Hi,
The offer stack presented on EMI reflects face value energy offers submitted by generators. However, these offers do not account for various system constraints that can limit the actual quantity of energy that can be cleared. These constraints include reserve requirements, ramping limits, market node constraints, and branch group (transmission) constraints.
These factors can significantly reduce the amount of energy that is actually available to be dispatched compared to what was offered. Here are a few examples:
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Reserve Constraint:
Suppose a generator has a total capacity of 100 MW and offers 100 MW of energy and 50 MW of reserve. If 25 MW of reserve is cleared due to tight reserve conditions, only 75 MW remains available for energy dispatch.
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Ramping Constraint:
If a generator offers 100 MW of energy but has a ramp-up capability of only 5 MW per minute, and its initial output is 0 MW, then in a 5-minute dispatch interval it can only ramp up to 25 MW. Therefore, only 25 MW can be cleared, not the full 100 MW offered.
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Market Node Constraint:
For system security—often related to frequency keeping—a constraint may be applied to a specific node or group of generators, limiting the total energy that can be cleared from that location.
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Branch Group Constraint:
N-1 security constraints on the transmission network may limit the amount of energy that can be exported from a region. This, in turn, restricts the total energy that can be cleared from generators within that region.
The attached image below shows an example of the offer stack for the 5-minute trading interval at the start of trading period 30 on 2025-08-21. You’ll notice that the adjusted offer quantities are significantly lower than the face value offers due to the constraints mentioned above. The marginal generation price for this interval was $186.86/MWh.
I hope this helps clarify your question.
Best regards,
Tuong Nguyen
Principal Analyst
