Reliability assessment of bulk electric systems containing large wind farms

Wind power is an intermittent energy source that behaves quite differently than conventional energy sources. Bulk electric system reliability analysis associated with wind energy conversion systems (WECS) provides an opportunity to investigate the reliability benefits when large-scale wind power is injected at specified locations in a bulk electric system. Connecting the WECS to different locations in a bulk system can have different impacts on the overall system reliability depending on the system topology and conditions. Connecting a large-scale WECS to an area which has weak transmission could create system operating constraints and provide less system benefit than connecting it to an area with stronger transmission. This paper investigates bulk electric system transmission constraints associated with large-scale wind farms. The analyses presented in this paper can be used to determine the maximum WECS installed capacity that can be injected at specified locations in a bulk electric system, and assist system planners to create potential transmission reinforcement schemes to facilitate large-scale WECS additions to the bulk system. A sequential Monte Carlo simulation approach is used as this methodology can facilitate a time series modeling of wind speeds, and also provides accurate frequency and duration assessments. An auto-regressive moving average (ARMA) time series model is used to simulate hourly wind speeds.     

Reliability-Based Transmission Reinforcement Planning Associated With Large-Scale Wind Farms

The reliability impacts of a highly variable energy source such as wind power is an important aspect that needs to be assessed as wind power penetration becomes increasingly significant. Bulk electric system (BES) reliability analysis associated with wind energy conversion systems (WECS) provides an opportunity to investigate the reliability benefits at potential BES connection points in close proximity to the wind power development area. A bulk electric system can encounter transmission capacity limitation problems when a large-scale WECS is connected to a weak transmission area. In this case, transmission reinforcement may be required in order to increase the system capability to absorb more wind power at specified locations. BES reliability analysis associated with large-scale wind farms as demonstrated in this paper can assist system planners to create potential transmission reinforcement schemes to facilitate large-scale WECS additions to a bulk system. Reliability cost/worth analysis is also incorporated in the examination of reinforcement alternatives. A sequential Monte Carlo simulation approach is used as this methodology can facilitate a time series modeling of wind speeds and also provides accurate frequency and duration assessments. An auto-regressive moving average (ARMA) time series model is used to simulate hourly wind speeds     

基于风速相关性风电场储能容量配置定量研究

为研究风速相关性对风电场储能容量的影响,首先建立了风速ARMA模型,并根据时移技术得到风电机组风速分别呈高度、中度和低度三种不同相关度的风电场的风速时间序列,最后对不同相关度下的风电场的储能容量进行定量分析。结果表明,合理布置风电场能在较好利用风能的情况下有效减少风电储能成本和土地资源的浪费。     

Capacitor voltage balancing in cascaded H-bridge multilevel inverter and its modelling analysis for grid integrated wind energy conversion system application

In this study, a comprehensive control scheme for cascaded H-bridge multilevel inverter (CHBMLI)-based grid integrated bulk wind energy conversion system (WECS) addressing the problem of deviation of wind speeds among windmills like a partial shading condition of PV system is presented. The proposed control scheme uses independent dc links with reduced voltages that makes such a topology an ideal candidate for high and medium power WECS with improved reliability. Inconsistency of wind speeds at each turbine causes distinct voltage conditions among the isolated dc links of the CHBMLI H-bridge cells (HBC) leading to unstable power generation. The projected scheme along with maximum and efficient energy conversion has the ability of dc link capacitor voltage balancing in each HBC of CHBMLI during the above said power generation mismatch conditions and also maintains power quality in the grid side voltage and injected currents as per standards. Moreover, the modelling analysis of the adopted CHBMLI for the grid integrated WECS application has also been derived. The system performance under inconsistent wind speed environment has been tested and analysed by using MATLAB simulation and validated by using FPGA-based real-time simulator (Opal-RT-OP5700).     

