Robust Hierarchical Control Strategy for Heaving Wave Energy Converters

Abstract

Numerous reference-based control strategies have been developed for enhancing the captured and converted energy of the heaving wave energy converters (WECs). This study developed a hierarchical control strategy (HCS) that provided an optimal reference signal considering the different sea-states and constraints on the heaving buoy’s elevation. The developed HCS is composed of a higher hierarchical controller (HHC) and a lower hierarchical controller (LHC). An optimal reference signal, for different combinations of wave specific heights and frequencies under elevation’s constraints, is provided by the HHC. The buoy’s output velocity is regulated by the LHC, based on the reference signal provided by the HHC. Robustness and tracking performance are achieved by the LHC. The present paper utilized a proportional-integral-derivative (PID) controller augmented with sliding mode control (SMC), known for its robustness capabilities, for implementing the LHC. The developed control strategy was validated using regular (monochromatic) and irregular (polychromatic) sea-states. Validation process also considered both nominal and perturbed and disturbed systems. The HCS showed effectiveness in enhancing the power train of the heaving WECs while respecting the physical limitations of the system.