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2009 State of the Market Report

Transmission Congestion

The top panel of the figure shows the quantities of scheduled energy curtailed by the TLR events. The bottom panel of the figure provides the hourly TLR activity by the various TLR levels. NERC’s active response TLR levels include:

  • Level 3 — non-firm curtailments;21

  • Level 4 — commitment or redispatch of specific resources or other operating procedures

to manage specific constraints; and

  • Level 5 — curtailment of firm transactions.22

In 2009 TLR activity as measured in flowgate-hours fell 46 percent compared to 2008. TLR curtailment volume decreased 44 percent. The more severe Level 4 and 5 TLRs have been largely eliminated since 2007. Although significant quantities of TLRs are still invoked to ensure that transactions external to the Midwest ISO are curtailed when contributing to congestion, the Midwest ISO relies primarily on economic redispatch for managing congestion.


Congestion Manageability

Congestion management is one of the most important activities of the Midwest ISO. The Midwest ISO monitors thousands of potential network constraints in real time throughout the footprint. As the flow over each of these constraints approaches its limit (or if it is anticipated to do so) in real-time, the constraint is “activated” in the market model. The Midwest ISO’s real- time market model will then manage the flow on the activated constraints to keep the flow below its operating limit on the facility while minimizing overall production costs.

The Midwest ISO’s real-time LMP-based energy market will redispatch generation subject to transmission constraints on the network. This process utilizes the redispatch capability of generators, especially those with high generation shift factors (“GSFs”) that have relatively large impacts on constraints. Constraints are at times difficult to manage if the available redispatch


Level 3a allows for the reallocation of transmission service by curtailing interchange transactions to allow interchange transactions using higher priority transmission service. Level 3b allows for the curtailment of interchange transactions to mitigate an SOL or IROL violation.


NERC’s TLR levels include four other levels: Level 1 (notification), Level 2 (holding transfers), Level 6 (emergency procedures) and Level 0 (TLR concluded).

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