DFIG-DVR系统在不平衡电网电压下的运行特性

针对双馈风电机组(DFIG)在电网电压不平衡时,二倍频扰动分量会造成定转子过电流、功率脉动、转矩脉动等一系列电气和机械的问题,提出了新型DFIG-DVR系统,即串联DVR始终维持DFIG定子端电压恒定,从根源上隔离电网不平衡故障的影响,从而在整个故障运行过程中,DFIG仍可以实现转子侧变换器功率解耦控制和网侧变换器维持直流电压恒定的目标。采用PSCAD/EMTDC建立DFIG-DVR系统模型,对比分析了电网电压不平衡时DVR的不投切与投切对DFIG的影响。结果表明,在电网电压不平衡条件下,所提控制方案可以实现DFIG的平衡运行。     

不平衡电压下双馈变速风电机组的控制策略

提出了不平衡电网电压下双馈发电机的控制策略,并建立了双馈发电机在正、反旋转坐标系下的数学模型。在此基础上推导和分析了电网电压不平衡条件下双馈发电机输出的瞬时有功、无功功率的组成。提出了4种可供选择的不平衡电压控制方案,并给出了不同控制目标下转子的正、负序电流目标值的计算原则。通过MATLAB/SIMULINK仿真验证了控制方案的有效性。     

DFIG wind generation systems operating with limited converter rating considered under unbalanced network conditions – Analysis and control design

This paper presents an analysis and operation of doubly-fed induction generator (DFIG)-based wind power generation systems under unbalanced network voltage conditions. The limited voltage and current ratings, and control capabilities of both grid side and rotor-side converters (GSC&RSC) for unbalanced network voltage compensation are investigated. A current control scheme consisting of a proportional integral (PI) controller plus a resonant (R) compensator is designed. The control scheme is implemented in the positive synchronous rotating reference frame and provides precise current control for both the GSC and RSC without involving the decomposition process of positive and negative sequence currents. By taking into account both the converters’ current ratings, a coordinated control strategy between the GSC and RSC is proposed and adjusted. Finally, simulation studies on a 1.5 MW DFIG generation system and experimental tests on a 1.5 kW prototype are carried out. The correctness of the analytical results as well as the feasibility of the proposed coordinated control scheme are confirmed when the network voltage is unbalanced.     

A versatile method for computation of power pulsations in DFIG under grid imperfections

In the context of a Doubly Fed Induction Generator (DFIG) connected to the utility grid under unbalanced voltage conditions, the controller design needs to ensure additional challenges such as restricting the Total Harmonic Distortion (THD) in grid current, minimizing the pulsations in generated power, torque, dc link voltage etc. apart from facilitating the generator power control. Thus the schemes for generating reference currents for rotor converters need to incorporate a measure of power pulsations to what is required for steady state power flow control. This paper proposes a versatile scheme for computing the power pulsations in a DFIG connected to grid under unbalance voltage conditions. The active and reactive power oscillations are computed in a simple and straight forward manner using the measured stator voltage and currents in the positive d-q frame without using flux estimation. The scheme is free from flux integration or differentiation, rotor position computation and independent of machine parameters. Further, the worst case error in computation is bound within 3% considering 30% voltage dip, 7% of harmonics, ±10°phase jump or ±10% dc offset in the grid voltage. The effectiveness of the scheme is validated through PSCAD/EMTDC simulations and experimental results for a 2.3 kW DFIG test setup.     

Co‐ordinated voltage control of DFIG wind turbines in uninterrupted operation during grid faults

Emphasis in this article is on the design of a co‐ordinated voltage control strategy for doubly fed induction generator (DFIG) wind turbines that enhances their capability to provide grid support during grid faults. In contrast to its very good performance in normal operation, the DFIG wind turbine concept is quite sensitive to grid faults and requires special power converter protection. The fault ride‐through and grid support capabilities of the DFIG address therefore primarily the design of DFIG wind turbine control with special focus on power converter protection and voltage control issues. A voltage control strategy is designed and implemented in this article, based on the idea that both converters of the DFIG (i.e. rotor‐side converter and grid‐side converter) participate in the grid voltage control in a co‐ordinated manner. By default the grid voltage is controlled by the rotor‐side converter as long as it is not blocked by the protection system, otherwise the grid‐side converter takes over the voltage control. Moreover, the article presents a DFIG wind farm model equipped with a grid fault protection system and the described co‐ordinated voltage control. The whole DFIG wind farm model is implemented in the power system simulation toolbox PowerFactory DIgSILENT. The DFIG wind farm ride‐through capability and contribution to voltage control in the power system are assessed and discussed by means of simulations with the use of a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk. The simulation results show how a DFIG wind farm equipped with voltage control can help a nearby active stall wind farm to ride through a grid fault, without implementation of any additional ride‐through control strategy in the active stall wind farm. Copyright © 2006 John Wiley &Sons, Ltd.     

