A fuzzy controller for improving Fault Ride-Through (FRT) capability of Variable Speed Wind Turbines (WTs) equipped with Doubly Fed Induction Generator (DFIG) is presented. The controller is designed in order to compensate the voltage at the Point of Common Coupling (PCC) by regulating the reactive and active power generated by WTs. The performances of the controller are evaluated in some case studies considering a different number of wind farms in different locations. Simulations, carried out on a real 37-bus Italian weak distribution system, confirmed that the proposed controller can enhance the FRT capability in many cases. 1. Introduction Wind turbines (WTs) are typically located in remote and rural areas. In these areas, the feeders are long and operated at a medium voltage level characterized by a high R/X ratio and unbalanced voltage situations. Furthermore, weak grids are usually referred to have a “low short-circuit level” or “low fault level.” In a weak network a change in the real and reactive power can cause a considerable change in the voltage. The impact relies on the strength of the network and the output power of the WTs [1]. Integration of WTs into weak grids can cause the steady-state voltage level to go outside of its acceptable limit. Therefore, it can limit the exploitation of wind energy resources. Another constraint is related to the effect of the power generated by WTs on the voltage quality. Voltage level limitations and accurate control systems are required to control the voltage variations as well as to improve the voltage quality [2], and variable speed WTs can be used as reactive power sources for voltage control. In recent times, many researches have been carried out in this field. A proportional-integral- (PI-) based control algorithm to control the reactive power produced by WTs has been proposed in [3]. In [4], the authors have proposed a mathematical model of the Doubly Fed Induction Generator (DFIG) for the analysis of active and reactive power performances of a wind farm (WF). In [5], the relation between reactive and active power to maintain the DFIG’s operation inside the maximum rotor and stator currents has been studied. In [6], the authors have proposed a fuzzy controller to manage the operation of a Flywheel Energy Storage System (ESS) connected to the DC bus. Recently, the penetration of WTs into the grids increased, and the performance of the WTs under faults has became an important issue, especially for DFIGs. Several grid codes prescribed, in fact, that WTs should remain connected to the network during and
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