Dynamic modeling and direct power control of wind turbine driven DFIG under unbalanced network voltage conditions

This paper proposes an analysis and a direct power control (DPC) design of a wind turbine driven doubly-fed induction generator (DFIG) under unbalanced network voltage conditions. A DFIG model described in the positive and negative synchronous reference frames is presented. Variations of the stator output active and reactive powers are fully deduced in the presence of negative sequence supply voltage and rotor flux. An enhanced DPC scheme is proposed to eliminate stator active power oscillation during network unbalance. The proposed control scheme removes rotor current regulators and the decomposition processing of positive and negative sequence rotor currents. Simulation results using PSCAD/EMTDC are presented on a 2-MW DFIG wind power generation system to validate the feasibility of the proposed control scheme under balanced and unbalanced network conditions.

PI-MPC Frequency Control of Power System in the Presence of DFIG Wind Turbines

For the recent expansion of renewable energy applications, Wind Energy System (WES) is receiving much interest all over the world. However, area load change and abnormal conditions lead to mismatches in frequency and scheduled power interchanges between areas. These mismatches have to be corrected by the LFC system. This paper, therefore, proposes a new robust frequency control technique involving the combination of conventional Proportional-Integral (PI) and Model Predictive Control (MPC) controllers in the presence of wind turbines (WT). The PI-MPC technique has been designed such that the effect of the uncertainty due to governor and turbine parameters variation and load disturbance is reduced. A frequency response dynamic model of a single-area power system with an aggregated generator unit is introduced, and physical constraints of the governors and turbines are considered. The proposed technique is tested on the single-area power system, for enhancement of the network frequency quality. The validity of the proposed method is evaluated by computer simulation analyses using Matlab Simulink. The results show that, with the proposed PI-MPC combination technique, the overall closed loop system performance demonstrated robustness regardless of the presence of uncertainties due to variations of the parameters of governors and turbines, and loads disturbances. A performance comparison between the proposed control scheme, the classical PI control scheme and the MPC is carried out confirming the superiority of the proposed technique in presence of doubly fed induction generator (DFIG) WT.

Small-Signal Stability Analysis of Wind Power System Based on DFIG

This paper focuses on the small-signal stability of power system integrated with DFIG-based wind farm. The model of DFIG for small-signal stability analysis has built;the 3-generator 9-bus WECC test system is modified to investigate the impacts of large scale integration of wind power on power system small-signal stability. Different oscillatory modes are obtained with their eigenvalue, frequency and damping ratio, the results from eigenvalue analysis are presented to demonstrate the small-signal stability of power system is enhanced with the increasing output of the wind farm.

Probabilistic Load Flow Algorithm with the Power Performance of Double-Fed Induction Generators

Probabilistic load flow(PLF)algorithm has been regained attention,because the large-scale wind power integration into the grid has increased the uncertainty of the stable and safe operation of the power system.The PLF algorithm is improved with introducing the power performance of double-fed induction generators(DFIGs)for wind turbines(WTs)under the constant power factor control and the constant voltage control in this paper.Firstly,the conventional Jacobian matrix of the alternating current(AC)load flow model is modified,and the probability distributions of the active and reactive powers of the DFIGs are derived by combining the power performance of the DFIGs and the Weibull distribution of wind speed.Then,the cumulants of the state variables in power grid are obtained by improved PLF model and more accurate power probability distributions.In order to generate the probability density function(PDF)of the nodal voltage,Gram-Charlier,Edgeworth and Cornish-Fisher expansions based on the cumulants are applied.Finally,the effectiveness and accuracy of the improved PLF algorithm is demonstrated in the IEEE 14-RTS system with wind power integration,compared with the results of Monte Carlo(MC)simulation using deterministic load flow calculation.

Analysis and Simulation of a Crowbar Protection for DFIG Wind Application during Power Systems Disturbances

The performance of a 1.5 MVA wind-power Doubly Fed Induction Generator （DFIG） under network fault is studied using simulator developed in MATLAB-SIMULINK. This paper investigates a new control method able to improve the fault-ride through capability of DFIG. In such generators the appearance of severe voltage sags at the coupling point make highlight important over currents at the rotor/stator windings, making the use of crowbar protection device necessary and inevitable in order to protect the machine as well as the rotor side power converter. The simulator consists of the DFIG analytical model, power transformer model and the detailed frequency converter model including crowbar protection device. Simulation results are carried out to show the transient behavior of the DFIG when a sudden voltage dip is introduced with and without the crowbar implementation.

