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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