A decade after dropping the Tennessee Valley Authority in search of cheaper electricity, Bristol Virginia Utilities has rejoined the TVA system. With the start of the new year, the 16,000-customer distributor became the 159th municipal supplier or electric cooperative to hook up with TVA, the nation’s largest public utility. Bristol had been buying its power from American Electric Power Co., one of the nation’s biggest investor-owned utilities. Bristol becomes the only distributor of TVA power based in Virginia and the first distributor to leave and later rejoin TVA in the 75-year history of the Knoxville-based federal utility. TVA rates under the new 20-year contract will be higher than what AEP charged, at least initially. But Bristol’s consultants project that TVA-generated power will be cheaper over the long run, even with an anticipated TVA rate increase in 2008.