The PJM Interconnection system can remain reliable with the addition of more natural gas and renewable resources, an analysis by the U.S. grid operator says.

However, the report notes that an increased reliance on any one generation type brings resilience risks not accounted for under traditional reliability standards.

The report, “PJM’s Evolving Resource Mix and System Reliability,” responded to questions about the effects of fuel diversity on reliability. PJM stakeholders had questioned whether the system is losing too many traditional resources as coal plants retire and nuclear owners consider their future. (Read the report.)

At an electric power conference in mid-April, Andrew Ott, PJM president and CEO, said that the share of natural gas-fired generation had grown from 4% of the mix in 2008 to around 30% today.

Similar reliability issues are in play in the adjacent MISO region where coal-fired generation's share has fallen from 80% to 50%, and natural gas has grown from an 8% share to around 30%. MISO grid operators also must contend with wind generation, which accounts for as much as 9% of total installed capacity, up from virtually zero a decade ago.

The PJM report analyzed the availability of generator reliability attributes essential to the grid under potential resource portfolios. Those qualities include frequency response, voltage control, ramp, fuel assurance, flexibility, black start, environmental restrictions and equivalent availability.

“We found that the risk to the system wasn’t that resources couldn’t necessarily provide reliability attributes but that the potential concentration of a single fuel source or low-probability, high-impact events could cause significant impacts to the system,” says Michael Bryson, vice president – Operations, who led the study.

PJM created a “composite reliability index” to assess the operational reliability of various resources across four states: normal peak conditions, light load, extremely hot weather and extremely cold weather.

The study found that a more diverse fuel portfolio isn’t necessarily more reliable. Certain resource blends that fall between the least and most diverse offer the greatest number of key generator reliability attributes. An adequate level of diversity fosters flexibility and adaptability in mitigating risks associated with equipment failure, fuel price volatility, supply disruptions, extreme weather and other unforeseen system shocks.

PJM’s current resource profile includes natural gas, coal, nuclear, renewables, demand response, and other generation types. The analysis identified no limit to the amount of natural gas-fired generation that could be added to the system before it affected reliability; however, highlighted the potential increased dependency on fuel infrastructure and the need for PJM to further explore grid resilience.

The report did not address the economics of resource types, factors that might impact a fuel’s deliverability or public policy issues such as environmental impact, including the use of subsidies.