Heng Nian

Direct Active and Reactive Power Regulation of DFIG Using Sliding-Mode Control Approach

This paper presents a new direct active and reactive power control (DPC) of grid-connected doubly fed induction generator (DFIG)-based wind turbine systems. The proposed DPC strategy employs a nonlinear sliding-mode control scheme to directly calculate the required rotor control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any synchronous coordinate transformations. Thus, no extra current control loops are required, thereby simplifying the system design and enhancing the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the designs of the power converter and the ac harmonic filter. Simulation results on a 2-MW grid-connected DFIG system are provided and compared with those of classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC. The proposed DPC provides enhanced transient performance similar to the LUT DPC and keeps the steady-state harmonic spectra at the same level as the VC strategy.

Dynamic Modeling and Improved Control of DFIG Under Distorted Grid Voltage Conditions

This paper presents a mathematical model of a doubly fed induction generator (DFIG) in the positive synchronous reference frame under distorted grid voltage conditions. The oscillations of the DFIGs electromagnetic torque and the instantaneous stator active and reactive powers are fully described when the grid voltage is harmonically distorted. Four alternative control targets are proposed to improve the system responses during grid harmonic distortions. A new rotor current control scheme implemented in the positive synchronous reference frame is developed. The control scheme consists of a proportional-integral (PI) regulator and a harmonic resonant (R) compensator tuned at six times the grid frequency. Consequently, the fundamental and the fifth- and seventh-order components of rotor currents are directly regulated by the PI-R controller without sequential-component decompositions. The feasibility of the proposed control strategy is validated by simulation studies on a 2.0-MW wind-turbine-driven DFIG system. Compared with the conventional vector control scheme based on standard PI current controllers, the proposed control scheme leads to significant elimination of either DFIG power or torque oscillations under distorted grid voltage conditions.

Improved Direct Power Control of a Wind Turbine Driven Doubly Fed Induction Generator During Transient Grid Voltage Unbalance

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.

Direct Power Control of Doubly Fed Induction Generator Under Distorted Grid Voltage

This paper presents a direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind power generation system under distorted grid voltage. By analyzing the six times grid frequency power pulsation produced by the fifth and seventh grid voltage harmonic components, a novel DPC strategy with vector proportional integrated (VPI) regulator has been proposed to implement the smooth active and reactive power output of DFIG. The performance analysis of the proposed DPC strategy, including the steady and dynamic state performance, closed-loop operation stability, and rejection capability for the grid voltage distorted component and back EMF compensation item has been investigated. The availability of the proposed DPC strategy with a VPI regulator is verified by experiment results of DFIG system under harmonically distorted grid condition.

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, is discussed. Finally, the experimental results of DFIG system verify the effectiveness of the proposed DPC strategy under unbalanced grid voltage conditions.

Independent Operation of DFIG-Based WECS Using Resonant Feedback Compensators Under Unbalanced Grid Voltage Conditions

This paper presents an independent operation of the rotor-side converter (RSC) and grid-side converter (GSC) for a doubly fed induction generator (DFIG)-based wind energy conversion system under unbalanced grid voltage conditions. In this paper, the RSC is controlled to achieve four different control targets, including balanced stator current, sinusoidal rotor current, smooth stator active and reactive powers, and constant DFIG electromagnetic torque. The GSC is commanded to keep the dc voltage at a constant value. Additional feedback compensators using resonant regulators for the RSC are employed, and the decompositions of the positive and negative sequence components and calculations of the rotor negative current references can be avoided. Another similar compensator is used in the GSC to suppress the dc voltage fluctuates and remove the GSC reactive power oscillations without the stator or rotor power information. The proposed method can make the RSC and GSC available to an independent operation with a simple implementation for higher reliability. The experimental results demonstrate the effectiveness of the proposed control strategy for both the RSC and GSC under unbalanced grid voltage conditions.

Collaborative Control of DFIG System During Network Unbalance Using Reduced-Order Generalized Integrators

The paper presents a collaborate control for the rotor-side converter (RSC) and grid-side converter (GSC) of a doubly fed induction generator (DFIG) generation system during network unbalance. In this study, the RSC is controlled to reduce the torque ripples, and three selectable control targets for the GSC, i.e., balanced total currents, and constant total active or reactive power into the grid from the overall system are identified to reduce the impacts of the negative-sequence voltage on the DFIG system performance. A reduced-order generalized integrator is employed, which is implemented in the positive synchronous reference frame. Based on the math model, the impacts of the limited dc voltage, as well as the rejection capability on negative-sequence voltage of the proposed control strategy, have been investigated. Finally, the simulation and experimental results are provided to demonstrate the effectiveness of the proposed collaborative control strategy.

Modularized Control Strategy and Performance Analysis of DFIG System Under Unbalanced and Harmonic Grid Voltage

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.

A three-phase programmable voltage sag generator for low voltage ride-through capability test of wind turbines

As the wind power penetration is increasing rapidly into the power grid, modern gird codes require the wind turbines (WTs) to possess the low voltage ride-through (LVRT) capability. In order to evaluate the performance of a WT under grid fault conditions, a voltage sag generator (VSG) is needed to simulate all kinds of grid faults. This paper investigates a three-phase programmable VSG based on a voltage source inverter (VSI). A novel dual-loop proportional integral resonant (PIR) controller with a zero sequence voltage compensation scheme is applied to control the VSI, so that the four kinds of possible grid voltage dips can be precisely simulated. The effectiveness of the proposed VSC is validated by the experimental results.

A novel DC grid connected DFIG system with active power filter based on predictive current control

The paper presents a novel DC grid connection technique for wind power generation system based on the double fed induction generator (DFIG), in which a diode bridge rectifier is directly connected to the stator side of DFIG to connect DC grid. The direct voltage control is applied for the DFIG system to achieve the sinusoidal stator voltage. In order to eliminate the harmonic currents produced by the diode rectifier, an active power filter is built between DFIG stator and the diode bridge, in which the predictive current control is applied to remove the control delay caused by computation and sampling. The availability of the proposed system and control strategy is validated by the simulation results on a 1.5-MW DFIG-based DC grid connection system.