Parallel operation conditions and dark lamp synchronizing

Parallel operation means connecting two or more alternators to the same bus-bars so that they share the load. Connecting an incoming alternator to live bus-bars is called synchronising.

Conditions for parallel operation

1. Equal voltage: the rms terminal voltage of the incoming machine must equal the bus-bar voltage.
2. Equal frequency: the frequency of the incoming machine must equal the bus-bar frequency.
3. Same phase sequence: the phase sequence (R-Y-B) of the incoming machine must be the same as that of the bus-bars.
4. In phase: the incoming voltage must be in phase with the bus voltage at the instant of closing the switch (zero phase difference).
5. The waveforms should be of similar shape (both nearly sinusoidal).

If these are not met, large circulating currents and heavy mechanical shocks occur at the instant of switching.

Dark lamp method

Three lamps are connected across the three poles of the open synchronising switch, i.e. each lamp is connected between a bus-bar phase and the same phase of the incoming alternator (R–R', Y–Y', B–B').

 Bus-bars      R         Y         B
               |         |         |
              (L1)      (L2)      (L3)   lamps across
               |         |         |     open switch
 Incoming      R'        Y'        B'
 alternator    |_________|_________|
                  G2 + prime mover

The voltage across each lamp is the difference of the bus voltage and the incoming voltage of the same phase.

Procedure

1. Start the incoming alternator with its prime mover and bring it near synchronous speed.
2. Switch on its field and adjust the excitation until its voltmeter reads the bus-bar voltage.
3. Watch the lamps:
   • If all three lamps become bright and dark together, the phase sequence is correct. If they flicker one after another (rotating effect), the phase sequence is wrong; interchange any two leads of the incoming machine.
   • The rate of flickering equals the frequency difference $(f_1 - f_2)$. Adjust the prime-mover speed until the flickering is very slow.
4. When all three lamps are dark (at the middle of the dark period), the voltages are equal and in phase. Close the synchronising switch at this instant.
5. After closing, the incoming machine is "floating" on the bus; it takes load when its prime-mover input (steam/water) is increased. The reactive load is shared by adjusting the excitation.

Merits and limitations

• Simple and cheap; it checks phase sequence as well.
• Lamps go dark at about one-third of rated voltage, so the exact instant of synchronism is uncertain; a voltmeter across a lamp or a synchroscope is used for accuracy in large stations.
• Filament failure may be mistaken for darkness.