Variations of marine current speed may lead to strong fluctuations in the power extracted by a marine current turbine (MCT). During a short-time period, swell effect is the main cause for the current speed variations. The conventional tip speed ratio maximum power point tracking (MPPT) algorithm will require the MCT to accelerate or to decelerate frequently under swell effect, which can cause severe fluctuations in the generator power. This paper focuses on power smoothing control of the grid-connected MCT system. In the first step, a modified MPPT algorithm with filter strategy is proposed in generator-side control to mitigate the fluctuation of generator power. In the second step, a supercapacitor (SC) energy storage system (ESS) is added to compensate the residual power fluctuations. Simulations of a 1.5-MW direct-driven grid-connected MCT system are carried out. The swell effect is calculated based on typical system location and sea state. Detailed control strategies and SC sizing are described. The results demonstrate that the association of the generator-side filter strategy with the SC ESS system achieves a smoothed power injected to the grid in case of swell disturbances.

This paper deals with power control strategies for a fixed-pitch direct-drive marine current turbine (MCT) when the marine current speed exceeds the rated value corresponding to the MCT nominal power. At overrated marine current speed, the MCT control strategy is supposed to be changed from maximum power point tracking (MPPT) stage to constant power stage. In this paper, flux-weakening strategy is investigated to realize appropriate power control strategies at high marine current speeds. During flux-weakening operations, the generator can be controlled to produce nominal or overnominal power for a specific speed range (constant power range). These two power control modes are compared, and the constant power range is calculated. The relationship between the expected constant power range and generator parameter requirements [stator inductance, permanent magnet (PM) flux, and nominal power coefficient] is analyzed. A Torque-based control with a robust feedback flux-weakening strategy is then carried out in the simulation. The proposed control strategies are tested in both high tidal speed and swell wave cases; the results validate the analysis and show the feasibility of the proposed control method.

Five-phase surface-mounted permanent-magnet machines can inherently produce a smooth electromagnetic torque which can be increased when using third harmonic current injection. To really take advantage of these characteristics, the rotor magnets can be shaped to obtain a back electromotive force with large third harmonic term. This is the scope of this paper. For the design specifications of a low-speed marine propulsion machine, the following objective must be achieved: to significantly mitigate the pulsating torque without reducing the average torque bearing in mind the solution where the rotor is made with full pole-pitch magnets. An analytical field computation, called equivalent coil method, is developed to quickly explore the magnet geometries. Thus a procedure to optimize small trapezoid notches at the surface of the pole magnets is performed. Referring to the classical fully pole-pitch magnet shape, the solution found allows a substantial reduction of the pulsating torque without reducing the torque density. Furthermore, with regard to an equivalent three-phase machine, for the same copper losses, the average torque of the optimized five-phase machine can be potentially higher if the third harmonic current injection is implemented.

In this paper, an original multi-phase Surface Mounted Permanent Magnet (SMPM) Machine designed for naval propulsion is proposed. The design objective of this high power low speed machine is twofold: to enhance the fault tolerance capability of the system and to optimize the quality of the torque by reducing the electromagnetic torque ripples which underlie the acoustic behaviour of the motor and of the global mechanical structure. A low level of ripple torques must also be ensured in faulty operations. To fullfill these constraints, the machine is equipped with a fractional-slot concentrated winding made up of four 3-phase windings each one being star-connected, each star being magnetically shifted by an angle of 15 degrees. This 4-star 3-phase configuration allows to reduce the cogging torque and to separate magnetically and physically the phase windings. The end-turns are also drastically reduced, which improves the compactness and the efficiency of the machine. This original multi-phase machine is supplied by four 3-phase PWM voltage source inverter with sinusoidal current law. The magnetic independences between the four star windings allow a very simple control of the four-star supply and a straightforward fault operating mode. Moreover, this 4-star winding configuration yields to very low torque ripples in nominal configuration (four stars connected) and in faulty operations if two magnetic non adjacent stars are disconnected. For all these reasons, this structure appears particularly suitable for naval propulsion application since it increases the machine performances in terms of compactness, reliability and quality of torque.