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Ahmed halim

on 5 December 2012

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POTENTIAL DISTRIBUTION OVER A STRING OF SUSPENSION INSULATORS B. The Effect of Stray Capacitance over Voltage Distribution The presence of stray capacitance causes unequal potential distribution over the string. The end unit of the string (which is the closest to the line) takes maximum potential difference and the upper units have a gradually decreased potential difference until the uppermost unit which has the lowest potential difference. The next proof illustrates this concept.
Let: C = Capacitance of each unit.
m = (Capacitance between each unit and the tower) / (Capacitance of each unit).
Then, mC = Stray Capacitance (capacitance to ground).
Fig. 3 illustrates the equivalent circuit of the string of suspension insulato Theoretical Background: A. Suspension Insulators and Stray capacitance Objectives: 1. To get the student acquainted with the different types of overhead insulators.
2. To study the potential distribution over a string of suspension insulators.
3. To show how the potential distribution can be improved. When the string is mounted on the tower, stray capacitance will appear between the metal links of the suspension insulator units and the tower, and there will be a capacitance between each metal link and the line conductor that has a very small value. Using KCL at each node it can be found that:
V2 = (1+m) V1
V3 = (1+3m+m2)V1
V4 = (1+6m+5m2+m3)V1
Vph = V1+V2+V3+V4
Where: Vph = The phase voltage across the subsequent units measured from the ground respectively. Generally, two main materials are used for manufacturing of overhead insulators, namely porcelain and toughened glass. The units are provided by metal links to obtain the insulating string which is mechanically strong. Electrically each unit between the metal links presents a capacitance, leakage resistance through the body of the insulator and a surface leakage resistance. The leakage resistances are generally high compared to the capacitance and thus the insulator unit can be electrically represented by an equivalent self-capacitance C. (for the unit shown in Fig. 1: C = 50-70 pF) It’s clear that the voltages across the suspension insulators are not equal. This causes a reduction in the effectiveness of the insulator usage, which is measured in terms of the string efficiency. C. Methods of improving the voltage distribution: 1. Control of m: Increasing the length of the cross arm of the tower will decrease the stray capacitance and thus uniform voltage distribution can be achieved. This method is limited by the strength and the cost of the tower. 2. Grading the units: The capacitance of the lowest unit is increased and the other units are gradually decreased, thus the voltage across the last unit is decreased and increased
across the higher ones. This method is very complex in erection and maintenance. 3. Guard ring or static shield
A guard ring which is connected to the life conductor surrounding the bottom unit creates another stray capacitance between the ring and metal caps of the insulator. The current passing through the newly initiated stray capacitance will minimize the difference of current drops along the suspension string. The guard ring and the equivalent circuit of the string with guard ring are shown in Fig. 4 and Fig.5 respectively. III. Experimental work A. Apparatus
1. A group of condensers connected in series is used to represent the string of suspension insulators
2. Another group of decade capacitors is used to represent the capacitances between the string and the tower.
3. A third group of decade capacitors is used to represent the capacitances between the string and the guard ring.
4. A digital voltmeter is used to measure the voltage on different units B. Procedure
1. Connect the condensers as shown in figure 4 to represent the string and the earth capacitances.
Take C = 2.25 μF and mC = 0.225 μF.
2. Connect a 110 V (or 220 V) A.C supply across the string and measure the voltage across each unit.
3. Repeat the above steps for various values of (m) and record for each value the voltage across each unit.
4. Adjust the decade condensers representing the guard ring at the values required for nearly uniform distribution of the potential
Where: Cn =n/N-n × mC
n = number of guard ring capacitance to be calculated
N = total number of insulating units.
5. Compare the results obtained experimentally with those calculated using the formulae. Thank you
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