Sycnhronization of Three Phase Alternators

The conditions to be satisfied for synchronization of three phase alternators are same as that for single phase alternators. But instead of saying that voltages must act in phase opposition, the phase sequence must be same i.e. phase must be connected in proper order of R, Y, B. Typical setup for synchronization of alternators is shown in the Fig. 1.

Fig. 1 Setup for Synchronization of Alternators

       In synchronizing three phase alternators, three lamps are connected as shown in the Fig.2, so that it can be use to indicate whether the incoming machine is running slow or fast. With symmetrical connection of lamps, they would dark out or glow up simultaneously provided that phase sequence is same for incoming machine and bus bar.

       Consider the two alternators A and B to be synchronized. The alternator A is already running at synchronous speed and its excitation is so adjusted that it builds up the rated voltage. The alternator A is connected to the bus bars of constant voltage and frequency. The alternator B is to be connected to bus bar i.e. it is to be synchronized with alternator A. The process or synchronization can be explained as below :

Step 1 : Start the prime mover of machine. Adjust its speed to a synchronous speed of machine B. This will rotate the rotor of alternator B. This will rotate the rotor of alternator B at synchronous speed.

Step 2 : The switch S₄ is then closed. By adjusting the rheostat R_x the excitation to the field is adjusted so that induced e.m.f. of B is equal to the induced e.m.f. of A. This can be verified by voltmeter.

Step 3 : To satisfy remaining conditions, the three lamps pairs are used which are  L₁, L₂ and L₃ as shown in the Fig. 2. These are connected in such a way that pair L₁ is straight connected while the pairs L₂ and L₃ are cross connected to understand the connection, the pairs are again shown in the Fig. 3.

Fig. 2

Fig. 3

       Now two supplies are supplying lamp pairs, E_RYB i.e. voltage supply of bus bar while  E_R'Y'B'  i.e. supply generated by alternator B. The switch S₃ is still open.

       Let the three bus bar voltages be represented by phasors OR, OY, OB rotating at angular speed of ω₁ rad/s. The incoming alternator voltage are represented by phasors OR', OY', OB' rotating at angular speed of ω₂ rad/s.

       The phasor E_RR',  joining the tips R and R' is voltage across lamp pair L₁. Similarly E_YB , and  E_BY, are voltages across lamps L₂ and L₃ respectively.

       If there is difference between the two frequencies due to difference in speeds of the twp alternators, the lamps will become dark and bright in a sequence. This sequence tells whether incoming alternator frequency is less or greater than machine A.

       The sequence L₁, L₂, L₃ tells that machine B is faster as the voltage star R'Y'B' will appear to rotate anticlockwise with respect to bus bar voltage RYB at a speed corresponding to difference between their frequencies shown in the Fig. 4. The sequence L₃, L₂, L₁ tells that the machine B is slower because voltage star R'Y'B' will appear to rotate clockwise with respect to bus bar voltage RYB. The prime mover speed can be adjusted accordingly to match the frequencies.

Fig. 4

       The synchronization is done at the moment when lamp L₁ is in the middle of dark period. If the lamps pair becoming dark and bright simultaneously, it indicates incorrect phase sequence which can be correct by interchanging any two leads either of the incoming machine or of bus bars.

Note ; For high voltage alternators it is not possible to use the lamps directly. In such cases lams are connected through potential transformers.

In this method when lamp L₁ is dark the other two lamp pairs L₂ and L₃ and equally bright. So this method of synchronization is called ''Lamps bright and dark'' method.