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 is now, in this circumstance, exactly the same for the two failures) then by event number. This means that when start dates are identical, the derating with the lower event number will be processed first.

However, this same situation of having derating events with the same start date/time can occur for other reasons (i.e., deratings that carry over from the end of one year and continue into the next year). NERC GADS requires that every event that carries over into the new year be "restarted" by entering the same start date/time of "01010000."

Therefore, overlapping deratings that carry over into the next year all have the same start date/time, similar to having two or more individual components failing at the same time. However, the "knowledge" as to which one started first is lost due to the restarting of the event data records.

Depending on company reporting policy, some companies artificially stop and then restart deratings that overlap full outage events. This was primarily done because older mainframe GADS software was incapable of dealing with deratings that overlapped full outages; so, the solution was to stop deratings at the same time that the outage started and create new derating events at the end of the outage.

The software options now available allow you to set a consistent methodology for calculating overlapping deratings when these situations occur. The options are as follows:

  1. Masking or Shadowing - the larger derating masks or shadows all smaller deratings
  2. Additive - each derating contributes to the unavailability of the unit
  3. Event Number Order - the NERC method previously described

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