Synchronising of Alternator or AC Generator

It may logically be assumed that one alternator is placed in parallel with one or more other alternators only when additional load requires it. Those alternators already carrying load are known as the running alternator, while that which is to be placed in the system is known as the incoming alternator. At the time of synchronizing the following conditions must be met.

The effective voltage of the incoming alternator must be exactly equal to all the other running alternators, or of the bus-bar connecting them. The phase rotation, or sequence of the running and incoming alternators, must be the same. The frequency of the running and incoming alternators must be the same, although it is more desirable that the frequencies at the instant of paralleling be almost, but not quite, identical. The individual phase voltages which are to be connected to each other must be in exact phase opposition. Which is very much likely to that DC generator must be connected + to + and - to -.

Synchronising single-phase alternators

Suppose alternator 2 is to be synchronized with the bus-bars to which alternator 1 is already connected. This is done with the help of two lamps method L 1 and L 2. This method is well known as Synchronizing lamps method. It should be noted that E 1 of alternator 1 and E 2 of alternator 2 are in-phase relative to the external circuit but are in direct phase opposition in the local circuit.

Steps for synchronising single-phase alternators

step 1: If the speed of the incoming alternator is not equal to the running alternator which is already connected to the bus-bars, then the frequency of incoming alternator and running alternator will differ.

step 2: As frequencies differ, then there will be a phase-difference between their voltages, as frequency is directly proportional to the voltage.

step 3: This phase difference will be continously changing with change in the frequency of the incoming alternator.

step 4: Our task is to bring down the phase difference between the incoming alternator and the running alternator to zero.

step 5: To bring down the phase differce to zero, the field current of the incoming alternator is varied.

step 6: By varying the field current of incoming alternator, the lamps connected to the bus-bars will 'glow up' and 'dark out' alternately.

step 7: When the phase difference between the incoming alternator and running alternator is in same phase, then there is a resultant current through the lamps, thus lamps are in glowing stage.

step 8:When the phase difference between the incoming alternator and running alternator is in same phase, then there is a resultant current through the lamps, thus lamps are in glowing stage.

step 9: Thus by using dark lamp method the incoming alternaters are get synchronised.

Synchronising three-phase alternators

To satisfy the first condition of paralleling i.e) Effective voltage be the same, a voltmeter can be used as shown below. For satisfication of the other conditions i.e) phase sequence, voltage opposition, and frequency may be determined by the use of the incandescent lamps connected between the two alternator. The following procedural steps must be followed for putting incoming alternator in parallel with running alternator which is already connected to the bus-bars.To satisfy the first condition of paralleling i.e) Effective voltage be the same, a voltmeter can be used as shown below. For satisfication of the other conditions i.e) phase sequence, voltage opposition, and frequency may be determined by the use of the incandescent lamps connected between the two alternator. The following procedural steps must be followed for putting incoming alternator in parallel with running alternator which is already connected to the bus-bars.

Steps for synchronising three-phase alternators

step 1: The primemover of the incoming alternator is started, and this incoming alternator is brought up nearer to the rated speed of running alternator.

step 2: By adjusting the field current, the terminal voltage of incoming alternator is made the same as that of the running alternator.

step 3: The lamps in the circuit will now flicker at a rate equal to the difference in frequency between the both incoming alternator and running alternator. As same as in synchronizing done in single-phase alternators, When the phase difference between the incoming alternator and running alternator is in opposite phase, then there is no resultant current through the lamps, thus lamps are in dark stage and When the phase difference between the incoming alternator and running alternator is in same phase, then there is a resultant current through the lamps, thus lamps are in glowing stage.

step 4: Further adjustment of the incoming primemover is now necessary, until all the three lamps flickers at a very low rate i.e) less than one dark period per second.

step 5: Final adjustment of the incoming voltage again made and the synchronizing switch is thrown in the middle of a dark period. The voltage across the lamps varies from zero to twice the phase voltage, and therefore, the lamps must be rated for this higher voltage. It is not convenient, but it is mandatory.

Advantages Of Lamp Dark And Bright Method

• Incandescent lamps are very cheap in price.
• Proper phase sequence is readily obtained.

Disadvantages Of Lamp Dark And Bright Method

• Higher rating of incandescent lamps are requires.
• Flicker of the lamps does not indicate which alternator has the higher frequency.