风能大规模利用对电力系统可靠性的影响

风是一种非常易变的能源,其表现与其它形式的能源有很大不同。该文使用时序蒙特卡洛模拟法,提出一种含风能电力系统发电容量充裕度评估的方法,使用一个有代表性的可靠性测试系统(reliability test system,RBTS)阐述了该方法,此系统既含有常规发电机组也含有风能转换系统(wind energy conversion system,WECS)。对含有一个风场和多个风场混合系统的可靠性进行分析计算,结果表明,风能利用具有很大的可靠性效益,使用所提方法,可以量化这些效益。     

A developed control strategy for mitigating wind power generation transients using superconducting magnetic energy storage with reactive power support

The fast variations of wind speed during extreme wind gusts result in fluctuations in both generated power and the voltage of power systems connected to wind energy conversion system (WECS). This paper presents a control strategy which has been tested out using two scenarios of wind gusts. The strategy is based on active and reactive powers controls of superconducting magnetic energy storage (SMES). The WECS includes squirrel cage induction generator (SCIG) with shunt connected capacitor bank to improve the power factor. The SMES system consists of step down transformer, power conditioning unit, DC–DC chopper, and large inductance superconducting coil. The WECS and SMES are connected at the point of common coupling (PCC). Fuzzy logic controller (FLC) is used with the DC–DC chopper to control the power transfer between the grid and SMES coil. The FLC is designed so that the SMES can absorb/deliver active power from/to the power system. Moreover, reactive power is controlled to regulate the voltage profile of PCC. Two inputs are applied to the FLC; the wind speed and SMES current to control the amount active and reactive power generated by SMES. The proposed strategy is simulated in MATLAB/Simulink®. The proposed control strategy of SMES is robust, as it successfully controlled the PCC voltage, active and reactive powers during normal wind speeds and for different scenarios of wind gusts. The PCC voltage was regulated at 1.0 pu for the two studied scenarios of wind gusts. The fluctuation ranges of real power delivered to the grid were decreased by 53.1% for Scenario #1 and 56.53% for Scenario #2. The average reactive power supplied by the grid to the wind farm were decreased by 27.45% for Scenario #1 and 31.13% for Scenario #2.     

Sliding mode control of a dual-stator induction generator for wind energy conversion systems

This paper presents a sliding mode control (SMC) associated to the field oriented control (FOC) of a dual-stator induction generator (DSIG) based wind energy conversion systems (WECSs). The DSIG has two sets of stator three-phase windings spatially shifted by 30 electrical degrees. The study of operation of the wind turbine leads us to two essential cases: optimization of the power for wind speeds lower than the nominal speed of the turbine and limitation of the power for higher speeds. Conventional electrical grid connected WECS present interesting control demands, due to the intrinsic nonlinear characteristic of wind mills and electric generators. The SMC is a robust nonlinear algorithm which uses discontinuous control to force the system states trajectories to join some specified sliding surface, it has been widely used for its robustness to model parameter uncertainties and external disturbances, is studied. In order to verify the validity of the proposed method, a dynamic model of the proposed system has been simulated, to demonstrate its performance.     

一种新颖的模糊自耦合PI在风力机MPPT控制中的应用

针对风速随机性给风能转换系统(wind energy conversion system, WECS)带来的非线性和参数不确定性,提出了一种模糊自耦合PI控制方案用于低风速的最大功率点跟踪。自耦合PI被用来完成基本的转速跟踪,以实现对风力机尖速比的最优化控制。而模糊控制器则被用来获取自耦合PI在不同工作点下的控制参数,以提高系统对风速变化的适应能力。为了验证所提方案的可行性,在Matlab/Simulink搭建的2?MW风能转换系统仿真模型中开展了与传统方法的对比实验。仿真结果表明,相比于传统PI、模糊PI以及GA-PI,所提出方法拥有更佳的转速跟踪性能、更平滑的响应曲线以及更多的电能输出。     

我国冬季风路径上风速与其影响因子的优化延迟相关性分析

为提高超短期和短期风速预测的准确率和可靠性,分析了我国冬季风主要路径上7个代表性地点的风速与气压、风向、气温等的相关性变化。首先,根据相关性/距离指标的性质,采用Pearson相关系数等来计算风速与其影响因子之间的相关性。采用样本交叉相关函数SCCF来分析各地风速之间的平均优化延迟时间。之后,计算了时间窗口上风速与其影响因子第一主成分之间的相关性及其优化延迟时间。结果表明:台湾海峡是特别适合空间相关性超短期预测的风能丰富区域。并且采用北京的气压、风速、风向和气温作为影响因子来预测澳仔的冬季风风速,预测误差对最优延迟时间的依赖性并不明显。北京对澳仔的气压差值,对预测误差的影响最为重要。     

