An improved direct predictive speed control for three-level NPC converter fed PMSG-based offshore wind turbine system with minimum computation

The objective of this study is to propose an improved direct predictive speed control (IDPSC) scheme for three-level neutral point clamped (NPC) converter-fed permanent magnet synchronous generator (PMSG)-based offshore wind turbine systems (OWTSs) with minimal computation. Conventional direct predictive speed control systems applied to OWTS machine-side converters face challenges due to the larger inertia. In these systems, the deviation between the predicted electromagnetic torque and load torque in different switching states has a reduced influence on the cost function because of the larger OWTS inertia. To address this, the IDPSC introduces additional weighting for torque deviations in the cost function, enhancing control performance and eliminating the need for integral action. Additionally, to reduce computation, the IDPSC incorporates revised switching state formulations based on the optimal switching states from the previous instant, which optimizes switching frequency without requiring a switching frequency constraint in the cost function. The proposed scheme is validated through simulations of a 1.5MW PMSG-based OWTS with a 3L-NPC converter, demonstrating effective speed control under step-varying and randomly varying wind profiles. Furthermore, the performance of the proposed IDPSC is compared with other relevant predictive control schemes, showcasing its improvements.