Retrofit Control of Interconnected Linear Systems

In this paper, we propose a method of designing low-dimensional retrofit controllers for interconnected linear systems. In the proposed method, by retrofitting an additional low-dimensional controller to a preexisting control system, we aim at improving transient responses caused by spatially local state deflections, which can be regarded as 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 low-dimensional retrofit controller, whose action is specialized to controlling 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 trade-off relation between the dimension of a retrofit controller and its control performance.