A robust power control strategy to enhance LVRT capability of grid‐connected DFIG‐based wind energy systems

This paper presents a new robust and effective control strategy to mitigate symmetrical voltage dips in a grid‐connected doubly fed induction generator (DFIG) wind energy conversion system without any additional hardware in the system. The aim is to control the power transmitted to the grid so as to keep the electrical and mechanical quantities above their threshold protection values during a voltage dip transient. To achieve this, the references of the powers are readjusted to adapt the wind energy conversion system to the fault conditions. Robust control strategies, combining the merits of sliding mode theory and fuzzy logic, are then proposed in this paper. These controllers are derived from the dynamic model of the DFIG considering the variations in the stator flux generated by the voltage drop. This approach is found to yield better performance than other control design methods which assume the flux in the stator to remain constant in amplitude. This control scheme is compliant with the fault‐ride‐through grid codes which require the wind turbine generator to remain connected during voltage dips. A series of simulation scenarios are carried out on a 3‐MW wind turbine system to demonstrate the effectiveness of the proposed control schemes under voltage dips and parameter uncertainty conditions.     

双馈感应风力发电系统低电压穿越控制

传统的双馈感应发电机(DFIG)矢量控制方案忽略了定子暂态磁通,导致当电网出现故障时控制性能恶化.为了提高电网故障下DFIG的不间断运行能力,将矢量控制与自抗扰控制器结合起来,利用扩张状态观测器估计出定子暂态磁通和电机参数误差对系统的影响并加以补偿.仿真结果表明该文提出的控制方法削弱了电网故障时DFIG的转子暂态电流峰值和电磁转矩波动,有效地保护了转子变频器和风力机机械结构,而且对电机参数误差具有鲁棒性.     

A novel line drop secondary voltage control algorithm for variable speed wind turbines

This paper addresses the design and implementation of the line drop secondary voltage control (LDSVC) for the doubly fed induction generator‐wind turbine (DFIG‐WT) complemented with reactive power allocation algorithm to achieve more efficient voltage regulation, reactive power compensation and to enhance the transient stability margin of the electric power system. The LDSVC is used to generate the local voltage reference, providing an improvement for overall voltage profile. The paper presents the influence of the integration of variable speed wind turbines‐based doubly fed induction generator (DFIG) while employing LDSVC for increasing the transient stability margin. This paper proposes an improved voltage control scheme, based on a secondary voltage controller complemented with an automatic gain controller (AGC). The scheme is applied to a wind energy system incorporating DFIG‐based wind turbines. The controller structure is developed and the performance of the self‐tuning AGC scheme is developed and analysed. The proposed controller is tested in response to system contingencies for different short circuit ratios. The performance of the secondary voltage control without and with AGC is verified. The influence of the AGC in improving the transient response and damping of voltage oscillations is verified. Copyright © 2010 John Wiley & Sons, Ltd.     

Reinforced Control and Operation of DFIG-Based Wind-Power-Generation System Under Unbalanced Grid Voltage Conditions

This paper proposes an enhanced control and operation of a doubly fed induction generator (DFIG) based wind power generation system under unbalanced grid voltage conditions. System behaviors of grid-side and rotor-side converters (GSCs and RSCs) are described. The RSC is controlled to eliminate the electromagnetic torque oscillation at double the grid frequency. Meanwhile, three selective control targets for the GSC, i.e., reducing the pulsations in the total active or reactive power, or unbalanced current outputs from the overall system, are identified and analyzed during voltage imbalance. A new current-control scheme is presented for the GSC and RSC without involving the decomposing of positive and negative sequence currents. The controller consists of a proportional (P) regulator and a resonant (R) one tuned at the grid frequency, which is implemented in the stator stationary reference frame. Finally, simulation studies are carried out on a 1.5-MW wind-turbine-driven DFIG system. The validity of the presented current controller and the feasibility of the proposed control targets are all confirmed by the simulated results.      

