Transmission Reliability Margin (TRM) is the amount of transmission transfer capability necessary to provide reasonable assurance that the interconnected transmission network will be secure. TRM accounts for the inherent uncertainty in system conditions and the need for operating flexibility to ensure reliable system operation as system conditions change.

ATC planning studies, except those required for MISO transmission service, will consider a 3 percent reduction in normal and emergency ratings for assessments within one year in the future and a 5 percent reduction for the assessments beyond one year in the future. However, the recommended timing of the resultant mitigation measures may be based on less than the 3 percent and 5 percent reductions.

In the planning horizon, anytime beyond 48 hours, MISO uses reservations from other transmission providers and control area generation merit orders to reduce uncertainty.  MISO will apply a 2 percent reduction in normal and emergency ratings for input uncertainties in the planning horizon. This is often referred to as the uncertainty portion of the TRM.

The operating reserve component of TRM is the amount of transmission transfer capability on a constrained interface to provide the amount of regional operating reserves associated with 100 percent of the greatest single contingency impacting the flowgate. For determining the operating reserve portion of TRM, MISO performs analyses to identify the required reserve for each flowgate. The worst case will be determined by tripping units (or line outages when a reserve sharing member can request emergency energy for the line trip) within the region and picking up each reserve sharing member’s share of the emergency energy to replace the unit that tripped. The distribution of each reserve sharing member’s share of the emergency energy among its individual generating units should be a realistic estimate for the conditions for which the TRM is being determined. The worst case will be the case that has the greatest incremental flow over the flowgate in the direction of the constraint. The highest incremental flow on the flowgates for the contingencies evaluated (generation and transmission) will be the amount of Automatic Reserve Sharing (ARS) TRM required to reserve transmission service for operating reserves.

All MISO transmission service studies use the summation of ARS TRM and the 2 percent uncertainty TRM in the flow based analysis of transmission service studies. The network analysis for transmission service studies does not use the ARS or 2 percent TRM, but requires for all network elements a 3 percent reduction in normal and emergency ratings for requests in the next 13 months and a 5 percent reduction in normal and emergency ratings for requests extending beyond the next 13 months.