Operating a generator connected to an infinite bus

If a generator is operated off an infinite bus:

  1. Terminal voltage and frequency (and hence speed) are constant
  2. The no-load settings of the mechanical system governor can be adjusted to supply more or less power
    variation of generator power with no-load set-point
    Fig. 1. variation of generator power with no-load set-point

Other impacts on operation can be best understood by considering the phasor diagram

To simplify analysis and aid understanding, neglect armature resistance

\[ \begin{aligned} P & = 3VI_A \cos \theta= \frac{3VE}{X_S} \sin \delta \\ Q & = 3VI_A \sin \theta= \frac{3VE}{X_S} \cos \delta - \frac{3V^2}{X_S} \end{aligned} \]

Since voltage is constant and assuming synchronous reactance is also constant, changing the power (with the mechanical system) will change \(E\sin\delta\) and \(|E|\) will control the reactive power. The phasor diagram is initially shown at rated operation (maximum power and apparent power). Reducing the mechanical power wihout adjusting the excitation impacts both the electrical power and reactive power.

Use the buttons below the phasor diagram to investigate the impact that changing the power and excitation have on the phasor diagram, and calculated power, reactive power and armature current.

Phasor Diagram

Fig. 2. Variation of power and excitation on an infintie bus