Monitoring the health of MOV arresters over the past few decades has been dominated by measurement of third harmonic currents, better known as leakage current. Unfortunately, this method is complicated by the fact that the arrester needs to be isolated from earth to accurately measure leakage current. The process is further complicated by the effects of internal and external currents. While this monitoring method may work well for arrester installations that have previously employed surge counters and where the arrester is already isolated from earth, it cannot be easily utilized for arresters that are solidly earthed.
In many installations, isolating the arrester base is not a practical option to allow for monitoring. Here, in place of leakage current, use of infrared scanning has become the method of choice. Thermal images of arresters are taken and then comparison is made to similar units in the same location. Still, in spite of its popularity, there is little guidance for users on what temperatures can be classified as being within a safe operating range and what temperature may be an indication that an arrester experiencing difficulties. There is a general rule of thumb that a 5°C difference between ‘partner’ arresters warrants some concern while a difference of 10°C is potentially serious. Typically, temperature differentials of greater than 10°C are not reported and it can be assumed that this is because any arrester that has reached such a point is either close to failure or will already have failed and been removed before the next thermal scan.
Even though IR scans are one of the best monitoring methods available, they are usually preformed on only a limited basis because they require a person to walk each sub-station to acquire the thermal images of each station arrester. Ideally, if the temperature of an arrester could be monitored continuously, a valued prediction of ongoing arrester health could be compiled.
Attend the 2017 INMR WORLD CONGRESS in Barcelona-Sitges from November 5 to 8 to hear a paper on this topic by arrester expert Jonathan Woodworth. He will also explore the variables that affect arrester surface temperature as well as the relationship between this parameter and watts loss of an arrester.