Synchronizing of Parallel operation.

A stationary alternator must not be connected to live bus bar Because the induce emf is zero at standstill and a short circuit will result.

the synchronizing procedure and the equipment for checking it are the same whether one alternator is to be connected in parallel with another alternator or an alternator is to be connected to infinite bus.

The following method are used for synchronization.

1. Synchronizing lamps
2. Synchroscope.

Synchronizing lamps

A set of three synchronizing Lamps  can be used to check the condition for paralleling the incoming machine with other machines. The dark lamp method along with a voltmeter used for synchronizing. It is used for synchronizing low power machines.

the prime mover of the incoming machine is started and brought up ti near its rated speed. the field current of the incoming machine is adjusted so that its terminal voltage becomes equal to the bus voltage . the three lamp flicker at a rate equal to the difference in the frequencies of incoming machine and the bus bar.If phases are properly connected , all the lamps will be bright and dark at same time. If this is not the case , then this means phase sequences are not correct.In order to correct the phase sequence, two leads of the line of the incoming machine should be interchange.the frequency of the incoming machine is adjusted until the lamps flicker at a very slow rate, less than one dark period per second. after the finally adjusting the incoming voltage , the synchronizing switch is closed in the middle of their dark period.since the voltage across the lamps varies from zero to twice the phase voltage , the lamps of suitable rating must be used.

Advantage of the dark lamp method

1. the method is cheap
2. the proper phase sequence is easily determined    

Disadvantages of the dark lamp method

1. the lamps become dark at about half their rated voltage, it is possible that the synchronizing switch might be closed where there is a considerable phases difference between machines. this may result in high circuilating current to damage the machines.
2. the lamps filament might burn out. 
3. the flicker of the lamps does not indicate which machine has the higher frequency.

Three Bright Lamp method
In this method lamps are connected across the phases , that is, A1 is connected to B2, B1 is connected to C2 and C1 is connected to A2. If all the three laps get bright and dark together , then the phase sequences are the same. the correct instant closing synchronizing switch in the middle of bright period. the brightest point in the cycle is easier to distinguish than the middle of a dark period and avoid confusing letter with a lamp filament failure.  

Two Bright one dark Lamp method

In this method, one lamp is connected between corresponding phases while the two others are cross-connected between the other two phases. that is, A1 is connected to A2, B1 to C2 and C1 to B2. The prime mover of the incoming machine is started and brought up to near its rated speed.

The excitation of the incoming machine is adjusted in such a way that the incoming machine induces the voltage EA1, EB2, EC3, which is equal to the Busbar voltages VA1, VB1 and VC1.The correct moment to close the switch is obtained at the instant when the straight connected lamp is dark, and the connected cross lamps are equally bright. If the phase sequence is incorrect, no such instant will take place, and all the lamps will be dark simultaneously.

The direction of rotation of the incoming machine is changed by interchanging the two lines of the machine. Since the dark range of the lamp extends to a considerable voltage range, a voltmeter V1 is connected across the straight lamp. The synchronising switch is closed when the voltmeter reading is zero.

Thus, the incoming machine is now floating on the Busbar and is ready to take up the load as a generator. If the prime mover is disconnected, it behaves as a motor. For paralleling small machines in power stations, three lamps along with the synchroscope are used. For synchronizing very large machine in power stations, the whole procedure is performed automatically by the computer.

Synchroscope 
The phase sequence of the generator is usually checked carefully at the time of its installation. Consider 1 and 2 are assumed by means of a synchroscope which compare the voltage from one phase of the incoming machine with that of the corresponding phases of the three phase system.

The position of a pointer of the Synchroscope shows the phase difference between the voltages of the incoming machine and the infinite bus. When the frequencies are equal, the pointer is stationary. When the frequencies differ, the pointer rotates in one direction or the other. The direction of motion of the pointer shows whether the incoming machine is running too fast or too slow. This means that whether the frequency of the incoming machine is higher or lower in comparison to the infinite bus.The speed of rotation of the pointer is equal to the difference between the frequency of the incoming machine and the frequency of the infinite bus. The frequency and phase positions are controlled by adjusting the prime mover input of the incoming machine.When the indicator of the Synchroscope moves very slowly and passes through the zero phase point or vertical up position, the circuit breaker is closed, and the incoming alternator is connected to the bus. In this case, the frequencies are almost the same. The Synchroscope checks the relationships only on one phase and does not give any information about phase sequence.

The  procedure is same as that for the large synchronous machine. The synchronous motor is started. As the motor approaches synchronous speed, direct current is applied to the field winding. If the load torque is not excessive, the motor pulls into synchronism with the system.