The lumbering coal-fired power stations facing closure because of age and air emissions have been the workhorses of the U.S. high-voltage electric transmission grid. When the grid was stressed and frequency dropped, they ran harder. Now, as tomorrow's grid is reshaped with more wind farms, solar arrays and gas-fired plants, experts warn that new regulation will be needed to ensure that these new resources provide the frequency support and other essential services that the coal plants delivered.