Operation of Grid-Connected Cross-Flow Water Turbines in the Stall Region by Direct Power Control

This paper proposes a new method of controlling cross-flow-water-turbine (CFWT)-based generation systems, having fixed pitch, by power regulation. The method consists of directly controlling the electrical generator power, as imposed by a dispatcher, without employing an inner rotational speed control loop. The proposed control approach represents the base for the operation framework (i.e., output power and rotational speed limitations, start-up, and stopping) for the entire operating range up to the cut-out water flow speed. A permanent-magnet synchronous generator is used in this paper. The system is operated at low rotational speed values, i.e., in the so-called hydrodynamic stall region, where mechanical and hydrodynamic stresses are lower. Given that this region corresponds to an unstable dynamic behavior, the power regulation relies upon an inner loop of rotational-speed stabilization. Consequently, the innermost (electrical machine current) control loop is fed by a reference having two components, namely, the driving and stabilization currents. The embodied control laws and the output power and rotational speed limitation sequences have been experimentally validated on a real-time simulator of CFWT-based generation systems.