TABLE OF CONTENTS The performance of surge arresters in power systems is determined on the one hand by the electrical and thermal properties of the varistors, and on the other by the design and installation of the arrester.
Compared with SiC gapped arresters, ZnO gapless metal oxide arresters offer a protection closer to the ideal. However, because these arresters contain no gaps, a leakage current flows through the material at working voltages which causes power losses and heating of the ZnO elements. This can be dangerous to the stability of the arrester, particularly in the low conduction regime where the V-I characteristic of ZnO material is very sensitive to temperature. Badly dimensioned arresters are exposed to the risk of thermal runaway. A further practical problem relating to power system usage concerns pollution of arresters, which can cause abnormal voltage and current distribution in the arrester in both steady state and transient conditions.
In surge arresters, the ZnO valve elements are generally located in an environment consisting of gas or solids which will limit cooling of the valve elements when submitted to their continuous operating voltage. As can be seen on Figure 5.3, for the same applied voltage level, any temperature rise would increase the current because of the high sensitivity to temperature of the V-I characteristic in the low conduction regime. A sort of positive feedback will occur and may cause...
