Stability Analysis And Controller Design Of Delayed Load Frequency Control Systems

Electrical energy is the necessary energy for human survival and development.Guaranteeing the quality of power and maintaining the safe and stable operation of the power grid is a major issue concerning the national economy and people’s livelihood.The instability of the power system can lead to serious consequences such as large-scale power outages,so studying the stability of the power system is crucial.Due to the widening and complication of power grid interconnection,the traditional communication channel relying on local feedback information is no longer suitable for modern power systems.With the development of communication technology and power technology,wide-area information needs to rely on open communication networks to achieve global control of the interconnected power system.However,the transmission of wide-area information based on such networks inevitably introduces a large number of transmission delays,and the existence of time-delay has a non-negligible effect on the stability of the power system.Therefore,the stability study of the delayed system has become hot issues.Load frequency control is an important control method to adjust grid frequency and improve power quality.In this thesis,the related research on the stability analysis and controller design for load frequency control is carried out.The main work of this thesis is as follows:The second chapter introduces the mathematical model of the governing system of the power plant.The modeling process of each component of the speed control system is introduced in detail.The basic structure and working principle of the load frequency control and the generation process of the network transmission time delay are discussed.Finally,the state feedback system model is transformed into static output feedback with time delay information,which lays a theoretical foundation for theoretical research for subsequent stability analysis and robust controller design.In the third chapter,aiming at the stability analysis problem of delayed load frequency control system,based on scaling integral inequality,a low-conservative delay dependent stability analysis method is established.By constructing an augmented Lyapunov functional,a relaxed integral inequality is used to effectively scale the time derivative of the functional,and a new delay-dependent stability criterion is derived,the conservativeness of the criterion is reduced without increasing the decision variables.By comparing with the existing research,it has the advantages of reducing conservative and computing burden.The simulation on a typical four-machine two-area power system model is exploited to verify its effectiveness.The fourth chapter studies the stability analysis method of PI type load frequency control considering the two delays,and the delay dependent stability criterion based on the extended reciprocally convex matrix inequality is proposed.In the modeling stage,the time delay from sensor to generator set is divided into two stages according to different transmission signals and different transmission characteristics,namely,the feedback time delay from sensor to controller and the forward time delay from controller to generator set of power plant.Based on Lyapunov method and linear matrix inequality technique,an augmented functional containing two delay information is constructed,which is effectively defined by using the extended reciprocally convex combination lemma,and the stability criterion related to two delays is derived.The numerical examples are used to analyze the relationship between two time delays,the influence of controller parameters on the delay margin is studied,and the influence of load disturbance on the system is also discussed.The accuracy of the method is verified by time domain model simulation..In the fifth chapter,the control problem of load frequency control system with two time delays under load disturbance is studied,and the design method of PI controller is proposed.In view of the previous research,the problem of system performance is studied under the premise of given PI controller parameters.In this chapter,the effective solution algorithm such as full-rank conversion is used to transform and process the coupled nonlinear term in linear matrix inequality,which solves the problem that the coupling term is difficult to design.The solvability of the PI controller parameters is achieved.In addition,two delay factors are taken into account to ensure the designed load frequency controller has time delay robustness in the modeling process,the introduction of H∞ performance realizes the disturbance robustness of the controller.The simulation of the designed controller gain is used to verify that the corresponding load frequency control system can achieve better control effects for both transmission time delay and load disturbance problems.