Integrating large quantities of variable energy resources into the North American bulk power system will require fundamental electricity system planning and operational changes to ensure continued reliability, the North American Electric Reliability Corporation (NERC) says in a new report that it prepared in collaboration with the California Independent System Operator Corporation (ISO).

The assessment, “Maintaining Bulk-Power System Reliability While Integrating Variable Energy Resources – CAISO Approach,” seeks to identify considerations that all system planners and operators must address to reliably integrate significant quantities of wind and solar photovoltaic resources into the bulk power system, and it showcases how CAISO is addressing these challenges as it prepares to connect more than 11 GW of variable resources to its grid over the next eight years.

Variable energy resources have different characteristics and respond differently on the system compared to fossil-fueled, large-scale hydro, and nuclear resources that have traditionally provided electricity supply, the assessment notes. “Industry has established reliability expectations with these generating technologies through knowledge accumulated over many years of experience,” it says. Less is known about how variable energy resources on a large scale will respond on the system, but it is certain that “As larger amounts of variable generation are added to the system, they will displace the traditional large, rotating machines and the operating characteristics those machines provided,” NERC concludes.

NERC has repeatedly noted the growing presence of wind and solar resources and their potential impact to reliability since 2012. In its latest Summer Assessment, it recommended that because weather plays a key factor in determining wind and solar output, “enhancing regional wind and solar forecasting systems can provide more accurate generation projections. Other methods include curtailment and limitation procedures used when generation exceeds the available regulating resources. In this respect, operating criteria, forecasting, commitment, scheduling, dispatch and balancing practices, procedures, and tools must be enhanced to assist operators in maintaining [bulk power system] reliability.”

In the new assessment, CAISO projects that more flexibility in accessing essential reliability services will be needed to reliably meet net load, manage approximately 3,000 MW of intrahour load-following needs, and provide nearly 13,000 MW of continuous up-ramping capability within a three-hour time period.

The report also concludes that as an electric system approaches a significant penetration in variable resources, technical aspects of the evolving resource mix must be given due consideration at state, federal, and provincial levels. Solutions to maintain reliability could come from market tools and rules, new technology integration, and standards or requirements. It notes, however, that unresolved cost implications could impede those solutions.