Page 258 - Mathematical Models and Algorithms for Power System Optimization

250 Chapter 7

Disturbance is given by a compensator:

C V ¼ GH C 1ð ÞV
If select
1
H C ¼
G
Then the output is completely independent of the perturbation. If the compensator cannot be
completely equal to 1/G for some reason, the disturbance output is approximately independent
of the disturbance. When the feedforward loop fails to fully compensate, it can be regulated by
the feedback loop, as shown in Fig. 7.14.
The disturbance quantity is

H C G 1
C V ¼ V (7.67)
HG +1

Fig. 7.14

Combined control structure diagram of Fig. 7.13B and C.

ItcanbeseenfromEq.(7.67)that,whenthefeedforwardregulationloopfailstofullycompensate
the disturbance influence, it is possible to adopt the feedback regulation, that is, the combined
feedforward-feedback system. It must be noted that estimation and forecasting with regard to
load disturbance ΔP L makes it possible to constitute the feedforward control loop. Failure to
know the disturbance signals means that the feedforward control loop cannot be constituted.

7.6.2 Load Frequency Control based on Invariance Principle

As mentioned earlier, the task of the load frequency controller is to find such a control law to
compensate for the effect of the load disturbance and to maintain the power system frequency
within the allowable range of the specified value. It is known that the integral load frequency
controller (see Fig. 7.15) can curtail the disturbance influence but cannot be completely offset.
Nevertheless, the feedforward controller (or compensator) based on the invariance principle,
under certain conditions (when the compensated transfer function is invertible and accurate