Optimal Shutdown Control

Optimal Shutdown Control (OSC) is one of the four major control components of the integrated wind turbine control concept SucCon, which is being developed in the SenterNovem research project number EOS LT02013 “Sustainable Control: A new approach to operate wind turbines”. The goal is to avoid (or minimize) the accumulation of damage during wind turbine shutdown as a result of a serious failure. To this end, condition-specific control towards standstill is required.

Serious failures and conditions that require shutdown of the wind turbine include failure of a pitch actuator or a sudden loss of the generator counter torque as a result of electric grid loss or generator/power electronics failure. In the case of pitch actuator failure, for instance, pitching the remaining two blades too fast to feathering position to stop the rotor will result in a very large asymmetric rotor load due to the resulting pitch imbalance between the blades, which can end up in additional damage. Wind turbines are designed to withstand such ultimate loads, but since these loads are design driving, implementing an OSC strategy that minimizes the loads during shutdown is expected to result in less material use. 

To achieve an OSC strategy, a cost function is defined that penalizes the weighted sum of rotor shaft and tower bending moments and generator speed. Due to the intrinsic nonlinear wind turbine dynamics in the presence of pitch actuator failures, a nonlinear model predictive controller is used to optimize this cost function online under the actuator constraints (pitch position and speed limits, maximal generator torque).