Stability Analysis And Robustness Design For Load Frequency Control In Large-Scale Delayed Power Systems

Load frequency control(LFC)plays a key role in maintaining the frequency of power systems at a fixed value or within a small range.In the LFC scheme,the collection and transmission of the measurement information and control signals require the communication networks,which inevitably induces communication delays,threatening the stable operation of power systems seriously.In fact,the closed-loop LFC scheme has some special characteristics,and it can be modeled as a time delay system with high-dimension,sparse,and group-symmetry properties.Taking these characteristics into account is the basis of analyzing the influence of time delay on the frequency stability of power systems and then designing robust control strategies.Moverover,with the increased integration of renewable energies,modern power systems have more randomness and security risks as well as less controllability.In order to enable the power systems to withstand the influence of the uncertainty on supply and demand side,it is necessary to design robust control strategies such that the balance between the fluctuated generation and load side can be achieved.This dissertation investigates the stability analysis and robustness design of large-scale load frequency control systems with communication delays.The main results are summarized in below.(1)A novel method is presented based on the sparsity of the delayed LFC system for investigating its stability analysis in large-scale power systems.Due to the high-dimension characteristics of power systems,the stability criteria established with the Lyapunov direct method and the original system model have heavy calculation burden.Thus,a novel method is presented to reconstruct the original system model by analyzing its sparse characteristics of the parameter matrices.Then,a Lyapunov–Krasovskii(LK)functional is constructed based on the reconstructed model,and its delay-related terms are dealt with the delay-related part of the reconstruction model.Next,a stability criterion is developed with the reduced computational complexity.Finally,employing the presented method calculates the delay margins for the two-area delayed LFC schemes under the traditional and deregulated environments,respectively.As a result,the calculation accuracy and efficiency are improved for the delay-dependent stability analysis of the delayed power systems.(2)A novel method is presented based on the group-symmetry characteristic of the delayed LFC system for investigating its stability analysis in large-scale power systems.When the Lyapunov direct method is employed to analyze the stability of the large-scale LFC systems with time delays,the established stability criterion is difficult to be solved.Therefore,a novel method is presented for the efficient stability analysis of the LFC schemes by investigating their structural characteristics.Compared with the existing methods,this method can achieve a significant increase in computational efficiency while sacrificing a little accuracy.This method is divided into two parts,including an improved model reconstruction method and a generalized symmetry-exploiting method.As only a few state variables are affected by the delayed states directly,the improved reconstructed model is presented and it helps establish a stability criterion with decreased computational complexity.Then,the generalized symmetry-exploiting method is obtained by analyzing the symmetry of the LFC systems.This technique helps constrain the structure of the weighting matrices required in the LK functional to further reduce the computational burden of the obtained stability criterion.The presented method is used to analyze the delay-dependent stability of the delayed LFC systems and the New England 39 bus system,respectively.It is shown that the presented method improves the calculation efficiency for the stability analysis of large-scale power systems with time delays,and it is applicable to the real-world power systems.(3)A novel method is presented for the stability analysis of large-scale LFC schemes with adjustable computation accuracy.Based on the Lyapunov direct method,the previous stability criteria are employed to calculate the delay margins,but the conservatism introduced into the obtained results is inconsistent considering different control gains.When the calculated results are used as the performance index to guide the controller design,the reasonability of the designed controller is degraded.Thus,a new delay-dependent stability analysis method is presented with a new LK functional and an integral inequality.This method can adjust the conservatism and computational complexity flexibly.Compared with the existing fixed stability criteria,this method has advantages in improving calculation accuracy and efficiency.The presented method is used to analyze the delay-dependent stability of the two-area LFC system and the New England 39 bus system,respectively.Under different controller gains,this method achieves the results with consistent conservatism.The obtained results are employed to guide the controller design while improving its reliability.(4)An effective method is presented for designing the robust controller in large-scale LFC schemes by considering their delayed dynamics equations.The bounded real lemma(BRL)directly established on the large-scale delayed LFC systems is too complex to effectively guide the design of robust controllers.A new CTODE model is presented by analyzing the physical meaning of each state participating into the LFC systems,and it is an equivalent form of the original system model.The time delay information is contained in the time-delayed ordinary differential equation whose order hardly changes with the scale of the system.Then,based on the new CTODE model,a BRL is established for the performance analysis.Compared with the method directly based on the original model,this method helps tune the PI controller in the large-scale delayed LFC systems effectively.The presented method is used to design the robust controllers for the three-area LFC system and the New England 39 bus system.Based on the proposed method,the designed controllers are verified to have the almost similar dynamic performances like thoese developed with the original model.(5)Two kinds of methods are presented for designing the robust LFC schemes in the large-scale wind power systems considering time delays.From the perspective of passive disturbance suppression and active disturbance suppression,two types of robust control strategies are designed respectively for the large-scale delayed LFC scheme integrated with the fluctuated wind power.First,based on the exponential decay rate,the delay-dependent stability criteria are established for wind power systems to design the output feedback controller.When the external disturbances are introduced,the settling time of system frequency is reduced.Then,a control strategy is constructed based on the equivalent input disturbance method for the wind power systems considering the demand response.These two types of controllers enable to achieve the balance between the randomly fluctuated generation and the load,ensuring the safe operation of the power systems.