From Equilibrium To Oscillation

Currently, the electric power utility industry in many countries is undergoing a tremendous change. Restructured power systems are expected to be operated at a greater variety of operating points and closer to their operating limits. Concerns for power system stability are prompting utilities to better understand stability problems so as to devise effective, efficient and economic solutions to the stability problems. In this dissertation an attempt is made to give deeper insights into topological characteristic of boundaries of Power Flow Feasibility Region (PFFR) and effects of load modeling on boundaries of Small Disturbance Voltage Stability Region (SDVSR), as well as the inherent meaning behind voltage oscillation damping coefficients and electromechanical oscillation damping coefficients. We propose a new boundary tracing method for PFFR and recommend two-step method for identifying SDVSR's boundaries more precisely. A novel online identification algorithm of damping coefficients of multimachine systems is also developed. All researches in this dissertation are conducted in the background of nonlinear dynamics. First, we decouple PFFR into two sub-regions: "Region of Load Injection Space" and "Region of Generator Injection Space". Then a new 'Hybrid' method for tracing boundary of PFFR is developed by adopting the basic idea of "predictor-corrector" and combining it with optimization technique. The hybrid method is also modified to account for concave boundary situation. In the subspace of load injection, we extend the hybrid method to calculate the closest boundary point related to current operating point in high-dimension injection space. In the subspace of generation injection, a new algorithm based on the hybrid method thoughts is presented to obtain the farthest boundary point under L2 and L1 norm definition respectively. It is noted that the topologies of boundaries in these two subspaces are different, especially the boundaries of PFFR in generation injection space can be approximated by hyper-planes within possible operation scope. Then the relationships between PFFR's and SDVSR's boundaries are analyzed with consideration of dynamic load models. We have observed that Saddle Node Bifurcation (SNB) points induced by stalling of motors always occur well before Fold bifurcation point and SNB does not coincide with Fold in general sense. At the same time, we point out that it is not all SNB caused by asynchronous motors stalling would lead to voltage instability and the system behavior at these SNBs depends on the system configuration and the system operating condition. After comparing CPF method, bifurcation method and two-step method, we show that two-step method is the most appropriate method for identifying precise voltage stability limit, and time domain simulations sometimes are also necessary when induction motors occupy major portion of power system's load. In order to reveal mechanisms of monotonic voltage instability mode and oscillatory voltage instability mode, a novel Phillips – Heffron (P-H) model based on Single Generator Single Motor system is proposed, and it is used to derive a voltage oscillation damping factor γ. Then we prove that induction motors in load buses of power systems can eliminate Hopf bifurcation points in boundary of SDVSR, and coexistence of slow-acting excitation systems and large shunt capacitor compensation will induce negative damping for voltage oscillation. All these conclusions are validated through simulations of a realistic power system. Since damping analysis is one of basic techniques in low-frequency oscillation studies, we give a series of theoretic explanation of damping torque coefficients. As the current identification algorithm of damping torque coefficient often fails when dealing with multimode oscillations, we propose a new effective algorithm based on Prony analysis and time domain data. To better understand what causes low-frequency oscillations in the interconnection of state power grid of China, we analytically compare damping torques of strong and weak connected systems. It has been shown that the relatively large impedance of tie-lines in weakly interconnected systems can greatly reduce the damping between these systems, and reinforcement of the connection between two bulk power grids is an effective way to mitigate low-frequency oscillations. At the same time, we clarify the doubt about the models of synchronous generators used in dynamic stability studies.