When frequency decreases but the Balancing Authority's ACE equals zero, what step is crucial for the system operator?

When the Balancing Authority's Area Control Error (ACE) is zero, it indicates that the total generation is equal to the total demand at that very moment. However, if the system experiences a frequency decrease, it signals that the system is not adequately balanced, likely due to an imbalance in generation and consumption in terms of instantaneous demand.

In such situations, the crucial step for the system operator is to increase generator output. This action aims to restore the frequency to its nominal level by adding more power to the system, thereby addressing the imbalance. The need for additional generation arises because a decrease in frequency typically correlates with a higher demand for power than what is currently being supplied. Therefore, by increasing generator output, the operator can effectively work to correct the frequency decline, ensuring a stable and reliable power system.

The other choices suggest actions that might either lead to further imbalances or are not appropriate responses to the situation at hand. For instance, decreasing generator output or stopping all generation would exacerbate the frequency issue, leading to potential instability and further deviations from the desired operational parameters. Idling generators is also unresponsive to the immediate demand for increased output, failing to address the underlying imbalance.