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What is String Efficiency of Suspension Insulator?
String Efficiency
The suspension insulator is the one which consists of a number of porcelain discs connected in series by metal links in the form of a string.
The voltage applied across the string of the suspension insulators is not uniformly distributed across the various discs, i.e. the disc nearest to the line conductor has much higher potential than the other discs. This unequal potential distribution is undesirable and it is usually expressed in terms of string efficiency. Therefore, the string efficiency of the suspension type insulator is defined as follows −
“The ratio of voltage across the whole string to the product of the voltage across the disc nearest to the conductor and the number of discs in the string is known as string efficiency.”
$$\mathrm{\therefore String\: Efficiency\mathrm{\, =\, }\frac{Voltage \: across \: the\: string}{Voltage\: across \: the\: disc\: nearest \: to\: conductor\: \times \mathit{n}}}$$
Where, n is the number of discs in the string of suspension insulator.
Hence, from the definition of the string efficiency, it is clear that the greater the string efficiency, the more uniform is the potential distribution across the string. In ideal case, the string efficiency is 100% for which the voltage across each disc will be exactly the same. However, it is impossible to achieve 100% string efficiency.
Methods of Improving String Efficiency
As we know, the potential distribution in a string of suspension insulators is not uniform, i.e. the highest potential appears across the insulator disc nearest to the line conductor and decreases progressively as the cross-arm is approached.
If the insulation of the disc nearest to the line conductor breaks down, the breakdown of the other discs will take place in succession. Therefore, it is required to equalise the voltage across the various discs of the string, i.e., needs to improve the string efficiency.
The following methods are employed for improving the string efficiency of a suspension type insulator −
By using the longer cross-arms
By grading the insulators
By using a guard ring
By Using Longer Cross-Arms
The value of string efficiency depends upon the ratio of shunt capacitance to the mutual capacitance (let K). That means, the lesser the value of K, the greater is the string efficiency and hence, more uniform is the potential distribution.
The value of K can de decreased by decreasing the shunt capacitance. For reducing the shunt capacitance, the distance between the line conductor and the tower must be increased, i.e., longer cross-arms should be used. In practice, K = 0.1 is the limit that can be achieved by this method. It is because the limitations of cost and strength of the tower do not allow the use of very long cross-arms.
By Grading the Insulators
In this method of improving string efficiency, the insulators of different sizes are so chosen that each has a different capacitance. Hence, the insulators used are capacitance graded, i.e., they are connected to form the string in such a way that the top disc has the minimum capacitance and it increases progressively as the disc nearest to the conductor is reached.
As the voltage is inversely proportional to the capacitance, hence the capacitance grading of the insulators tends to equalise the potential distribution across the various discs in the string. The main disadvantage of this method is that it requires a large number of different sized insulators.
By Using Guard Ring
The guard ring is a metal ring which is electrically connected to the conductor and surrounding the bottom insulator disc as shown in Figure-2. It equalises the potential across each disc in the string.
The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is designed in such a way that the shunt capacitance currents 𝑖1,𝑖2, etc., are equal to the metal fitting line capacitance currents 𝑖1′, 𝑖2′, etc. The result is that the same charging current (I) flows through each disc of the string. As a result, the potential distribution across the discs of the string will be uniform.
Numerical Example
In a 33 kV overhead transmission line, there are three discs in the string of insulators. If the voltage across the disc nearest to the conductor is 8 kV. Then, determine the string efficiency.
Solution
$$\mathrm{Voltage\: across\: string\mathrm{\, =\, }\frac{33}{\sqrt{3}}\mathrm{\, =\, }19.05 \: kV}$$
The string efficiency is given by,
$$\mathrm{String\: Efficiency\mathrm{\, =\, }\frac{Voltage \: across \: the\: string}{Voltage\: across \: the\: disc\: nearest \: to\: conductor\: \times \mathit{n}}}$$
$$\mathrm{\therefore \% \: String\: Efficiency\mathrm{\, =\, }\frac{19.05}{8\times 3}\times 100\mathrm{\, =\, }79.4\%}$$
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