1 power systems. An arrester with some kind of failure is not able to actuate properly during an electrical surge. As a consequence it will present an excessive heating which can lead to a thermal runaway [1, 2, 3]. Thermographic inspection is a widely used method to monitor surge arresters preventing their failure. When an arrester is substituted due to some abnormality detected by means of thermal inspection, no further analysis is done, so that the causes of the problem cannot be evaluated. In addition, there is no study indicating which are the main causes of failures in ZnO arresters or if there is another alternative to the substitution of the. This work presents a study done with 96 kV ZnO arresters presenting the most common failures detected in substations and their effects on the thermal image. Index Terms—Arrester, Diagnosis, Failures, Monitoring, Thermal Image, ZnO. I. INTRODUCTION 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 [4]. This work had the financial support of the Companhia Hidro Elétrica do São Francisco – Chesf, by means of a Research & Development project with the Universidade Federal de Campina Grande, and of the Federal government by means of CAPES. E. T. Wanderley Neto and E. G. da Costa are with the Universidade Federal de Campina Grande, Campina Grande, Brasil. (e-mail: estacio@ee.ufcg.edu.br, edson@ee.ufcg.edu.br). M. J. A. Maia is with the Companhia Hidro Elétrica do São Francisco, Recife, Brasil. (mjamaia@chesf.gov.br). 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. II. OBJECTIVES This study has the main purpose of indicating the most common failures in arresters and their influences over their thermal images. This is the starting point for the creation of an analysis system based in thermal images and artificial intelligence techniques. In order to achieve this goal, a series of tests simulating failures in actual 96 kV ZnO arresters was conducted. These simulated failures correspond to the most common problems found in arresters in operation. Their analysis will allow the creation of a more safe criteria to the replacement of defective arresters. Besides that, arresters manufactures will be more concerned about the main problems occurring with their products. III. FAILURES IN ZNOARRESTERS The usage of ZnO arresters in power systems has begun at about 30 years ago. They were created as an evolution of semi conductive ceramics for electronics and became an alternative more efficient in relation to the silicon carbide ceramics used in power surge arresters. Besides the problem related to failures in ZnO arresters, there is also too little information about the natural ageing of ZnO varistors. That’s because the 1-4244-0288-3/06/$20.00 ©2006 IEEE 2006 IEEE PES Transmission and Distribution Conference and Exposition Latin America, Venezuela Failure Analysis in ZnO Arresters Using Thermal Images E. T. Wanderley Neto, E. G. da Costa, T. V. Ferreira, and M. J. A. Maia