Page 189 - Modern Control of DC-Based Power Systems

Control Approaches for Parallel Source Converter Systems 153

R f L f I L PCC

d·E C f V R L I d

Figure 5.27 Decentralized model including the virtual disturbance.

u d
x nom, nom Setpoint +
U
trajectory + u + u
generator e LQR lqr
–
Î d x ^
Figure 5.28 LQG controller with two degree of freedom structure.

V
P = Constant

R P1

R P2

I
Figure 5.29 Illustration of the set-point generation.

Essentially, the trajectory generation is responsible for the linearization
of the system because the nonlinearities of the loads that generate the dis-
turbance current are not included in the model but hidden in the distur-
bance variable I d . Hence, the system is handled by definition as a linear
system, where the trajectory generation will handle the deviations of the
“linear assumed operating point” from the actual operating point.
Strictly speaking the LQR controller is unaware, from a global point
of view, where the system operating point is at the moment.
This global point of view, the entire CPL curve, compared to the lin-
earized small-signal region (e.g., R P1 ) is illustrated in Fig. 5.29. The set-
point trajectory generation is the component which determines in which
operating point the system is. Then, the LQR controller is responding to
disturbances around this point.