Optimal control of a wind turbine with digital fluid power transmission

Digital fluid power (DFP) technology may lead to a paradigm shift in large-scale transmission systems in, e.g., wind and wave energy. Therefore, the development of applicable control algorithms is of major importance, but is complicated by the non-smooth behavior of the DFP displacement machines. The power throughput of a full stroke operated digital displacement machine is quantized by the number of pressure chambers. The dynamics of each pressure chamber may be described by highly nonlinear continuous differential equations, whereas the input is discretely updated and binary (active or inactive). This paper contributes with a feedback control strategy for a digital displacement machine, where the binary inputs are handled by a pulse density modulator. The paper presents a linearization method of handling the many nonlinearities and thereby enabling the use of Discrete Linear Time Invariant (DLTI) control theory. The control strategy is validated for control of a digital fluid power wind turbine transmission, where both a deterministic and a stochastic optimal controllers are synthesized. The study is based on the NREL 5-MW reference wind turbine, where its model is combined with a nonlinear model of the DFP transmission and full-field flow wind profiles are used for a realistic performance evaluation scenario. By simulation, it is found that the performance of the optimal controllers using the DFP transmission is similar to that of the NREL controller using a conventional transmission.