Arresters, Woodworth on Arresters

It is a challenge to accurately describe what arresters will be like 35 years from now. But some things can be predicted with near certainty. One is that they will be smart arresters. In fact, the smart arrester is just around the corner and its ‘brain’ already exists in the form of the leakage current monitors now available on the market. However, these systems still lack good communication skills and, until they learn how to better inform the external world of their status, arresters will continue to only be ‘semi smart’. In the meantime, we need to find ways to better assess their condition in service. In a day when we increasingly need information right away, the best two methodologies for assessing the condition of arresters are thermal imaging and 3rd harmonic leakage current monitors. While much has been written about surge monitors, there is still lack of practical guidance on applying thermal imaging to asset management of surge protection.

The reason thermal imaging can assess the health of a surge arrester is because these components dissipate very little energy during steady state operation and seldom exhibit a temperatures much above ambient. Even the largest MOV arresters, e.g. 4 to 5 meters in height, dissipate less than 50 watts. With an arrester of that length, this does not create a discernible temperature rise and makes any effort to measure temperature gradient above ambient a challenge.

Infrared thermographer, Bill Hoth in Clearwater, Florida has been scanning substations thermally for over 30 years and has developed some interesting methods. Over the course of a typical year he scans over 700 substations, visiting each four times and reporting any anomalies he discovers on a daily basis. His workday starts around midnight and for the 6 hours or so before sunrise he focuses on arresters. Through his long experience, Bill has found that he can achieve a better assessment of arrester temperature in an environment without sunlight and can more easily see less than a degree difference from ambient in an arrester. While he preferred not to share the specific settings he uses for arrester analysis and which constitute his proprietary knowledge, it is a multi-tiered approach.

For example, if an arrester displays a difference of even a few °C from similar nearby arresters, it is placed on a watch list. If the arrester displays from 4 to 6°C difference, then it is tagged to be checked again in a few weeks. If the temperature gradient is greater than 7-10°C, then the arrester is placed on an emergency change list. This same type of approach is used by other utilities across North America. One point that is almost certain is that seldom does any surge arrester become hotter than 15°C in comparison to other units in its vicinity. Once temperature starts to rise that much, leakage currents will also increase which in turn causes even faster heating so this means it will likely not survive much longer.

Arrester in center is about 9°F (4°C) hotter than that at far left and would be checked again in a few weeks.
Arrester in center is about 9°F (4°C) hotter than that at far left and would be checked again in a few weeks.

If an arrester is in the process of failing or slowly reaching an end-of-life event, it is usually because a dialect failure is taking place within it. MOV disks are extremely stable devices under steady state conditions. However, if moisture finds its way inside, the resulting voltage stress along the arrester’s length can lead to dielectric breakdown at the edges of disks. It is this leakage current along the edges that dissipates the energy that is converted into heat and becomes a telltale sign of eventual failure. Unfortunately, partial discharge does not itself generate enough of a heat signature to be detectable with thermal imaging.

The benefits of thermal imaging are significant, starting with speed of data collection. Indeed, there is no faster way at present to tell if an arrester is near end-of-life than a scan of its temperature. Accuracy from a distance is also excellent, especially if using a long-range camera lens. The risk that an arrester is in the process of failing without also generating some heat is very low. At the same time, it is worthy of note that if an arrester is damaged by lightning strike or switching surge only days after its last thermal scan, it may well fail before the next scheduled scan. This potential for failure between successive scans is perhaps the only major negative of thermal imaging. Another drawback is that there are currently no thermal imaging devices on the market that can transmit data remotely. As such, obtaining a thermal profile for an arrester requires an individual to physically go to the site and collect data. Still, in spite of these negatives, thermal imaging remains the best option to determine the health of a surge arrester while energized – at least until a completely smart arrester becomes available.

Jonathan Woodworth