Page 231 - Mathematical Models and Algorithms for Power System Optimization

Optimization Method for Load Frequency Feed Forward Control 223

steady state. By means of various controls, the effects of such disturbances will be counteracted.
The most important types of control include the following four:

(1) The prime mover torque of the system generator.
(2) Excitation current of generator excitation coil.
(3) Adjustment and control of the transformer tap changer.
(4) Various switches or circuit breakers.

This chapter studies Scenario (1) only, that is, active power control.
The active power control of the power system in normal operation is divided into two branches,
specifically economic dispatch and load frequency control, both of which are currently closed.
Since 1945–1950, particularly the 1950s when the computer came into practical application,
both branches have achieved substantial developments. In recent years, there have been
new advances, such as the application of optimal controls. These new research results can
be applied to existing devices.

Voltage control, related with reactive power distribution, is another branch of normal
operation condition, which is not discussed in this chapter based on the following: (1) the
unbalance of active power mainly affects the system frequency, with little impact on the
voltage and (2) the unbalance of reactive power mainly affects the voltage, with weak
impact on the frequency.
In the early days, the integral feedback control method was used to keep the
frequency stable. However, the essence of feedback control is that the feedback system
plays the role of detecting the correcting the deviation after the disturbance occurs, that is,
the feedback system works after the deviation occurs. However, because some objects are of
serious inertia or hysteresis, the controlled variable will not vary until a long time after
the disturbance and is not easily or rapidly corrected once it varies, with poor adjustment
effects.
If the disturbance can be predicted and the change can be preadjusted to the system, then the
effects of these disturbances can be suppressed as long as the controlled quantity changes, this
control method is called feedforward control. However, feedforward control requires that
the disturbance is measurable or predictable. But some of the disturbance is not measurable or
difficult to measure, people at that time did not grasp the theory of the estimation and
forecasting system, thus such feedforward method was not further studied.

Later, many literatures utilized the modern control theory to study the load frequency
control problem, trying to solve load frequency control with the modern linear control
theory. Its basic idea is the state feedback, which can be used to improve the system
performance by means of linear quadratic optimal control or pole assignment, etc.