Q3Electrical Machines II

Before an incoming alternator can be safely connected in parallel with a live grid or another running alternator, it must be exactly synchronized. The conditions for synchronization are: 1. Equal terminal voltages. 2. Exactly equal frequencies. 3. The same phase sequence. 4. Zero phase difference between respective phases at the instant of connection.

1. Three Dark Lamps Method: Three lamps are connected across the switch blades of the corresponding phases of the incoming machine and the grid. If frequencies slightly differ, the lamps will slowly flicker on and off. The prime mover speed is adjusted to slow this flickering. The exact moment to close the switch is in the middle of the dark period when all three lamps are completely unlit, indicating zero voltage difference.

2. Synchroscope Method: A synchroscope is a precision instrument with a rotating pointer. It compares the frequency and phase of the incoming machine to the grid. If the incoming machine is too fast, the pointer rotates in the "Fast" direction; if too slow, in the "Slow" direction. The operator adjusts the prime mover until the pointer rotates very slowly and stops at the exact 12 o'clock (vertical) position, indicating perfect phase alignment, at which point the breaker is closed.