电网电压不对称故障时双馈风电机组多目标优化控制

文章首先对电网不对称故障对双馈风电机组造成的影响进行分析,采用转子侧和网侧变换器的比例积分谐振控制器(PIR)来简化传统低压穿越控制结构;通过引入两个可调参数,从而对前馈量进行调节来得到不对称故障期间双馈电机的多目标优化控制策略;在仔细研究模型的基础上,通过对电网电压不对称故障时双馈风电机组的多目标优化控制的PSCAD模型进行仿真试验,仿真结果表明,该方案对于电网电压不对称故障时的LVRT有一定的可行性。     

Ridethrough of wind turbines with doubly-fed induction generator during a voltage dip

In this paper, a solution is described that makes it possible for wind turbines using doubly-fed induction generators to stay connected to the grid during grid faults. The key of the solution is to limit the high current in the rotor in order to protect the converter and to provide a bypass for this current via a set of resistors that are connected to the rotor windings. With these resistors, it is possible to ride through grid faults without disconnecting the turbine from the grid. Because the generator and converter stay connected, the synchronism of operation remains established during and after the fault and normal operation can be continued immediately after the fault has been cleared. An additional feature is that reactive power can be supplied to the grid during long dips in order to facilitate voltage restoration. A control strategy has been developed that takes care of the transition back to normal operation. Without special control action, large transients would occur.     

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.     

Nonlinear control of the doubly-fed induction generator in wind power systems

This paper describes the models of a wind power system, such as the turbine, generator, power electronics converters and controllers, with the aim to control the generation of wind power in order to maximize the generated power with the lowest possible impact in the grid voltage and frequency during normal operation and under the occurrence of faults. The presented work considers a wind power system equipped with the doubly-fed induction generator and a vector-controlled converter connected between the rotor and the grid. The paper presents comparative results between proportional-integral controllers and neural networks based controllers, showing that better dynamic characteristics can be obtained using neural networks based controllers.     

Specification of rotor side voltage source inverter of a doubly-fed induction generator for achieving ride-through capability

The ride-through capability of a doubly-fed induction generator under three-phase balanced voltage sags is examined, under the condition that the machine should have the capability of real and reactive power control during the external fault. Mathematical formulae for the peak rotor fault current and the required rotor voltage output under vector control are derived. Moreover, the DC link dynamics are incorporated into the analysis and it is shown that they can have a decisive impact on fault behaviour of the machine during voltage sags. Combined, a design methodology for the rotor side voltage source inverter aiming to achieve a ride-through capability at the lowest cost is described. Simulation results in PSCAD/EMTDC show very good agreement with the theoretical analysis.     

双馈感应风力发电系统电流控制策略研究

由双馈发电机q-d坐标系下基本数学模型,推导出双馈发电机的转子电流和转子电压、磁链之间的关系模型。文章研究了双馈风力发电系统转子侧电流控制策略框图,计算选择PI控制器参数。利用MATLAB/SIMULINK平台自行建立了1.5 MW风力发电系统整体仿真模型。运行该系统模型,得到了双馈发电机转子电流、电压,滤波器输出电流及定子电压等信号波形曲线。仿真结果验证模型及控制器的正确性,证明PI控制器能够满足双馈风力发电系统的稳、准、快等性能指标。     

孤岛双馈异步风力发电机组自抗扰控制研究

针对孤岛运行双馈异步风力发电机输出电压易受负载切换影响、PI控制器对电机参数依赖性高等问题,建立了包含励磁电流动态过程与电机参数误差的数学模型。将电机参数误差和励磁电流动态过程分别视为系统的内扰与外扰,应用分离性原理和零极点配置方法设计双闭环自抗扰控制器替代PI控制器,扩张状态观测器可实时估计系统总扰动,并通过前馈进行补偿。建立兆瓦级DFIG孤岛运行仿真模型进行验证,结果表明文章控制方法能够有效抑制负载切换引起的电压扰动和电机参数变化的影响,提高了系统抗扰动性能。     

