Over excitation most often occurs during which of the following situations?

Over excitation typically occurs during startup and complete load rejection because this is a critical time for generators and synchronous machines. When a generator is started and brought online, it must reach synchronization with the grid. If the machine experiences a rapid change in load demand, such as complete load rejection, it can lead to conditions where the machine is producing excessive reactive power, resulting in over excitation.

During the startup phase, the machine's excitation system increases the field current to energize the rotor, enabling it to generate the necessary output voltage. If, at this point, the load is suddenly disconnected or rejected, the machine continues to generate power without an opposing load. This can cause the machine to enter an over excitation state since the field current remains high, leading to an increase in voltage output beyond the design limits.

In contrast, during normal operations, shutdown procedures, or frequent voltage adjustments, while excitation levels may change, they typically do not result in the same rapid conditions that lead to over excitation as seen in startup and load rejection scenarios. These situations generally allow for more stable adjustments and control of excitation levels, reducing the risk of over excitation.