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Need for parallel operation and dark lamp method

Short answer

Parallel operation ensures continuity of supply, allows efficient loading of machines, permits maintenance without shutdown, and enables easy expansion. In the dark lamp method, three lamps connect across the open synchronizing switch. When all lamps flicker together and become dark simultaneously, voltages are equal and in phase; the switch is closed at mid-dark period.

Why we need to perform parallel operation of two alternators? Explain dark lamp method of synchronizing an alternator with the Busbar.

Need for parallel operation

Generating stations use several alternators connected in parallel to common bus-bars instead of one large machine because:

  1. Continuity of supply: if one machine fails or needs maintenance, others keep supplying the load.
  2. Efficiency: machines are most efficient near full load; units are switched in or out as load varies, so each runs near full load.
  3. Maintenance and repair can be done one unit at a time without shutting down the station.
  4. Future expansion: new units can be added as demand grows.
  5. Reserve capacity: only a small spare unit is needed instead of a spare of the full station capacity.
  6. Interconnected grids require all generators to run in parallel.

Dark lamp method of synchronising

Three lamps are connected across the open synchronising switch, each between a bus-bar phase and the same phase of the incoming machine (R–R', Y–Y', B–B').

 Bus-bars      R         Y         B
               |         |         |
              (L1)      (L2)      (L3)
               |         |         |
 Incoming      R'        Y'        B'
 alternator    |_________|_________|

Procedure

  1. Run the incoming alternator up to about synchronous speed by its prime mover.
  2. Adjust its field current until its voltage equals the bus voltage (voltmeters).
  3. Observe the lamps. If they flicker together (all bright, all dark at the same time), the phase sequence is correct; if they glow one after another, interchange two leads of the incoming machine.
  4. The flicker rate equals the frequency difference; adjust the speed until flickering is very slow.
  5. Close the switch in the middle of the dark period of all three lamps; then the voltages are equal and in phase.
  6. Increase prime-mover input to make the machine take active load; adjust excitation to share reactive load.

Limitation: lamps are dark over a range of voltage (up to about 1/3 of rated), so the exact moment is uncertain; a voltmeter across a lamp or a synchroscope gives better accuracy.

✓ Answer checked by Claude Sonnet 5.5

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