风速时间序列的符号化描述

利用统计分析技术,对已知的时间序列外推,可以克服短期风速预测中缺乏因果关系的困难。但在选择外推模型、参数及学习样本等方面存在主观认识模糊性的挑战。为降低主观认识模糊性对分类预测效果的影响并提高样本分类效率,提出按变化特征来定义符号,以及用符号串描述风速时间序列的粗粒化概念。在此基础上,引入趋势特征,完善风速时间序列的符号化过程,提出单元窗口特征和趋势特征相结合的两层符号化方法。利用甘肃酒泉风电基地一年的实际数据验证了该粗粒化方法的有效性。     

ESTIMATION OF ROTOR VOLTAGE VECTOR ON THE DOUBLE EXCITED INDUCTION MACHINE USED IN WECS

ABSTRACT

The aim of this work is to estimate and control the rotor voltage of a double excited induction machine DEIM that is necessary to operate it as a generator in wind energy conversion systems (WECS), with speed range extending from the sub synchronous range to double the synchronous speeds. For efficient power generation, the DEIM is to be operated at its rated electric torque which is realised by having both rotor and stator currents magnitudes constant throughout the whole operating range. Also the angle between the rotor current and stator current is kept constant irrespective of the rotor speed.     

含具有风速相关性风电场的发输电系统可靠性评估

提出了一种改进相关矩阵蒙特卡罗模拟法与拉丁超立方采样法相结合的多维相关风速抽样方法,并把该方法应用于含多个风电场的发输电系统可靠性评估。通过实际风速数据对模型的验证表明：提出的风速相关模型能够充分保持实际风速数据的概率分布,基本统计量和相关特性。将该模型和方法应用到 MRTS可靠性测试系统中,证明了该方法适用于含具有风速相关性的风电场的发输电系统可靠性评估。并通过算例,对风速相关性程度、风电场接入位置和风能渗透水平三个因素对含风电场的发输电系统可靠性的影响进行了分析。     

Effective MPPT technique and robust power control of the PMSG wind turbine

Wind generator performance improvement requires sophisticated and robust control techniques to overcome various constraints and achieve optimal aerodynamic energy conversion. Because of the continuously changing nature of the wind, the output power of a wind energy conversion system (WECS) can be maximized if the wind rotor is driven at an optimal rotational speed for a particular wind speed. This is achieved with a maximum power point tracking (MPPT) controller. Over the years, a variety of MPPT studies have been made, but very few provide guidelines to single out the most suited MPPT technique. In this paper, a comprehensive comparison of the four most used schemes has been made in relation to a permanent magnet synchronous generator (PMSG) wind turbine system. These techniques can be classified into various categories. The obtained results show clearly the superiority of the fuzzy logic control (FLC) technique. Using this MPPT approach, the generated power by the turbine is considered to be, in terms of power control, an auxiliary source feeding a grid. To achieve smooth regulation of the active and reactive power exchange between the PMSG and the grid, direct power control, using space vector modulation DPC‐SVM strategy combined with sliding mode control (SMC) is applied in the grid‐side converter. Simulation results show the efficiency and reliability of the control strategy proposed in this paper. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

直驱六相风力发电系统无速度传感器研究

采用六相永磁同步发电机(PMSG)结合双变频器并联方案以提高直驱式风电系统(WECS)的机组容量和可靠性.为消除系统因测速码盘带来的故障,采用无速度传感器矢量控制方案.给出基于模型参考自适应系统的永磁电机转速和转子位置估计方法,提出一种将最优叶尖速法与爬山法相结合的风力发电最佳风能捕获方法.采用六相电机矢量控制策略,对风速突变时六相电机的最佳风能追踪过程进行仿真并在此基础上,设计了一套风电变流器控制系统,实验结果表明,该变流器控制系统具有良好的动、静态控制特性,为整个风电控制系统提供了基础.     

