An Investigation of Subsynchronous Oscillation of AC/DC Power Systems: Modeling and Analysis

This dissertation, "An investigation of subsynchronous oscillation of AC/DC power systems: modeling and analysis" by Chang, Yu, 余暢, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled "An Investigation of Subsynchronous Oscillation of AC/DC Power Systems: Modeling and Analysis" Submitted by Yu Chang for the degree of Doctor of Philosophy at The University of Hong Kong in July 2006 Subsynchronous Oscillation (SSO), which can result from torsional interactions with series compensation capacitors and/or various fast acting power system controllers, is a complex stability issue in power systems. The development of HVDC transmission and thyristor switching devices in power systems has stimulated considerable research into device-dependent SSO. This thesis seeks to present comprehensive mathematical models and small signal stability analysis of SSO in ac/dc power systems. Both eigenvalue analysis and complex torque coefficient (CTC) calculation methods are discussed. Traditional eigenvalue analysis is implemented based on system linearized ordinary differential equation (LODE) models with algebraic variables eliminated. However, the system physical transparency laid in the original differential and algebraic equations (DAE) can be destroyed after reduction. In addition, it may be inconvenient to conduct eigenvalue sensitivity analysis w.r.t. certain parameters based on LODE models. This thesis proposes linearized differential and algebraic equation (LDAE) models for SSO study. The DAE formulation process facilitates equation assembly efficiently by breaking down the modeling issue into tow steps, viz. element modeling and system integration. Both generalized eigenvalue analysis and CTC calculation methods are derived directly based on the proposed LDAE model. The LDAE-based SSO study model and its corresponding analysis methods have the potential to jointly work with appropriate thyristor switching device models for SSO study and provide deep insight about their effects on SSO. For SSO study in a power system with thyristor-switching devices, the time variations and nonlinearities caused by the switching transients pose a challenge to the eigenvalue analysis and CTC calculation methods. The quasi-steady-state models of converter-based thyristor-switching devices such as HVDC converters are widely used to investigate their impacts on SSO. The quasi-steady-state model with the switching transients ignored, theoretically, is accurate only for the fundamental component at the steady state. So far this approach has not yet been verified. This thesis reports a comprehensive study of the dynamic modeling of a generator/rectifier system, using the modeling technique in power electronics. A nonlinear detailed model of the electromechanical ac/dc system was constructed, based on the description of the commutation and non-commutation stages with the switching transients modeled in detail. In order to use the CTC calculation method to analyze SSO with impacts of rectifier controls, a precisely linearized model is derived by calculating the transition from the begin to the end of the period with piecewise accurate linearized system models. The computer case studies reported in this thesis show that the conventional quasi-steady-state model will result in more conservative simulation results than those from the proposed precisely linearized model. Hence SSO damping controllers designed from the quasi-steady-state model have to posses more control energy to retain system stability under SSO. Finally a stand