The Electric Reliability Council of Texas, which oversees the high voltage distribution system for about 85 percent of the state, has started to pull in power from neighboring grids and is asking for conservation until 7 p.m. Friday. Thursday about 5,000 megawatts of power plant capacity were offline, more than 7 percent of the state’s total capacity, pushing the state to the verge of rolling blackouts.More mechanical breakdowns occur during prolonged periods of hot weather, according to power plant operators, as units tend to run for longer hours.