If balancing authority A is importing 225 MW while only scheduled to import 200 MW, what does the ACE indicate if frequency is 59.99 Hz?

In this scenario, the Balancing Authority (BA) is importing more power than it had previously scheduled. Specifically, it's importing 225 MW while only planning to import 200 MW, resulting in a difference of 25 MW. Since this BA is importing more power than expected, it indicates an imbalance in supply and demand.

The term ACE, which stands for Area Control Error, is a critical metric used to gauge the balance between generation and the load or demand that a balancing authority must maintain. ACE is calculated based on the actual net power flow into the area compared to the scheduled power flow, adjusted for frequency. When the frequency is below the nominal 60 Hz, as indicated with 59.99 Hz, it suggests that the system is under-generating relative to its demand.

In this case, since the BA is importing 225 MW instead of the scheduled 200 MW, and the frequency is below normal, the ACE reflects that the Balancing Authority is 25 MW under generation based on the imbalance created by the higher import and the lower frequency. Therefore, the ACE indicates that there is a need to compensate for this imbalance, reinforcing the notion that the correct answer is identifying the Balancing Authority as being under-generated by 25 MW.