MISO Releases Renewable Integration Impact Assessment

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MISO Releases Renewable Integration Impact Assessment

February 16, 2021

Editor’s note: While the wider situation is no doubt known to our readers, it should be noted that the following report is made with a large swath of the country’s midsection, from Texas to the northern plains, suffering electrical outages. While identification of causes awaits another in an increasingly long line of post-outage postmortems, the proximate causes seem to be unusual weather and the demands placed on the electrical grid.

While policymakers call for 100 percent renewable electricity and utilities shift to provide it, the Midcontinent Independent System Operator (MISO) recently cautioned that “as renewable energy penetration increases, so does the variety and magnitude of the bulk electric system need and risks.”

That statement came in February’s release of an updated Renewable Integration Impact Assessment (RIIA). The assessment finds that managing the regional electrical grid beyond a 30 percent system-wide renewable level “is not insurmountable” but will require “transformational change.”

The RIAA examines renewable penetration levels up to 50 percent, far short of the 100 percent currently being called for by many Minnesota policymakers.

The report steers clear of overt policy recommendations but notes the need for “transformational thinking and problem-solving.” 

There are mighty problems to wrestle with.

“The grid’s ability to maintain stable operation is adversely impacted, primarily when renewable resources are clustered in one region of the transmission system,” notes the report. As inverter-based (digital) resources displace conventional (physical) generators, the grid loses stability.

To maintain stability with increased renewable resources, MISO foresees a combination of multiple technologies — such as high-voltage direct current (HVDC) lines, synchronous condensers, motor-generator sets and emerging technology such as grid-forming inverters. This transition will need to be accompanied by “operational and market changes to identify and react to this risk as it occurs.”

The periods of highest stress on the transmission system shift from peak power demand to times when renewables supply most of the energy and long-distance power transfers increase. As renewable resources supply most of the energy, the system becomes more dependent on the stability attributes of the remaining conventional generators, increasing the system risk associated with unexpected outages of those generators. The direction and magnitude of power flows change rapidly due to the output of renewable resources that vary with weather conditions, resulting in new and shifting periods of stress.

The risk of not having enough generation to meet demand shifts from the historic times of peak power demand to other periods, specifically hot summer evenings and cold winter mornings, when low availability of wind and solar resources is coincident with high power demand. These shifts are regional in nature, noted the RIAA. The colder and windier northern states exhibit different patterns than the hotter and sunnier southern states. To address this changing risk, the system needs to ensure (1) sufficient visibility of locational risk and (2) that other energy-supplying resources are available during these new times of need, with adequate transmission to deliver across regions. 

The ability of resources to provide system flexibility will be challenged. Current flexibility is needed primarily around the morning as energy demand increases and again during the evening as demand decreases. This risk shifts as variable renewables are added. As solar resources meet a larger share of the mid-day generation needs, non-solar resources are needed to ramp down in the morning and ramp up again in the evening. Similarly, non-wind resources will ramp up and down to balance wind patterns, which change daily.

The current transmission infrastructure, said the report, becomes unable to deliver energy to load. This is especially true if renewables are concentrated in one part of the footprint while serving load in another. Given how much time is typically needed to build transmission, proactive planning is necessary.

RIIA found when the percentage of system-wide annual load served by renewable resources is less than 30 percent, the integration of wind and solar will require transmission expansion as well as significant changes to current operating, market, and planning practices — all of which appear manageable within MISO’s existing framework.

Beyond 30 percent, “transformative thinking and coordinated action between MISO and its members are required to prepare for the significant challenges that arise.”

The RIAA also pointed out that renewable growth does not happen uniformly across the MISO footprint, or the broader interconnected system. Growth occurs fastest in areas with high quality wind and solar resources, available transmission capacity, and favorable regulatory environments. For example, when MISO reaches 30 percent renewable energy penetration, some Local Resource Zones are likely to be approaching 100 percent renewable energy penetration. Locations which experience the fastest renewable growth experience challenges first, but beyond 30 percent renewable penetration the system as a whole faces “new and shifting risks” rather than simply local issues.

Today, MISO’s renewable fleet accounts for 13 percent of MISO’s system-wide energy. Nearly 80 percent of MISO’s renewable resources are in the northwest region of MISO, concentrating the current integration challenges to one area. 

Looking ahead, as the significant pipeline of generators with executed Interconnection Agreements reach commercial operation (6 gigawatts (GW) of new wind, 10 GW of new solar), renewables are expected to account for approximately 20 percent of the system-wide annual energy mix. Beyond that, the 30 percent milestone could occur as soon as 2026.