What must the phase sequence of the incoming alternator match?

The phase sequence of the incoming alternator must match the system phase sequence to ensure proper operation and synchronization with the existing power system. When connecting multiple generators or connecting a generator to the grid, it's essential that the phases align correctly. If the phasing does not match, it can lead to serious operational issues, including potential damage to equipment due to phase mismatch.

A correct phase sequence ensures that power can flow smoothly without causing short circuits or excessive currents, which can occur when misaligned. Additionally, synchronization procedures often include checking phase sequence using phase sequence indicators to prevent such problems.

The other options, while important to various aspects of power engineering, do not directly relate to the requirement of matching the phase sequence for safe and effective operation of the alternator within the existing system. For instance, while system frequency is crucial, it is a separate consideration from phase alignment; battery connections and generator capacity also do not pertain to phase sequence specifications.