The objective of voltage control is to have the root mean square (RMS) value of the terminal voltage of a generator (Va ) tracking a reference. As we have discussed from the electromagnetic model of a synchronous generator, the rotor excitation voltage vF is treated as the input to the synchronous generator model. If we know the plant model V a v F (if the relationship is nonlinear, we need to obtain the linearized model Δ V a Δ v F , we can then design the feedback control and test the controller accordingly. In the real world, design has to be carried out in multiple stages. Starting from the simplest plant model, we then add layers of complexity to see how those features will influence our design. In this particular case, we may need the following stages of model complexity for design.

Ignore all dynamics, only consider the steady-state relationship of vF to Va . At this stage, we examine two scenarios with the first as the simplest and the second with more complexity.

First scenario: stator side is open.

Second scenario: stator side is not open. We will consider a case of a single-machine infinite-bus (SMIB) system.

This stage of plant model investigation helps to determine whether negative feedback or positive feedback will be employed.

Ignore electromechanical (EM) dynamics, only consider electromagnetic (EMT) dynamics. This includes multiple sub-stages.

124Consider only rotor flux dynamics. This model is termed “flux decay model.”

Consider rotor flux dynamics and stator flux dynamics for a generator with only an excitation circuit on the rotor. The three-order EMT model derived in Chapter 4 adopts the same assumption and can be used.

Consider rotor flux dynamics (linked to the rotor excitation circuit), rotor damping circuit dynamics (linked to D and Q damping circuits), and stator flux dynamics. In this case, the dynamic model will have five orders. There will be five state variables, λF, λD, λQ, λd, λq . This model is a sophisticated model and is the base of the subtransient model.

Final stage: include both EM and EMT dynamics.