Transient Response Improvement for Interconnected Linear Systems: A Low-Dimensional Controller Retrofit Approach

In this paper, we propose a method of designing low-dimensional controllers for interconnected linear systems. In the proposed method, by implementing an additional low-dimensional controller, which we call a retrofit controller, to a pre-existing control system, we aim at improving transient responses caused by a local fault occurring at a specific subsystem. It is found that a type of state-space expansion, called hierarchical state-space expansion, is the key to systematically designing a retrofit controller, whose action is specialized to control the corresponding subsystem. Furthermore, the state-space expansion enables theoretical clarification of the fact that the performance index of the transient response control is improved by appropriately tuning the retrofit controller. The efficiency of the proposed method is shown through a motivating example of power system control, where we clarify the tradeoff relation between the dimension of a retrofit controller and its control performance.