Optimal control switching of thyristor controlled braking resistor for stability augmentation

In this paper, the thyristor controlled system dynamic braking resistor and the nonlinear optimal control theory are approached simultaneously within a hierarchical framework. This creates a multiple local feedback controllers that can be realistically implemented using only local measurements and whose performance is consistent with respect to changes in network configuration, loading and power transfer conditions. Following a major disturbance, the rotor angle and rotor speed of each generator unit are determined and the firing angle of the thyristor switch associated with the braking resistor is calculated by the local controllers. By controlling the firing-angle of the thyristor, braking resistor controls the accelerating power in each generator and thus enhances the stability margins and damping oscillations. Since the local controllers rely only on information particular to their own subsystem, interconnection effects and the nonlinearities introduced by them, are accounted for by a supervisory controller. The proposed control strategy was tested on the IEEE Western States Coordinating Council (WSCC) test system. Results show that the method is capable of bringing the system under control when starting with inherently unstable conditions, even when the severity of the disturbances is increased.