  Wind turbines are prone to structural degradation, particularly in offshore
locations. Based on the structural health condition of the tower, power
de-rating strategies can be used to reduce structural loads at the cost of
power losses.This paper introduces a novel closed-loop switching control
architecture to constrain the thrust in individual turbines. By taking
inspiration from developments in the field of reference governors, an existing
demanded power tracking controller is extended by a thrust tracking controller.
The latter is activated only when a user-defined constraint on fore-aft thrust
force is exceeded, which can be set based on the actual damage status of the
turbine. Having a down-regulation with monotonic aerodynamic load response, a
simple linear thrust tracking controller is proposed. Such a scheme can reduce
aerodynamic loads while incurring acceptable losses on power production which,
in a wind farm setting, can be compensated for by other turbines. Large eddy
simulations demonstrate the performance of the proposed scheme on satisfying
thrust constraints.
