Parallel operation conditions and three lamp method

Connecting an incoming alternator to live bus-bars (synchronising) is safe only when the following conditions are met:

1. Equal voltage: rms terminal voltage of the incoming alternator = bus-bar voltage.
2. Equal frequency: frequency of the incoming alternator = bus-bar frequency.
3. Same phase sequence: R-Y-B of the machine must match R-Y-B of the bus.
4. Phase agreement: at the instant of closing, corresponding voltages must be in phase.

Otherwise heavy circulating current and mechanical shock occur.

Three lamp (two bright, one dark) method

One lamp $L_1$ is connected between corresponding phases (R–R'), while the other two are cross-connected: $L_2$ between Y and B', and $L_3$ between B and Y'.

 Bus-bars     R          Y          B
              |           \        /
            (L1)          (L2)  (L3)
              |              \  /
              |               \/
              |               /\
 Incoming     R'          B'      Y'
 alternator   |___________|_______|

Procedure

1. Run the incoming machine near synchronous speed and adjust its field so its voltage equals the bus voltage.
2. The lamps brighten and darken one after another in a rotating sequence. The direction of rotation shows whether the machine is fast or slow, and the rate shows the frequency difference.
3. Adjust the prime-mover speed until the sequence is very slow.
4. Close the switch at the instant when $L_1$ is dark and $L_2$, $L_3$ are equally bright.

Justification (phasors)

Let the bus phase voltages be $V_R, V_Y, V_B$ (each $V$) and the machine voltages $V_{R'}, V_{Y'}, V_{B'}$.

          VR = VR'
           ^
           |          lamp L1 : VR - VR' = 0  (dark)
           |
          O
         / \          lamp L2 : VY - VB' = sqrt3 V
        /   \         lamp L3 : VB - VY' = sqrt3 V
     VB=VB'  VY=VY'   (equal brightness)

• When the two systems are in phase, $V_R - V_{R'} = 0$, so $L_1$ is dark.
• $L_2$ sees $V_Y - V_{B'} = V_Y - V_B$, which is a line voltage of magnitude $\sqrt{3}V$; similarly $L_3$ sees $\sqrt{3}V$. Both are equally and fully bright.
• When the machine phasors rotate relative to the bus (frequency difference), the voltages across $L_1, L_2, L_3$ reach their maxima at different instants (120° apart), so the lamps glow in sequence. This gives the "rotating light" effect that tells fast/slow.
• If the phase sequence were wrong, all lamps would not follow this pattern (they would brighten/darken together), so the method also checks phase sequence.

Advantage over dark-lamp method: the exact instant is easier to judge, because the moment of one dark lamp with two equally bright lamps is sharper than "all dark", and the direction of rotation tells whether to speed up or slow down.