When two or more alternators are operating in parallel, it is important that certain parameters of the alternators are identical to ensure proper synchronization and stable operation of the system. The three primary factors that should be identical for two alternators running in parallel are:

1. Phase sequence: The phase sequence, or the order in which the voltages of the three-phase system change, should be the same for both alternators. This ensures that the voltages are in phase with each other, and prevents any circulating currents between the two alternators.

2. Voltage: The voltage of both alternators should be the same, to prevent any imbalance in the system and ensure proper power sharing between the two alternators.

3. Frequency: The frequency of both alternators should be identical to ensure that the loads connected to the system receive stable and reliable power.

Overall, all three factors - phase sequence, voltage, and frequency - should be identical for two alternators running in parallel to ensure stable and reliable operation of the system.