并网DFIG多通道附加阻尼控制器设计及其控制参数整定

针对双馈感应发电机(DFIG)并网电力系统中多低频振荡(LFO)抑制的问题,提出一种基于DFIG的多通道附加阻尼控制器(MBSDC)设计及其参数整定方法,用于增加系统多LFO模式的阻尼。建立DFIG并网系统状态矩阵,由特征值分析筛选出待抑制的LFO模式,并设计了MBSDC控制策略。在DFIG-MBSDC系统线性化模型基础上,推导目标LFO模式对相应抑制通道传递函数灵敏度的解析表达,以量化多LFO模式与MBSDC传递函数间的联系,应用于控制参数整定。特征值分析与时域仿真结果表明,所提DFIG-MBSDC能抑制DFIG系统中多LFO模式。基于传递函数灵敏度对控制参数进行整定,提高多LFO的抑制效果。

基于PMU实测数据的DFIG风电场等值模型鲁棒性与适应性分析

对传统聚合方法无法解决风电场长期运行过程中产生的参数变化问题,该文基于实测数据建立双馈感应发电机(DFIG)风电场详细等值模型与初始化方法,分析时变参数的轨迹灵敏度,提出首先将非时变参数固定为聚合值,然后利用风电场公共并网点相量测量单元(PMU)数据针对时变参数采用基因学习粒子群(GLPSO)混合算法进行参数辨识的策略。采用WECC标准算例分析不同风速、尾流效应、部分风电机组离线、风速未知、不同短路故障位置与电压暂降深度情形下等值模型的鲁棒性与适应性。仿真算例表明所提出的参数辨识方法全局寻优能力远高于标准粒子群与遗传算法。而且,对于高灵敏度参数,参数辨识结果与真实值的最大偏差小于10%,远优于目前技术水平。

A Sliding Mode Approach to Enhance the Power Quality of Wind Turbines Under Unbalanced Voltage Conditions

An integral terminal sliding mode-based control design is proposed in this paper to enhance the power quality of wind turbines under unbalanced voltage conditions. The design combines the robustness, fast response, and high quality transient characteristics of the integral terminal sliding mode control with the estimation properties of disturbance observers. The controller gains were auto-tuned using a fuzzy logic approach.The effectiveness of the proposed design was assessed under deep voltage sag conditions and parameter variations. Its dynamic response was also compared to that of a standard SMC approach.The performance analysis and simulation results confirmed the ability of the proposed approach to maintain the active power,currents, DC-link voltage and electromagnetic torque within their acceptable ranges even under the most severe unbalanced voltage conditions. It was also shown to be robust to uncertainties and parameter variations, while effectively mitigating chattering in comparison with the standard SMC.

Analysis of Short-circuit Characteristics and Calculation of Steady-state Short-circuit Current for DFIG Wind Turbine

Large-scale doubly-fed induction generator(DFIG)wind turbines are connected to the grid and required to remain grid-connection during faults,the short-circuit current contributed by the generation has become a significant issue.However,the traditional calculation methods aiming at synchronous generators cannot be directly applied to the DFIG wind turbines.A new method is needed to calculate the short-circuit current required by the planning,protection and control of the power grid.The short-circuit transition of DFIG under symmetrical and asymmetric short-circuit conditions are mathematically deduced,and the short-circuit characteristics of DFIG are analyzed.A new method is proposed to calculate the steady-state short-circuit current of DFIG based on the derived expressions.The time-domain simulations are conducted to verify the accuracy of the proposed method.

Protection of Large-scale Wind Power Integration

More and more large capacity wind power will be integrated into power system in the future,and certain technical challenges will emerge due to the fluctuation characteristics of wind power and the complex control of power electronic devices inside the wind turbines(e.g.,low voltage ride through(LVRT)).By comparing a wind power integration grid with a hydropower integration grid,the special transient phenomena caused by the wind power integration is studied and simulation results are presented.Furthermore,the potential impacts on the traditional protection are discussed.Results show that the special transient phenomena can decrease the sensitivity,reliability and operation speed of conventional protections.

