Criteria for synchronizing two alternators

Synchronising (paralleling) is the process of connecting an incoming alternator to a live bus or to another running alternator. If this is done incorrectly, large circulating currents and mechanical shocks occur.

Conditions (criteria) to be satisfied

1. Equal terminal voltage: the rms voltage of the incoming machine must equal the bus voltage. It is adjusted by the field current (excitation).
2. Equal frequency: the frequency of the incoming alternator must equal the bus frequency. The prime-mover speed (governor) is adjusted until the difference is nearly zero.
3. Same phase sequence: the phase order (R-Y-B) must be the same as that of the bus. A wrong sequence causes a short circuit.
4. Same phase angle (in phase): corresponding phase voltages must be in phase at the instant the switch is closed.
5. Same waveform (sinusoidal for both) is also desirable.

Detailed explanation

• Voltage: if the voltages differ, a circulating current $I_c = \frac{E_1 - E_2}{Z_1 + Z_2}$ flows, which is mostly reactive. It is avoided by matching the voltages using a voltmeter.
• Frequency: if the frequencies differ, the phase difference changes continually and the machines go in and out of phase at the beat frequency. A synchroscope or lamps show the beat. The switch is closed when the beat is very slow.
• Phase sequence: checked once at installation with a phase sequence indicator, or with lamps (in the dark-lamp method all lamps should brighten and darken together). Interchanging any two lines corrects a wrong sequence.
• Phase angle: the switch is closed when the slowly rotating synchroscope pointer reaches the vertical (12 o'clock) position, or when the dark lamps are fully dark.

Methods

• Three dark lamps method
• Two bright, one dark lamp method
• Synchroscope (most common)

 Bus ----+----+----+----
         |    |    |
       [S]--(Lamps / Synchroscope)--
              |
    Incoming alternator