A robust direct current control of DFIG wind turbine with low current THD based predictive approach

This paper presents a simple and robust direct current control based predictive approach for rotor side converter (RSC) of the doubly fed induction generator (DFIG), which operates at a constant switching frequency and has a fast dynamic response. First, sector of required rotor voltage vector is predicted in this strategy, and according to this predicted sector, two active vectors and two zero vectors are elected in each switching period. Derivatives of rotor current in the synchronous frame are determined for each predicted voltage vector in every period. These derivatives are used to compute the duration of the vectors in such a way that the current error at the end of the switching period gets minimized. The accuracy of the proposed control strategy under variation of rotor speed is evaluated in Matlab/Simulink environment for a 2 MW DFIG. Moreover, the impact of parameter variations on the system is examined for this suggested technique. Furthermore, the dynamic response and stator current total harmonic distortion (THD) of proposed strategy is compared with traditional vector control (VC), direct power control (DPC) and predictive direct power control (PDPC) methods. Finally, the performance of the proposed method is evaluated under disturbance voltage. The results demonstrate that suggested control technique has the lowest stator current THD and operates perfectly near the synchronous speed and under grid voltage dip. Copyright © 2015 John Wiley & Sons, Ltd.     

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。     

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.     

基于Crowbar保护的双馈感应发电机组的低电压穿越研究

分析了双馈感应发电机组在电网电压跌落时Crowbar阻值变化对电网的影响。根据双馈感应发电机(Doubly-FedInduction Generator,DFIG)的数学模型,推导出在发电机机端发生对称故障时,定子、转子电流的表达式,通过故障期间的最大转子电流,给出Crowbar阻值估算值。在EPSCAD/EMTDC软件中仿真分析不同的Crowbar阻值对系统的影响,验证公式推导的正确性,并通过仿真试验确定合理的Crowbar切除出时间。     

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.     

Direct power control of wind‐turbine‐driven DFIG during transient grid voltage unbalance

In this paper, a direct power control (DPC) of a wind‐turbine‐driven doubly fed induction generators (DFIGs) under unbalanced network voltage conditions is studied. Variations of the stator output active, reactive and electromagnetic powers are fully deduced in the presence of negative sequence supply voltage. The rotor side converter is controlled on the basis of DPC to eliminate the electromagnetic torque oscillations at double supply frequency under unbalanced stator supply. The rotor voltage references estimation requires only simple calculations without any integral operation. The proposed control scheme removes rotor current regulators and the decomposition processing of positive and negative sequence rotor currents. The performance of the proposed and conventional DPC schemes is compared under the same operating conditions. Simulation results using Matlab/Simulink are carried out for a 1.5 MW DFIG wind generation system to show the validity of the proposed scheme during unbalanced voltage supply. Copyright © 2013 John Wiley & Sons, Ltd.     

改进的双馈风电机组故障穿越控制策略研究

基于传统矢量控制技术,该文提出一种改进的矢量控制技术来抑制双馈感应电机故障期间的转子过电流,以提高双馈风电机组故障穿越能力。从分析传统矢量控制技术入手,提出一种改进的矢量控制技术,其主要特点是反映定子电压瞬态对转子电流的影响,但并未增加控制的复杂程度。从理论的角度对所提方案能提高双馈风电机组故障穿越能力的机理进行深入分析。最后,对基于PSCAD/EMTDC软件环境下搭建的2 MW双馈风电机组模型进行仿真研究。仿真结果表明所提方案能有效抑制双馈感应电机故障期间的转子过电流,从而提高双馈风电机组的故障穿越能力。     

双馈感应风力发电机三相短路电流分析与仿真研究

基于DFIG精确暂态数学模型,深入研究了机端三相短路所激起的双馈电机电磁过渡过程产生的机理及定、转子电流中各频率成分之间的依存关系,并详细推导了三相短路电流的解析表达式, 在此基础上,深入分析了定子和转子绕组阻值对短路电流幅值的影响规律,得到抑制短路电流最佳阻值,实际仿真验证了分析结论的有效性。此研究对于深入探索风机crowbar保护的电阻值整定及短路电流抑制措施都有十分重要的意义。     

