On the load-frequency control problem
Bratton, Timothy Lee
Pearson, J. Boyd
Master of Science
This thesis reports an investigation of tie-line bias load-frequency control. This type of control is currently used on many electric energy supply systems, but has recently been the subject of much controversy in the literature. Claims have been made that typical industry values of the load-frequency controller parameters result in smaller "stability margins" than other gains which minimize integral quadratic performance indices which weight only tie-line power and frequency deviations. Through analog simulation studies of the dynamic model of two interconnected steam-electric areas, it was found in this research that the "optimum" LFC gains resulted in rates of change of generated power which were beyond the capabilities of typical industry generating units. A digital-computer parameter optimization routine was written which demonstrated that typical industry LFC gains also satisfy an integralquadratic performance index -- one which weights speed-changer rate more heavily than frequency or tie-line power. A case is then made against the use of parameter optimization as a tool for designing control systems. The eigenvalues of the Two-Area Model were studied and it was found that the LFC gains have little effect on any interconnection poles other than those which are characteristic of the motion of the speedchanger. In particular, the synchronizing oscillations become less damped as the LFC integrator gain increases from zero. When the two areas of the Model were identical and a decentralized control strategy was implemented, it was found that half the poles of the interconnection were fixed as the interconnection coefficient increased from zero. These fixed-pole modes were not observable in the transfer function between an area's load change and its angle deviation. Classical root-locus techniques were used to study the observable modes. It was found that as the interconnection coefficient went to infinity, the synchronizing oscillation poles approached a vertical asymptote whose position depends only on the inertia and damping of each area, not on any feedback of an area's state variables to its governor input. It is indicated that further improvement in interconnection response may be difficult to achieve via a decentralized control strategy which acts through the governor.