Nonlinear Quadratic Regulator Design With a Hybrid Switching Scheme for Wind Turbine Generators

Abstract

This article proposes a new control scheme with a nonlinear quadratic regulator (NQR) associated with a hybrid switching stage for permanent magnet synchronous generator-based wind turbines (WTs). The proposed method shows a new approach combining a nonlinear optimal control method with a hybrid switching stage for WT generations (WTGs). As the WTG is a nonlinear system, NQR control design is directly employed to deal with the nonlinearity while a hybrid switching scheme can deal with constrained control inputs as discrete nonlinear characteristics of switch-mode power converters. A polynomial-based dynamic model is derived for the new NQR design. Unlike the previous state-dependent Riccatti equation (SDRE) technique where the solution of the nonlinear optimal controller is approximated, this technique finds the exact online solution of SDRE with nonlinear feedback gains flexibly depending on the tracking error. Then, the optimal control laws are implemented via a hybrid switching stage ensuring a smooth current regulation under a reduced switching frequency while avoiding any complicated modulator or finite state search. The proposed control design is investigated via comparative studies with linear quadratic regulator (LQR), direct switching, and space-vector pulse-width-modulation stages (SV-PWMs) implemented in both platforms of MATLAB/Simulink and real-time OPAL-RT. Comparative results exhibit flexible optimal performances via online hyper-parameter tuning to reduce the speed tracking error and low current ripples under a reduced switching frequency.