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As the wind power technology develops, the doubly fed induction generator (DFIG) based wind power system, when connected to a weak network with large impedance, may suffer resonances, i.e., subsynchronous resonance or high-frequency resonance when connected to the series- or parallel- weak network. Besides these two resonances, a middle frequency resonance (MFR) between 200 and 800 Hz may appear when the phase-locked loop (PLL) with fast control dynamics is applied. In order to analyze the MFR, the DFIG system impedance considering the PLL is studied based on the vector-oriented control strategy in rotor-side converter and grid-side converter. On the basis of the established impedance modeling of the DFIG system, it is found that the PLL with fast control dynamics may result in the occurrence of MFR due to a decreasing phase margin. The simulation results of both a 7.5 kW small-scale DFIG system and a 2 MW large-scale DFIG system are provided to validate the theoretical analysis of the MFR. [ABSTRACT FROM PUBLISHER]

The impedance-based model of doubly fed induction generator (DFIG) systems, including the rotor part (rotor side converter (RSC) and induction machine), and the grid part (grid side converter (GSC) and its output filter), has been developed for analysis and mitigation of the subsynchronous resonance (SSR). However, the high-frequency resonance (HFR) of DFIG systems due to the impedance interaction between the DFIG system and parallel compensated weak network is often overlooked. This paper, thus, investigates the impedance characteristics of DFIG systems for the analysis of HFR. The influences of the rotor speed variation, the machine mutual inductance and the digital control delay are evaluated. Two resonances phenomena are revealed, i.e., 1) the series HFR between the DFIG system and weak power grid; 2) the parallel HFR between the rotor part and the grid part of DFIG system. The impedance modeling of DFIG system and weak grid network, as well as the series HFR between DFIG system and parallel compensated weak network has been validated by experimental results. [ABSTRACT FROM AUTHOR]

When operating in a micro or weak grid which has a relatively large network impedance, the doubly fed induction generator (DFIG)-based wind power generation system is prone to suffer high-frequency resonance due to the impedance interaction between the DFIG system and the parallel compensated network (series RL + shunt C). In order to improve the performance of the DFIG system as well as other units and loads connected to the weak grid, the high-frequency resonance needs to be effectively damped. In this paper, the proposed active damping control strategy is able to implement effective damping either in the rotor-side converter or in the grid-side converter, through the introduction of virtual positive capacitor or virtual negative inductor to reshape the DFIG system impedance and mitigate the high-frequency resonance. A detailed theoretical explanation on the virtual positive capacitor or virtual negative inductor has been given, and their parameters are also optimally designed. The proposed DFIG system damping control strategy has been validated by experimental results. [ABSTRACT FROM PUBLISHER]

This paper presents the control strategy of achieving sinusoidal stator current of doubly fed induction generator using improved repetitive control (RC) regulator under generalized harmonic grid voltage. The improved RC regulator, with the larger magnitude response at the higher control frequency due to the magnitude response compensation unit, is proposed to restrain the stator current higher harmonic sequence more effectively. Besides, the appropriate phase leading compensation can also be automatically introduced by the improved RC regulator. The steady-state control performance and the closed-loop operation stability using the improved RC regulator are discussed in this paper. The availability of the proposed strategy under generalized harmonic grid voltage is verified by experiment results. [ABSTRACT FROM AUTHOR]

The paper presents a modularized control strategy of doubly fed induction generator (DFIG) system, including the grid-side converter (GSC) and rotor-side converter (RSC), under unbalanced and harmonic grid voltage. The sequence decomposition process and complicated control reference calculation can be avoided in the proposed control strategy. From the perspective of power grid friendly operation, the control targets of DFIG system in this paper are chosen as: 1) smooth active and reactive power injected into the power grid; 2) balanced and sinusoidal current injected into the power grid. The RSC and GSC can work as two independent modules and the communication between RSC and GSC can be removed. Furthermore, the third harmonic current component, dc-link voltage fluctuation, and electromagnetic torque pulsation under the different control targets are theoretically analyzed. Finally, the availability of the proposed modularized control strategy of DFIG system under unbalanced and distorted grid voltage is verified by experiment results. [ABSTRACT FROM AUTHOR]

This paper proposes an improved direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind power generation system under unbalanced grid voltage dips. The fundamental and double grid frequency power pulsations, which are produced by the transient unbalanced grid faults, are mathematically analyzed and accurately regulated. Five selectable control targets, with proper power references given, are designed for different applications during network unbalance. In order to provide enhanced control performance, two resonant controllers, which are tuned to have large gain at the power pulsation frequencies, are applied together with the proportional-integral controller to achieve full control of the DFIG output power. The effectiveness of the proposed DPC strategy is verified by the experimental results of a 5-kW DFIG system under different unbalanced voltage dips, which are generated by a specially designed voltage dip generator. [ABSTRACT FROM PUBLISHER]