Load reduction potential of variable speed control approaches for fixed pitch tidal current turbines

Abstract

The present paper compares control strategies for fixed pitch tidal current turbines and introduces the underspeed control technique. The state of the art approach of overspeed control for fixed pitch turbine increases the tip speed ratio to limit the power output of the turbine. This leads to a high fatigue loading as the number of load cycles increases and requires a specific rotor design not to increase the thrust coefficient as well.The underspeed controller on the other side reduces the tip speed ratio resulting in a reduced thrust coefficient and a reduced number of load cycles. However, this point of operation is unstable based on the characteristic curves of the rotor and needs to be stabilised by the controller. The basic structure of the controller as well as the calculated stability limits will be shown in the paper.Further, the performance of the underspeed controller regarding loads is compared to state of the art fixed and variable pitch controllers. The paper concludes with an analysis of the impacts on the turbine design and further potential and issues arising with the underspeed control approach, especially the stochastic stall.