Tips & News - January 2012

TIPS NEWS &

manufacturing issue or mishandling or partial discharge damage or some other thing,” says the consulting engineer. “Arresters can help eliminate momentary outages caused by lightning. (Note: It won’t prevent all momentary outages, like ones caused by falling tree branches.) And the way to do that is to install arresters at a fairly high population rate. GTC has done that on their lines. On the lines GTC has chosen to most protect, they have gone from seven, eight or nine outages per year— down to zero. They did this by installing arresters on every phase of every tower,” says the consultant. Playing with Line Designs In 1997, it was widely believed at GTC that a 115-kV line (based on its BIL and other characteristics) should be able to withstand a lightning strike of 30,000-Amps or less and not flash over. But, if the lightning strike was more than 30,000-A, engineers expected that the strike would invariably lead to an unpreventable flash-over. “Of course, we would try to reground lines to prevent problems caused by lightning strikes of less than 30,000-A, but we were not making much headway. Shortly thereafter, we began using The Electric Power Research Institute, Inc.’s (Palo Alto, CA) T-Flash lightning performance software to model its typical line configurations,” says Maddox. To get the best results, the model requires a lot of information, including type of terrain, tree cover, size and height of the poles, size of the wires, types of insulators assemblies, etc. The software then models the transmission line and predicts that there will be ‘X’ number of momentary outages per year (based on the lightning density of this particular part of the state and line modeling). Once the current line configuration is entered, transmission engineers and designers can experiment by modifying the line’s design; changing the shielding, the grounding, the insulators, the lightning arrester patterns, etc. They discovered something interesting. “When we modeled the lines with three lighting arresters on each structure (one on every phase), T-Flash predicted no momentary outages due to lightning—even if the strike was greater than 30,000-A. On the other hand, when we modeled the lines with two lightning arresters on each structure (one on the top phase and one on the bottom phase), T-Flash predicted one momentary outage every 16 years--due to lightning. So, GTC chose the top and bottom configuration as its standard, because it would reduce costs and one momentary outage every 16 years would be tolerable,” Says Maddox. So, for the last ten years, the GTC has been installing lightning arresters on its transmission lines, focusing on the load serving lines first (46-kV, 69-kV and 115-kV). Typically, the results have been excellent. What about that ‘terrible’ line—the one that had 10 momentary outages per year? “We went for broke and installed lightning arresters on all three phases at every structure. Momentary outages due to lightning went from ten outages per year to none for the next two years. For the last two years, that line has

This significant aluminum oxidation is due to moisture ingress in the Zinc Oxide Disks of a MOV type arrester.

After extensive modeling and virtual experimentation, GTC chose the top and bottom installation as its stan- dard lightning arrester configuration. 4 www.hubbellpowersystems.com