to the line conductor is highly stressed and have maximum voltage. On the other hand, disc farther to the line conductor is least stressed and have minimum voltage.

String efficiency of Suspension insulator is defined as the ratio of voltage across the whole string to the product of the number of insulator discs and the voltage across the insulator disc nearest to the line conductor.

Another way to define string efficiency is in terms of flashover voltage.

String efficiency of Suspension insulator is defined as the ratio of flashover voltage of string to the product of the number of insulator discs and the flashover voltage of one disc.

Question. What is the importance of the string efficiency?

Answer. String efficiency gives valuable information like

1. String efficiency decides the potential distribution along the string. The greater the string efficiency, the more uniform is the voltage distribution.

2. The potential distribution along the string determines the flashover voltage and voltage at which radio interference and corona take place.

3. 100% string efficiency is an ideal case for which the voltage across each disc will be exactly the same. Practically, it is impossible to achieve 100% string efficiency.

Methods of improving string efficiency

1. By using longer cross-arm

2. Capacitance grading

By grading the insulators, this method tends to equalize the potential distribution across the units in the string. Good results can be obtained by using standard insulators for most of the string and larger units for that near to the line conductor.

3. Use of Grading rings or Static Shielding

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. 

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