Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> This paper develops a framework for analysis of harmonics in a doubly fed induction generator (DFIG) caused by nonsinusoidal conditions in rotor and unbalance in stator. Nonsinusoidal rotor voltages are decomposed into harmonic components and their corresponding sequences are identified. Induced harmonics in stator are analyzed and computed, from which the torques produced by these interactions between stator and rotor harmonic components can be found. During unbalanced stator conditions, symmetric component theory is applied to the stator voltage to get positive-, negative-, and zero-sequence components of stator and rotor currents. The steady-state negative-sequence equivalent circuit for a DFIG is derived based on the reference frame theory. Harmonic currents in the rotor are computed based on the sequence circuits. In both scenarios, the harmonic components of the electromagnetic torque are calculated from the interactions of the harmonic components of the stator and rotor currents. Three case studies are considered, namely: 1) nonsinusoidal rotor injection; 2) an isolated unbalanced stator load scenario; and 3) unbalanced grid-connected operation. The analysis is verified with results from numerical simulations in Matlab/Simulink. For illustration, the second case is verified using experiments. The simulation results and experimental results agree well with the results from analysis. <s> BIB001 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> Renewable energy harvesting is of prime interest in lieu of the increasing demand for energy and high price of fossil fuels, albeit the diminishing resources of petroleum and natural gas have given a renewed attention to the topic. Wind energy is, by far, the most accessible and efficient form of renewable energy given the state of technology in other renewable resources, i.e., solar, fuel cells, etc. Over the past decade, there have been significant advances in capturing wind energy using doubly fed induction generators (DFIGs). The versatility of the DFIG in providing a controllable flow of active and reactive power to the system, as well as its inherent low cost and minimum-size power electronic converters, has been the driving force in superior adequacy of this family of generators. However, as it has been noted in the literature, voltage unbalance and system harmonics can deteriorate the performance of DFIGs by introducing unwanted torque harmonics and inaccuracy in the generation of commanded active/reactive power. This paper will address this phenomenon from an electromagnetic and system analysis point of view. Simulation results are given to validate the claims. <s> BIB002 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> This paper proposes a new control strategy of effective fifth and seventh harmonic elimination in the stator output voltage at the point of common coupling for a stand-alone doubly fed induction generator (DFIG) feeding a three-phase diode rectifier. This load regularly causes such harmonic distortions, which harmfully affect the performance of other loads connected to the DFIG. In order to allow the DFIG to deliver a pure sinusoidal stator output voltage, these harmonics must be rejected. The proposed elimination method is investigated based on the rotor current controller employing a proportional integral and a resonant controller, which is implemented in the fundamental reference frame. In this frame, both positive seventh and negative fifth voltage harmonic can be eliminated by using only single resonant compensator tuned at six multiples of synchronous frequency in the rotor current controller. The control scheme is developed in the rotor-side converter for the control and operation of the DFIG. Simulations and experimental results with 2.2-kW DFIG feeding a nonlinear load are shown to verify prominent features of the proposed control method. <s> BIB003 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> Voltage harmonics in the grid can introduce stator current harmonics in a doubly fed induction generator (DFIG) wind turbine system, which may potentially impact the generated power quality. Therefore, wind turbine current controllers need to be designed to eliminate the impact of grid voltage harmonics, especially low-order harmonics. This paper proposes a stator current harmonic suppression method using a sixth-order resonant controller to eliminate negative sequence fifth- and positive sequence seventh-order current harmonics. A stator current harmonic control loop is added to the conventional rotor current control loop for harmonic suppression. The overall control scheme is implemented in dq frame. Based on a mathematical model of the DFIG control system, the effects on system stability using the resonant controller, an analysis of the steady-state error, and the dynamic performance, are discussed in this paper. Taking these effects into account, the parameters of the resonant controller can be designed and effectively damp the influence from the grid voltage harmonics. As a result, the impacts of the negative sequence fifth- and positive sequence seventh-order voltage harmonics on the stator current as well as the electromagnetic torque are effectively removed. Simulations and experiments are presented to validate the theoretical analysis. <s> BIB004 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> Doubly fed induction generator (DFIG)-based stand-alone variable-speed constant-frequency systems reported so far cannot supply nonlinear unbalanced load current containing cophasor components. This paper presents three topologies of a stand-alone DFIG which can handle this type of load. The harmonic/imbalance component of the load current in each case is supplied by the stator-side converter. In the first option, a Δ/Y transformer with neutral connection supplies the load. However, the harmonic current circulating in the delta winding still distorts the load phase voltage beyond the acceptable limit. As a solution, the zero-sequence load voltage is actively controlled through the stator-side converter with the load neutral connected to the midpoint of the dc-link capacitor. However, this configuration also fails when the load current contains dc component as in half-wave rectifiers. Finally, a configuration with an additional inverter leg connected to the load neutral is proposed in conjunction with the zero-sequence stator voltage controller. Simulation and experimental results from this last configuration demonstrate its excellent load voltage regulation property while supplying various nonlinear and unbalanced loads. The control algorithm is