Increasing reliance on renewable generation poses challenges

By Steve Downer, MMUA’s The Resource, January 2020 edition; reprinted with permission

Minnesota electric utilities are ready to serve peak load reliably for winter 2019-2020, but incorporating increased renewable generation may lead to reliability issues. Those were among the conclusions to be drawn from a Midcontinent Independent System Operator (MISO) quarterly update meeting held Dec. 13 at the Minnesota Public Utilities Commission (MPUC).

According to a presentation from MISO staff, adequate resources are available this winter to meet normal load under typical conditions. However, over recent years, MISO has seen an increase in total outages during the winter, particularly in January, while the base of renewable “limited use resources” has grown.

Total generation outages, including forced outages, have almost doubled from January 2015 to January 2019, from 24,681 megawatts (MW) to 44,411 MW. Much of this is due to “significant challenges” presented by increasing reliance on wind generation, which typically goes off-line at -22 degrees Fahrenheit.

The capacity adequacy of intermittent wind and solar generation is “always a concern,” said MISO’s Jordan Bakke. “Sun is not well matched to load.”

“Transmission solutions” are needed to further utilize renewable resources, and to significantly reduce curtailment. At 30 percent renewable penetration, MISO’s Renewable Integration Impact Assessment (RIAA) “indicates integration complexity increasing sharply.” At 40 percent renewable penetration, “the transmission system is insufficient to further facilitate renewables and access the diversity in renewables and load.”

The MISO presentation contained information from its RIIA, which studies capacity and energy adequacy and operating reliability, including the ability to withstand unanticipated component losses or disturbances, such as occurred during last winter’s coldest days.

MISO’s RIIA found that integration complexity increases sharply beyond 30 percent renewable penetration. The probability of losing load is “expected” to be one day in 10 years, over all penetration levels.

The effects of a high percentage of wind in the MISO North geographic footprint include a reduced effective load carrying capability (ELCC) during winter.

From its Base to 50 percent renewable “milestone,” MISO assumed the majority of its thermal fleet remains available to maintain adequacy (with 17 gigawatts retired), and 100 gigawatts of renewable capacity added.

As renewable penetration increases, the change in fuel mix at various electric demand points drives changing reliability risks. Increased percentages of wind generation coupled with decreased natural gas generation leads to increased reliability risk. Constantly shifting wind and solar generation leads to the need for increased ramping up and down of thermal (fossil-fueled) generation.

System-wide voltage stability is the main driver of dynamic complexity starting at 40 percent and worsens at 50 percent, which requires transmission technologies equipped with dynamic-support capabilities.

After the presentation from MISO, utility industry representatives responded to the presentation and Commissioners added some comments and questions. The discussion at this point revolved around batteries, electricity storage and various other technologies that, it was generally acknowledged, are not yet commercially available.

Electronic aids will likely be applied in various upcoming situations, but the ultimate electronic solution is not yet identified and MISO staff warned that without thermal, rotating generation, the grid “loses ability to remain stable.”

As the region’s generation portfolio becomes more weather dependent, the transmission system gets more stressed. Given our “current tool set,” said an ITC representative, more wind means more 345-kilovolt lines. And, significant transmission upgrades “won’t happen overnight.”