  In this paper, we develop a modular design method of decentralized
controllers for linear dynamical network systems, where multiple subcontroller
designers aim at individually regulating their local control performance with
accessibility only to their respective subsystem models. First, we derive a
constrained version of the Youla parameterization that characterizes all
retrofit controllers for a single subcontroller, defined as an add-on type
subcontroller that manages a subsystem. The resultant feedback system is kept
robustly stable for any variation in the neighboring subsystems, other than the
subsystem of interest, provided that the original system is stable prior to
implementing the retrofit control. Subsequently, we find out a unique internal
structure of the retrofit controllers, assuming that the interaction input
signal from the neighboring subsystems is measurable. Furthermore, we show that
the simultaneous implementation of multiple retrofit controllers, designed by
individual subcontroller designers, can improve the upper bound of the overall
control performance. Finally, the practical significance of the method is
demonstrated via an illustrative example of frequency regulation using the IEEE
68-bus power system model.
