2009 State of the Market Report
of generation that can be utilized in the five-minute horizon, given ramp limitations. The figures also show the SMP and the average change in net imports.
Figure 27: Real-Time Prices and Headroom by Time of Day Winter 2009
$120 $100 $80 $60 $40 $20 $0
System Marginal Price (As-Offered)
12 AM 1 AM 2 AM 3 AM 4 AM 5 AM 6 AM 7 AM 8 AM 9 AM 10 AM 11 AM 12 PM 1 PM 2 PM 3 PM 4 PM 5 PM 6 PM 7 PM 8 PM 9 PM 10 PM 11 PM
Change in Net Imports SMP Effective Headroom
1,600 1,400 1,200 1,000 800 600 400 200 0
In winter and summer 2009, as in prior years, prices fluctuate most when load is ramping up or down near the peak load hours of the day (afternoon in the summer, and dual morning and evening peaks in the winter). Changes in real-time prices are directly related to changes in effective headroom, which often changes significantly at the top of the hour when hourly import schedules change and the commitment and de-commitment of units most often occurs. The sharp upward price movements that result from these patterns indicate a short-term system scarcity and are generally caused by generator operating constraints such as binding ramp constraints. Ramp constraints are limits to how quickly the system’s generation can change in response to system conditions. These ramp constraints are exacerbated by generator inflexibility arising from decreases in offered ramp capability or dispatch range.