Retrofit control is a new control concept towards the control of large-scale network systems that are subject to change due to extension and reconstruction of local subsystems by multiple independent subsystem operators. The proposed approach is a control method that, rather than an entire system model, requires a model of the subsystem of interest for controller design. The retrofit controller can guarantee robust stability in the sense that the entire closed-loop system is stable for any variations of neighboring subsystems (assembly of which is called an environment), other than the subsystem of interest, as long as the preexisting network system (the feedback system of the environment and the subsystem of interest) is originally stable. This enables the modular design of decentralized control systems, i.e., multiple subsystem operators can independently plug in, plug out, and reconstruct respective local controllers without concerning the instability of the entire network system.

As theoretical achievements, we have given the parameterization of all the retrofit controllers in terms of a constrained version of the Youla parametrization. Furthermore, providing a particular design method of retrofit controllers, we have analyzed resultant system properties when multiple retrofit controllers are designed and implemented in parallel. As further extension of theory, we have developed a design method of a generalized version of retrofit controllers where system identification (data-based learning) of the unknown environment is incorporated for better performance improvement. The major advantage of this extension is that the stability of the resultant control system is robustly assured regardless of not only the stability of approximate environment models, but also the magnitude of modeling errors. This has actually good compatibility with existing system identification methods because the accuracy of identified models may neither be reliable nor assurable in reality.

Smart use of renewable energy is one of the key tasks towards reductions in greenhouse gas emissions. To push up the penetration level of renewable energy resources, it is crucial to devise a practical control method to improve the degree of power system stability because renewable power generation, such as photovoltaic and wind power generation, is highly volatile due to weather change. With this background, we have developed a plug-in-type local control method for renewable-integrated power systems based on retrofit control theory. In particular, we have conducted detailed simulation analysis of a wind-integrated IEEE 68-bus test system to show that the proposed plug-in-type local control can properly enhance system stability in a modular fashion. Furthermore, using a modified Japanese bulk power system model, called PV-integrated EAST30 model, we have shown that the proposed method is practically effective also for the volatility of photovoltaic power generation.