Page 236 - Mathematical Models and Algorithms for Power System Optimization

228 Chapter 7

The load frequency compensator (i.e., feedforward controller) is designed based on this
^
principle. The state of the unit is estimated by the local estimator, by which data ΔP Tj and Δ^ ω j
are estimated and then to be the input of the center estimator, to estimate the state variable of
the equivalent machine and the load forecast value ΔP L . ΔP L then is taken as the disturbance
value in the next step. This process is repeated with the value ΔP L as the next disturbance value.
In this process, the compensation controller is used to compensate for the disturbance
and control the system state, thereby forming an entire tracking control process.

7.2.4 Considerations of Transformation Methods for Linear Models

When addressing the load frequency control problem of the power system, this chapter tries to
apply a series of system theories in identification, estimation, and control, however some
problems cannot be solved by the existing linear model transformation theory. This chapter has
made innovation and improvement to the existing linear model transformation theory to solve
some never-encountered problems, which may bring great convenience.

Because the time interval (60s) for measuring load disturbance is different from the time
interval (1 or 4s) adopted by the controller, which should be taken into consideration for the
formulating of mathematical models for discrete state equations of load disturbance, this study
employs two methods to solve the new problem: ① transforming the discrete model into
an equivalent continuous model, based on which desired discrete model will be derived, and ②
directly using the discrete transformation to develop the desired discrete model. This has
brought a question of mutually transforming the continuous and difference models. When
the model is of a higher order, based on the computer control, the problem of numerical solution
of transforming the differential transfer into difference transfer type is solved.

7.3 Identification of Load Disturbance Model ΔP L

The load disturbance of a power system is a complex disturbance, and when addressing the load
frequency control, it is required to first understand the variation of the disturbance, then use a
proper mathematical model to fit it.
The operation process of the power system is a stochastic process; users can cut off or put on the
loadsanywhereandanytime,sotheloaddisturbanceisubiquitous,anditschangerulesarenoteasy
to grasp. However, the power system frequency can be taken as an indicator whether the power
system is in balance operation, that is, a balance indication between the generation and load. The
load disturbance is bound to cause frequency changes, and in turn, if the mathematical model
of load disturbance has been developed, the changes in load disturbance can be estimated based on
frequency changes. Therefore, to carry out effective control on the load and frequency, it is
imperative to develop the mathematical model for load disturbance and master its essence.