Harmonics and instabilities in thyristor-based switching circuits

This thesis investigates nonlinear dynamics, harmonic distortions and bifurcation instabilities in thyristor switching circuits. The analysis is directed towards the study of a Thyristor Controlled Reactor (TCR) which consists of a fixed reactor and two oppositely poled thyristors. The dependence of the thyristor switching times on the system states causes the circuit nonlinearities and is the focus of much of the thesis. New concepts for instability, dynamic response and damping for TCR circuits are introduced. These concepts are general and can be extended to other switching circuits. Useful TCR circuit examples such as the 230 kV Kayenta advanced series compensator and the 230 kV Rimouski static Var system are used to numerically verify these concepts. We have found new instabilities in both the Kayenta and the Rimouski systems in which switching times change suddenly, or bifurcate as a system parameter varies slowly. Switching time bifurcations are associated with large distortions of the TCR current or voltage waveforms leading to a new earlier TCR current zero, the disappearance of a current zero, or a thyristor misfire. The instabilities are explained and their mechanisms are illustrated by analysis, simulation and experiment. The TCR is a periodically operated, nonlinear circuit which may be studied by sampling the system states once per period and by means of the Poincare mapping from the dynamical systems theory. We compute the Jacobian of the Poincare map to understand switching time and conventional bifurcations and also to study the small signal dynamic response of TCR switching circuits. The computation includes the effects of synchronization and the nonlinearity due to thyristor switching. Eigenvalues of the small signal dynamics response are computed and used to study the dynamic response of the Kayenta system using different methods of synchronization. In addition, the Jacobian formulas show how and why a TCR circuit can damp small perturbations even when the circuit resistances are ignored and the thyristors are assumed ideal.