双馈风力发电机网侧变换器的统一模糊控制策略

双馈风力发电机网侧变换器通常工作在单位功率因数状态下,未充分发挥出其无功调节潜能。文章分析了双馈风力发电机网侧变换器的无功功率调节极限,针对网侧变换器非线性、时变d-q系统,提出了一种统一模糊控制策略,即利用网侧变换器进行无功补偿,同时抑制因风速变化引起的发电机出口电压波动。在PSCAD/EMTDC环境下,通过无穷大电网末端接入1台2 MW双馈风力发电机进行仿真,验证了该统一控制策略的有效性。     

无刷双馈风力发电系统双空间矢量调制矩阵变换器励磁控制

矩阵变换器具有低频性能好、功率因数可调节、四象限运行等特点,以矩阵变换器为无刷双馈风力发电机的励磁控制器,可建立交流励磁变速恒频风力发电系统。采用双空间间接矢量控制策略控制矩阵变换器输出电流的幅值、频率、相位,从而调节发电机控制绕组电流的幅值、频率、相位,实现无刷双馈风力发电系统的变速恒频运行及功率因数控制。采用C语言编写数字信号处理器控制系统程序,实现双空间间接矢量调制策略,复杂可编程逻辑器件实现四步换流和产生双向开关矩阵控制信号。通过实际风力发电机组平台进行了相关试验,结果验证了控制方案的有效性。     

基于dSPACE的电网谐波环境下双馈风机的适应性分析

文章以双馈电机(DFIG)机组及以含有5,7,11,13次谐波电压的电网为研究对象,针对风电机组应对电网谐波带来的扰动并维持正常运行等问题进行了研究。给出了一种基于多重谐振控制器的谐波抑制方法,并在dSPACE仿真平台上建立了一台2 MW风机模型,借助dSPACE系统提供的ControlDesk环境对其进行实时仿真分析。实时仿真结果表明,该方案能有效减少5,7,11,13次电网谐波电压带来的影响,使DFIG定子电流波形更加正弦化,改善直流母线电压波动问题,提高了电网谐波环境下风机系统的适应性。     

变风速下双馈异步风机在微电网中的电压频率协调控制

微电网线路阻抗比值较大,当微电网孤岛运行时,负荷频繁投切导致微电网电压、频率波动较大。文章对双馈感应风电机组与柴油机协调控制共同为微电网提供电压、频率支撑,提出了DFIG在微电网中的电压、频率协调控制策略。DFIG有功功率控制采用虚拟惯量控制与超速减载控制,并采用f-P下垂控制与柴油机配合对微电网频率进行调节。通过控制DFIG转子侧换流器的无功功率,并采用V-Q下垂控制与柴油机配合对微电网电压进行控制。最后在DIgSILENT中搭建了风光柴中压微电网模型,仿真结果验证了文章所提控制策略的有效性。     

Transient state analysis of a doubly-fed induction generator underthree phase short circuit

The doubly fed induction generator (DFG) is a variable-speed constant-frequency generator operating in either subsynchronous or supersynchronous mode. The transient behavior of a symmetrically loaded DFG after a three-phase short circuit is presented. Both speed and rotor excitation voltage and frequency remain unchanged during short circuit. The complete mathematical model of the transient state and experimental results are given, along with the transient state equivalent circuit     

Transient performance study of a brushless doubly fed twin stator induction generator

This paper presents theoretical, simulation, and experimental study of the brushless doubly fed twin stator induction generator (BDFTSIG) dynamics under vector control based on the orientation on the power machine stator flux. A complex transfer function is derived which links the control current and power winding current space vectors in the field coordinates. Based on this result, the transient response of the BDFTSIG to step changes in the control current is examined theoretically. The oscillatory transients are explained in detail and linked to control flux transients triggered whenever operation point of the generator is changed. Furthermore, BDFTSIG operation with closed loop control of the power machine active and reactive powers is examined theoretically and experimentally. It is shown that in the closed loop operation, the system damping may be reduced so that the PI controller gains must be properly selected to achieve a good transient response.     

