When there is a loss of external load, what happens to frequency and ACE?

When there is a loss of external load in a power system, the immediate effect is an imbalance between generation and demand. The power system is designed to maintain a balance, and a sudden reduction in load means that there is more generation in the system than is required at that moment.

In this situation, the frequency of the system will increase because the generators continue to operate at their set output levels, but there are fewer customers (loads) consuming that energy. This surplus generation causes a rise in frequency, as frequency is inherently linked to the rotational speed of generators; when the load decreases, the speed increases.

Moreover, the Area Control Error (ACE), which is a measure of the difference between scheduled and actual interchange or generation, will typically not change instantaneously. The ACE reflects adjustments based on both frequency deviations and interchange power agreements. As frequency increases due to the loss of load, ACE initially stays the same because the system has not yet responded to the disturbance to bring the generation back into balance.

Thus, when there is a loss of external load, the correct interpretation is that frequency increases while ACE remains unchanged initially.