A new approach for low voltage ride through capability in DFIG based wind farm

Protection against voltage dips is very important for transient stability in a Doubly Fed Induction Generator (DFIG). Conventional crowbar circuits have been used for protection of DFIGs; however, they may be insufficient in some transient situations. Therefore, the Low Voltage Ride Through (LVRT) capability was enhanced by a Demagnetization Current Controller (DCC) for the purpose of transient analysis. In addition, both stator and rotor circuit electromotive forces were modeled in a DFIG. The performances of the DFIG models with and without the DCC were compared. Modeling was carried out in a MATLAB/SIMULINK environment. A comparison of system behaviors was made between three-phase faults with and without a stator–rotor dynamic. Parameters for the DFIG including output voltage, speed, electrical torque variations and d–q axis rotor–stator current variations in addition to a 34.5 kV bus voltage were examined. It was found that in the DFIG model the system became stable in a short time when using the DCC.     

A fuzzy logic supervisor for active and reactive power control of a variable speed wind energy conversion system associated to a flywheel storage system

In this paper, we propose a Wind Energy Conversion System (WECS) at variable speed using a Doubly Fed Induction Generator (DFIG) controlled on the rotor side through converters. A Flywheel Energy Storage System (FESS) is connected to the studied wind generator at the DC bus in order to evaluate its capacity to participate to the ancillary services. We study the improvement of the active and reactive power quality produced by the wind generator and its effect on the load voltage regulation connected to the wind generator. For that, a fuzzy logic supervisor is established to control the FESS operation and the DC bus voltage in order to smooth the active power fluctuations due to the random wind speed variations. A control law is also described to smooth the reactive power at the connection node to the grid.     

基于电压闭环的双馈风力发电软切入控制

传统的风力发电并网技术在并网瞬间会产生很大的冲击电流,严重威胁发电机及电力系统的安全.为研究空载并网技术,设计了一套30 kW变速恒频双馈异步风力发电机励磁控制实验系统.根据交流励磁变速恒频风力发电的运行特性,采用背靠背变流器作为双馈异步发电机(DFIG)的励磁源.分析了基于旋转坐标系统下的DFIG的数学模型,采用定子...     

双馈风力发电机功角暂态行为及其控制策略

为研究电网扰动下双馈风力发电机(DFIG)暂态行为和抵御电网扰动的技术措施.针对单机无穷大系统,定义了DFIG功角并分析其暂态行为.由于DFIG的功角与转子位置无关,其暂态行为是电磁暂态过程,故具有快变特点.在电网故障扰动下,DFIG的功角可能大幅变化,通过相量图分析发现功角大幅变化是导致机端电压严重跌落和转子过载的主...     

一种新型的能量回馈型DFIG低电压穿越Crowbar拓扑

为了双馈感应风力发电机(DFIG)满足低电压穿越(LVRT)要求,同时降低硬件成本,提出了一种新型的能量回馈型撬棒电路(Crowbar Circuit)。新型电路将传统电路的直流侧通过一个合理设计的反馈回路,将Crowbar直流电压反馈嵌位到转子变流器的直流母线,降低了由于较长的转子电缆造成的传输线效应对Crowbar电路耐压的影响,并对反馈回路的特性进行了分析。在2MW双馈风力发电机组上进行了仿真和实验验证。仿真和实验表明,回馈型拓扑可以有效改善Crowbar电路电压余量,其具有较好的理论和工程应用价值。     

Controller saturation nonlinearity in doubly fed induction generator‐based wind turbines under unbalanced grid conditions

Doubly fed induction generator (DFIG) is widely used in wind energy generation systems due to its cost‐effective, partially rated back‐to‐back power converters, variable rotor speed operation, and maximum wind power capture. The conventional design assumes balanced grid voltage and utilizes power protection for the power converters. The DFIG wind turbine is naturally one of the major components in distributed generations of the smart grid system. However, newly developed smart grid system is rich in unbalanced loads. This paper summarizes the limiter settings of controllers and explores the nonlinear behaviors of the DFIG‐based wind power generation system with unbalanced loads. The generator rotor speed and an unbalanced load resistance are chosen as variation parameters. An emerging low‐frequency linear‐modulated oscillation at line second harmonic frequency with DC drifting is identified on the DC link voltage of the back‐to‐back power converters. In terms of second harmonic and the usually reported hazardous low‐frequency oscillation, the saturation nonlinearity and over‐modulation of the back‐to‐back power converter and its power flow are investigated and analyzed. The built‐in detailed model of the DFIG wind energy generation system in Matlab with SimPowerSystems library is used in this study. Copyright © 2015 John Wiley & Sons, Ltd.     

