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.Index Terms-Direct power control (DPC), doubly fed induction generator (DFIG), grid voltage unbalance, resonant controller, wind turbine.

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 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 (RSC) or in the Grid Side Converter (GSC), 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.Index Terms -DFIG system impedance; series RL + shunt C network; high frequency resonance damping; virtual positive capacitor; virtual negative inductor.The authors are with the

-The wind power generation techniques are continuing to develop and increasing numbers of Doubly Fed Induction Generator (DFIG)-based wind power systems are connecting to the on-shore and off-shore grids, local standalone weak networks, and also micro grid applications. The impedances of the weak networks are too large to be neglected and require careful attention. Due to the impedance interaction between the weak network and the DFIG system, both SubSynchronous Resonance (SSR) and High Frequency Resonance (HFR) may occur when the DFIG system is connected to the series or parallel compensated weak network respectively. This paper will discuss the SSR and the HFR phenomena based on the impedance modeling of the DFIG system and the weak networks, and the cause of these two resonances will be explained in details. The following factors including 1) transformer configuration; 2) different power scale of DFIG system with different parameters; 3) L or LCL filter adopted in the Grid Side Converter (GSC); 4) rotor speed; 5) current closed-loop controller parameters and 6) digital control delay will be discussed in this paper. On the basis of the analysis, active damping strategies for HFR using virtual impedance concept will be proposed.

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As the wind power generation develops, the Doubly Fed Induction Generator (DFIG) based wind power system are more and more likely to operate in the emerging weak network rather than the conventional stiff network. Due to the comparatively large impedance of the weak network than the stiff grid, the DFIG system may be subject to the resonances due to the impedance interaction between the DFIG system and the weak network. Especially, when connected to a series π sections weak network, the Multiple High Frequency Resonances (MHFR) may occur and require careful studies. The impedance modeling of the DFIG system and the series π sections weak network is firstly demonstrated in this paper. Then, due to the multiple magnitude peaks of the series π sections of the weak network, the MHFR will be produced and can be theoretically explained based on the impedance modeling results. For the purpose of mitigating the MHFR, an active damping strategy which introduces a virtual impedance, including a phase leading compensation unit and a virtual positive resistance, is proposed and demonstrated. Simulations are conducted to validate the DFIG system MHFR as well as the proposed active damping strategy.Index Terms -DFIG system; series π sections weak network; multiple high frequency resonances; virtual impedance; active damping.The authors are with the

The paper presents the control strategy of stator current harmonic distortion performance improvement for doubly fed induction generator (DFIG) using bandwidth based repetitive control (BRC) under generalized harmonic grid voltage. The control target is to eliminate the DFIG stator current 6n±1 harmonic components, thus sinusoidal stator output current can be injected into the power grid. Considering that the frequency deviation always occurs in the practical grid, the BRC regulator is designed on the basis of conventional repetitive control (RC) regulator with the introduction of control bandwidth. The closed-loop operation stability considering different bandwidth and gain parameter is also analyzed. Finally, the availability of the proposed BRC control strategy under generalized harmonic grid voltage is verified by experiment results.Index Termsbandwidth based repetitive control (BRC); doubly fed induction generator (DFIG); generalized harmonic grid voltage; closed-loop operation stability. where he is currently working toward the Ph.D. degree. His current research interests include motor control with power electronics devices in renewable-energy conversion, particularly the control and operation of doubly fed induction generators for wind power generation under adverse grid condition.Heng Nian (M'09, SM'14) received the B.Eng. degree and the M.Eng. degree from HeFei University of

-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 Sub-Synchronous Resonance (SSR). However, the High Frequency Resonance (HFR) of DFIG systems due to the impedance interaction between 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.Index Terms -DFIG system impedance modeling; weak network impedance modeling; high frequency resonance.

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