The Arizona Corporation Commission voted to exempt a 200-megawatt power plant expansion in Mohave County from environmental review, upending the way the commission has regulated power plants for decades. State law requires utilities to obtain a certificate of environmental compatibility before building power plants larger than 100 megawatts. But UniSource Energy asked regulators to rule the company should not have to undergo an environmental impact assessment for a proposed 200-megawatt expansion of the company’s Black Mountain Generation Station in Mohave County.