Abstract: - The electric field distribution in high voltage plant and insulation systems is governed by the shape and location of the metallic electrode as well as the type of insulation media. Surge arresters are common examples of configurations giving rise to non-uniform electric field distribution. In this paper effect of grading rings and spacers on potential distribution of ZnO arresters are presented.
Key-words: -Grading ring, Spacer, ZnO Arrester, Potential
The voltage distribution on a ZnO surge arrester under normal operating conditions has been observed to be non-uniform. The discs at the top are subjected to a higher voltage and hence thermal stresses, leading to a faster thermal ageing of these highly stressed discs [1,2,3,4].
In this paper with the aid of PC-Opera-8.7 software  two surge arresters (230 kV and 400kV) are simulated and effects of grading rings and spacers are investigated.
In the present paper two surge arresters (220 kV & 400 kV) are simulated . 220 kV surge arrester is composed of two units and 400 kV surge arrester is composed of three units. These units are connected together. The housing of arresters is made of porcelain.
Fig. 1 shows the simulated 220 kV surge arrester.
Fig. 2 shows simulated 400 kV surge arrester.
3. Effect of Grading Position
- Effect of Grading Position
The grading ring is used to make the voltage distribution uniform. Its efficiency is clearly dependent on its relative position. Therefore position of grading rings on 200 kV and 400 kV surge arresters are changed. Potential distribution on arresters is shown in figs. 3 and 4.
The maximum of voltage gradient for different height of grading ring is shown in table 1.
- Effect of Grading Ring Diameter
In this section we assumed the position of grading ring at 85% of height of arrester and the effect of diameter of grading ring on maximum of voltage gradient is investigated and results a are shown in table 2.
- Effect of Spacers
The height of the porcelain housing is dependent on the voltage rating of the arrester and the environmental pollution conditions . Thus for given voltage rating, the total height of the discs will be less than the height of housing and hence metallic spacers are employed to make up the extra height. To ascertain the effect of these spacers, three cases were considered and effect of positions of spacers on the maximum of voltage gradient are shown in table 3.
From results of this paper we can conclude:
- For both arresters the best position of grading ring is at the 75% to 80% of height of arrester.
- By increasing the grading ring diameter, the maximum value of voltage gradient will decrease. 3-Best position for spacers is bottom.