State feedback control assisted by a gain scheduling scheme for three-level NPC VSC-HVDC transmission systems

In this work, a feedback linearization controller together with a gain scheduling function is proposed in order to control a high-voltage direct-current transmission system (VSC-HVDC), composed by two three-phase three-level neutral point clamped converters. The described dq0 model has the advantage of simplifying the mathematical analysis and does not disregard the zero-dynamics, against other reported solutions. In addition, based on the input-output linearization a direct current control (DCC) for the whole VSC-HVDC system is presented, where the control tasks are distributed between both converters. Moreover, a control loop to balance, in an independent way the voltage between capacitors at each converter is designed. Additionally, to the unbalance compensator, a gain scheduling scheme is proposed to assigning new tuning parameters for the PI controller, considering the system states as scheduling variables. The real-time simulator OPAL-RT® is employed as rapid prototyping and test the performance of the proposed DCC scheme.