What unit type experienced a trip that caused a negative ACE in the scenario described?

A negative Area Control Error (ACE) indicates that the system is generating more power than is being consumed, causing an excess of generation in relation to the demand. In this scenario, the trip of a unit would mean that it unexpectedly ceased operation, which typically results in a decrease in generation.

The coal-fired steam unit is the most applicable choice because this type of unit, when it experiences a trip, can lead to a significant loss in generation capacity due to its relatively large output compared to other generation units like gas or renewable sources. Furthermore, coal-fired plants often have longer ramp-up and ramp-down times, meaning that a trip can have a pronounced effect on the supply-demand balance, quickly leading to a negative ACE.

In this context, the other unit types may not typically cause such a drastic shift in power generation when they trip. For instance, while a hydroelectric unit can impact ACE when it trips, its generation can be more easily managed with reservoir levels and other generation sources, making the effect less pronounced than that of a coal-fired steam unit. Wind turbines are generally variable and often operate at lower capacities, therefore their trips tend not to register significant changes in ACE unless they are a substantial portion of the grid's generation capacity. Gas-fired units are