The failure of an arrester almost always results in a complete short circuit inside its housing.
In most scenarios, failure occurs due to dielectric breakdown, whereby the internal structure has deteriorated to the point where the arrester is unable to withstand applied voltage, whether normal system voltage, temporary power frequency overvoltage (e.g. following external line faults or switching) or lightning or switching surge overvoltages.
There are a variety of reasons why an arrester might reach such a state. This article, contributed by longtime expert, Michael Comber, discusses the most typical modes by which arresters fail.
Perhaps the most common cause of arrester failure is moisture entering its interior. This implies that the arrester was:
• not well designed, or
• not properly manufactured, or
• damaged by some external force resulting in a compromise to its sealing system.
While the underlying cause in each case may be the same, the way this progresses to eventual failure can vary significantly.
For a hollow core arrester where there is gas space around the column of MOV blocks (typically dry air or nitrogen), even a tiny leak can result in what is referred to as ‘seal pumping’ due to pressure differentials. For example, during the day suns heats the arrester such that the internal pressure increases relative to ambient and outward gas leakage occurs. When the arrester cools at night, this process reverses, with internal pressure dropping below ambient and external air (with all its moisture content) being drawn into the arrester. Such a cycle can repeat itself over many days, months or even years before the moisture inside builds to the point where there is a problem with reduced dielectric integrity.
In a solid core arrester design (with little to no internal gas space) this process will not take place. However leakage can still occur through imperfect end seals. In this case, moisture ingress is more due to ‘wicking’ – a process whereby moisture gradually finds its way down through interfaces between the MOV blocks and the materials in contact with them.
The manner in which dielectric integrity is degraded due to moisture ingress can also vary. The mere presence of moisture, if concentrated only within the gas inside a hollow core arrester, will not have significant impact on dielectric strength. Rather, it is how this moisture interacts with internal surfaces and materials that becomes the issue.
It has been noted, for example, that moisture related failures of porcelain-housed arresters tend to occur more in the evening than during the heat of day. This is attributed to accumulated moisture condensing on the inside walls of the porcelain when it cools after sunset. Electrical strength across the wall is then progressively reduced until internal flashover occurs from end to end.
Moisture will typically not condense on the MOV blocks of an energized arrester because these generate enough heat to keep their temperature slightly higher than that of the surrounding gas. However, if the material used to coat or collar the blocks is hygroscopic, it can absorb moisture thereby causing some blocks to become more conductive on their outer surfaces. This essentially shifts voltage to other blocks and leads to higher conducted currents, external to some blocks but internal to others. Ultimately, the complete stack can no longer withstand the applied voltage. (Note: this scenario can be avoided by ensuring only non-hygroscopic collaring materials, such as glass, are used.)
In the case of solid core arresters, moisture that has wicked into internal interfaces along either a portion or the complete length of the arrester can result in dielectric breakdown and failure.