What is the concern about “overlapping” deratings?
The starting date/time for a derating is normally defined as the time the system, major component, or piece of equipment became unavailable for service, effecting an actual or potential loss of unit capacity. Deratings often overlap each other in duration. NERC GADS considers all deratings “additive” except those which are masked (shadowed) by an outage or a larger derating for their entire duration. This means the derating that started first is assumed to be the primary cause of the load reduction until it terminates or a full outage begins. Deratings that are masked (shadowed) for their full duration by outages or larger deratings are considered non-curtailing in nature; that is, they do not affect the available capacity of the unit. Occasionally, two or more individual components will fail at the same time. According to the NERC Data Reporting Instructions, you can report each component failure as a separate derating. NERC processes the data first sorting by start date/time (which i
