Research On Static Voltage Stability With Improved Continuation Power Flow Based On GMRES

Continuation power flow(CPF) is an important analytic method of power system static voltage stability. Continuation power flow can overcome the singularity of Jacobian matrix at collapse point. It can trace the track of the power flow solutions, draw a power system PV curve, and get the information of a power system stability margin. Continuation power flow is an effective tool to analyse the security and stability of a power system. Contingency screening and ranking is one of the main components of voltage stability assessment. Researching on the contingency ranking method and index has an important meaning in the aspect of analyzing the security and stability of a power system. According to regional power grid interconnection and the expansion of a power system, many researchers have started to pay attention to voltage stability analysis in a large-scale power system. A large power system has more nodes and more complex network structure than a small-scale power system. How to ensure the security and stability of a large power system and avoid voltage instability and collapse is one of the hot issues in the field of power system.This dissertation analyses the characteristics of conventional continuation power flow method and points out the drawback of conventional continuation power flow method in a large power system. In this dissertation, an improved continuation power flow is presented based on the generalized minimal residual (GMRES) method. Based on the improved CPF with GMRES, two-stage contingency screening and ranking method is proposed in this dissertation. Voltage drop method, reactive power loss method and the improved CPF with GMRES/direct method are all used in the two-stage contingency screening and ranking method.The main contents and contributions of this dissertation are summarized as follows:(1) This dissertation discusses the contents of the static voltage stability, analyses the characteristic curves and common methods of static voltage stability, points out the inadequacies of conventional power flow method and continuation power flow method, studies the elements of continuation power flow and GMRES theory, and summarizes the common contingency screening and ranking methods.(2) An improved continuation power flow is presented based on the generalized minimal residual (GMRES) method. The new method is mainly used in large power systems. Nonlinear predictor with Lagrange quadric interpolation is used in the process of predictor which is more efficient. Double-layer iterative method with ILU preconditioner is used in the corrector which consists of Newton method and GMRES method. This method has an advantage in terms of computation speed. The new method adjusts step-size according to the gradient of the PV curve. The number of the converged solutions to draw a PV curve decreases. As a consequence, the new continuation power flow enhances the efficiency. The result obtained with the new method for an actual power system demonstrates the validity and efficiency of the proposed continuation power flow method.(3) In this dissertation, two-stage contingency screening and ranking method is proposed. In the first stage, voltage drop method and reactive power loss method are used to obtain the critical contingency set by means of power flow (before and after a contingency). An insecure contingency set is made up of the contingencies which cause power flow solution divergence. In the second stage, the improved CPF with GMRES or direct method is used in the obtained critical contingency set according to the scale of a power system. As a consequence, a dangerous contingency set is obtained. The new method divides the whole process of contingency screening and ranking into two stages, balances the speed and accuracy, reduces the computational time and enhances the efficiency. The proposed method is illustrated with an actual power system. The result shows that the new method is efficient and feasible.