Parallel Operation of Generators
In modern power systems isolated generators are very rare. Power systems are highly
interconnected and many generators share the load. The first problem of an engineer is
connecting a synchronous generator on an existing bus.
CONNECTING a GENERATOR to a BUS
The above figure 4-1 illustrates a generator G1 which is already connected to a power
grid under load. Generator 2 has to be connected or “brought on line”
1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator.
2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor.
3. Observe the lights which are connected across the switches: they should beat, first
get brighter and then dim as the phasors for generator and bus respectively shift.
If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat
out of phase, one pair of phases should be reversed.
4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2.
One should observe a slow beat of the light brightness.
5. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum). Parallel Operation of Generators 2
Recap:
· Phase sequence must be the same
· Voltages must have same magnitude
· Frequency must be the same
· Phasors must be aligned
Note that in modern installations a ”synchroscope” is used. The synchroscope will
instruct the governor of the prime mover to set the speed, and instruct the exciter to
produce a voltage. When the phasors are detected within 5 degrees match, the
synchroscope will close the switch.
In modern power systems isolated generators are very rare. Power systems are highly
interconnected and many generators share the load. The first problem of an engineer is
connecting a synchronous generator on an existing bus.
CONNECTING a GENERATOR to a BUS
The above figure 4-1 illustrates a generator G1 which is already connected to a power
grid under load. Generator 2 has to be connected or “brought on line”
1. The prime mover of the generator has to bring the speed of the shaft close to the rated speed of the generator.
2. The excitation of the generator has to be increased to give a no-load output voltage as close as possible to the existing bus voltage We want to create a phasor rotation for generator 2 output similar to the bus voltage phasor.
3. Observe the lights which are connected across the switches: they should beat, first
get brighter and then dim as the phasors for generator and bus respectively shift.
If the 3 lights beat concurrently, the phase sequence is correct, else if lights beat
out of phase, one pair of phases should be reversed.
4. Adjust now the prime mover to slowly increase/decrease the speed of generator 2.
One should observe a slow beat of the light brightness.
5. When the lamps beat slowly, the switches should be closed when the lights are extinguished (line-line voltage at minimum). Parallel Operation of Generators 2
Recap:
· Phase sequence must be the same
· Voltages must have same magnitude
· Frequency must be the same
· Phasors must be aligned
Note that in modern installations a ”synchroscope” is used. The synchroscope will
instruct the governor of the prime mover to set the speed, and instruct the exciter to
produce a voltage. When the phasors are detected within 5 degrees match, the
synchroscope will close the switch.
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