Abstract :
THE component units of an insulator string do not carry equal voltages when the string is in service, because of stray capacitances from each unit of the string to line and to ground. The voltage across a particular unit in a string may be determined by placing a calibrated spark gap across that unit and noting the total string voltage required to make the gap spark. Obviously, the presence of the gap should not change the potential distribution of the string by introducing capacitance or ionization. Fig. 1 shows the potential distribution curves for strings of 18 and 21 units, and it can be seen that the line end units are the most highly stressed. The voltage across these units can be reduced by fitting grading rings at the line end of the string, which increase the capacitance of these units to the line. Several shapes of grading ring have been used, but probably the most common is that shown in Fig. 2. Experiments have demonstrated that with this form of ring, the best position is as high, relative to the string, and as close to it as is practicable. This fact is also of appreciable significance in the design and positioning of grading rings of various other shapes.
