Why are Midwest grid operators turning away wind power?
Note: Midwestern United States is the area overseen by the MISO and the SPP balancing authorities.
As wind generation capacity has grown in the Midwest of the United States, grid operators have increasingly restricted wind generation because of both oversupply and congestion on the grid.
Grid operators in the areas overseen by the Southwest Power Pool (SPP) and Midcontinent Independent System Operator (MISO) curtailed an hourly average of 800 megawatts (MW) of wind generation in the Midwest last year, compared with less than 200 MW in 2019. Curtailment is when electricity generation is deliberately reduced below its maximum generation potential. Wind generation increased by 42% between 2019 and 2023 and makes up a substantial proportion of the energy mix in the Midwest. Hourly average Midwest wind curtailments decreased last year from a 2022 record of almost 1,000 MW because of less generation brought on by slower-than-normal wind speeds. Grid operators also made changes in 2023 to how they planned for congestion and oversupply.
Curtailments can be necessary for grid operators to balance supply and demand. Curtailments occur either when generation exceeds electricity demand (oversupply) or when insufficient transfer capacity is available to transmit electricity over its preferred path to meet demand (congestion). Wind is curtailed before other resources in the Midwest because:
- It is cheaper and faster to both shut down and restart wind (and solar) plants than other types of generation.
- On very windy days in particular locations, transmission capacity is often insufficient to receive the large amount of wind power generated.
In 2023, wind generation made up 36% of SPP’s total generation and 15% of MISO’s. Average hourly wind curtailment increased from 136 MW in 2019 to 1,097 MW in 2023 in SPP and from 242 MW to 508 MW in MISO. Fewer curtailments occurred in 2023 than in 2022, largely due to decreases in wind generation.
In MISO, congestion often occurs during winter storms because of associated high wind speeds and transmission constraints. Curtailments were lower in winter 2023–24 compared with the past two winters partly because of better resource planning and congestion management strategies during winter storms. Although wind generation within MISO set a record of almost 26 gigawatts (GW) within one hour and averaged 16 GW per hour during Winter Storm Heather in January 2024, MISO curtailed wind generation 12% less throughout winter 2023–24 than in the previous winter and 47% less than in the winter of 2021–22.
Limiting transmission congestion
In the Midwest, curtailments are more common because of congestion, when the amount of generation in one part of the grid exceeds the capacity of transmission lines to bring it to areas of greater demand. In MISO, the market monitor found that high wind output caused almost half of MISO’s transmission system congestion in 2022, resulting in average hourly wind curtailments of 726 MW per hour, including curtailments of as much as 5.9 GW in some hours of 2022.
Managing oversupply
Operators sometimes curtail wind generation because of oversupply.
The generation chart from April 7 shows how operators in SPP use wind curtailments in the early morning hours to reduce electricity supply closer to demand. In the evening, when system demand is higher, the need to curtail wind generation is eliminated entirely.
Congestion in either SPP or MISO can affect the other because of the more than 6,000 MW of interties between the two.
Principal contributors: Lori Aniti, Susanna Smith
Tags: generation, electricity, Midwest, wind