The U.S. Southeast’s biggest utilities have filed plans for a Southeast Energy Exchange Market, an electricity trading platform that they say could reduce power prices for about 50 million people and better integrate the region’s growing share of clean power in years to come. But clean-energy advocates and big corporate energy buyers worry that the plan submitted to North and South Carolina regulators on Friday could divert efforts in both states to explore wider-ranging electricity system planning and integration across the region. Friday’s announcement from Duke Energy marks the first official step in a plan first revealed this summer. The modern, technology-enabled 15-minute energy trading market being proposed would replace the bilateral electricity trading now conducted by the region’s utilities, including Southern Company, Dominion Energy South Carolina and the Tennessee Valley Authority.