When a generator operates in parallel with a system, the power supplied by the generator is controlled by the torque control on the prime mover, for instance the rack of a diesel engine fuel injection pump.

The power factor is controlled by the field or excitation current of the generator. By increasing the field current the phase angle of the stator current progressively lags that of the terminal voltage, and conversely a reduction in field current results in the stator current phase angle leading that of the terminal voltage.

The power supplied by the generator will remain constant during this process if the prime mover torque is kept constant. The operator or automatic controller must be aware of the limits of the generator otherwise excessive current may occur in the field or stator windings.

To this end, an operating chart is often provided by the manufacturer (sometimes referred to as the capability chart). An example is given in Fig. 5.13.

The vertical axis represents output power and the horizontal axis represents output kVAr. The output voltage is shown as OV and the output current is represented by OB, shown here lagging the voltage by φ.

The field current is represented by AB. The output power is given by OD (or VI cos φ), and the output VAr is represented by OG (VI sin φ). The limits for generator operation are represented by the shaded area, where:
● BH represents the field current temperature limit
● KB represents the power limit of the prime mover
● KJ represents the practical limit of stability. Reducing field current reduces the peak torque capability of the generator, and the rotor load angle moves progressively towards the peak output angle.

Any fluctuations in voltage or load can then lead to pole slipping. Automatic relay protection of generators will produce an alarm if the generator operation comes outside this area.

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