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Powerline Outage Spotlights Strength of GELD Grid

by Russell Harris


   Last Sunday evening about 10 p.m. Groton Electric Light Department confronted a serious outage when one of Groton’s two 69,000 volt transmission lines maintained by National Grid failed. Thanks to GELD planning and system design, most people experienced the serious outage as ‘a couple flashes’ rather than half the town being without power for 18 hours while one of the two transmission lines feeding the town’s electrical grid was being repaired by National Grid. 

   GELD Manager Kevin Kelly explained that one of Groton’s 69,000-volt lines coming into Groton through Ayer [near Tiny’s Restaurant], and then continuing onto Pepperell and Dunstable, failed. He said that the exact cause of the failure is unknown, but that it was probably due to structural failure of one of the towers or the electric line itself coming into contact with something. 

 Two transmission lines provide power to the town. One line and one transformer serve roughly half the town while the other half is served by a separate transmission line and transformer. There are four sub-circuits in town with each transformer servicing two circuits. 

    Kelly explained that Groton’s equipment is programmed to detect automatically a transmission line failure.  If a power-line failure is detected, the town’s electrical grid automatically reconfigures itself from two circuits into a single circuit running on the one ‘good’ transmission line.  The ‘flashes’ experienced at the outset of Sunday’s outage were National Grid’s protective gear operating in Ayer.  After the outage occurred, Groton’s grid automatically converted itself to single circuit mode.

      For 18 hours all of Groton was running on one transmission line with one transformer on a single circuit.  If anything had happened during the time when there was only the one transmission line, everybody in town would have been out of power. Fortunately, that did not happen.

   GELD Manager Kevin Kelly said, “We at GELD are doing everything we can - at a reasonable cost, of course, - to maximize the redundancy in the system. This is a perfect example of how and why we build in redundancy when planning out the system.

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