TABLE OF CONTENTS Figure 5.27 reproduces the procedure of arrester selection specified by the standards [166]. During the selection process of a metal oxide surge arrester for a particular application, the following arrester parameters need to be determined: the continuous operating voltage, the rated voltage, the nominal discharge current and the protective characteristic, the line discharge class, the pressure relief class, the pollution withstand and live washing capabilities and the mechanical properties [166]. The following are particularly important.
Continuous operating voltage U c: must be higher than the maximum attainable power frequency system voltage including any harmonic content. It is proposed to use a safety factor of 1.05 to account for harmonic content. The continuous operating voltage level to be used depends on the system earthing and the earth fault clearing arrangements.
Rated voltage: is determined from the temporary overvoltage levels where the arrester is to be installed. The power frequency voltage characteristic of the arrester should be higher than the temporary overvoltage characteristic of the system. The standards recommend considering temporary overvoltages due to earth fault overvoltages, load rejections, resonance effects and Ferranti effect. Table 5.6 reports current rating for the whole range of standard rated voltages.
Nominal discharge current: is normally selected according to the lightning discharge current through the arrester. Table 5.7 summarises the recommended values of arrester nominal discharge current for the range of voltages in...
