31-08-2017, 01:13 PM
ALTERNATOR SYNCHRONIZATION
Two or more alternators can be operated in parallel, with each alternator carrying the same proportion of the load. However, certain precautions must be taken and several conditions must be met before connecting an alternator to a bus with another alternator.
Synchronization, or parallelism, alternators is somewhat similar to dc parallel generators except that there are more steps with alternators.
To synchronize (in parallel) two or more alternators to the same bus, they must have the same sequence of phases, as well as equal voltages and frequencies. The following steps are a general guide for synchronizing an alternator and connecting it to a bus system in which one or more alternators are already operating.
1. Phase sequence check. The standard phase sequence for three-phase AC circuits is A, B, C. The phase sequence can be determined by observing two small indicator lights, connected as shown in Figure 9-55. If a lamp is lit, the phase sequence is A, B, C. If the other lamp lights up, the phase sequence is A, C, B. If the light indicates an incorrect phase sequence, reverse the two wires to alternator. To parallelize or synchronize two alternators with an erroneous phase sequence would be equal to the short-circuit of two conductors and to configure dangerous circulation currents and magnetic disturbances within the alternator system, which could overheat the conductors and loosen the windings of the coil.
2. Checking the voltage. The voltage of the alternator to be connected to the bus must be equal to the bus voltage. It is set by a control rheostat located on the switch panel. This rheostat controls the current in the coil of the voltage regulator and causes the alternator's magnetic field to increase or decrease, in turn controlling the alternator voltage.
3. Frequency check. The frequency of an alternator is directly proportional to its speed. This means that the speed of the alternator connected to the bus must be equal to the speed of the alternators already connected. By observing the frequency gauge and adjusting the rheostat on the switch panel, the frequency of the incoming alternator can be raised to the correct value. By observing the sync lamp, shown in Figure 9-56, and by fine-tuning the speed control rheostat, the frequencies can be brought into almost exact synchronization. The sync lamp will blink when the two frequencies approach the same value; when they are almost the same, the lamp will flash slowly. When the flashing decreases to a flashing or less per second, close the circuit breaker while the lamp is dark and connect alternator number 2 to the bus. The dark lamp indicates that there is no voltage between phase A of the bus and phase A of the incoming alternator. During the period when the lamp is switched on, there is a voltage difference between the A phase of the bus and the A phase of the alternator to be connected to the bus. To close the circuit breaker when the synchro lamp is on would be similar to short circuit two conductors and cause serious voltage and magnetic disturbances inside the alternators.
Two or more alternators can be operated in parallel, with each alternator carrying the same proportion of the load. However, certain precautions must be taken and several conditions must be met before connecting an alternator to a bus with another alternator.
Synchronization, or parallelism, alternators is somewhat similar to dc parallel generators except that there are more steps with alternators.
To synchronize (in parallel) two or more alternators to the same bus, they must have the same sequence of phases, as well as equal voltages and frequencies. The following steps are a general guide for synchronizing an alternator and connecting it to a bus system in which one or more alternators are already operating.
1. Phase sequence check. The standard phase sequence for three-phase AC circuits is A, B, C. The phase sequence can be determined by observing two small indicator lights, connected as shown in Figure 9-55. If a lamp is lit, the phase sequence is A, B, C. If the other lamp lights up, the phase sequence is A, C, B. If the light indicates an incorrect phase sequence, reverse the two wires to alternator. To parallelize or synchronize two alternators with an erroneous phase sequence would be equal to the short-circuit of two conductors and to configure dangerous circulation currents and magnetic disturbances within the alternator system, which could overheat the conductors and loosen the windings of the coil.
2. Checking the voltage. The voltage of the alternator to be connected to the bus must be equal to the bus voltage. It is set by a control rheostat located on the switch panel. This rheostat controls the current in the coil of the voltage regulator and causes the alternator's magnetic field to increase or decrease, in turn controlling the alternator voltage.
3. Frequency check. The frequency of an alternator is directly proportional to its speed. This means that the speed of the alternator connected to the bus must be equal to the speed of the alternators already connected. By observing the frequency gauge and adjusting the rheostat on the switch panel, the frequency of the incoming alternator can be raised to the correct value. By observing the sync lamp, shown in Figure 9-56, and by fine-tuning the speed control rheostat, the frequencies can be brought into almost exact synchronization. The sync lamp will blink when the two frequencies approach the same value; when they are almost the same, the lamp will flash slowly. When the flashing decreases to a flashing or less per second, close the circuit breaker while the lamp is dark and connect alternator number 2 to the bus. The dark lamp indicates that there is no voltage between phase A of the bus and phase A of the incoming alternator. During the period when the lamp is switched on, there is a voltage difference between the A phase of the bus and the A phase of the alternator to be connected to the bus. To close the circuit breaker when the synchro lamp is on would be similar to short circuit two conductors and cause serious voltage and magnetic disturbances inside the alternators.