Power Quality Improvement Using Grid Side Converter of Wind Energy Conversion System

This paper presents the control ofa WECS （wind energy conversion system）, equipped with a DFIG （doubly fed induction generator）, for maximum power generation and power quality improvement simultaneously. The proposed control algorithm is applied to a DFIG whose stator is directly connected to the grid and the rotor is connected to the grid through a back-to-back AC-DC-AC PWM （pulse width modulation） converter. The RSC （rotor side converter） is controlled in such a way to extract a maximum power, for a wide range of wind speed. The GSC （grid side converter） is controlled in order to filter harmonic currents of a nonlinear load coupled at the PCC （point of common coupling） and ensure smooth DC bus voltage. Simulation results show that the wind turbine can operate at its optimum energy for a wide range of wind speed and power quality improvement is achieved.

计及低电压穿越及故障全过程动态的双馈风电场等值方法

在分析双馈感应发电机(DFIG)故障各阶段动态的基础上,提出了一种计及低电压穿越控制的双馈风电场等值方法。首先,讨论了低电压穿越控制策略,给出了含双馈风电场的电力系统故障稳态潮流计算方法。在考虑电压暂态过程的条件下,分析了外部系统短路故障后DFIG转子电流的变化机理,指出电压跌落深度及风电功率是影响短路电流的主要因素,可根据动作分界线判断撬棒是否动作。然后,根据初始风速及撬棒状态将风电场中的DFIG分为3群,并对等值DFIG及集电网络进行聚合。为提高等值模型在恢复阶段的精度,根据故障稳态电压对双馈风电场功率恢复曲线进行预估,并在此基础上对等值DFIG恢复速率进行分段修正。最后,对含双馈风电场的算例系统进行了等值计算。仿真结果表明,等值模型较好地保持了原系统在故障各阶段的动态,在保持较高精度的前提下有效简化了系统,提高了计算速度。

DFIG风电机组串联耦合补偿低压穿越研究

双馈风电机组(doubly fed induction generator,DFIG)在电网电压故障以及故障恢复作用下的磁链变化将导致转子过电压或过电流,威胁转子侧的变换器。采用串联耦合补偿(series coupled compensation,SCC)的新型电压穿越方案,SCC通过耦合变压器串接在发电机输入端,它能克服电网多种故障对风电系统的影响。提出的控制策略有如下优势:能削弱故障下机组定子、转子电流暂态成分,克服了定、转子绕组不平衡发热的问题,延长了风电机组使用寿命;在电网故障下发电机组仍能平稳输出有功、无功功率;在整个故障运行过程中,DFIG系统转子侧变换器始终可控,从而为故障电网提供无功功率支持成为可能。在电网对称和非对称故障条件下,采用PSCAD/EMTDC建立了1MW DFIG风力发电系统模型,仿真结果表明该控制方案可以提高DFIG的故障穿越能力。

基于撬棒控制策略的DFIG风电场动态等值建模研究

为研究双馈风电机组(doubly fed induction generator,DFIG)低电压穿越对风电场动态等值的影响,提出了一种基于撬棒控制策略的双馈风电场动态等值建模方法。首先通过对DFIG在电网故障时的转子电流分析,给出了时域下转子暂态电流表达式。然后研究撬棒电路两种常用控制策略在不同电压跌落程度及DFIG运行状况下对风电机组低电压穿越的影响,仿真结果表明两种控制策略均有各自的适用范围。在此基础上,文中提出了撬棒电路投切曲线及控制策略选择区域曲线,以此判断电网故障时撬棒电路投切以及投切时控制策略选择情况。考虑故障前一时刻风电场机群划分情况,在电网故障时以实测风速与撬棒电路是否动作共同作为分群指标,将撬棒电路动作机组从故障前原"N"个机群中独立出来形成"N+1"个机群,利用容量加权法计算各机群参数,建立多机等值模型。最后在MATLAB/Simulink平台上搭建算例,验证了该等值方案的有效性和准确性。

