Mohammed Reza Meshkatoddini - Power and Water Institute Of Technology, Tehran, Iran

In this paper the different causes of the failures or misfunctionning of ZnO surge arresters in power system are studied. These problems can be due to the thermal or electrical design, the quality of used materials, the manufacturing process or the utilization method. The first group of problems are created by the incorrect electrical and thermal design. Low threshold voltage comparing to the TOV level of the network, insufficient thermal exchange capability of the varistors and their housing with the embiemt atmosphere , to dissipate the energy of the electrical straints such as lightning impulses, can lead to a thermal runaway.The second group of problems are due to the poor quality of the varistors, the metalization layer on their opposite faces or the insulating layer on thir lateral side.The third group of the arrester problems are because of pollution on the arrester surface, presence of partial discharges in its housing, loose or oxided contacts in the arrester's external circuit and finally.a long electrical distance between the arrester and the protected device (transformer).These problems can cause perturbations in ZnO arrester's function, result damages in the arrester or the protected device , or cause problems in both the arrester and the transformer. In this work we have accomplished several experiments to study the above-mentioned problems and have presented some solutions to these problems.