Doubly fed induction generator‐based variable‐speed wind turbine: Proposal of a simplified model under a faulty grid with short‐duration faults

The aim of this study is to provide a simplified model of a variable‐speed wind turbine (VSWT) with the technology of a doubly fed induction generator (DFIG), which operates under faulty grid conditions. A simplified model is proposed, which consists of a set of electrical and mechanical equations that can be easily modeled as simplistic electrical circuits. It makes it an excellent tool to achieve fault ride‐through capability of grid‐connected VSWT with DFIGs. Both symmetrical and unsymmetrical grid faults, which cause symmetrical and unsymmetrical voltage sags, have been applied to the system in order to validate the model. The proposed simplified model has been compared with the traditional full‐order model under multiple sags (different durations and depths), and the results reveal that both models present similar accuracy. As the idea is to reduce the computational time required to simulate the machine behavior under faulty grid conditions, the proposed model becomes suitable for that purpose. The analytical study has been validated by simulations carried out with MATLAB .     

计及RSC控制的DFIG短路电流直流分量解析表达

现有文献针对计及转子变流器(RSC)控制的双馈感应风电机组(DFIG)定子短路电流解析表达,将定子磁链当作一阶直流衰减分量或忽略功率外环控制。基于DFIG电压、磁链和RSC控制方程,得到定子电流关于定子电压和定子功率的传递函数,提出定子电流的精确解析表达式。基于RSC内、外环PI参数关系,推导直流分量衰减时间常数和角频率关于PI参数的表达式。分析了RSC内外环PI参数对定子电流直流衰减分量的影响。仿真结果验证了解析表达式的准确性,为PI参数选取和保护装置测量、整定提供依据。     

Effectiveness evaluation of passive resistive element placement on a fault ride‐through enhancement in a DFIG‐based wind energy conversion system

Over the last few decades, doubly fed induction generators (DFIG) have become one of the most successful and preferred types of wind energy generators (WEG). The DFIG has the advantages of a wide range of speed operations, a high efficiency, and partial rated converters. However, direct coupling of the stator with the grid makes the system more prone to grid disturbances. The consequences of grid disturbances, such as a rotor overcurrent, stator overcurrent, electromagnetic torque oscillations, and direct current (DC) link overvoltage, are the predominant considerations that affect the rotor circuit, stator circuit, mechanical components, and DC‐link capacitor of the DFIG, respectively. These uncertainties affect the operation of the generator and may lead to the generator to be shutdown. In this paper, a novel position for the placement of a passive resistive element (PRE) is illustrated. This position of the PRE placement is compared with all other possible locations for the PRE. The different locations for PRE placement are the stator side, rotor side, across the DC‐link capacitor, and between the rotor side converter (RSC) and grid side converter (GSC). This paper aims to determine a cost‐effective solution among all possible locations of the PRE placement. The novel position of the PRE, ie, between the RSC and GSC, is the best position among the other possible locations of the PRE, when the performance, cost, and loss are taken into consideration. The effectiveness of the PRE is further compared with the resistive‐type superconducting fault current limiter (R‐SFCL). The PRE performs better and has a lower cost than the R‐SFCL.     

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。     

双馈风力发电系统的最大风能控制策略

在分析变速恒频风力发电系统最大风能捕获策略的基础上,提出了一种通过直接控制双馈发电机转子电流就可实现系统低于额定风速下的最大风能捕获又可使得电机铜耗最小化运行的控制策略.首先,在考虑风力机特性和双馈发电机基本电磁关系的基础上,分别推导了双馈发电机定子铜耗最小化运行和最大风能捕获的数学模型.其次,根据双馈发电机最优转子电流的数学模型,建立了双馈风力发电机系统最大风能捕获的控制策略.最后,利用Matlab/Simulink对不同风速下双馈发电机系统的运行性能进行了分析和比较,结果验证了该控制策略的正确性和可行性.     

