The three-lamp synchronisation method is used to synchronise an alternator with an infinite bus. In which of the following conditions will all the three lamps darken and brighten in step?

Concept:

Synchronisation of Alternator : 3 lamp method:

Let  frequency of alternator 1 = ω₁

frequency of alternator 2 = ω₂

frequency of grid = ω_r

Case 1: ω1 > ω2  bulbs glow in clockwise fashion i.e. L₃, L₂, L₁.

Case 2: ω1 < ω2  bulbs glow in anti-clockwise fashion i.e. L₁, L₂, L₃.

Application:

For the incandescent lamp method, the frequencies of alternators have to be different, which means alternator 2 should move slower or faster w.r.t alternator 1.

Voltages and phase sequence of both alternators have to be the same for synchronization in order to ensure that when the circuit breaker is closed by 3 pole switch, that time the voltages

appearing across three phases of breaker is zero. Suppose,

V_g1 = V_g1(t) = V_mg sin (ω_g1t + φ_g1)           

V_a1 = V_a1(t) = V_ma sin (ω_a1t + φ_a1) 

The voltage across the switch is V_s = V_g1 - V_a1, where g1 represents grid and a1 is alternator 1.

For, V_s = 0, phase difference should zero i.e. φ_g1 = φ_a1. So, the same phase sequence is a must.

And V_mg = V_ma, i.e.peak voltages should be the same.

 Key Points
To connect two alternators in parallel, the two alternators must be synchronized. The synchronization process must be performed also when connecting an alternator to the grid.

Asymmetrically connected lamps indicate whether the incoming machine is running slower or faster.