引入比例谐振调节器的双馈电机转子电流无速度传感器控制

双馈风力发电机转子电流模型参考自适应系统(MRAS)的转速估算方法中,由于定、转子电压、电流采样信号的漂移及AD转换的偏差,致使所估计的双馈电机转速幅值中呈现不同程度的交流脉动。针对此问题,提出了在自适应实时调节机构中,采用PI并联比例谐振(PR)控制器共同作用的控制策略。该方法可消除转速估计中角频率和转差频率的交流脉动,实现转速精确估计,提高双馈电机发电质量。基于10 k W的双馈风力发电模拟平台进行了实验研究,通过与PI控制对比验证了PI并联比例谐振控制的优越性。     

新型联合仿真在双馈风电机组电网不平衡故障下的应用研究

由于双馈电机的定子绕组与电网相互连接,双馈电机对电网的故障较为敏感。在电网所有故障类型中,不平衡故障发生的频率更高。文中建立了新型联合仿真方法,利用Ansoft Maxell,MATLAB/Simulink以及Ansoft Simplorer三个软件联合仿真搭建了1.5 MW双馈发电系统,这三部分的联合仿真,既可以考虑电机模型真实因素,也能完全实现模拟双馈发电系统。模拟了双馈电机在B型单相接地故障下的暂态过程,得出了在B型单相接地故障下双馈电机的定子磁链以及转子电压等电气量的变化过程。最后,联合仿真结果与理论分析一致,验证了该仿真方法的有效性。     

基于宽频带转子附加阻尼的双馈型风机次同步振荡抑制策略

针对双馈型风电场并网引发的次同步振荡问题进行了研究,建立了双馈电机以及变流器的等效模型。基于动态等值阻抗解析式分析了并网阻抗特性的关键影响因素。结果表明控制器转子侧内环增益对次同步频带阻抗特性影响较大。在此基础上,提出了一种基于宽频带转子附加阻尼控制的次同步振荡抑制措施。该抑制措施实施于双馈型风机转子侧变流器控制中,无须附加装置,利于工程实现。最后,通过时域仿真验证了该抑制措施的有效性以及在暂态工况下的适用性。     

含双馈风机的配电网模糊谐波潮流计算

针对配电网中存在着大量的不确定因素对接有双馈风机的配电网节点进行基波和谐波模糊建模.并考虑节点内部存在的关系:在节点基波建模中,考虑并联补偿器和功率因数的关系可以提高理论收敛程度;在节点谐波建模中,考虑风机负荷、无源负荷和非线性负荷的关系可以极大简化模糊谐波建模的复杂程度.最后把双馈风机接入典型33节点辐射型配电网进行...     

改善双馈感应发电机无功特性的变阻值撬棒保护方案

撬棒保护的接入及其电阻值会改变低电压穿越期间双馈感应发电机(DFIG)的无功功率动态特性,由此将影响风电场周边区域电网电压稳定性。针对这一问题,详细分析不同电网电压跌落水平和低电压过渡不同时期撬棒保护接入对DFIG无功特性的影响机理及其变化规律。提出了一种改善机组无功特性的变阻值撬棒保护方案,制定了该方案的撬棒阻值整定方法及其投切控制策略,并进行仿真对比研究。仿真结果表明,相比以限流为目标的传统撬棒保护和以尽快恢复机组可控性的主动式撬棒保护,所提撬棒保护方案不仅能在故障持续期间缩短撬棒投入时间,减少撬棒投入期间DFIG吸收的无功功率,提升出口电压。同时也能最大限度降低故障切除时的DFIG无功振荡峰值,加速电网电压恢复,有助于系统区域电压稳定性的提升。     

