Power Smoothing Control in a Grid-Connected Marine Current Turbine System for Compensating Swell Effect

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.