Wind turbines across the Great Plains states produced, for the first time, more than half the region’s electricity. The power grid that supplies a corridor stretching from Montana to the Texas Panhandle was getting 52.1 percent of its power from wind at 4:30 a.m. on Sunday. As more and more turbines are installed across the country, Southwest Power has become the first North American grid operator to get a majority of its supply from wind. That beats the grid’s prior record of 49.2 percent and the 48 percent that a Texas grid operator reached in March, Derek Wingfield, a spokesman, said in an e-mail.