Zinc oxide (ZnO) surge arresters are part of the protection system of power substations and transmission and distribution networks. They actuate dumping overvoltages generated by atmospheric or switching surges, avoiding them to reach equipments like power transformers. To work properly, an arrester absorbs part of the surge energy converting it to thermal energy. A defective arrester may not be able to work in a satisfactory way when an electrical surge occurs, presenting an excessive heating. As a consequence, a thermal runaway can begin, when the leakage current and the temperature of the arrester increases till its explosion or break up, risking the power system and people nearby. To avoid this kind of problem, power companies have been more concerned with the operation of their arresters, searching and improving techniques for their monitoring and maintenance. The leakage current monitoring and thermal inspections have been pointed as the most effective techniques for monitoring and predictive maintenance of ZnO arresters.
Thermal image analysis is a non-invasive monitoring technique that has been used for a long time in transmission lines and power equipments [5]. However, in the case of surge arresters there is no specific methodology for the application of thermal monitoring. Each company follows its own methodology and determines its own parameters to be analyzed, based on the experience and observation of their technicians and engineers. Besides that, when an arrester is pointed as defective by means of thermal inspection, no further study is conducted to point the causes of failure or an alternative solution to the replacement of the equipment. So, the simple acquisition of a thermal image does not lead to a precise diagnostic. A complete analysis could be done with more detailed study of the thermal image and the conditions to which the arrester was submitted. A complete case study and the creation of a failures database can become a very important tool to aid the analysis of suspicious arresters thermal images. Beyond that, it could help in a precise diagnosis of the problem of the equipment. This paper presents an study of failures in ZnO arresters was conducted in the High Voltage Laboratory of the Universidade Federal de Campina Grande.