Understanding Parallel Operation of Alternators

Running two or more alternators in parallel is a common practice in power systems to meet increased load demands, improve reliability, and allow for maintenance. However, connecting alternators in parallel requires careful synchronization. Several conditions must be met before an incoming alternator can be connected to a busbar already supplied by other alternators.

Conditions for Synchronizing Alternators

For successful parallel operation, the incoming alternator must be synchronized with the busbar (or the alternator already connected). The main conditions are:

• The RMS voltage of the incoming alternator must be approximately equal to the busbar voltage.
• The frequency of the incoming alternator must be approximately equal to the busbar frequency.
• The phase sequence of the incoming alternator must be identical to the phase sequence of the busbar.
• The phase angle between the voltage of the incoming alternator and the busbar voltage must be zero at the instant the synchronizing switch is closed.

Why Phase Sequence Must Be Identical

Among the conditions listed, the phase sequence is the most critical and must be exactly identical. Here's why:

• Phase sequence refers to the order in which the phase voltages (e.g., R, Y, B) reach their maximum positive values. For a three-phase system, this is typically R-Y-B or R-B-Y.
• If the phase sequences of the incoming alternator and the busbar are different (e.g., R-Y-B vs. R-B-Y), the phase voltages will be opposing each other across the synchronizing breaker at certain instants, even if the frequencies are the same.
• When the synchronizing breaker is closed, a large circulating current will flow between the alternators due to the significant voltage difference caused by the mismatched phase sequence. This can lead to severe damage to the alternators or the power system.

Other Factors (Power Factor, Power, Reactive Power)

The other options listed (Power factor, Power, Reactive Power) are outcomes of the parallel operation and load sharing, not pre-conditions for synchronization. Once synchronized, alternators share the total load (active power) and reactive load (reactive power). The amount of active and reactive power shared depends on the prime mover input and excitation level, respectively. The power factor is determined by the reactive power shared relative to the active power shared and the load's characteristics. These quantities are generally not identical for parallel alternators unless they are identical units sharing the load equally.

Conclusion

Therefore, the factor that must be identical for two alternators running in parallel, specifically during the synchronization process, is the phase sequence.