Instabilities due to bifurcation of switching times in a thyristor controlled reactor

The authors describe two bifurcation instabilities of a thyristor controlled reactor (TCR) circuit in which switching times suddenly change and system stability is lost. The instabilities are unexpected because they are quite different from what might be expected from conventional theory in that they occur without the usual indications such as eigenvalues of a Jacobian matrix crossing the unit circle. The instabilities are explained and their mechanisms are illustrated by the simulation of a static volt-ampere-reactive (VAr) example with realistic parameters. In particular, it is shown how distortion of voltage and current waveforms can cause a thyristor switch-off time to disappear or a new thyristor switch-off time to suddenly appear. The consequence of the sudden change in switch-off times is that stable periodic operation of the circuit is lost and a transient will occur until the circuit settles down to a new steady state