SAN JUAN, Puerto Rico — A big fire erupted at an electricity plant that powers most of Puerto Rico on Wednesday, causing a blackout that swept across the U.S. territory of 3.5 million people. The Electric Power Authority said two transmission lines of 230,000 volts each failed. Executive Director Javier Quintana told reporters that he expected most power to be restored by Thursday morning. Puerto Rico’s fire department said it had extinguished the blaze at the southern power plant, which serves a majority of customers on the island. Gov.