Criteria for synchronizing two alternators

Synchronizing (paralleling) means connecting an incoming alternator to a live bus (or another alternator) without disturbing the system. If the conditions are not met, a large circulating current and a mechanical shock result when the switch is closed.

Conditions (criteria) for synchronizing

1. Equal terminal voltage (rms). The terminal voltage of the incoming machine must equal the bus voltage. Adjust its field current to match. If the voltages differ, a (reactive) circulating current I = ΔE / (Z_s1 + Z_s2) flows, which heats the windings.
2. Same phase sequence. The phase sequence (RYB) of the incoming machine must match that of the bus. Otherwise the phase voltages are not in step and a short circuit results. It is checked with a phase-sequence indicator, or with the three-dark-lamp method (all lamps going dark and bright together shows the correct sequence).
3. Equal frequency. The frequency of the incoming machine must equal the bus frequency. The prime-mover speed (governor) is adjusted until the frequency difference is very small. With unequal frequencies, the voltages beat and the phase difference between the machines changes continuously.
4. Phase coincidence. The voltages must be in phase at the instant of closing the switch (zero phase-angle difference). This is checked with a synchroscope or the lamps.
5. Same waveform (sinusoidal), to avoid circulating harmonic currents.

Methods of checking

Three dark lamp method: three lamps are connected between corresponding terminals of the incoming alternator and the bus. If the sequence is correct and the frequencies are close, the lamps flicker (dark and bright) together at the difference frequency. The breaker is closed in the middle of the dark period, when a voltmeter across a lamp reads zero.

Two bright, one dark lamp method: one lamp is connected straight and the other two are cross-connected. With the correct sequence the lamps light up one after another (a rotating light), and the direction of rotation shows whether the incoming machine is fast or slow. The breaker is closed when the straight-connected lamp is dark and the two cross-connected lamps are equally bright.

Synchroscope: its pointer rotates at the difference frequency, clockwise if the incoming machine is fast. The breaker is closed when the pointer is at the 12 o'clock (vertical, zero phase angle) position.

Procedure

1. Start the prime mover and bring the speed close to rated speed.
2. Adjust the field current until the voltmeter shows a voltage equal to the bus voltage.
3. Adjust the speed to bring the frequency to the bus frequency (slow pointer rotation).
4. Close the breaker at the synchronising instant.
5. Increase the prime-mover input so that the machine takes up load.

  Incoming       Bus
  alternator --[ 3 lamps / synchroscope ]-- bus
              \__ breaker closed when dark / pointer at 12