Facility rating methodology

Equipment Thermal Loadability Ratings

ATC maintains criteria to establish ratings for substation equipment, overhead transmission lines and underground transmission lines for use in planning and operating the ATC network. These criteria are applied to all components and elements of the ATC network and facility ratings are determined and managed in the Substation Equipment and Line Database (SELD) application. Procedures are in place to govern the application of those criteria and the process for updating the facility ratings database for modifications to existing facilities and the addition of new facilities. For some non-Bulk Electric System facilities, ATC continues to use ratings from the previous transmission facility owner’s planning and operations models. ATC is actively reviewing these facilities and applying ATC ratings criteria.

(Applicable NERC Standards: FAC-0081-R1)

Voltage Ratings

The standard voltage percentage ratings (limits) of being within 95% to 105% of nominal system voltage for normal conditions and within 90% to 110% of nominal system voltage for emergency condition will be applied, except when special voltage limits are observed. Special voltage level limits are those acceptable to the affected transmission customers or needed to address specific ATC equipment limitations. Special voltage level limits, derived from a list maintained by ATC, are incorporated into the standard PSSE voltage monitor file and available for application in other analytical tools.

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