Many changes have transpired in the arrester industry over the past 25 years, but the one word that perhaps best summarizes the most important is, ‘polymers’. The transition to polymeric housings started a few years before 1990 and fundamentally transformed not only the form of the housing but also its functionality. With this, new markets opened up, standards had to be rewritten and, most important, the substation and pole top became much safer places.
In April 1987, U.S. Patent No. 4,656,555 – titled Filament Wrapped Electrical Assemblies & Method of Making Same – was issued to Donald E. Raudabaugh, at the time Engineering Director at Ohio Brass. The concept behind the patent proved a ‘game changer’ for the arrester industry that started first with distribution models but, by the early 1990s, had started to find its way into most designs. Interestingly, the patent did not focus on the polymeric housing. Rather, this was assumed while introducing a novel, cost-effective means of shifting the arrester’s mechanical strength to a new component – the internal module. Until that time, porcelain insulators had provided the arrester’s seal and strength; however, the era of the polymeric-house arresters was set to take off.
It might seem that the changeover from porcelain was little more than a shift in housing material. But not so! The fundamental strength characteristic of the arrester was being transferred to internal components since the polymeric housing itself offered none. The polymeric housing did however offer other important functions, as porcelain, including weather protection, and elongated leakage. But there was also an additional function of great importance – namely a fast-responding pressure relief system. The difference in this regard is subtle yet substantial. A polymeric arrester housing does not allow pressure to build up and relieves it at relatively low levels. Indeed, eliminating the risk of fragmentation of porcelain was what triggered the rapid shift in technology. Polymeric designs typically remain intact during an overload event and it’s rare that internal components are ejected into the substation as still seems to happen with porcelain, in spite of extensive testing to certify high-pressure relief capability.
The strength function of the housing in polymeric arresters was picked up by the internal module. Variations to the original Raudabaugh concept are still being produced but today one can also find several other designs, e.g. a ‘cage’ whereby rods clamp the MOV disks between two end pieces. The module must also be designed so that it cannot allow pressure to build-up. This was accomplished in a durable, cost-effective way by creating openings or weak points along the module’s side. Incidentally, early designs of polymeric station class arresters had insufficient cantilever strength compared to porcelain. Therefore, for taller units, a hollow core composite insulator housing allowed for significant increase in mechanical performance.
Besides being safer, polymeric arresters also offered much lower surface leakage when the arrester was wetted. Moreover, because of less power losses over the long-term and lower energy costs to produce, polymeric arrester definitely proved a ‘greener’ product than what they replaced. In addition, since polymeric housings could be molded with thinner sheds than porcelain, it became possible to get longer leakage distances. This, in turn, allowed shorter arrester designs for most applications and also units that could perform well even in service environments marked by high pollution levels.
When polymeric housings were first introduced, it was quickly assumed that they would be the solution to moisture ingress problems that had traditionally plagued porcelain designs. This, however, did not prove to be so and considerable care is still needed in designing and manufacturing for units to remain unaffected by ambient moisture. Finally the polymeric housing on arresters reduced the weight of arresters to the point that they could easily be suspended from lines and poles and installed in even the most demanding locations. This made the transmission line arrester a practical reality.
Service life expectancy was of course a concern at the start of this ‘new arrester era’. The only assurance designers had was the solid track record of experience offered by composite insulators – something that also positively influenced acceptance of this technology. History now tells us that life expectancy of silicone rubber housings is outstanding and, with the exception of some poorly formulated models, these housings seem to last indefinitely. The 1987 vintage OB arrester, for example, seems to show no sign of end-of-life, even after over 25 years of service.
Now, what will come next in this technology?