Ride-through improvement of wind-turbines via feedback linearization

As the grid code specifies, wind turbines have to remain connected to the grid at voltage levels far below the nominal values. The improvement of wind turbine performance under such conditions has become a problem of general concern. However, this performance usually relies on conventional linear controllers that operate at network faults far off the nominal point for which they were designed. As a consequence, wind turbines should operate with increasing converter currents, which may result in converter damage. This paper proposes a nonlinear controller for converter-based wind turbines that ensures that the currents are maintained within the design limits, even at deepest voltage dips, thus improving the wind turbines fault ride-through capability. The controller is based on feedback linearization theory and linearizes the system by a control approach that takes into account the energy stored on the converter dc-link capacitor. This controller has few nonlinear terms and is feasible to implement into modern floating, or fixed point, digital signal processors.