Design of a Robust Nonlinear Fuzzy Controller for HVDC Systems

The design and stability assessment of a new robust nonlinear fuzzy controller that may be applied in the control loops of HVDC systems is presented. A new simplified nonlinear dynamic model is developed for HVDC systems that can be used to design the controller. The proposed model decomposes into several linear systems around important equilibrium points. These local linear models describe the plant dynamic behavior at different operating points. An evaluation is performed by simulation using the Cigré benchmark HVDC model. The simulation results show that, compared to conventional controllers, the new controller gives an improvement in AC/DC/AC system performance during severe faults. Stable behavior is shown to be obtained during a sudden change in effective short ratio (ESCR) when using the proposed controller within a very weak AC/DC system. This is compared with the operation of a conventional controller under similar conditions, during which the HVDC-system becomes unstable.