First and High Order Sliding Mode Control of a DFIG-Based Wind Turbine

Abstract

Wind energy has many advantages, because it does not pollute and is an inexhaustible source of energy. In this paper, we study the dynamical behavior of the Horizontal Axis Wind Turbine (HAWT) with MPPT (Maximum Power Point Tracking) algorithm and a pitch control, using a Doubly Fed Induction Generator (DFIG). The DFIG stator is directly linked to the grid; the rotor is connected via a back-to-back converter. To ensure a maximal power extraction, we use the MPPT algorithm combined with a PI controller to generate the maximum power and to force the system to work at the maximum operation point. The pitch control is used to protect the Wind Turbine (WT) against high wind speed. Field Oriented Control (FOC) based on a Sliding Mode (SM) first order and high order (super-twisting) are compared. These schemes control the stator active and reactive powers as well as the stator power factor and improve the power quality injected in the grid. Simulations illustrate the active and reactive power control of the HAWT by adjusting the rotor voltages of the DFIG. We choose the two rotor currents to define the sliding surfaces, in order to avoid the measurement of the electromagnetic torque and stator active and reactive powers. Adequate simulations results with Matlab-Simulink of the whole system including the MPPT algorithm, pitch control and SM controllers are presented and discussed.     

双馈型风力发电系统低电压穿越策略仿真

针对双馈风力发电机组的低电压穿越能力的问题,介绍了风力发电在电网电压跌落时的并网要求,分析了目前已有的各种应对策略,提出了一套应对电网电压跌落时的控制策略。对于电网严重短暂跌落,通过对转子电流和直流侧电压滞环比较来控制Active Crowbar和直流侧卸荷电路,以卸荷多余能量并保护变流器,并保持风电机组的并网。对于电网的长时间跌落,还进行电网电压闭环发送无功,以支持电网进行恢复。通过仿真模型验证了所提出的控制策略能很好地抑制转子侧电流和直流侧电压的上升,并对电网提供无功支持。     

双馈发电机定子PQ输出数值区间研究

为实现双馈发电机定子侧有功、无功功率调节时确保转子变流器处于安全工作区,并实现最大限度的风能利用,通过双馈电机等效电路中电压与电流方程的建立,推导得出转子励磁电流有效值与定子侧有功、无功功率输出数值区间的映射关系,通过旋转坐标系中电压方程与磁链方程的建立及推导对上述映射关系进行验证.并提出了一种基于该映射关系的优化无功输出控制方法.仿真结果表明:在维持转子励磁电流有效值恒定的条件下,通过无功功率负方向调节可以实现发电机定子侧有功功率的正方向调节;在维持发电机定子侧有功功率输出恒定的条件下,通过无功功率负方向调节可以实现转子励磁电流有效值的负方向调节.在转子励磁变流器安全工作区内,可通过有功、无功工作点位移提高双馈发电系统极限条件下的稳定性和对原动机机械能最大限度的电能转换.     

双馈感应发电机的直接功率控制策略

建立了单机与无穷大系统相连的双馈感应发电机(DFIG)数学模型,并以定子电压综合矢量恒定为控制约束,提出了一种双馈感应发电机的励磁控制策略。该控制策略仅需测量定子侧电压、电流信号,简化了控制系统。利用该策略对并网双馈感应发电机有功、无功稳态调节特性及机端三相对地突然短路的过渡过程进行了仿真研究。结果表明,该方法能够实现有功和无功的独立控制,且具有良好的动态性能。     

风力发电机运行特性分析研究

介绍了不同类型的风力发电机组的数学模型,采用PSASP软件,以WSCC3机9节点为例,分析了不同类型风电接入系统的暂态稳定性,通过仿真结果表明,对于小干扰稳定,风电接入后对系统的阻尼特性均有不同程度的改善;系统发生故障时,双馈风电机较直驱风电机的功率输出平滑,但双馈风电机的在电压恢复上更为有利。     

