Modelling and control of thyristor-controlled series compensators

A linearised discrete-time model of a thyristor-controlled series-compensated transmission line is presented, in which a thyristor-controlled reactor (TCR) is used as the compensating device. The discretisation is performed at the peaks of the capacitor voltage of the series compensator. Since these peaks are broad, rather than sharp spikes, they are quite good indicators of variations in the fundamental component. During transients this will result in variation from uniform sampling. However, the system degenerates to a uniform sampled system in the steady state, and thus all the standard tools for the stability analysis of a standard discrete-time model can be applied. As a part of the linearised model, two different output equations are determined. They are the line current, which is in quadrature with capacitor voltage, and the real power flowing through the line. These output equations are then used for control design. The model developed is validated through digital computer simulation studies, in which the robustness of the closed-loop system is investigated through eigenvalue plots as a function of the operating points