Compensation of network voltage unbalance using doubly fed induction generator-based wind farms

A control strategy for compensating AC network voltage unbalance using doubly fed induction generator (DFIG)-based wind farms is presented. A complete DFIG dynamic model containing both the rotor and grid side converters is used to accurately describe the average and ripple components of active/reactive power, electromagnetic torque and DC bus voltage, under unbalanced conditions. The principle of using DFIG systems to compensate grid voltage unbalance by injecting negative sequence current into the AC system is described. The injected negative sequence current can be provided by either the grid side or the rotor side converters. Various methods for coordinating these two converters are discussed and their respective impacts on power and torque oscillations are described. The validity of the proposed control strategy is demonstrated by simulations on a 30 MW DFIG-based wind farm using Matlab/Simulink during 2 and 4% voltage unbalances. The proposed compensation strategy can not only ensure reliable operation of the wind generators by restricting torque, DC link voltage and power oscillations, but also enable DFIG-based wind farms to contribute to rebalancing the connected network.     

双馈风力发电机组并网控制策略及性能分析

为了实现双馈风力发电机组无冲击电流并网,基于电网电压定向矢量控制技术,提出了一种考虑转子电流动态调节特性的双馈风力发电机组空载并网控制策略。基于Matlab/Simulink仿真平台,建立了双馈风力发电机系统及其并网控制的数学模型,并对不同初始运行转速的双馈风力发电机组的自动并网运行特性进行了仿真。仿真实验结果证明无论初始转速为同步转速,还是超、亚同步转速,利用提出的并网控制策略,双馈风力发电机组能很好快速地建立定子电压,并网过渡过程定子电流基本没有冲击。     

DFIG sliding mode control fed by back-to-back PWM converter with DC-link voltage control for variable speed wind turbine

This paper proposes an indirect power control of doubly fed induction generator (DFIG) with the rotor connected to the electric grid through a back-to-back pulse width modulation (PWM) converter for variable speed wind power generation. Appropriate state space model of the DFIG is deduced. An original control strategy based on a variable structure control theory, also called sliding mode control, is applied to achieve the control of the active and reactive power exchanged between the stator of the DFIG and the grid. A proportional-integral-(PI) controller is used to keep the DC-link voltage constant for a back-to-back PWM converter. Simulations are conducted for validation of the digital controller operation using Matlab/Simulink software.     

Low voltage ride through of doubly-fed induction generator connected to the grid using sliding mode control strategy

Wind Energy Conversion System (WECS) based on Doubly Fed Induction Generator (DFIG) connected to the grid is subjected to high transient currents at rotor side and rise in DC-link voltage during voltage sag at stator/grid side. To secure power system operation wind turbines have to meet grid requirements through the Low voltage ride through (LVRT) capability and contribute to grid voltage control during severe situations. This paper presents the modeling and control designs for WECS based on a real model of DFIG taking into account the effect of stator resistance. The non-linear control technique using sliding mode control (SMC) strategy is used to alter the dynamics of 1.5 MW wind turbine system connected to the grid under severe faults of grid voltage. The paper, also discusses the transient behavior and points out the performance limit for LVRT by using two protection circuits of an AC-crowbar and a DC-Chopper which follow a developed flowchart of system protection modes under fault which achieved LVRT requirements through results. The model has been implemented in MATLAB/SIMULINK for both rotor and grid side converters.     

Fault ride through and voltage regulation for grid connected wind turbine

High penetration of wind generation challenges wind turbine operators to supply reliable power and extract optimum power from the wind. Hence, the fault ride through (FRT) capability of wind turbine together with the optimum power tracking and regulation of wind turbine output voltage due to fluctuating nature of the wind becomes essential. In this paper, a method is proposed to ensure that the double fed induction generator (DFIG) wind turbine continues to operate during severe grid faults and maintains a constant output voltage, irrespective of the fluctuating wind. The proposed controller also allows the DFIG wind turbine to track optimum power from the wind. Extensive simulation is performed using PSCAD/EMTDC software and results obtained show that the DFIG output voltage fulfills the grid code requirements. The results also show that the proposed method is able to track the optimum power, regulate the DFIG output voltage and perform fault ride through of wind turbine.     

