Modularity in design of dynamical network systems: Retrofit control approach

Takayuki Ishizaki, Hampei Sasahara, Masaki Inoue, Takahiro Kawaguchi, Jun Ichi Imura

Research output: Contribution to journalArticlepeer-review

8 Citations (Scopus)


In this article, 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.

Original languageEnglish
Pages (from-to)5205-5220
Number of pages16
JournalIEEE Transactions on Automatic Control
Issue number11
Publication statusPublished - 2021 Nov 1


  • Modularity in design
  • Youla parameterization
  • power system stabilizer (PSS)
  • retrofit control

ASJC Scopus subject areas

  • Control and Systems Engineering
  • Computer Science Applications
  • Electrical and Electronic Engineering


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