Small-signal Stability Analysis And Load Frequency Control For Delayed Power Systems

The modern power systems usually require an open communi-cation network to transmit information related. This network can achieve the exchanging of large information within a big range, which enables the analysis and control of power systems from the view of the whole system. However, the usage of such network inevitably introduces delays into the transmitted signals, and those delays may degrade dynamic performances and even cause the instability of the system, thus the impacts of the delays should be considered during the analysis and control of the power systems. The theoretical research of time-delay systems has attracted much attention and lots of methods for the analysis and design of the systems have been developed, while there are only a few investigations on the application of those methods to practical engineering problems. Modern delayed power systems provide an application platform for the theoretical researches. The effectiveness of theoretical methods would be verified, and the further improvements required for easy application to practical problems would be found. This dissertation investigates the small-signal stability and load frequency control (LFC) for the delayed power systems, which not only solves practical problems concerned but also improves theoretical reseasch of time-delay systems.(1) Delay-dependent small-signal stability analysis for the delayed power systems based on a free-weighting matrix (FWM) techniqueFor the small-signal stability analysis of power systems with time-varying delays, a misunderstanding in frequency-domain-based researches is pointed out, namely, the delay margin of time-varying delay is wrongly considered as the same as that of constant delay. Based on delay-dependent stability criteria derived by Lyapunov direct method, an FWM-based delay-dependent small-signal stability analysis method is proposed, which can discuss time-varying or random delay. For a single machine infinite bus (SMIB) power system equipped with different excitation controllers, the delay margins of the closed-loop system are calculated via the proposed method, and the relationship between the parameters of controllers and the delay margins is discussed. The relationship can provide an extra guideline for the choice of the controller gains.(2) Delay-dependent robust stability analysis for PID-type LFC in the one-area power system based on an improved FWM techniqueFor the stability analysis of PID-type LFC in one-area power system, a time-delay system model with structured uncertainties is constructed by taking communication delays and parameter uncertainties into account. A delay-dependent robust stability analysis method is derived based on an improved FWM technique, which, compared with the existing methods, has less conservativeness and is calculated more easily. For the one-area LFC equipped with PI/PID controller, the relationship between the controller gains and parameter uncertainties and delay margins is investigated by the proposed method, and how to use the obtained delay margins to guide the design of the controllers is also discussed.(3) Delay-dependent stability analysis for PID-type LFC in the inter-connected power system via a novel functional and an integral inequalityFor the stability analysis of PID-type LFC for the interconnected power system, multiple-delay linear system models for the traditional and deregulated environment are developed by combining one-area LFC model and the power exchanges among different control areas. A new delay-dependent stability analysis method is presented by constructing a novel Lyapunov functional and using Jensen’s integral inequality. The method is dependent upon the criterion with less conservativeness and computing complexity. Thus, it can increase the accuracy of the calculated results and is more suitable to deal with large scale power systems with high dimensions. For two-area and three-area power systems equipped with PID-type LFC, the delay margins are calculated by the proposed method, the interaction effect of the delays appearing in different areas is investigated, the stability regions of the systems are calculated, and the usage of the obtained stability regions to guild the presetting of some operating conditions for a practical LFC is discussed.(4) Robust PID-type LFC design of power systems based on delay-dependent H∞output feedback control design method For the PID-type LFC design for the power systems considering both communication delays and load disturbances, a time-delay system model based on static output feedback control is constructed after treating coupling part of power exchanges in different areas as load disturbances, and a delay-dependent H∞control design method is developed. The improved FWM technique is applied to derive the design conditions, in which the delay-dependent characteristic and the H∞performance index respectively guarantee the robustness against the time delays and the load disturbances. Meanwhile, a new algorithm for tuning the gains of the output feedback controller is proposed by following the procedure of calculation and verification, and it provides a feasible idea to design the output feedback controller, which is known as a challenging problem in the research of time-delay systems. For two types of three-area systems, the corresponding robust PID-type LFC are designed such that the closed-loop systems have desired robustness against the communication delays, load disturbances, and parameter uncertainties.(5) Analysis and design of control systems with sampled-data and its application to the LFC working in discrete-modeFor the analysis and design of sampled-data systems via Lyapunov direct method, a novel idea for reducing the conservativeness of analysis results is developed by relaxing the positive conditions of the functional, and a more effective stability analysis method is proposed. Meanwhile, based on the procedure of calculation and verification, an improved control design method based on parameter-tuning technique is given. In consideration of the fact that the practical LFC usually uses the feedback signals updated non-continuously, the proposed method is applied to analyze some existing state-feedback controllers to check whether they can work well in such discrete-mode, and it is also used to design a state-feedback-based LFC to ensure that the closed-loop power system can achieve the objective of the LFC just using sampled feedback signals.