123 Transmission Loading Relief TLR

Overloaded transmission lines are incrementally shut down to prevent voltage collapse. The evolution of this somewhat automated procedure can be traced to efforts by various utilities seeking solutions to inadvertent or parallel path flows in electricity transmission and the FERC's functional unbundling of transmission and generation in Orders 888 and 889. Prior to these orders, transmission system overloads were typically handled by the affected control areas by first curtailing their wheeling services for third parties and, if that proved inadequate, redispatching generation.

Prior to the system-wide application of NERC TLR procedures, overloads were handled primarily by local procedures, whereas TLRs are regional. TLR relies on multiple control-area coordinators curtailing transmission flows over a much wider area (based on model-generated measures of their impacts on the constrained facilities). The TLR approach is a flow-based approach that curtails transactions based on actual power flows over the transmission system and their estimated impacts on the overloaded facilities. Historically, utilities instituted local curtailments based on contract path flows. According to NERC, this has proven inadequate to deal with the nature and increased volume of transactions on the transmission grid in recent years.

This response system has become a daily obstacle in the evolution of the wholesale/retail power market. It was a significant contributor to the Mid-America Interconnected Network (MAIN) price spikes of June 1998.

FIGURE 1.2

North American Electrical Council Reliability (NERC) regions: Western States Coordinating Council (WSCC), Mid-Continent Area Power Pool (MAPP), Mid-America Interconnected Network (MAIN), Southeastern Electric Reliability Council (SERC), East Central Area Reliability (ECAR), Electrical Reliability Council of Texas (ERCOT), Florida Reliability Coordinating Council (FRCC), Mid-Atlantic Area Council (MAAC), Northeast Power Coordinating Council (NPCC), Southwest Power Pool (SPP).

FIGURE 1.2

North American Electrical Council Reliability (NERC) regions: Western States Coordinating Council (WSCC), Mid-Continent Area Power Pool (MAPP), Mid-America Interconnected Network (MAIN), Southeastern Electric Reliability Council (SERC), East Central Area Reliability (ECAR), Electrical Reliability Council of Texas (ERCOT), Florida Reliability Coordinating Council (FRCC), Mid-Atlantic Area Council (MAAC), Northeast Power Coordinating Council (NPCC), Southwest Power Pool (SPP).

ECAR ERCOT FRCC MAAC MAIN MAPP NPCC SERC SPP WSCC ASCC

FIGURE 1.3.

Projected U.S. capacity margins by NERC reliability region (Borbely, 1999).

Additional capacity was available from Ontario, Southeastern Electric Reliability Council (SERC), and Mid-Continent Area Power Pool (MAPP), but TLR procedures reduced the import capacity to MAIN. Additionally, PJM (Pennsylvania/New Jersey/Maryland) Interconnection used TLR to curtail all electricity exports during the heat wave.

It is a legal, but potentially devastating, procedure for reliability regions to interrupt contract electricity transfers across their territories if their native load is jeopardized. Dwindling capacity margins are expected to impact the frequency of TLR curtailments.

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