基于量测信号扰动的DFIG变流器控制参数辨识方法

双馈感应发电机(DFIG)的变流控制器模型是整个机组模型中的关键部分。变流控制器的参数众多且内外环比例—积分(PI)控制器之间存在级联,常规辨识方法或遇到参数可辨识性问题,或工程实现难度较大。文中提出一种以"分步辨识、交互迭代"为特点,基于二次侧量测信号扰动的控制器参数辨识方法。该方法分别在变流控制器中各PI控制器输入端所用的量测信号上施加扰动,从而突出某个PI控制器在特定量测信号扰动下的作用,并将该PI控制器的可测量控制目标作为观测变量,进而重点辨识其参数。对于存在级联的两个PI控制器,先对后一级PI控制器所用量测信号施加扰动,同时屏蔽前一级PI控制器所用量测信号的变化,从而可实现两个级联PI控制器参数的解耦辨识。该方法可以使变流控制器中每个PI控制器的两个参数在特定的量测信号扰动下被单独辨识,有效保证了参数的可辨识性;而对于全部待辨识参数,可通过多轮交互迭代辨识进一步提高整体辨识精度。

应用于DFIG风电场的VSC-HVDC控制策略

随着风力发电的快速发展,如何连接风电场和电网成为至关重要的问题。研究了基于双馈感应电机(DFIG)的风电场连接方式及控制策略,通过与交流输电和电流源型高压直流输电(CSC- HVDC)的比较,说明电压源型高压直流输电(VSC-HVDC)技术在连接风电场与电网方面具有独特优势。在换流站的dq解耦控制基础上,对DFIG与VSC-HVDC的换流站的控制设计进行了研究。利用PSCAD/EMTDC平台对该方案进行了仿真研究。仿真结果表明,该方案可以实现一个换流站连接多台DFIG的结构,并具有良好的控制效果。

适用于全故障类型的双馈风电场送出线负序电流纵联保护方案

随着双馈风电机组(DFIG)并网容量的增加,给风电场送出线继电保护的整定带来新的挑战,保护动作性能下降甚至会出现拒动和误动的风险。为此,首先分析双馈风电机组负序电流分量特征;然后进一步分析风电场经交、直流并网时交流送出线两侧电流的特征差异,提出一种适用于双馈风电场送出线的负序电流分量纵联保护新方法。该方法利用双馈风电场送出线两侧负序电流的幅值比和相位差构成保护动作判据,同时,利用数据延时在对称故障下构造出负序分量,使其能适用于风电场送出线的各种故障类型,且具有灵敏度高、耐受过渡电阻能力强等优点。最后,在Matlab/Simulink和PSCAD/EMTDC中搭建双馈风电机组电磁暂态仿真模型,仿真分析保护方案在交直流送出线上的适应性。

采用串联网侧变换器的DFIG风电系统低电压穿越控制

双馈感应发电机(DFIG)因其对电网故障的敏感性,其低电压穿越运行控制技术备受关注。文中针对定子侧加装串联网侧变换器的新型DFIG风电系统,详细研究了其低电压穿越运行特性,提出了一种适用于新拓扑下发电系统的运行控制方案:通过控制串联网侧变换器抑制定子磁链暂态直流分量及电网电压负序分量对发电机转子侧的不良影响;通过控制转子侧变换器进一步限制故障时发电机的定、转子电流,从而实现发电系统的低电压穿越运行。仿真结果表明:采用所提出的控制方案,可实现电网故障时DFIG风电系统的零电压穿越运行;采用新拓扑的DFIG风电系统具有优良的电网对称及非对称短路故障穿越能力。

配置STATCOM的DFIG风电场在不对称电网故障下的控制策略

该文研究在不对称电网故障穿越的第2阶段,双馈式(doubly-fed induction generator,DFIG)风电场如何实现"自身不脱网安全运行并同时满足并网导则无功支撑要求"的问题。由于受限于变流器的容量,DFIG机组无法保证在所有故障情况下实现既定的控制目标。为此,通过引入最优化的方法,分析了变流器容量限制条件下DFIG机组的可控性问题,得到了以电网电压不平衡度所量化表示的可控区,指出为确保在不对称跌落故障较严重时能满足并网导则要求,有必要在DFIG风电场层面配置静止同步补偿器(staticsynchronous compensator,STATCOM)以降低场内DFIG机组的机端电压不平衡度。最后,设计了STATCOM所需容量最小的电压补偿控制策略,并给出了相应的容量匹配方法。仿真结果验证了该文提出的在不对称低电压穿越(lowvoltage ride-through,LVRT)第2阶段DFIG机组可控区的正确性、STATCOM辅助控制策略的有效性以及相应容量匹配方法的可行性。