Short‐circuit analysis of a doubly fed induction generator wind turbine with direct current chopper protection

Modern doubly fed induction generator (DFIG) wind turbines can ride through a symmetrical fault in the network by using a chopper protection on the direct current (DC) link without triggering a crowbar protection. A novel method to model the DC link system of such wind turbines as an equivalent resistance during symmetrical faults is presented in this paper. The method allows looking at the DFIG with chopper protection as to one with an equivalent crowbar protection and, hence, to apply to the former type of DFIG short‐circuit calculation methods developed for a DFIG with crowbar protection. This may be a valid help in short‐circuit calculations, for example, for protection settings. It also allows simulating for short‐circuit studies a DFIG with chopper protection, often not available in a standard power system simulation software, by using an equivalent DFIG with crowbar protection, which is a standard model in power system simulation software. The results for the short‐circuit current obtained through the proposed method are compared with simulations of a detailed model of a DFIG with chopper protection under different conditions, which showed good agreement. It is also shown that the DFIG with chopper protection delivers lower short‐circuit current than a DFIG with standard crowbar protection, especially for low initial loading. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Modelling analysis of transient stability simulation with high penetration of grid‐connected wind farms of DFIG type

This paper shows a model of a doubly fed induction generator (DFIG) including a simplified model of a wind turbine for the purpose of transient stability analysis of large‐scale power systems with great wind farms penetration. The wind turbine model and the DFIG model are systematically deducted in this paper. Specially, the improved model of rotor‐side converter and the simplified grid‐side converter model are considered in our work. The corresponding machine–network interface solution based on the synchronously rotating common x‐y reference frame is elaborately issued in this paper. Furthermore, a method is proposed to calculate the DFIG initial conditions as well. A simplified model of the turbine is used excluding among other components the pitch control. Copyright © 2007 John Wiley & Sons, Ltd.     

Reactive power compensation and active filtering capability of WECS with DFIG without any over‐rating

This paper presents a novel approach for reactive power compensation and active filtering capability of a variable speed wind energy conversion system (WECS) with doubly fed induction generator (DFIG), without any over‐rating. First, the WECS is capable of capturing maximum wind power under fluctuating wind speed. Second, depending on the available wind power value versus nominal WECS power, power quality can be improved by compensating the reactive power and the grid harmonic currents, without any system over‐rating. The proposed rotor side converter (RSC) control manages the WECS function's priorities, between main active power generation and power quality management. To ensure high filtering performances, we used an improved harmonic isolator in the time domain, based on a selective pass band filter (SPBF) developed in our laboratory. Moreover, we took advantage of the high amplification effect of the rotor side‐controlled DFIG to compensate harmonic currents. Consequently, no over‐rating is necessary for the proposed additional active filtering capability. Simulation results for a 2 MW WECS with DFIG confirm the effectiveness and the performances of the proposed approach. Copyright © 2009 John Wiley & Sons, Ltd.     

Synchronous mode operation of DFIG based wind turbines for improvement of power system inertia

Inertia emulation methods exist to compensate for the reduced inertial support provided by doubly fed induction generator (DFIG) based wind turbines. Instead of emulating inertia, this paper proposes to temporarily convert DFIGs to synchronous generators, enabling supply of real inertia to the system. In order to achieve this, the voltage supplied to the DFIG rotor needs to be made independent of the grid frequency. Feeding the rotor with a fixed dc voltage while it is rotating at synchronous speed enables the DFIG to operate in synchronism with the grid and couple the inertia of its rotating mass to the power system. The rotor side converter of a DFIG can be controlled to function as the dc voltage source, allowing convenient switching between the two operation modes according to system requirements.     

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     

Theory and performance of series connected self-excited synchronousgenerators

The series connected self-excited synchronous generator is a slip ring type induction machine with stator and rotor windings connected in series along with excitation capacitors. The machine, when self-excited, will yield an output voltage with a frequency equal to half the rotor angular speed. This paper presents analytical as well as experimental investigations into machine performance. The analysis is based on a deduced phasor diagram. The suggested method of analysis is simple and makes it possible to study the effect of machine parameters on its performance. Useful conclusions are given showing proper design considerations to be accounted for to allow the machine to develop acceptable output levels