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Change in Net Imports




Effective Headroom



To determine whether price volatility in the Midwest ISO is excessive, Figure 29 shows the average percentage change in real-time prices between five-minute intervals for several hubs in other RTO markets. Within the Midwest ISO, the Cinergy Hub exhibited the least volatility interval-to-interval because it was the least affected by congestion. WUMS is historically the most congested location and, in turn, it exhibited the largest average interval price change. The figure shows that average volatility dropped significantly in 2009 when compared to 2008 at all four representative locations in the Midwest ISO, which is likely due in part to the AS markets. These markets have led to substantial improvements in supply flexibility in the Midwest ISO.

The figure also shows that the Midwest ISO and the NYISO have the most price volatility, and ISO-NE has the least. These differences can be explained by software and operational characteristics of the various markets. The Midwest ISO and NYISO are true five-minute markets with a five-minute dispatch horizon. Ramp constraints are more prevalent in these markets due to the shorter time to move generation. However, NYISO’s real-time dispatch is a multi-period optimization that looks ahead one hour, so it can anticipate ramp needs and begin

2009 State of the Market Report

Market Performance

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$120 $100 $80 $60 $40 $20 $0

Figure 28: Real-Time Prices and Headroom by Time of Day Summer 2009

System Marginal Price (As-Offered)

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