Design of GEMSTAR tidal current fixed pitch rotor controlled through a Permanent Magnet Generator (PMG) de-fluxing technique

This work reports a study on a tidal current system named GEMSTAR, composed of two hydrokinetic turbines supported by a floating submerged structure. A flexible mooring cable links the floating support to the seabed allowing the free rotation of the system and its alignment to changing current direction. The work is mainly focused on the design of the turbine blade with two objectives: keeping a constant power output above the rated power condition and limiting the thrust increase between rated and maximum operating speed. A fixed pitch blade was considered, in order to reduce the costs associated to a complex pitch control mechanism. An analytical procedure for blade design is proposed and validated by comparison to other numerical methods. Above the rated current speed, the power is held constant while the rotational speed of the turbine and of the generator increases, assuming an over-speeding control strategy. The adopted design configuration relies on the possibility of the generator to operate in de-fluxing working conditions. As reported in the last part of this study, a numerical model of the generator has also been developed and applied to a test case in order to analyse the electric behaviour of the system