Power limitation control for a PMSG-based marine current turbine at high tidal speed and strong sea state

This paper deals with the control strategies for a fixed-pitch marine current turbine (MCT) when the marine current speed exceeds the rated value corresponding to the rated power of generator and converter. Over-rated marine currents occur at large spring tides or under strong sea states; in these cases maximum power point tracking (MPPT) strategies must be changed to power limitation strategies for limiting the generator power at rated value. In this paper, two power limitation strategies with flux-weakening control are investigated. A feedback flux-weakening control is applied to extend the rotor speed operating range over the nominal MPPT tracking speed during high speed marine currents. In the speed control strategy, the turbine power characteristics are used to generate the rotor speed reference and the power limitation can be obtained at steady-state. In the torque control strategy, the generator torque is directly calculated by the power limitation and the rotor speed; this method enables to control the generator power at the limited value at dynamic process. Both strategies are investigated and compared at high tidal speeds without and with swell effect.