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 DFIG's 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.

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.

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 DFIG's 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.

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.

Given the high complexity of the conventional three-level space-vector pulse-width-modulation (3L SVPWM), simplified SVPWM algorithms are usually adopted instead. However, existing simplifications for 3L SVPWM still have a large computation burden, which limits its applications, e.g., in high-switching-frequency silicon-carbide (SiC) converters with a short computation period. In this letter, based on a typical simplified algorithm known as the center-aligned 3L SVPWM, a simplified analytical algorithm in abc coordinate is proposed. The reference-voltage-vector adjustment and the calculation of modulation waves are unified based on the center-aligned SVPWM. The modulation waves can be analytically derived by only 8 simple equations, without sector determination, calculation and assignment of dwell times. Consequently, the complexity of implementing the 3L SVPWM is significantly reduced.

When faced with unbalanced dc-link voltages of three-level inverters, extra harmonics will be generated with reduced linear modulation region if the conventional pulsewidth modulation (PWM) for balanced dc-link voltages is adopted. In this article, in order to suppress output harmonics and maximize the linear modulation region, as well as to reduce switching losses, the discontinuous space-vector PWM (DSVPWM) is investigated and improved for three-level inverters with unbalanced dc-link voltages. First, subsectors in the vector diagram with unbalanced dc-link voltages are optimally redivided for low switching losses, and concisely determined by a cutting-line method. Then, the modulation waves are analytically derived based on the calculated dwell times of the vector states. And an analytical DSVPWM for dealing with unbalanced dc-link voltages is proposed and implemented by a carrier-based algorithm, which can significantly reduce the computation burden. Finally, the proposed analytical DSVPWM is verified through comparative experiments, presenting its merits of high linear modulation region, high efficiency, and relatively low output harmonics.