Before operating an alternator or synchronous generator in parallel to share the load with other already existing alternators. The connections (in aspects of terminal voltage, frequency, phase sequence) of the newly added alternator with the busbars of other alternators in parallel operation must be checked properly, this process is called 'Synchronization of Alternators'.
- By varying the field excitation the generated terminal voltage of the generator or alternator can be varied.
- The synchronization of frequency is done by controlling the speed of the prime mover that drives the alternator.
- For synchronizing the phase sequence of the alternator the following methods can be used,
- By using a phase sequence indicator.
- By lamps method.
- By using a synchroscope.
- By using a phase sequence indicator.
- By lamps method.
- By using a synchroscope.
Synchronization by Lamps Method :
Dark Lamp Method (For Single-phase Alternators) :
Consider alternator B is to be connected to bus-bars, to which alternator A is already connected as shown in the figure below. The prime-mover of alternator B is brought up to its rated speed. The alternator is then excited and voltage is raised to that of bus-bars or alternator A voltage.
If the frequencies of the alternators A and B are same and their terminal voltages are in phase opposition, no resultant voltage act across the lamps L1 and L2, and therefore these lamps remain dark.
If the frequencies of the alternators A and B are not equal, the current through the lamps and local series circuit (shown with dotted line) will be changing, resulting in the flickering of lamps. The frequency will be changing, resulting in the flickering of lamps. The frequency of flickering is equal to (fA ~ fB). At this condition, lamps will glow up alternately. In the middle of the dark, the two voltages will be in-phase opposition with respect to the local circuit.
The speed of the alternator B is adjusted until the flickering of lamps is very slow. The voltage is also made equal to the incoming bus-bar voltage by changing field excitation. Now the switch S1 is closed in the middle of the dark period of the flickering lamps. Hence it is known as the dark lamp method.
Bright Lamp Method :
It is somewhat easier to judge the middle of the bright period than the middle of the dark period and some engineers prefer to synchronize with the bright lamp method. This necessitates the crossing over of the lamp connections, as in the case of another alternator C. Now, the lamps will glow brightest when the two voltages are in phase with the bus-bar because then the voltage across them is twice the voltage of each alternator.
Two Bright One Dark Lamp Method (For Three-phase Alternators) :
In addition to the condition discussed for single-phase alternators, it is necessary that the phase sequence of the incoming three-phase alternator must be the same as that of bus-bars (RYB) and, synchronization of one phase results in automatic synchronization of the other two phases.
In synchronization of three-phase alternators, the connection of three lamps (for each phase) must be done as shown in the below figure. Once the lamp is connected between the same phases while the other two are in cross-connection with the bus-bar phases. The phase sequence is synchronized on a particular sequence of alternatively varying brightness of three lamps.
For perfect synchronization, the switch is closed when lamp L1 is dark while lamps L2 and L3 are equally bright.
When the incoming alternator B is in synchronism with alternator A or bus-bars, lamps L1, L2 are bright and L3 is dark (since L1 is connected between YB', L2 between BY' and L3 between RR').
Since near the synchronism, the brightness of L1, L2 increases and of other L3 decreases, so the instant at which the incoming alternator is in synchronism with the bus-bars can be accurately determined, and switch S should be closed at this instant. This method of synchronization is known as the 'Two Bright and One Dark Lamp' method.
The voltage star RYB refers to the bus-bars and R' Y' B' to the incoming alternator B. Then the instantaneous voltages across the three lamps are given by the vectors YB', Y'B, and RR'. When the frequency of the incoming alternator is same as that of the bus-bars, both vector diagrams rotate in space with the same angular velocity.
Suppose the incoming alternator is too slow, then diagram R' Y' B' will rotate more slowly than RYB so that at the instant represented in below phasor figure YB' is increasing, Y'B is decreasing and RR' is increasing. If the incoming machine is too fast, then YB' is decreasing, Y'B increasing and RR' is decreasing.
Hence, if the three lamps are placed in a ring fashion a wave of light will travel in a clockwise or counter-clockwise direction around the ring according to the incoming alternator is fast or slow and also determines whether the speed must be decreased or increased.
The switch is closed when the changes in light are very slow and at the instant L1 is dark. Lamp synchronizers are suitable only for low voltage alternators due to the limitation of lamp ratings.
