On the electromechanical behavior of hydrokinetic turbines

• Control scheme for hydrokinetic systems based on variation of resistive load.
• Ideal resistive load to power extraction maximization.
• Assessment of control system under variable water stream speeds.
• Influence of ideal tip speed ratio on the energy production.
• Impact of characteristic rotor curve shape on HECS dynamic.

A stand-alone hydrokinetic energy conversion system (HECS), comprising a permanent magnet synchronous generator (PMSG), coupled to a horizontal axis hydrokinetic turbine through a mechanical transmission. The rotor performance is given by a characteristic curve of power as function of the rotation, given in non-dimensional quantities. The transmission is assumed to be of single stage with known mechanical efficiency. Park’s transform is applied to obtain the PMSG model, which is connected to resistive and inductive loads. A new method for the rotor angular speed control underwater speed variations, consisting on changing a resistive load connected to the generator, is presented. We present an analytic expression for the value of the resistive load, which keeps the HECS in the optimal operational condition. In addition, the numerical model in used to perform an investigation on the influence of the rotor power curves on the generation system stability and conversion efficiency. The generation system is submitted to variations of terminal load and water speed in order to assess its response in several situations of practical interest. It is shown that rotors which sharp characteristic curves are most likely subjected to severe stopping in the case of stream speed variation or demand variation oscillation, when compared to rotors with more smooth power curves.