are interesting solutions to control unstable or integrating processes with significant dead times and subjected to strong disturbances in the inner loop. Because of this, in the past few years this subject has attracted the attention of several researchers [42, 80–82]. However, the control structures involve many controllers, and the design methods are difficult to be used.
The LFC scheme has evolved over the past few decades and is in use on interconnected power systems. There has been continuing interest in designing LFC with better performance to maintain the frequency and keep tie-line power flows within prespecified values using various control method- ologies. Operating the power system in a new environment will certainly be more complex than in the past, due to the considerable degree of interconnection, and to the presence of technical constraints to be considered together with the traditional requirements of system reliability. In response to the new challenges, novel modeling and control approaches are required to get a new trade off between efficiency and robustness. At present, the power system utilities participate in LFC task with simple, heuristically tuned controllers. In response to the new technical control demands for large scale power systems, the main motivation of the present thesis is to develop new LFC synthesis methodologies for multi-area power systems based on fundamental LFC concepts and generalized well-tested traditional LFC scheme, to meet all or a combination of following specifications: robustness, decentralized prop- erty, simplicity of structure, cover all the specified LFC objectives, formulation of uncertainties and constraints.
1.5 Contributions of this Thesis
improved disturbance rejection response. The control structure gives two-degrees-of-freedom control and correspondingly the setpoint and load disturbance responses can each be tuned conveniently by a single control parameter. Robustness studies on the stability and performance are provided, with respect to the uncertainties in the process model parameters. The effectiveness of the proposed tech- nique is verified by simulation results.
II. Enhanced Series Cascade Control Structure for Unstable Delay Processes
A series cascade control structure with modified Smith predictor is presented for controlling open- loop unstable time delay processes. The proposed structure has three controllers of which one is meant for servo response and the other two are for regulatory responses. An analytical design method is de- rived for the two disturbance rejection controllers by proposing the desired closed-loop complementary sensitivity functions. These two closed-loop controllers are considered in the form of PID controller cascaded with a second order lead/lag filter. The direct synthesis method is used to design the set- point tracking controller. By virtue of the enhanced structure, the proposed control scheme decouples the servo response from the regulatory response in case of nominal systems i.e., the setpoint tracking controller and the disturbance rejection controllers can be tuned independently. Internal stability of the proposed cascade structure is analyzed. Kharitonov’s theorem is used for the robustness analysis.
The disturbance rejection capability of the proposed scheme is superior as compared to some existing methods.
III. Improved Series Cascade Control Structure for Integrating Delay Processes
Unlike self-regulating processes, cascade control strategies for control of integrating processes with time delay are limited. A novel series cascade control structure to enhance the closed loop performance is proposed for integrating and time delay processes. It realizes two-degree-of-freedom control in the primary loop. The proposed control structure has only two controllers and a setpoint filter. The inner loop controller is designed based on IMC approach and the primary setpoint filter is based on optimal performance index. The primary load disturbance rejection controller, a PID controller in series with a lead/lag compensator, is designed on the basis of the desired closed-loop complementary sensitivity function. The robustness analysis is carried out using Kharitonov’s theorem. Simulation results demonstrate the efficacy of the proposed method by showing satisfactory nominal and robust performances.
To improve the dynamic performance of a control system, parallel cascade control strategies have been proposed earlier mainly for control of stable processes. In this thesis, further results are presented for a new parallel cascade control structure and controller design for controlling stable, unstable or integrating processes with time delay. The design of the disturbance rejection controller and the setpoint tracking controller are based on loop shaping and ISE performance measures, respectively. A modified Smith predictor scheme is used in the primary loop to enhance the closed-loop performance of the system. Based on the Nyquist stability theorem, the stabilization of typical time delay processes is investigated. For each process, the maximum stabilizable normalized time delay for different controllers is derived. The robustness and performances of time delay processes are analyzed. Examples are given to illustrate the usefulness of the proposed method and its superiority over some parallel cascade control schemes.
V. A New Parallel Cascade Control Scheme for Unstable Delay Processes
A new parallel cascade control scheme is proposed for controlling stable and unstable processes with time delay. The two main features of the proposed scheme are: the primary process output completely tracks the primary setpoint and the servo response decouples the regulatory response in the nominal system. The control structure has only two controllers. The inner loop controller is designed based on IMC approach. The outer loop controller is a PID controller in series with lead/lag filter which is designed based on the desired complementary sensitivity function. Significant improvement in the load disturbance rejection performances are obtained when compared to some recent methods in the literature.
VI. A New Control Scheme for PID Load Frequency Controller of Power Systems A new control structure with a PID load frequency controller for power systems is presented.
Initially, the controller is designed for single-area power system, then it is extended to multi-area case. The control strategy is based on the desired complementary sensitivity function. The controller parameters are obtained by expanding the controller transfer function using a Laurent series. Re- lay based identification technique is adopted to estimate the power system dynamics. Robustness studies on the stability and performance are provided, with respect to the uncertainties in the plant parameters. The proposed scheme ensures that the overall system remains asymptotically stable for all bounded uncertainties and for system oscillations.