Two-layer supervisory controller-based thyristor-controlled braking resistor for transient stability crisis

In this paper, a supervisory two-layer control structure is synthesized with network elements as control means for thyristor-controlled braking resistor (BR) of multimachine power system operating in transient emergency state. This creates a multiple local feedback controller 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 BR is computed by the local controllers. By controlling the firing angle of the thyristor, the BR controls the accelerating power in each generator and, thus, enhances the stability margins and damping oscillations. Since the local controllers rely only on measurements particular to their own subsystem, interconnection effects and the nonlinearities introduced by them are accounted for by the supervisory layer. The two-layer controller was tested on the IEEE Western States Coordinating Council (WSCC) test system. Results show that the controller is capable of bringing the system under control when starting with inherently unstable conditions, even when the severity of the disturbances is increased.