The operation of connecting the alternator in parallel with another alternator or with common bus bar is known as synchronizing.In an actual power system,a number of generators are connected in parallel according to consumers demand. If the load on a generator station becomes so large that the rating of the running generators is exceeded, it becomes necessary to add another generator in parallel to increase the power available from the generating station.For the two or more synchronous generators may be connected in parallel; the following conditions must be fulfilled:
»Their phase sequence must be the same (phase 1 to phase 1 ,phase 2 to phase 2 & phase 3 to phase 3)
»Terminal voltage if both alternators must be equal.
»The waveform of emf generated by both alternator should be in phase.
»The frequency of the both alternator must be equal.
»The percent impedance of both alternators should be same.
The two alternators are operating in parallel so that the above requirements are satisfied, they are said to be synchronous. The operation of getting the machine into synchronism is known as synchronization.
Above figure shows an arrangement for synchronizing two alternators. M/C A is running & M/C B is required to be connected in parallel with M/C B.
At First,the phase sequence is checked to confirm the respective phases are connected. Phase sequence is checked by phase sequence indicator.
Which is small 3 phase induction motor that rotates in one direction for one phase sequence and in opposite in another phase sequence.
As shown in the figure,M/C Bis rotated by prime mover approximately up to its synchronous speed keeping circuit breaker CB-2 open.Excitation ofM/C B is adjusted so that so that it's generated voltage V2 is same as that of V1.
The lamp L1 is connected across R & R1, L2 is connected to Y & B1 & L3 is connected across B & Y1. Physically these three lamps are installed in a triangular format.
For frequency check,there may be following three cases:
When phasors R1, Y1& B1 rotate with the same speed as R,Y & B phasors.
Then, L1 becomes dark.
L2 becomes bright
& L3 becomes bright.
When phasors R1, Y1& B1 rotate faster than R,Y & B phasors.
L1 starts to glow.
Brightness decreases in L2 .
& Brightness increase in L3.
In such a case, the speed of generator G2 has been reduced until f1 =f2.where L1 is dark and L2 & L3 glow with equal brightness.Then CB-2 is closed.
When phasors R1, Y1& B1 rotate slower than R,Y & B phasors.
L1 starts to glow.
The brightness increases in L2 .
& Brightness decrease in L3.
In such a case,the speed of generator G2 has to be increased until f1=f2.Where, L1 is dark abd L2 & L3 glow with equal brightness.
Hence,The same frequency condition of that alternator can be checked by the three lamps as shown in the figure above.
The infinite bus system is system behaves like a large generator having virtually zero internal impedance & infinite rotational inertia.Such a system of constant voltage & constant frequency regardless of load.
An alternator connected to an infinite bus have following operating characteristics:
The lamp L1 is connected across R & R1, L2 is connected to Y & B1 & L3 is connected across B & Y1. Physically these three lamps are installed in a triangular format..