A Sea-State-Dependent Control Strategy for Wave Energy Converters: Power Limiting in Large Wave Conditions and Energy Maximising in Moderate Wave Conditions

Conventional control strategies for wave energy converters (WECs) maximise power capture of the WEC by amplifying its responses, but this exacerbates hardware constraint violations not generally taken into account, causing undesirable shutdown of electrical systems in adverse wave conditions. When WECs operate close to power take-off (PTO) capacity, the primary control objective is to limit peak power for hardware protection purposes, enabling longer continuous electricity generation time. In this paper, we propose a sea-state-dependent control strategy based on model predictive control to maximise the annual energy production of a WEC with a realistic PTO: in small to moderate sea states it adopts a conventional energy-maximising objective function to increase output power, while in higher sea states a speed-limiting objective function may be utilised to enable longer generating time before shutdown becomes necessary. While this control strategy applies to a wide range of WECs, here we carry out the case study on an attenuator WEC called M4, with gearbox transmission and a permanent magnet synchronous generator (PMSG) as its PTO, which is being designed for a 1/4 scale ocean test in Albany, Australia. Simulation results show that compared with a benchmark passive damping controller, a 66% increase in annual energy production can be expected at the targeted site.