Tips & News - January 2012

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from 50 to 75% of momentary outages are due to lightning at GTC,” says Maddox. Lightning Arresters Versus Moisture

With so much riding on the lightning arresters, GTC keeps a close watch on them and sends failed arresters to a consultant for analysis. A consulting engineer in surge protection, says, “GTC has been having some problems. They wanted to know if the arrester failures were a one-time thing or a systemic problem that they need to worry about. GTC is very progressive in terms of reliability: I think the company is way ahead of the crowd in terms of protecting their system with arresters.” Analysis of failed arresters from GTC points to the usual suspect. The consulting firm found that moisture ingress is the predominate cause of arrester failures at GTC (which is consistent with the industry in general) and that there was evidence of moisture ingress in older arresters that had not yet failed. “Of course, moisture ingress is only one source of failure. Mechanical failure due to swinging is another,

Lightning is a common occurrence in Georgia and impacts a utility’s momentary outage index.

but moisture is the most common problem and it is the arrester’s biggest enemy. Moisture is the nemesis of the arrester designer (both in porcelain and polymer arresters). Designers continuously strive to make lightning arresters water insensitive,” says the consulting engineer. Choosing Protecta*Lite® Arresters To reduce problems associated with moisture ingress, GTC is now installing Protecta*Light ® arresters from Hubbell Power Systems (Ohio Brass ® brand), and has found them to be a reliable solution with a very low failure rate. So far, the few Ohio Brass failures have been due to installation problems. The consultant continues, “Moisture ingress can lead to failures in more than one way. For example, if lightning strikes a pole or line… I’ve seen arresters installed on the un-faulted phase fail during the line to ground fault. Often times, if there is a fault on phase ‘A,’ the line to ground voltage on Phases B and C increase and, depending on the configuration of the system and the impedance of the neutral, the voltage can rise high enough to cause arrester failures on the un-faulted phases. Lightning didn’t actually pass through the arrester but caused the event that lead to the failure. It would appear like a lightning failure and would most likely be recorded as such, but it was a temporary overvoltage (TOV) failure. If the arrester was already compromised by moisture, then I would say neither the lightning nor the overvoltage stress was actually the cause of the failure. Moisture ingress was the root cause.” It is a common misconception that lightning is the biggest reason for arrester failures, but that is not the case. Modern arresters are tough and are capable of surviving many lightning strikes over time. “Fortunately, when lightning of any magnitude (30 or 50-kA), passes through an arrester, it leaves a footprint. It actually modifies the characteristics of the semi-conductor and if you look at its characteristics with an oscilloscope, you can see that it experienced a high current impulse. Over the years--maybe 10% of the time—the arresters I’ve examined have failed due to a very high current surge caused by a direct lightning strike. But, usually the failures are due to moisture ingress or a

Lightning and Momentary Outages Lightning can cause momentary outages in three distinct ways. The first is the classic overvoltage flash from a high amperage strike. The voltage builds-up and flashes from the insulators to ground. The second is a ‘shielding’ failure. This occurs when lightning hits a line from the side, bypassing the over- head grounding (shield) wire. Lastly, a momentary outage can occur due to ‘back-flash,’ when lightning hits the pole. As the cur- rent passes down the pole ground, a voltage of sufficient magnitude can be created. This voltage can flash over to the conductors. In this case, improving pole ground- ing will provide some protection. Despite grounding, really high magnitude strikes will still cause a back-flash. But, properly installed arresters can prevent all types of momentary outages caused by lightning.

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