Transmission reliability margin methodology
TRM is the amount of transmission transfer capability necessary to provide reasonable assurance that the interconnected transmission network will be secure during changing system conditions, particularly during Reserve Sharing events such as the loss of a critical single unit. TRM accounts for the inherent uncertainty in system conditions and the need for operating flexibility to ensure reliable system operation as system conditions change.
In the planning horizon, anytime beyond 48 hours, MISO uses reservations from other transmission providers and Balancing Authority generation dispatch to reduce uncertainty. MISO will apply a two percent reduction in normal and emergency ratings for input uncertainties in the planning horizon. This is often referred to as the uncertainty component of the TRM.
The ARS component of TRM is the amount of transmission transfer capability required on a flowgate to deliver Contingency reserves. These Contingency reserves are defined as 100 percent of the impact of the greatest single Contingency impacting the flowgate. The worst single Contingency is determined by tripping units (or transmission elements) within the region and replacing the lost resource with a realistic dispatch for each reserve sharing member’s share of the emergency energy. The worst case is the case that has the greatest incremental flow across the flowgate. The highest incremental flow on the flowgate for the Contingencies evaluated (generation and transmission) will be the amount of ARS TRM required.
MISO uses the summation of the ARS and two percent uncertainty components of TRM in the network analysis for Long-Term Transmission Service Requests. Please reference the MISO Transmission Reliability Margin Identification (TRMID) methodology for a description of the application of TRM to all Transmission Service Requests.
Other ATC planning studies screen at a five percent reduction in normal and emergency ratings for thermal loading criteria and a two percent reduction for steady state under voltage criteria, except for studies that consider a wide range of system conditions (e.g., load, dispatch, transfers), such as 10-year assessments. MISO generator interconnection studies shall utilize a five percent reduction in normal and emergency ratings for all facilities inside the ATC footprint. The different methodologies are to accommodate inherent uncertainty and the number of system conditions considered in the study methodology. The recommended timing of the resultant mitigation measures may be based on less than the five percent reduction.