Multivariable control design of multifunctional unified power flow controller

One of the most important features of a unified power flow controller (UPFC) is its multiple control functions which may be implemented by multiple controllers. The dynamic interactions among the multiple controllers were investigated. Examples demonstrate that due to the interactions, separately designed and individually implemented stable controllers for the UPFC may result in poor control performance and even closed-loop system instability when they are in joint operation. Hence it was suggested that the multiple functional UPFC is treated as a multi-input and multi-output control system. A single multivariable controller can be designed for the UPFC to perform the multiple control functions, which takes full account of the control interactions and guarantees the closed-loop system stability.