Paralleling Generators

In the diagram, a generator is to be added to an existing power system that is connected to a load. Before the switch, S, can be closed, there are a number of steps which must be taken:

1. The rms line-line voltages must be equal
2. The phases must be sequenced in the same order (i.e. Phase A - Phase A; Phase B - Phase B; Phase C - Phase C)
3. The phase angles must be equal
4. The no-load frequency of the oncoming generator must be slightly higher than the existing system frequency

The importance of closing the switch when the generator is at a slightly higher frequency than the system can be seen when you consider the speed droop and \(f-P\) equations. The power from the supplied from a generator to bus after the switch is closed in Fig. 1 can be found using

If \(f_{nl} \lt f_{sys}\) then the power supplied from the generator will be negative. This implies the generator has become a motor, and both the mechanical system and electrical system will try to accelerate the rotor up to synchronous speed. This can have significant negative effects on the prime mover. Simmilarly, if \(f_{nl} \gg f_{sys}\) there will be a sudden step change in the power supplied and the power and current flowing in the bus, which can also cause negative impacts to hte power system and the prime mover.

Paralleling Procedure

1. With the generator running, adjust the field current of the oncoming generator such that the measured line-line voltages of the system and the generator are equal
2. Check the phase sequencing. This can be done in two ways:
   1. Connecting small motors to each three-phase system and observing the direction of rotation. If the phases are sequenced correctly, the two motors will rotate in the same direction. If the sequencing is different, the motors will rotate in opposite directions.
   2. Connect bulbs across the switch. If the phases are sequenced correctly, the bulbs will light up/go dim together. If the phases are not correctly sequenced, the lights will flash one after the other
3. Check the phase angles. If the frequency of the oncoming generator is slightly higher than the system frequency, the phase angle between the system and generator voltages will slowly change at a frequency equal to the difference between system and generator frequencies. If the voltages and sequencing are correct, the oncoming generator can be connected to the system when the phase difference is zero. This can be test using two approaches:
   1. Using a synchrometer. A synchrometer measures the phase angle between 2 voltages. If the meter needle rotates clockwise, the generator frequency is greater than the system. Counterclockwise rotation indicates that the oncoming generator frequency is lower than the system. When the needle is vertical, the phase angle is zero. In the ideal case, the needle should be rotating slowly in the clockwise direction and the switch should be closed when the needle is vertical.
   2. Using bulbs connected across the switch. This lower cost approach does not provide information about the relative frequency between the oncoming generator and the system. If there is a phase difference between the voltages, the lights will be bright. When the lights go out, there is no voltage across the switch and the switch can be closed.