What does ACE indicate if Scheduled Interchange is 200 MWs and Actual Interchange is 225 MWs, given a frequency drop?

In this scenario, ACE stands for Area Control Error, which measures the difference between the actual interchange and the scheduled interchange, adjusted for system frequency. To find ACE, you subtract the Scheduled Interchange from the Actual Interchange.

Here, the Scheduled Interchange is 200 MW, and the Actual Interchange is 225 MW. Subtracting these values, you get:

ACE = Actual Interchange - Scheduled Interchange = 225 MW - 200 MW = 25 MW.

Given that the frequency is dropping, it indicates that the system is experiencing a generation shortfall compared to the demand. However, since the actual interchange is greater than the scheduled interchange by 25 MW, it reflects that there is an excess of generation—hence, it is categorized as over-generating.

Therefore, having calculated the ACE as 25 MW over the scheduled value, the correct conclusion is that there is 25 MW over-generating, which aligns with the provided correct answer.