Multi-level controller design for UPFC

This paper presents a two-level controller design for UPFC, i.e., internal level controller and external level controller. A switch is designed in the external level to determine the functional operation of UPFC. The external level controller is a conventional PI controller to provide the references signals for the internal level. A robust controller is designed for the internal level to provide dynamic control input signal for the GTOs, making the output of converters to dynamically trace the reference values set by external level. In the internal level controller, a conventional PI controller of regulating the dynamics of capacitor is adopted as the modulation of the shunt real current. The reference values of shunt reactive current and series active and reactive current should be taken into account to determine the shunt active current reference value. All the variables are locally measurable. The simulation results verify the effectiveness of the designed controller.