一种提高系统频率响应特性的风储协调控制策略

风电并网规模的不断扩大削弱了电力系统的惯量水平,给频率稳定带来巨大挑战.通过分析不同风速下双馈风机(DFIG)参与惯性响应的能力,给出了一种风速分段方法,从而确定DFIG参与调频的风速范围.在此基础上,提出了一种DFIG与储能技术联合的调频控制策略,根据系统惯性响应和频率恢复2个阶段的频率变化特点,制定风储协调出力模式:在惯性响应阶段,通过虚拟惯性控制使DFIG释放转子动能以阻止频率跌落,并采用超速控制将DFIG转速变化分配至最大功率点跟踪控制运行点两侧以改善调频效果,同时逐渐增加储能系统的输出功率对DFIG后期的调频功率下降进行补充;在频率恢复阶段,将DFIG退出调频模式以避免虚拟惯性控制从系统索取能量,主要依靠储能系统出力辅助同步发电机加快完成一次调频.算例仿真结果表明所提方法能够有效改善系统的频率响应特性,避免二次频率事故的发生,提高了系统的频率稳定性.     

扩展MPPT运行区间的双馈风电机组智能双模控制策略

为扩展双馈型感应发电机(DFIG)最大功率点跟踪运行区间,提高其在低风速区间的发电效率,首先,文中提出了一种智能双模控制策略,通过机侧变流器控制策略的调整以及必要的开关切换,DFIG可根据风速不同自动地运行于两种模式:传统DFIG模式及DFIG定子短路模式。然后,针对模式切换过程时间短的需求以及冲击大、影响久的特点,重点研究了文献鲜有提及又极为重要的模式柔性切换控制方法,特别是双向模式切换的具体流程、机侧变流器控制策略以及发电机负载的选取等。最后,基于RT-lab的硬件在环仿真结果表明,所提控制策略较好地发挥了DFIG的成本优势与全功率型发电机的宽变速范围优势,并实现了两种模式的快速、柔性切换。     

Short circuit fault analysis in a grid connected DFIG based wind energy system with active crowbar protection circuit for ridethrough capability and power quality improvement

In power industry, improvement of the short circuit Fault-ride through (FRT) capability of grid integrated Doubly Fed Induction Generators (DFIGs) for the wind power system is an important issue. In this paper an Active Crowbar Protection (ACB_P) system is proposed to enhance the Fault-ride through (FRT) capability of DFIG so as to improve the power quality of the system. The protection scheme proposed here is designed with a capacitor in series with the resistor unlike the conventional Crowbar (CB) having only resistors. The importance of ACB_P is to maintain the connection of DFIG with the grid during fault conditions to provide uninterruptable power supply to the loads. The major functions of the capacitor in the protection circuit are to eliminate the ripples generated in the rotor current and to protect the converters as well as the DC-link capacitor. The main objectives of the proposed approach are: minimisation of magnitude of rotor fault currents, maintenance of constant DC-link voltage, reduction in crowbar operation time to avoid disconnection between the DFIG and the Rotor side converter (RSC), improvement in the short circuit response of terminal voltage and enhancement in the dynamic responses of the DFIG. These objectives are achieved through the incorporation of ACB_P scheme between the rotor of the DFIG and RSC. The proposed scheme is validated on different types of fault conditions and it is observed that there is significant improvement in the objectives. Simulation results are carried out on a 1.7 MVA DFIG based WECS under different types of short circuit faults in MATLAB/Simulation and functionality of the proposed scheme is verified.     

Optimal tracking and robust power control of the DFIG wind turbine

In the present paper, an optimal operation of a grid-connected variable speed wind turbine equipped with a Doubly Fed Induction Generator (DFIG) is presented. The proposed cascaded nonlinear controller is designed to perform two main objectives. In the outer loop, a maximum power point tracking (MPPT) algorithm based on fuzzy logic theory is designed to permanently extract the optimal aerodynamic energy, whereas in the inner loop, a second order sliding mode control (2-SM) is applied to achieve smooth regulation of both stator active and reactive powers quantities. The obtained simulation results show a permanent track of the MPP point regardless of the turbine power-speed slope moreover the proposed sliding mode control strategy presents attractive features such as chattering-free, compared to the conventional first order sliding technique (1-SM).