In this topic, you study Synchronization of Generator (or Alternator).
The process of connecting an alternator in parallel with others already in operation is known as synchronizing. When once paralled, an alternator continues to run in synchronism. Any tendency of it to drop out of synchronism is immediately counteracted by a torque called synchronizing torque which is set up due to circulating current flowing through the alternators.
Method of synchronization of Alternator
An alternator can be synchronized i.e. put in parallel with others already in operation using the following methods:
(a) Lamp methods:
- Three dark lamps’ method.
- ‘One dark and two equally bright lamps’ method.
(b) Synchroscope method.
One dark and two equally bright lamps method
Here, again let us assume that Alternator-I is already connected to the bus-bars and
Alternator-2 is to be synchronized with it by using this method.
Procedural Steps :
Step-I to Step-4 : These are identical to those dark lamp method.
Step-5 : In this method the proper instant Of synchronizing is determined again by the use Of three identical incandescent lamps connected across the synchronizing switch asymmetrically as shown in Fig. 4.4. In this case, the lamp L1 is connected directly across the terminals in phase R of the synchronizing switch. On the other hand, the lamps L2 and L3 are cross-connected across the terminals of the synchronizing switch in the phases Y and B. To be more clear, the lamp Ll is connected between the terminals Of the synchronizing switch leading to bus-bar R and the terminal r of the incoming alternator. The lamp L2 is connected between the terminals of the synchronizing switch leading to bus-bar Y and the terminal b of the incoming alternator. On the other hand, the lamp L3 is connected between the terminals of the synchronizing switch leading to busbar B and the terminal y Of the incoming alternator. If these lamps are mounted at the three corners Of a triangle (Fig. 4.5), then it will be observed that they glow brightly in rotation.
Fig. 4.5 : Direction of rotation of the glow
(a) When incoming alternator is faster
(b) When incoming alternator is slower
The direction of rotation of the glow indicates whether the incoming alternator is running faster or Slower than the Other alternator/s already connected to the bus-bars. The speed Of rotation of the glow is decided by the difference in frequencies of the incoming alternator and bus-bars. The small adjustment in the speed Of the prime mover Of the incoming alternator is finally made until the glow rotates at a very low speed. The synchronizing switch is then closed at the instant when the directly connected lamp L1 is dark and cross-connected lamps L2 and L3 are equally bright.
Explanation :
Fig. 4.6 shows the relevant phasor diagram for this method under the condition when frequency difference exists between the frequencies of the incoming alternator and bus-bars.
Fig. 4.6 : Phasor diagrams for ‘One dark and two equally bright lamps’ method
Here, as in the phasor diagram for the previous lamp method, the phasors Vr(bus) , Vy(bus) and Vb(bus) represent the bus-bar phase voltages. Similarly the phasors Vr(alt) , Vy(alt) and Vb(alt) represent the phase voltages of the incoming alternator. These two sets of voltages rotate in the anticlockwise direction at unequal speeds when the frequencies Of the incoming alternator and bus-bars are different. The voltages across the lamps L1, L2 and L3 are shown by the phasors VL1, VL2 and VL3. At the instant when the incoming alternator voltage and bus-bar voltage will be in phase, the voltage across the directly connected lamp L1 will be zero and it will be dark. On the other hand, the voltages across the cross-connected lamps L2 and L3 will be equal in magnitude. Therefore, these lamps will be equally bright. The synchronizing switch should be closed at this instant. If the incoming alternator is running faster than the other alternator/s already connected to the bus-bars, the phasors Vr(alt) , Vy(alt) and Vb(alt) will appear to rotate anticlockwise with respect to the phasors Vr(bus) , Vy(bus) and Vb(bus) at a speed corresponding with the difference between the frequencies of the incoming alternator and bus-bars. Then at the instant under consideration (Fig. 4.6), the voltage across the lamp L1 will be approaching zero, that across the lamp L3 will be approaching its maximum, while that across the lamp L2 will be decreasing, having just through its maximum. Hence, the lamps will attain maximum brightness in the order L2— L3— L1 or L1— L2— L3. If on the other hand, the incoming alternator is running slower in comparison with the other alternator’s connected to the bus-bars, the relative rotation of the phasors and will be clockwise and it will be seen that the lamps attain maximum brightness in the order L2— L1— L3 or L1— L3— L2. Hence, the direction of rotation in which the lamps glow brightly indicates whether the incoming alternator is running faster or slower than the other alternator/s already connected to the bus-bars.
Advantages :
(i) Needs inexpensive equipment.
(ii) Direction of rotation in which the lamps glow brightly indicates whether the incoming alternator is running faster or slower than the other alternator/s already connected to the bus-bars.
Disadvantages :
(i) Even though this method is more accurate as compared to dark lamp method, still the accuracy depends upon the sense of correct judgement of the operator.