风/柴/储能系统发电容量充裕度评估

提出一种用于含风能和能量储存设备的小型孤立电力系统(SIPS)发电容量充裕度评估仿真方法。该仿真方法根据每小时计算的随机事件模拟发电系统的运行历史记录,考虑现场风资源的时序性、系统中发电机组的故障和修复特性。使用几个样例系统说明了该方法的应用,该系统的充裕度取决于许多因素,它们是能量储存容量、系统负荷需求、风能注入水平、发电机组强迫停运率(FOR)和其地理位置。能量储存设备对该系统可靠性能有积极的影响;SIPS随着系统负荷的增加,充裕度减少;风力发电机组FOR的变化对系统充裕度的影响不大;增加风能注入水平可以改善可靠性;位于高平均风速处的系统明显比低平均风速处的系统可靠性高。     

Maximum power tracking control for a wind energy conversion system based on a quasi‐ARX neural network model

By itself, a wind turbine is already a fairly complex system with highly nonlinear dynamics. Changes in wind speed can affect the dynamic parameters of wind turbines, thus rendering the parameters uncertain. However, we can identify the dynamics of the wind energy conversion system (WECS) online by a quasi‐ARX neural network (QARXNN) model. A QARXNN presents a problem in searching for the coefficients of the regression vector (input vector). A multilayer perceptron neural network (MLPNN) is an embedded system that provides the unknown parameters used to parameterize the input vector. Fascinatingly, the coefficients of the input vector from prediction model can be set as controller parameters directly. The stability of the closed‐loop controller is guaranteed by the switching of the linear and nonlinear parts of the parameters. The dynamic of WECS is derived with given parameters, and then a wind speed signal created by a random model is fed to the system causing uncertainty parameters and reducing the power that can be absorbed from wind. By using a minimum variance controller, the maximum power is tracked from WECS. From the simulation results, it is observed that the proposed controller is effective in tracking the maximum power of WECS. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

基于随机响应面法的含风电电力系统小扰动稳定性分析

风力发电的不确定性对电力系统小扰动稳定性造成一定影响,而地理位置相近的风电场间的风速具有一定的相关性。本文建立双馈风电机组的动态模型,利用Nataf变换建立考虑风速相关性的风电场群模型,运用随机响应面法(SRSM),以风速作为输入,系统特征值、阻尼比、频率作为输出,分析了系统的小扰动稳定性,同时应用基于线性无关原则的概率配点选取法,合理选择有效配点,提高了SRSM的计算效率。在10机39节点系统中将SRSM与蒙特卡罗法比较,结果显示SRSM计算结果准确可靠,计算时间远远少于蒙特卡罗法的计算时间,能够实现系统小扰动稳定性分析的快速评估。在是否考虑风速相关性两种情况下进行对比,结果表明:考虑风速相关性时系统小扰动分析结果的波动更小,对系统安全稳定运行具有更明确的指导意义。     

基于Johnson分布体系的含风电场发电系统可靠性评估

提出了一种基于Johnson分布体系的多维变量建模方法,并将其应用于含风电场发电系统年度可靠性指标计算中。该方法利用风速历史数据和持续负荷曲线构建风速和负荷与标准正态变量的关系,即模拟风速和负荷的概率分布特征,进而通过模拟相关多维标准正态变量来实现对风速相关性的模拟,避免了因假设风速概率分布模型可能导致模拟精度低的缺点。通过算例分析,验证了算法在模拟负荷和风速分布时的精度以及应用于含风电场发电系统可靠性评估中的适用性。在此基础上分析了风电场风速相关性对含风电场发电系统可靠性指标的影响。     

计及电网频率波动敏感特性的双馈风电机组优化发电控制策略

为协调优化提高风电机组发电量目标和限制机组功率波动目标,提出一种带滤波器的改进转矩控制策略.在风力机空气动力学、机组转子运动学和风机控制原理的基础上,考虑自然风频谱特征和电网调频敏感频段,推导并建立了机组转速和输出功率对风速的小信号分析模型.基于工作点处转速和功率对风速的传递函数,对比分析了常规转矩控制和改进转矩控制的...     

使用无速度传感器的PMSG单位功率因数控制

使用永磁同步发电机(PMSG)的直驱风电系统目前发展很快,PMSG的控制对系统性能非常重要,其中无速度传感器控制也受到了广泛关注。为推进此一技术,采用单位功率因数控制方法控制永磁同步发电机以降低永磁直驱风电系统变流器的功率等级;采用基于锁相环的无速度传感器控制策略对电机转速和相位进行观测。实验结果表明在永磁直驱风电系统中,单位因数控制可以有效降低变流器容量,无速度传感器控制可以替代常规编码器参与控制并获得良好效果,从而证实了所采用的单位因数控制和无速度传感器控制策略的有效性。