并网双馈风力发电机矢量控制的改进及实现

分析了传统的双馈风力发电控制策略,并在网侧变换器和转子侧变换器中分别使用了电网电压定向和转子侧定子磁链定向的矢量控制.传统的控制策略比较复杂,对其提出了改进方案,改进后的控制方法大大简便.利用PSCAD/EMTDC平台进行仿真表明,改进后的控制方案是正确的.     

Voltage-frequency control of a self-excited induction generator

A new strategy for controlling voltage and frequency of a self excited induction generator (SEIG) is presented. The SEIG operates in the linear region of the core magnetizing curve, so that efficiency and performance are upgraded. An external excitation circuit, comprising permanently connected capacitors and electronically switched inductances is used. The external circuit allows to compensate for the generator reactive demand. A detailed analysis is performed, showing some salient aspects related to the connection of the external excitation circuit on the control performance. Asynchronous switching is used, but some important considerations must be taken into account related to the instantaneous phase angle between stator voltage and external inductor current at the switching instant, if good transient response is desired. Sliding mode controllers are proposed, showing good dynamic response and robust behavior upon changes in load and generator parameters. Computer simulations are used to demonstrate the validity of the proposed scheme     

Wind-driven self-excited induction generator with voltage and frequency regulated by a reduced-rating voltage source inverter

This paper discusses the regulation of the voltage and frequency of a stand-alone fixed-pitch wind energy conversion system (WECS) based on a self-excited squirrel-cage induction machine. A shunt connected voltage source inverter (VSI) and a controllable dump load are used for regulation purposes. A battery bank is included in the dc side of the VSI so that it can absorb and inject active power thus increasing the efficiency and availability of the system. A control scheme for the VSI with independent control of active and reactive power allows the state of charge of the batteries to be kept in a safe range while maximizing the voltage regulating capabilities of the VSI. The characteristics of the wind turbine, self-excited generator, and the ratings of the VSI are considered in order to determine the load range for which voltage and frequency can be regulated for a given wind speed range. The feasibility of the proposed system is verified by simulations.     

一种改进的双馈感应发电机低电压穿越控制策略

为提高双馈感应发电机的低电压穿越能力,对转子侧换流器考虑了电网故障时定子磁链变化对有功、无功解耦的影响,并将反映电流耦合及定子磁链变化的附加量作为前馈分量加入电流指令值,对传统矢量控制策略进行了改进。对网侧换流器,提出一种考虑直流环节两侧功率不平衡及电网电压突变的改进控制策略。仿真验证了改进方案对转子过电压、过电流及直流母线电压波动均有很好的抑制作用,有效提高了双馈机的低电压穿越能力。     

Evaluation of the effects of rotor harmonics in a doubly-fed induction generator with harmonic induced speed ripple

This paper is concerned with the low-frequency harmonics which originate from the rotor inverter of a doubly-fed induction generator (DFIG). By including the mechanical speed response, it expands the transformer approach previously taken to analyze the harmonic transfer in the machine. A numerical method is proposed to calculate the stator current sidebands, which can be used to predict the voltage fluctuation at the system busbar. It is shown that the pulsating torque associated with the rotor harmonics can induce speed ripple depending on the inertia, causing a significant change in the stator current spectrum. Experiment and simulation verify the analysis and the proposed calculation method.     

Fault-tolerant control of an open-winding brushless doubly-fed wind power generator system with dual three-level converter

To improve the fault redundancy capability for the high reliability requirement of a brushless doubly-fed generation system applied to large offshore wind farms, the control winding of a brushless doubly-fed reluctance generator is designed as an open-winding structure. Consequently, the two ends of the control winding are connected via dual three-phase converters for the emerging open-winding structure. Therefore, a novel fault-tolerant control strategy based on the direct power control scheme is brought to focus in this paper. Based on the direct power control (DPC) strategy, the post-fault voltage vector selection method is explained in detail according to the fault types of the dual converters. The fault-tolerant control strategy proposed enables the open-winding brushless doubly-fed reluctance generator (BDFRG) system to operate normally in one, two, or three switches fault of the converter, simultaneously achieving power tracking control. The presented results verify the feasibility and validity of the scheme proposed.