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.
According to the North American Electric Reliability Corp. (NERC), those cleaner, greener sources of power provide a "significantly lower level of essential reliability services than conventional generation."
The Federal Energy Regulatory Commission (FERC) also said there is a "significant risk" that primary frequency support -- the initial source of backup power for the grid -- could be weakened if gas, wind and solar don't offer the services the shuttered coal plants provided.