Area control error, commonly called ACE, is a quantity used in operating bulk electric systems. ACE is defined as the instantaneous difference between a balancing authority’s net actual and scheduled interchange with all adjacent interconnected balancing authority areas. ACE is measured in megawatts (MW). In actual application, the quantity is slightly adjusted to account for: known meter error; frequency bias setting obligation (a quantity related to the area’s obligation to support frequency); and, in some cases, time error correction.
While supply in a balancing area is scheduled to match forecasted load, actual loads and actual supply vary during operation (load forecasts are based on consumer behavior that cannot be known with certainty ahead of time, loads vary during the hour, dispatchable generating units may not produce at scheduled levels, and variable generation fluctuates based on uncontrollable factors such as weather). So a system operator cannot know how closely actual supply and demand are balanced until they are measured in real time.
Metering actual unit output and consumer usage in real time is not practical. Instead the supply/demand balance is monitored by tracking flows at the balancing authority’s boundaries, which are their transmission interconnections to neighboring systems. The actual flows are compared to scheduled flows using the single quantity ACE. This requires limited data collection, processing, and monitoring, thus allowing it to be done in near real time.
A positive ACE means that more power is flowing out of an area than scheduled, which tells the system operator that there is more supply than demand in that area and that generation should be ramped down. Conversely, a negative ACE means that more power is flowing into the area than scheduled and that generation in the area should be ramped up to meet demand. The North American Electric Reliability Corporation (NERC) requires that all system operators maintain ACE within specific tolerances so that any one balancing area does not negatively affect operations of interconnected areas.