implemented without any speed/position sensor. <s> BIB005 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> This paper proposes a new control strategy of doubly fed induction generators (DFIGs) under unbalanced grid voltage conditions. The proposed controller includes a model predictive direct power control (MPDPC) method and a power compensation scheme. In MPDPC, the appropriate voltage vector is selected according to an optimization cost function, hence the instantaneous active and reactive powers are regulated directly in the stator stationary reference frame without the requirement of coordinate transformation, PI regulators, switching table, or PWM modulators. In addition, the behavior of the DFIG under unbalanced grid voltage is investigated. Next, a power compensation scheme without the need of extracting negative stator current sequence is developed. By combining the proposed MPDPC strategy and the power compensation scheme, distorted currents injected into the power grid by the DFIGs can be eliminated effectively. <s> BIB006 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> The integration of wind power into national grids introduces a unique set of challenges. The wind-turbine generators are incapable of supplying the voltage and frequency control and they absorb significant amounts of reactive power particularly when there is a fault in the system. The utilities, the world over, are making their own Grid Codes which must deal with these new challenges being imposed by the integration of wind power with their respective grids. <s> BIB007 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> This letter proposes a novel method to reject $6n \pm 1$ harmonics in the stator voltage at the point of common coupling (PCC) for a stand-alone doubly-fed induction generator (DFIG) supplying a balanced three-phase diode rectifier. These harmonics are rejected by using the rotor current controller in the fundamental synchronous reference frame. The proposed control algorithm is based on a proportional integral controller and a bank of resonant filters. In this frame, each resonant filter in the rotor current controller has the possibility of rejecting one pair of the positive and the negative stator harmonic voltages at the PCC. Experimental results with 2.2-kW DFIG are shown in this letter. <s> BIB008 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> This paper describes a wind power system which controls the active and reactive generated powers as well as it performs the function of filtering the harmonic components of the grid currents. From the grid side converter, the harmonic filtering is achieved by an algorithm proposed by compensation of harmonics. This technique ensures the improvement of power quality. The machine side converter controls the active and reactive powers that are delivered to the electric grid by the stator flux oriented control. The design methodology of the controllers used is presented. This paper is distinguished by three key contributions. The first contribution of this article is the tutorial character, it should assist in the development of future work. The second is the analysis of the harmonic filtering behavior for some operating points of the DFIG/APF system. The third is the application of the precise model of the DC link voltage dynamics, allowing verifying the stability of the system control for each DFIG/APF (active power filter) operating point. Simulation and experimental results confirm the effectiveness of the proposed research. <s> BIB009 </s> Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review <s> C. POWER QUALITY IMPROVEMENT <s> This paper presents a stator harmonic currents suppression method for doubly fed induction generators (DFIGs) under distorted grid voltage. In the proposed control strategy, the feed-forward regulator instead of the resonant regulator is employed to eliminate the negative impacts on the stator current caused by the distorted grid voltage. The leading phase compensator is applied to compensate the system delay when the sampling frequency is low. This approach can provide both the good dynamic response and strong rejection ability against the stator harmonic voltages. Based on the stator current model of DFIG, the feed-forward regulator is designed in detail. The comparison between the resonant controllers and the feed-forward regulator is also made on the basis of Bode diagram. Then, the robustness against the frequency variations and the parameter deviations is analyzed. Finally, the simulation and experimental results are presented to validate the effectiveness of the proposed control strategy. <s> BIB010
The DFIG based WECS is traditionally used to generate active power, yet it can also operate as active power filter which can enhance power system quality by reducing the total harmonic distortion (THD) at the point of common coupling (PCC). Figure 13 shows how the DFIG system can act like an active power filter. There are few research articles that address power quality improvement using DFIG based WECS. Reference BIB006 proposes a power compensation and a model predictive direct power control (MPDPC) technique to effectively eliminate distorted currents injected into the power grid by the DFIGs. Reference BIB010 applies feed-forward regulator instead of the resonant regulator to remove the DFIG stator current distortions rose by disfigured network voltage. Three schemes that can handle nonlinear unbalanced load current containing co-phasor components of a standalone DFIG are introduced in BIB005 . Reference BIB003 proposes a single resonant compensator tuned at six multiples of synchronous frequency in the rotor current controller to eliminate the DFIG stator voltage fifth and seventh harmonic. Reference BIB001 also proposes a 6n±1 harmonics resonant filter integrated to rotor current controller to reject both negative and positive stator voltage harmonics for stand-alone application. Reference BIB008 investigates voltage harmonics in DFIG caused by unbalance in stator and nonsinusoidal conditions in rotor. Reference BIB004 suggested to add a sixth-order resonant stator current harmonic controller to remove positive sequence seventh order and negative sequence fifth order DFIG system current harmonics. The effects of system harmonics and unbalanced voltages in DFIG connected to a network are explored in BIB002 . Other references such as BIB007 considered the power quality issues and improvements of DFIG wind farms. A detailed evaluation of advanced and recent active power filters that considers size, weight, and cost of electric converters used in wind energy conversion systems is presented in BIB009 .