Improving network voltage unbalance levels by controlling DFIG wind turbine using a dynamic voltage restorer

This paper proposes a method to compensate the network voltage unbalance using the doubly fed induction generator (DFIG) wind turbine connected to a dynamic voltage restorer (DVR). In the configuration employed, a DVR based on a voltage-source converter is connected in series between the generator and the grid. The proposed control uses symmetrical components to calculate the negative sequence current to be injected in the network to compensate the voltage unbalance at the Point of Common Coupling (PCC), and the voltage to be applied at the stator, to avoid torque oscillations. The contribution of this paper consists on proposing a coordinated strategy for compensating the voltage unbalance at PCC and avoiding torque oscillations in the machine. Due to the coordinated use of all resources available for compensation, the proposed method increases the operational range of the existing methods. Basically, the strategy of the proposed method consists on using the remain capacity of the grid side converter of the DFIG for injecting compensating current and, when it reaches its limit, the rotor side converter is controlled to inject compensating current through the machine stator to the grid. The torque oscillations due to the unbalanced operation of the machine is avoided by applying a series voltage by the DVR. Simulation results have validated the proposed method, which maximizes the compensation at the PCC voltage unbalance and guarantees the machine operation with low oscillation in the electromagnetic torque.