Power System Stability Including Shaft and Network Dynamics

Abstract

<p>This thesis describes dynamic stability modelling and analysis of balanced multimachine power systems where in addition to detailed generator, governor-turbine and excitation system simulation, the dynamics of the mechanical shaft system and electrical network are included. Eigenvalue and complementary eigenvalue sensitivity methods are used in the investigation of stability or lack thereof.</p> <p>A unified structure is presented in the formulation of a state space model for the complete system in terms of subsystem models. A wide variety of subsystem types and complexities can be accommodated including all present industry standard models. In addition, general models describing the dynamics of the mechanical shaft system and electrical network can be accommodated. The formulation approach further preserves the identity of the various subsystems, thus allowing ease of system modification or update.</p> <p>The overall modelling concepts are applied to a number of practical situations to demonstrate their applications. In particular, situations involving insufficient synchronizing torque, insufficient damping torque, interaction between turbine-generator and control equipment, interaction between turbine-generator and network dynamics and interaction between turbine-generator and asynchronous motor loads are examined.</p> <p>The concepts developed are also applied in the analysis of the interaction between the shaft dynamics of closely coupled identical generators. A method is presented for determining the shaft natural frequencies for an arbitrary number of identical units, in terms of two equivalents. The extent to which the shaft modes may be stimulated in feasible on-line experiments is investigated.</p> <p>Results and comparisons for alternative dynamic models are presented. It is shown that a single high order 'Benchmark' model which embraces all important dynamic effects, can be systematically reduced to models of reduced complexity - the basis for the reduction being the approximations normally applied in practice.</p> <p>The main contributions are:</p> <p>1. A systematic state-space equation structure which can include shaft and/or network dynamics has been developed. The formulation structure, which includes facility for eigenvalue sensitivity evaluation, can be applied in general problems involving electrical machine and system dynamic stability.</p> <p>2. A technique has been developed for deriving the multiplicity of normally considered low order power system models, from a single high order system model (Benchmark model). This permits the evaluation of a wide range of alternative models and also, the identification of sources of instability.</p> <p>3. The fundamentals of interacting shaft dynamics in closely coupled turbine-generators, have been examined. It has been shown that only two equivalents are necessary for the prediction of natural frequencies and mode shapes in a general N unit situation.</p> <p>4. Insights have been presented into the interpretation of eigenvalues as they reflect the various aspects of power system stability.</p>