| dc.description.abstract | The lightning activity and switching operations cause overvoltages in power systems. These overvoltages are detrimental to the insulation of power system equipment. It is therefore essential to limit these overvoltages. Zinc Oxide (ZnO) surge arresters are employed for this purpose. These devices clamp the amplitude of surges that exceed a critical level.
Under normal power frequency operation, the voltage distribution along the surge arrester is reported to be non uniform. The discs at the top portion are found to be stressed more than the remaining discs. As a result, they suffer faster thermal ageing, which may eventually lead to premature failure of the arrester. Therefore, it is essential to make the voltage distribution in the arrester as uniform as possible.
The voltage distribution can be studied either experimentally or theoretically. The experimental approach is expensive and tedious, and therefore the theoretical approach is generally preferred. Different numerical methods such as the Finite Element Method (FEM) and Charge Simulation Method (CSM) have been employed in the literature for voltage distribution studies. However, these studies are limited to capacitive field distribution, and thus the conduction in the discs as well as surface conduction originating from pollution deposits have not been considered. In addition, the influence of geometrical parameters-such as height and cross section of the grading ring, position of the spacer, and type/diameter of discs-has not been studied.
The present work attempts a field theoretical approach for voltage distribution studies addressing the above shortcomings. For the field solution, the surge arrester poses a semi open geometry with a large aspect ratio. Considering this, the Boundary Element Method (BEM) is adopted for field computation. Attention has been given to BEM formulations for singularities and multiple boundary junctions. The large condition number of the BEM matrix for the floating conductor is also considered. BEM based computer codes have been written to solve for the axisymmetric field. Validation of the method and code is carried out with published experimental results of a 288 kV class arrester.
Voltage distributions are computed for arresters of both 220 kV and 400 kV systems. The influence of the geometry of various components is studied in detail. The position and cross sectional diameter of the grading ring are found to have the maximum influence on voltage distribution. The position of the spacer is also found to affect the maximum stress. Optimum positions for these two are obtained. Voltage distributions are studied for arresters with 3 kV and 6 kV varistors. Due to the lesser spacer requirement with 3 kV varistors, the resulting voltage distribution is better. The voltage distribution is also found to improve with increasing disc diameter. The type and diameter of the container and the height of the pedestal are found to have insignificant influence on voltage distribution.
Environmental pollution forms a conductive coating on the porcelain housing under wet conditions. The resulting conduction can significantly alter voltage distribution. Considering this, voltage distribution studies are also undertaken for uniformly polluted conditions. The governing BEM equations are first derived. Studies are carried out for different pollution levels. Only under light pollution and dry conditions does the voltage distribution remain non uniform. Furthermore, the geometry of the different components is found to have a very similar influence on the voltage distribution as that under clean conditions.
In summary, the present work attempts a field theoretical study on voltage distribution in surge arresters. The boundary element based approach is developed for studying the prevailing capacitive–resistive fields arising due to bulk and surface conduction. Studies are made with practical geometries of 200 kV and 400 kV class arresters for both clean and uniformly polluted conditions. The influence of the geometry of various components is studied in detail, and optimum values are identified. | |
