What does it indicate when a system operator increases generators and ACE becomes positive after initially being zero?

The correct interpretation when a system operator increases generators and the Area Control Error (ACE) becomes positive after initially being zero is that it signifies a response to a balance issue in the system. When ACE is positive, it typically indicates that the generation has exceeded the load, which can lead to an increase in system frequency.

In this context, the choice related to costing the company money is accurate as increasing generation can lead to high operational costs. If the system operator has to increase generation to handle excess supply without corresponding demand, it may necessitate purchasing external resources or incurring additional operational expenses to manage the surplus.

High frequency is indeed a consequence of excess generation but is not directly indicative of a financial outcome. The relationship between a deficient Balancing Authority (BA) and positive ACE could suggest operational complexities but is less straightforward in terms of cost implications. Finally, unscheduled power flow generally refers to unplanned transmission of power across boundaries, which does not directly connect to the assessment of cost or financial impact but rather to system stability and flow management.

Thus, while various effects follow from increasing generation, the direct financial consequence makes it clear that there are costs associated with such an operational decision.