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Coordinated Direct Power Control of DFIG System without Phase Locked Loop under Unbalanced Grid Voltage Conditions

This paper proposes a coordinated direct power control (DPC) scheme for the rotor-side converter (RSC) and the grid-side converter (GSC) of the doubly fed induction generator (DFIG) under unbalanced grid voltage conditions. In order to eliminate the coupling interactions between the phase locked loop (PLL) and the local unbalanced network, a virtual phase angle is used to replace the actual one acquired by the PLL. Thus, the PLL is removed out of RSC and GSC in the proposed DPC scheme. During network unbalance, the RSC is controlled to reduce torque ripples, while three selectable control targets are identified for the GSC, i.e., constant total active power, constant total reactive power and balanced currents.

A single-side resonant controller with the frequency discrimination between the positive and negative sequence signals of the same frequency is employed in the coordinated DPC scheme to avoid the complex calculations of the power compensating components. Meanwhile, the sequential separations of the voltages and currents are also eliminated. Then, the control performance, including the limits of the dc-link voltage, the dc-capacitor power oscillations, the impacts of the frequency deviations and the grid synchronization of the proposed DPC strategy, are discussed. Finally, the experimental results of DFIG system verify the effectiveness of the proposed DPC strategy under unbalanced grid voltage conditions.

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