A Study on the Use of Internal Grading to Improve the Performance of Insulators

As transmission line voltages keep increasing, the length of insulator strings has to keep pace. Owingto the non-linear voltage distribution along a string as a result of stray capacitances, the marginal effectiveness of each additional unit of insulator decreases. With an anticipated transmission voltage of up to 1200 kV in the not too distant future, it is likely that the detrimental effects of non-uniform voltage distribution may pose as the ultimate limiting factor. By studying the effect of stray capacitances along a string, it is found that this nonuniform voltage distribution can be reduced by increasing the intrinsic capacitance of each insulator unit. Computer models have been developed as a first step to investigate the effectiveness of using internally capacitivelygraded insulators. Results from these computer studies are presented. These models together with experimental data have been integrated into a computer aided general design procedure which is discussed here. A practical measuring technique for mapping the equipotential field is also described.