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When bringing a synchronous generator online, we are faced with the necessity of paralleling it with the rest of the power system. Figure 1 illustrates the problem.
Conditions for Synchronizing the Generator with the Grid (Power System)
Once the generator is operating, we would like to close the switch to connect it to the power system. The question, then, is what conditions should be present before the switch can be closed?
The phase sequence is particularly important. If the generator is producing an ABC sequence and the bus is operating with a CBA sequence, then closing the switch creates a rotating magnetic field that opposes the rotation of the machine. With the rotor spinning in the opposite direction of the stator magnetic field, extremely high pulsating torques are created, and there have been cases where the shaft of a machine was actually fractured due to these high braking torques.
Clearly, we would like to have an indication that there are no voltages across the poles of the three-phase switch in Figure 1 before we close the switch.
In the three-bulb method, a light bulb is placed across each pole of the three-phase switch that connects the generator to the power system, as shown in Figure 2(a).
The objective is to close the switch when all of the lights are dark, as that would indicate that each pole of the switch has nearly zero voltage across it.
Consider each of the four criteria we have listed. If the generator and the bus are operating at significantly different voltage levels or if they are separated by a constant phase angle, then all three poles of the switch will have voltages across them and all three bulbs will be on all of the time.
If the generator and the bus have different frequencies, then the phasor voltages will rotate at different speeds. Figure 2 (b) shows the two sets of phasor voltages. At some point, the two sets will come into phase with each other and the bulbs will go dark, but then they will drift out of phase due to the different rotating speeds. Thus, if the bus and generator have the same phase sequence, as shown, the bulbs will go light and dark together.
On the other hand, if the phase sequences are different, as shown in Figure 2(c), then only one of the phases can be in phase at any one time. Thus, the lights would go dark sequentially, one at a time. Note that the bulbs will be dark for a period of time when the voltage is too low to light them but not zero.
The synchroscope is a device that indicates the instantaneous angle between two voltages of the same frequency or that shows the frequency difference between them if they are not the same.
Figure 5 is a drawing of a phase angle meter/synchroscope. Obviously, if the frequencies are different, the angle between the voltages is constantly changing, which means the pointer will rotate.
The generator would be connected to the left terminals and the bus to the right (transformers may be required to step the voltages down). If the generator’s frequency is too high, the needle will rotate clockwise; if it is too low, the needle rotates counterclockwise.
Clearly, we would like to have an indication that there are no voltages across the poles of the three-phase switch in Figure 1 before we close the switch.
