2009 State of the Market Report
Generation capability is consistently greater in the real-time market than in the day-ahead. This occurs because some resources are self-scheduled by participants after the day-ahead market and because generation is committed by the ISO after the day-ahead market. On a market-wide basis, the Midwest ISO commits generation after the day-ahead market when load is higher than expected; when load is under-scheduled in the day-ahead markets; or when net virtual supply scheduled in the day-ahead market must be replaced in real time. In addition, the Midwest ISO often commits additional generation to manage congestion or satisfy the local reliability needs of the system.
The figure further shows that load was considerably lower in 2009 than in prior years, but it was more fully scheduled. This chart also shows that the average dispatchable range (the range between each online unit’s economic maximum and economic minimum) was seven percentage points lower in the real-time (29 percent) compared to the day-ahead (36 percent). This difference can result from an increase in a unit’s dispatch minimum or a decrease in its dispatch maximum. Figure 38 shows there was substantially more dispatch flexibility in 2009 than in prior years, although it remains well below the actual physical flexibility of the resources.
Average Dispatchable Range (MW)
Dispatchable Range (MW) Share of All Units Share of Dispatchable Units Commercial Flexibility
Figure 38: Real-Time Dispatchable Range 2008 – 2009
Share of Online Capability
All Unit Types
Hydro & Pumped Storage