Consider a string or suspension insulators. The number of porcelain insulators. The number of porcelain discs are connected in series with the help of metal links. The Fig. 1 shows string of 4 porcelain disc suspension insulators.
The porcelain portion which is an insulator is in between the two metal fittings. Thus it forms a capacitor. This sis called ''self capacitance'' or ''mutual capacitance''. Hence the whole string shown in the Fig.1 will consist of 4 such self capacitors in series. If only such self or mutual capacitors exist alone in series, the voltage across them would have been equal and series charging current through them would have been same.
But in addition to the self capacitances, there will be capacitance between each metal fitting and the earth i.e. tower. The air acts as a dielectric. Such a capacitance is called ''shunt capacitance''.
Due to shunt capacitors, the charging current no longer remains same.
The different currents, mutual capacitors and shunt capacitors are shown in the Fig. 2. The mutual capacitors are denoted as C while the shunt capacitors are denoted as C1. Assuming the design of each section of the string same, the mutual capacitors are assumed equal. Similarly all shunt capacitors are also assumed equal.
There will be capacitance between metal fittings and the line conductor also. But its value is very small and generally it is neglected.
The currents I1, I2, I3 and I4 are charging currents flowing through mutual capacitors while I1, I2, I3and I4 are the currents flowing through the shunt capacitors.
Due to the different charging currents, each capacitor will get charged to different potential. Hence the voltage across each section of the string will be different. It is shown as V1, V2, V3 and V4 in the Fig. 2.
As the charging current is highest nearest to the line conductor, the voltage across the capacitor nearest to the line conductor will be maximum. Thus V4 will be maximum, for the case considered. Hence the insulator adjacent to the line conductor is under maximum electrical stress and is liable to puncture. The graphically such potential variation can be shown as in the Fig. 3.
The following observations can be made related to the voltage distribution over a string of suspension insulators :
1. The voltage distribution is not uniform due to shunt capacitors.
2. The charging currents through various mutual capacitors are different.
3. The voltage across the top unit farthest from the line conductor is lowest.
4. The voltage across the bottom unit which is adjacent to the line conductor is maximum.
5. Due to maximum voltage impressed on the insulator nearest to the line conductor, it is under maximum electrical stress.
6. Due to maximum electrical stress, the insulator nearest to the line conductor is likely to puncture. Hence practically the efforts are made to have uniform voltage distribution as far as possible.
7. In case of d.c. voltage, the capacitors do not play any role and the voltage distribution is obviously uniform.