In the event of a loss of external generation, what occurs to frequency and ACE?

When there is a loss of external generation, it leads to a decrease in the amount of available electrical power to meet the demand on the system. This imbalance causes a drop in system frequency. In a power system, frequency is directly linked to the generation and load balance; when generation decreases without an equivalent decrease in load, frequency will decline.

Alongside this, the Actual Control Error (ACE) is a measure that reflects the difference between the scheduled and actual frequency and also the net power flow into or out of the area. When external generation is lost, if the load remains constant, the decrease in frequency results in an increase in ACE because the system is not meeting its scheduled generation level. Therefore, while frequency decreases due to the loss of power, ACE registers this deviation and reflects that the system is not in balance.

This context clarifies the dynamics between frequency and ACE under the conditions specified in the question, supporting the conclusion that frequency decreases while ACE demonstrates a similar behavior, staying constant in specific situations.