2009 State of the Market Report
capability of the generators that affect the flow on the constraint is limited. The available redispatch capability is reduced when:
Generators that are most effective at relieving the congestion are not online;
Generator flexibility is reduced (i.e., generators set operating parameters, such as dispatch range or ramp rate, lower than actual physical capabilities); or
Generators are already at their limits (e.g. operating at their maximum point of their dispatch range, the “EcoMax”).
When available redispatch capability is insufficient to reduce the flow below the transmission limit in the next five-minute interval, we refer to the transmission constraint as “unmanageable”. Importantly, the presence of an unmanageable constraint does not mean the system is unreliable. The Midwest ISO performance criteria for most constraints require control within the limit in 20 minutes. If control is not obtained within 30 minutes, a reporting criterion to stakeholders is triggered. The small subsets of constraints that can lead to cascading outages are controlled to limits that are more stringent than the actual security limits. When a constraint is unmanageable in the Midwest ISO market, an algorithm is used to “relax” the limit of the constraint for purposes of calculating a shadow price for the constraint and the associated LMPs. While an unmanageable constraint is not necessarily a reliability concern, it nonetheless warrants evaluation.
Figure 57 and Figure 58 evaluate the manageability of constraints by month and by voltage level, respectively. The first figure shows the frequency with which constraints were unmanageable in each month in 2008 and 2009. Overall, total constrained hours increased in 2009 compared to 2008, but manageability improved. In 2009, 21 percent of internal congestion costs were unmanageable, which is a decrease of more than one quarter from 2008. The introduction in 2009 of ASM and the PVMWP led to substantial increases in generation flexibility, which the real-time market can use to better manage congestion. In addition, the Midwest ISO has also modified a number of real-time modeling parameters in response to prior IMM recommendations that have increased the amount of congestion relief available to the real-time market.