Responding to concerns that the Clean Power Plan could jeopardize electric-system reliability and risk power outages, U.S. EPA included a much-sought-after "reliability safety valve" to protect critical electric generating units.
The mechanism gives plants a 90-day period to exceed carbon limits during emergencies.
EPA air chief Janet McCabe said, however, that her agency expects regulatory relief for reliability purposes will be a rarity.
Regulators and industry interests had expressed concerns when the rule was prosed in June 2014 that the shift from coal-fired power to natural gas and renewable energy would require a rapid buildup of gas and electric infrastructure, including pipelines and power lines that might take years to construct.
The most prominent warnings about the Clean Power Plan's effects on reliability came from the North American Electric Reliability Corp. in November 2014.
The issue pulled the Federal Energy Regulatory Commission into a highly charged debate about what happens when environmental policy to combat global warming forces the closure of dozens of baseload coal plants and reconfigures the electricity system.
After soliciting stakeholder feedback in a series of technical conferences across the country, FERC suggested EPA include a backstop tool like a safety valve to address unanticipated events. But FERC stopped short of asking EPA to require a broad review of how state compliance plans might interact.
FERC said adequate processes for review already exist, and grid operators, overseers and reliability organizations could predict problems.
McCabe said grid operators and regulators have options if another extreme weather event like the polar vortex of 2013 and 2014 happens again. Short-term needs for coal- and gas-fired plants to run harder can be "smoothed out over these long averaging times," she said. The safety valve is "a place to go in a truly unexpected situation."
The rule would require states to justify the need for the safety valve exception and get sign-off from the grid operator or coordinator in their region. "While the initial 90-day period is in use, the emissions of the affected [generators] that exceed their obligation will not be counted against the state's overall goal or emission performance rate," the rule states.
But if conditions require a plant to continue operating under the safety valve provisions beyond 90 days — for example, if a nuclear plant were forced out of commission for a lengthy period — a state would have to revise its compliance plan to make up for the excess carbon emissions.
McCabe noted that electric reliability concerns are also addressed by the extra time in the final rule for states to prepare compliance strategies and to start cutting emission in 2022.
FERC Chairman Norman Bay praised EPA for its efforts to address reliability concerns "by adding time and flexibility for compliance, adopting a reliability safety valve, and requiring state plans to be reviewed for reliability."
"Much work lies ahead," Bay said as he released a "coordination document" reached among FERC, EPA and the Energy Department pledging to work together on the implementation of the Clean Power Plan.
"The three agencies will meet frequently, no less than quarterly, to discuss what they are learning about the developing state plans and any potential reliability concerns," the document said.
Others charged with overseeing the nation's electric grid reserved early judgment, promising further analysis to gauge the rule's effect on reliability.
PJM Interconnection LLC, the nation's largest grid operator, plans to analyze the final rule "for potential impacts to the power grid."
The Midcontinent Independent System Operator Inc. is launching its stakeholder process to "fully assess its impact," it said in a statement.
Also in a statement, the North American Electric Reliability Corp. said it expects to issue an updated reliability assessment in the second quarter of 2016.