Yikang He

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 DFIG-Based Wind Turbine During Network Unbalance

This paper proposes an improved coordinate direct power control (DPC) strategy for the doubly fed induction generator (DFIG) and the grid side converter (GSC) of a wind power generation system under unbalanced network conditions. Two improved DPC schemes for the DFIG and the GSC are presented, respectively. The DPC for DFIG is to eliminate the torque and stator reactive power pulsations, while the DPC for GSC is to compensate for the pulsations of stator active power. In order to provide enhanced control performance of the overall system, the resonant controllers tuned at the fundamental and double grid frequency are applied in the improved DPC to eliminate the torque and power pulsations produced by the transient unbalanced grid faults, so that the output power of DFIG and GSC can be directly regulated without any necessity of the positive and negative sequence decomposition. The effectiveness of the proposed DPC strategy is verified by the simulation results of a 2-MW DFIG system under unbalanced 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.

Operation of Wind-Turbine-Driven DFIG Systems Under Distorted Grid Voltage Conditions: Analysis and Experimental Validations

This paper presents enhanced control strategies for doubly fed induction generator (DFIG)-based wind power generation systems under distorted grid voltage conditions. The mathematical model of DFIG, in view of the fifth- and seventh-order components of grid voltage harmonics, is proposed and analyzed in detail. Based on the analytical model, further studies are conducted on the distortions of stator/rotor currents and the oscillations in the stator active/reactive powers as well as the electromagnetic torque, where the impact of DFIGs load conditions is considered. Meanwhile, alternative rotor current references are calculated to enhance the uninterruptable operation capability of the wind-turbine-driven DFIG systems under distorted grid conditions. An improved software PLL is designed, which is capable of accurately and rapidly tracking the frequency and phase angle of the fundamental grid voltage under distorted grid conditions. A proportional integral plus resonant (PI-R) current controller in the synchronously rotating (dq) reference frame is employed to simultaneously regulate the fundamental and harmonic components of rotor currents without any sequential component decomposition. Experiment results on a 3-kW DFIG prototype demonstrate the correctness of the analytical results and the effectiveness of the software PLL and PI-R current controller when the grid voltage is distorted.

Coordinated Control of DFIG's RSC and GSC Under Generalized Unbalanced and Distorted Grid Voltage Conditions

This paper proposes a coordinated control of the rotor-side converter (RSC) and grid-side converter (GSC) of a doubly fed induction generator (DFIG)-based wind-turbine generation system under generalized unbalanced and/or distorted grid voltage conditions. The system behaviors of the RSC and GSC during supply imbalance and distortion are investigated. To enhance the fault ride-through operation capability, the RSC is properly controlled to eliminate the torque oscillations, whereas the GSC is carefully designed to ensure constant active power output from the overall DFIG generation system. To achieve simultaneous regulation of the positive-/negative-sequence currents and fifth-/seventh-order harmonic currents for both the RSC and GSC, a novel current controller, consisting of a conventional proportional-integral regulator and a dual-frequency resonant compensator, is proposed and implemented in the positive (dq)+ reference frame. The simulation and experiment studies demonstrate the correctness of the developed model and the effectiveness of the suggested control strategy for DFIG-based wind-turbine systems under such adverse grid conditions.

Integrated Modeling and Enhanced Control of DFIG Under Unbalanced and Distorted Grid Voltage Conditions

This paper focuses on the enhanced control and improved fault-ride-through capability of a doubly fed induction generator (DFIG)-based wind turbine system under both unbalanced and distorted grid voltage conditions. A more integrated mathematical model of DFIG is first set up with both the negative-sequence and the low-order harmonic grid voltages considered. Based on the developed model, the instantaneous active/reactive powers and electromagnetic toque are redefined in detail. Besides, four alternative control targets and their corresponding rotor current references are calculated and assigned. A novel current controller, consisting of a conventional PI regulator and a dual-frequency resonant (DFR) compensator, tuned at twice and six times the grid frequency, named PI-DFR controller, is designed to regulate the fundamental and the fifth- and seventh-order harmonic components simultaneously. Experiment studies verify the correctness of the developed model and the effectiveness of the suggested control strategies in improving the fault-ride-through capability of a DFIG system under such adverse grid conditions.

Fuzzy logic direct torque control for permanent magnet synchronous motors

An outstanding feature of the conventional direct torque control (DTC) for permanent magnet synchronous motors (PMSM) is its fast dynamic response. However, large torque and flux linkage ripples are generated because of the use of the hysteresis controllers and the crude position signals based on six 60/spl deg/ angular regions for selecting the space voltage vectors. This paper proposes a fuzzy logic based PMSM DTC system, in which the torque error, stator flux linkage error and the stator flux vector angular position are all properly fuzzified into several fuzzy subsets, so that a more suitable space voltage vector can be selected to minimize the torque and flux linkage ripples. Both the simulation and the experimental results show that the modified DTC system has a much better steady state performance while retaining the good dynamic performance.

Control and Protection of a DFIG-Based Wind Turbine under Unbalanced Grid Voltage Dips

This paper proposes a new direct power control (DPC) strategy and a novel crowbar protection technique for the doubly fed induction generator (DFIG) used in the wind power generation systems. The main difficulty for a DFIG to ride through severe unbalanced grid voltage dips is the large transient currents induced in the rotor windings, which may damage the ac excitation converter. The proposed DPC is capable of suppressing the transient oscillations of fault currents and electromagnetic torque. The crowbar protection is able to limit the peak values of the fault rotor currents under a preset threshold. Simulation results of a 2MW DFIG system prove that with this control scheme the DFIG is capable of riding through the severe unbalanced grid voltage dips.

Sensorless Operation of an Inset PM Bearingless Motor Implemented by the Combination Approach of MRAS and HF Signal Injection

On the basis of analyzing the mathematic model of high frequency signal injection and model reference adaptive system, a novel rotor position estimation scheme using the combined rotor position self-sensing method was suggested, which will meet the need of sensorless operation for an inset permanent magnet type bearingless motor. The rotor position detecting principle, such as the saliency-tracking technology with the injection of high-frequency carrier signal at low speed, the observer estimation at high speed, the smooth switching of these two detecting schemes for rotor position estimation and their implementation technique were all discussed in this paper. And then, the sensorless vector control system of an inset permanent magnet type bearingless motor was set up based on the aforementioned position estimation approach. Simulation study demonstrates that the proposed combined self-sensing method is capable of precisely estimating the rotor space position in the full speed regions for the inset permanent magnet type bearingless motor and is capable of achieving stable sensorless suspension operation at the low or high speeds with heavy load disturbance

Predictive direct power control of grid-connected voltage-sourced converters under unbalanced grid voltage conditions

This paper proposed a modified predictive direct power control (P-DPC) strategy for a grid-connected voltage-sourced converter under unbalanced grid voltage conditions. The proposed P-DPC strategy is implemented in the stationary reference frame without the necessity of either the space vector modulation or the synchronous speed d-q transformation. A compensation method is adopted in the conventional P-DPC to eliminate the negative sequence component of gird current for the purpose of obtaining the symmetrical and sinusoidal grid current under unbalanced grid voltage conditions. Positive and negative sequence components extraction is also avoided in the proposed method. Simulation results verified that the modified P-DPC strategy is effective to obtain symmetrical and sinusoidal grid currents under unbalanced grid voltage conditions.