1470 | Page VOLTAGE DISTRIBUTION ACROSS STRING INSULATORS BY SPHERE GAP METHOD Prof. Ch.Kavitha Chenna Reddy 1 , Dr.S.Sujitha 2 Assistant professor, EEE Department,New Horizon College of Engineering ABSTRACT Insulators for overhead lines are considered to be of basic importance to the transmission system, through their ability to insulate the power lines as well as their function in carrying the weight of the line conductor. For higher voltages, a string of suspension insulators is used where the number of insulator units used depends on the voltages of the lines. The voltage is not equally shared between the units in a suspension insulator string. The unequal distribution of the voltage is due to the presence of stray capacitance of the hardware to earth and to line. Hence in this paper we are calculating the voltage distribution by sphere gap methods and comparing the results and we are also trying to make the voltage distribution across the insulators as uniform as possible by Guard rings. In this paper, we are also calculating the voltage distribution across the string insulators theoretically and comparing the results with practical values. I. INTRODUCTION Power transmission by overhead lines is the means of transportation of electrical energy from source to load centers. The insulator must also provide the necessary mechanical supports for the conductor against the worst likely mechanical loading conditions. The presence of pollution on the surface of insulators considerably modifies the voltage distribution across the insulator string. The pollution flashover poses serious threat to reliability of the system. The main reasons for the failure of insulators appear to be the insulator surface erosion and ion migration within its volume across each insulator in the string. This implies that the voltage distribution across the string is responsible for the uneven accumulation of the pollutants on the surface of insulators. Non uniform adherence of the surface of insulator surface has been reported both under laboratory and field conditions. The uneven voltage distribution across the string insulator will increase the probability of the puncture or shattering of insulators, if the condition such as surface erosion and increase in sodium ion concentration in the insulator volume due to pollution dominate. From the point of view of accumulation of the pollutant on the surface of the insulator, uneven voltage distribution and the failure of insulators occur.Thus the necessity to measure the voltage distribution along the insulator string has opened an interesting and challenging field of research. Hence simulation of voltage distribution calculation methods over a string of suspension insulators in case of clean - dry condition is presented. The voltage distribution across an insulator is not uniform, and the unit nearest to the line end is stressed to their maximum allowable value. Therefore, we have discussed a method in calculating the voltage distribution and have compared the string efficiency.