Design and analysis of nonlinear hierarchical controllers for electric utility industry

This paper suggests a control strategy of coordinating multiple dynamic-braking units during the transients ensuing major disturbances. The control strategy considered in this study is a two-level hierarchy. The proposed two-level structure results from the decomposition of the overall problem into parallel sub-problems. This allows the retention of the closed-loop controls associated with each subsystem, which together constitute the lower level (Level I). The central coordinating controller forms the upper level (Level II). The coordination of the local controllers by the central controller, accounts for nonlinear terms and interconnections, and yields the global optimization of the overall system transient performance. The local controllers are not dependent on one another and are robust to any changes in the network configuration, due to the feedback or closed-loop control formulation inherent in the proposed strategy. To ensure physical realizability of the local controllers, the input was restricted to locally measurable signals. The methodology was implemented into a prototype software program, which was tested on a single machine connected to a very large network approximated by an infinite bus, and then on the IEEE four-generator test system. These studies considered fault clearing times greater that the critical, assuring an unstable condition. The well-damped optimal state and control trajectories illustrate the successful solution of the problem, indicating that the technique is a valuable tool dealing with transient control problems for large-scale systems.