一种转子串联可变电阻与卸荷电路配合的低电压穿越方法

针对双馈感应发电机(DFIG)转子串联固定电阻在低电压穿越(LVRT)时,应对故障时的灵活性较低,且低穿效果过于依赖制动电阻的问题,文章提出了转子串联可变电阻与直流侧卸荷电路配合的改进方案。该方案根据转子故障电流的时域表达式对串联阻值进行整定并形成策略表。基于PSCAD/EMTDC仿真不同电压跌落情况下,改进方案的低电压穿越特性。结果表明,文章所提出的方案改善了风电机组的暂态稳定性,有效降低无功损耗,总体上低电压穿越效果较好。     

基于双环控制的双馈风力发电机组控制策略

在分析了双馈风力发电机组运行特性的基础上,提出一种基于双环控制的变换器控制策略。在转子侧,变换器采用定子磁链定向矢量控制技术,推导出了用转子有功电流和无功电流独立解耦控制有功功率和无功功率的策略,并实现了风能的最大跟踪;在电网侧,变换器采用电网电压定向矢量控制技术,构建了电流内环、电压外环的双闭环PI控制系统。利用PSCAD/EMTDC软件,构建了双馈风力发电机组仿真模型。仿真结果验证了所提控制策略的有效性和合理性     

Microgrid Dynamic Performance Improvement Using a Doubly Fed Induction Wind Generator

This paper describes a control approach applied on a doubly fed induction generator (DFIG) to provide both voltage and frequency regulation capabilities, and hence, an improvement in the dynamic behavior of a microgrid system. The microgrid system is assumed to be a portion of a medium voltage distribution feeder and is supplied by two distributed generation (DG) units, i.e., a gas-turbine synchronous generator and a variable-speed wind turbine with DFIG. A control approach algorithm is proposed for the DFIG unit to improve both voltage and primary frequency controls. Two distinct operation modes, i.e., grid-connected and islanding mode, are used in the proposed approach for proper transfer from normal to islanding operation. Case studies are simulated based on both planned and unplanned islanding scenarios to evaluate the performance of the control approach. The study results show that the proposed control approach for DGs in the microgrid increase the microgrid system's dynamic performance, reduce frequency changes, and improve bus voltages regulation during islanding and autonomous operations.      

Enhanced Control and Operation of DFIG-Based Wind Farms During Network Unbalance

This paper investigates the control and operation of doubly fed induction generator (DFIG)-based wind generation systems under unbalanced voltage conditions. DFIG system behaviors under unbalanced voltage are analyzed and different control targets are discussed. A new rotor current control strategy containing a main controller and an auxiliary controller is proposed. The main controller is implemented in the positive $(dq)^{+}$ frame without involving positive/negative sequence decomposition, whereas the auxiliary controller is implemented in the negative $(dq)^{-}$ frame with negative sequence current extracted. The impact of providing unbalanced control on converter voltage rating is investigated. Simulation results using EMTDC/PSCAD are presented for a 2-MW DFIG wind generation system to validate the proposed control scheme and to demonstrate the enhanced system operation during “small” steady state and “large” transient unbalances.      

Dynamic contribution of variable-speed wind energy conversion system in system frequency regulation

Frequency regulation in a generation mix having large wind power penetration is a critical issue, as wind units isolate from the grid during disturbances with advanced power electronics controllers and reduce equivalent system inertia. Thus, it is important that wind turbines also contribute to system frequency control. This paper examines the dynamic contribution of doubly fed induction generator (DFIG)-based wind turbine in system frequency regulation. The modified inertial support scheme is proposed which helps the DFIG to provide the short term transient active power support to the grid during transients and arrests the fall in frequency. The frequency deviation is considered by the controller to provide the inertial control. An additional reference power output is used which helps the DFIG to release kinetic energy stored in rotating masses of the turbine. The optimal speed control parameters have been used for the DFIG to increases its participation in frequency control. The simulations carried out in a two-area interconnected power system demonstrate the contribution of the DFIG in load frequency control.     

An improved fault ride-through strategy for doubly fed induction generator-based wind turbines

Keeping the generators operating during transient grid faults becomes an obligation for the bulk wind generation units connected to the transmission network and it is highly desired for distribution wind generators. A proposed scheme is implemented to keep the wind-power DFIG operating during transient grid faults. Challenges imposed on the generator configuration and the control during the fault and recovering periods are presented. A comprehensive time domain model for the DFIG with the decoupled dq controller is implemented using Matlab/Simulink software. Intensive simulation results are discussed to ensure the validity and feasibility of the proposed fault ride through technique. The scheme protects the DFIG components, fulfills the grid code requirements and optimises the hardware added to the generator.