WAMS-based Dynamic Stability Analysis And Control Of Power Systems

Along with the application of modern power electronic equipments and the continual scale extension of the power system,the structure and the dynamic behaves of the interconnected power power system become more and more complicated.Lots of off-line simulation analysis and on-line monitoring results show that there are serious dynamical stability problem in the interconnected power grid. The deterioration of whole dynamical stable performances of the interconnected power grid makes the local disturbances evoke the collapse of the whole grid,so as that the power -transmitting capacities are limited in transmission lines between regions.The emerging of wide area measurement systems(WAMS) offers new opportunities for the monitoring and control of wide area power systems. WAMS can offer the synchronous information of the state and algebraic variables of a wide area power system in real time.Therefore,with the information from WAMS, the synchronous monitoring and the on-line dynamical process analysis and even the on-line urgent or optical control of a wide area power can be done.This dissertation introduces the concept of WAMS firstly,and then illustrates the research status of the WAMS-based dynamical stability analysis and control of power systems,classifying into two categories:the open-loop and closed-loop dynamical stability analysis and control of power systems based on WAMS.And then the dissertation unfolds the main work from the two aspects of the open-loop and closed-loop systems,respectively.In the aspect of WAMS-based dynamical stability analysis and control of closed-loop power systems,the following work is done in this dissertation.In the second chapter,based on the argument principle,a method is proposed to derive delay-dependent stability criteria without any conservatism for LTI (linear time invariant) systems with multiple time delays.Because the characteristic polynomial of a LTI system with multiple time delays is analytic in the whole complex plane,the argument principle is used to judge whether the characteristic equation has roots in the right-half complex plane.With a straightforward form,the criteria can be considered as a generalized Nyquist criterion.The criteria are sufficient and necessary for the stability of the system. The method involves no symbol calculation so as that can deal easily with the systems of which the orders and time delays are more than 3.The example case study shows that the criteria can judge whether a LTI system is stable exactly,for any given time delays.In the third chapter,the relation is discussed between the topology of stable time-delay regions and the stability analysis of a linear system with multiple time delays.In order to analyze the topology of stable time-delay regions,this dissertation constructs a function of which the value is equal to zero for the time-delay points on the boundaries of stable time-delay regions.The function is continuous and differentiable in the whole defining field with the global minimum of zero so that can be used to locate the boundaries by minimizing the value of the function.Based the above topology analysis,GA(Genetic Algorithm) and the LM(Levenberg-Marquardt) algorithm together with the Filled Function Method are used to find an appropriate feedback gain matrix that can stabilize the system using feedback signals with randomly varying time delays.As a method without any conservatism,the method based on the topology analysis is simple and reliable so that can deal with a LTI(linear time-invariant) system with high order and multiple time delays(more than 3) easily.The example case study shows that the above method can work reliably.The fourth chapter analyzes the dynamical process of time-delayed power systems losing stability and the robustness of the controllers based on linearized models.The analyzing results show that the time delays in feedback signals deteriorate the robustness of controllers,and the instability is in the form of the voltage losing stability;and the stability of a closed-loop time delayed power system following a disturbance is determined by whether the initial state following the disturbance has aωlimit set.Based on the above analysis,a control strategy is proposed.The strategy is in essence a kind of switch stabilization of nonlinear systems.Numerical results form a tow-area four-machine power system show that the control strategy can deal with the instability caused by the weak robustness of controllers,and is simple and reliable so that can be easily implemented in power systems.In the fifth chapter,based on the theory analysis of the 2nd～4th chapters,a controller is designed that is not sensitive to the asynchronous variations of time delays,considering closed-loop power systems as the nonlinear dynamic systems with multiple time delays and varying time delays.Numerical results form a tow-area four-machine power system demonstrate that the controller derived from the above method can damp low-frequency oscillations in the system for the feedback signals with multiple time delays and varying time delays.In the aspect of WAMS-based dynamical stability analysis and control of closed-loop power systems,this dissertation gives a method in the sixth chapter to monitor and control Hopf bifurcations in multi-machine power systems using the information from wide area measurement systems(WAMS).Power Method (PM) is adopted to compute the pair of conjugate eigenvalues with the algebraically largest real part and the corresponding eigenvectors of the Jacobian matrix of a power system.The distance between the current equilibrium point and the Hopf bifurcation set can be monitored dynamically by computing the pair of conjugate eigenvalues.When the current equilibrium point is close to the Hopf bifurcation set,the approximate normal vector to the Hopf bifurcation set is computed and used as a direction to regulate control parameters to avoid a Hopf bifurcation in the power system described by differential algebraic equations (DAEs).The validity of the proposed method is demonstrated by regulating the reactive power loads in a 14-bus power system.