Addressing and assessing the issues related to connection of the wind turbine generators to the distribution grid

Integration of wind power generation into power distribution grid may result in stability and power quality challenges. In this way, this paper first reviews the grid integration issues of wind power generation. Then, as the main contribution, the paper develops in-depth theoretical analyses for examination the grid integration issues such as voltage rise, output power fluctuations, and static and dynamic voltage variations created due to grid integration of wind turbines and corresponding aerodynamic power fluctuations. Besides, the paper addresses these issues by proposing elaborate control approaches and by using the static compensator (STATCOM). The paper first addresses the problem of voltage rise by active power curtailment through pitch control. However, this method may result in a large waste of wind energy that may not be cost effective. Next, the STATCOM is implemented and efficient control approaches are proposed for addressing the problem of voltage rise, improvement of voltage profile and suppression of voltage and power fluctuations. At the end, simulation studies for the study distribution grid are given. The study system is an actual distribution grid in Iran with approximately 7.9 MW wind power generation and 21.9 MW load.     

Harmonic current prediction by impedance modeling of grid-tied inverters: A 1.4 MW PV plant case study

Grid-tied inverters are the main responsible for the successful integration of photovoltaic (PV) plants in the power grid. Two important challenges are the integration of power plants based-inverters and their contribution to the power system quality. This work analyzes the features of inverter impedance models when used in harmonic integration studies. It is aimed to estimate the harmonic current contribution as a function of the background harmonic voltages components. Time domain simulations based on detailed and average models are compared with the impedance model developed in frequency domain. In grids with harmonic voltages, impedance models can predict the current distortion for all active power injection scenarios. Furthermore, measurements in a 1.4 MW PV plant connected in a distributed grid are used to validate the simulation based on impedance models during different power injections and harmonic voltage profiles. Results reinforce that impedance models can represent with relatively accuracy the harmonic current emitted by the PV plants at the point of common coupling (PCC). Lastly, a stress test is performed to show how a variation in the harmonic voltage phase angle impacts the PV plant harmonic emission.     

Design and control of LCL filter interfaced grid connected solar photovoltaic (SPV) system using power balance theory

In this paper solar photovoltaic (SPV) system connected to the utility grid is designed and simulated. The utility grid and SPV system are coupled with current controlled voltage source converter (VSC) and LCL filter. The design of LCL filter, MPPT algorithm and power quality improvements are discussed and simulation results are shown for the performance analysis of grid-coupled PV system under different load condition. The system is controlled through power balance theory method. The principle behind the control implementation is to evacuate the solar power generated during the daytime and the reactive power demand for the load should be supplied by the PV. The grid coupled system consists of SPV system, dc–dc boost converter, maximum power point tracking (MPPT), voltage source converter (VSC), LCL filter, different loads and three phase utility grid. This system is capable of eliminating harmonic and load balancing by supplying unbalanced current from the PV as a compensator. The system is simulated with 10 kW SPV array using indirect current control scheme.     

电网低次谐波电压下双馈风电系统定子谐波电流抑制

IEEE相关标准对分布式发电系统注入电网的谐波电流作出了严格的限制.通过分析电网低次谐波电压对双馈风力发电系统定子电流总谐波畸变率(THD)的影响发现,传统基于定子电压定向的双馈风力发电系统并网发电矢量控制策略在大容量变流器控制中存在着缺点,即因为控制带宽低而无法有效抑制电网低次谐波电压引起的定子谐波电流,从而在较大工...     

干扰性用户并网电能质量评估与可视化软件设计

随着新能源、电力电子设备、非线性负荷接入比例的快速增长,现有的电能质量评估方法存在无法适应不同应用需求、尚未对长时间尺度进行评估等问题。本文提出了一种干扰性用户并网电能质量评估方法,包括区域电网建模、潮流仿真分析以及影响因素评估,能够实现多时间尺度下对干扰性用户接入区域电网的电能质量规范化评估。在此基础上,建立基于B/S架构设计干扰性用户并网电能质量评估可视化软件系统,最后应用该系统对某风电场开展并网评估,结果表明接入电网后该风电场的电压偏差、电压不平衡、各次谐波电流以及电压总谐波畸变率均满足国标要求,该系统能够实现对干扰源用户模型接入电网后的电能质量标准化进行仿真。     

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.     

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.     

双馈风电机组接入地区电网后的电压稳定分析

利用PSASP软件的UPI接口程序模块,在PSASP中建立了双馈风电机组模型,以两个实际的地区电网为例,研究双馈风电机组接入地区电网后对电网电压的影响,结果表明：双馈风电机组提高了地区电网的静态电压水平;双馈风电机组在故障后能够减少系统所需的无功储备,从而有利于地区电网的电压稳定;风速的波动对地区电网的电压有影响,但一般不会影响其暂态稳定性。     

负序及谐波畸变电网电压下双馈风力发电系统的改进直接功率控制策略

由于定子直接连接到电网,电网电压中的负序和谐波分量会严重恶化双馈风力发电机(DFIG)系统的运行性能,导致系统输出总电流三相不对称及谐波畸变、总输出有功功率及无功功率波动等,使得DFIG系统无法安全稳定可靠运行,且输出风电质量下降。同时考虑负序和谐波电网下DFIG系统机侧变流器和网侧变流器的运行状态,以改善DFIG系统总输出电流或功率质量为目标,研究基于二阶矢量积分器(SOVI)的DFIG系统网侧和机侧变流器改进直接功率控制(DPC)策略,改善DFIG系统的运行性能。实验结果验证了所提出的负序和谐波畸变电网电压下DPC策略的正确性及有效性。     

双馈风机参与系统调频对系统暂态功角稳定性的影响分析

以含双馈风力发电机(Double-Fed Induction Generator,DFIG)的扩展双机系统为研究对象,研究DFIG参与系统调频对系统暂态功角稳定性的影响.将含DFIG的两机系统等值为含DFIG的单机无穷大系统,计及风电比例、风机并网位置、故障位置、负荷接入位置等四种影响因素.运用等面积法则对风机参与系统...     

大规模双馈风电机组并网频率稳定控制策略

风电在电网中的渗透率逐年升高,尽管其在优化电源结构、节能减排方面有颇多贡献,但是由于自然风的波动性和不确定性,大规模风电并网势必会对电网频率稳定等方面产生影响。针对此问题,对装机量较高的双馈型风力发电机组（doubly-fed induction generator,DFIG）控制系统进行研究,制定大规模双馈型风电机组并网下的频率稳定控制策略。在现有附加下垂控制及虚拟惯性控制的基础上,根据输入风速以及电网侧频率波动,对下垂控制环节进行变系数处理,改善下垂控制特性。将DFIG运行工况进行分类,并采用变减载系数,使DFIG在限功率运行的同时减少弃风。采用具有大规模风电并网接入的实际电网模型,仿真验证了所提频率稳定控制策略的有效性。     

Power Quality Enhancement Using Linear Quadratic Regulator Based Current-controlled Voltage Source Inverter for the Grid Integrated Renewable Energy System

This paper presents an optimal control strategy using linear quadratic regulator (LQR) applied in current-controlled voltage source inverter (CCVSI) to control the real and reactive power flow between the renewable energy system (RES) and the grid. It also compensates harmonic current components drawn by the load from the grid terminal. A simplified equivalent circuit is used to develop the reduced order state space model of the three-phase grid connected renewable energy system. This makes the analysis and design of control law simpler by reducing the number of weighing variables used in LQR. The extension real–reactive power (p–q) method implemented in a–b–c frame is used to generate the reference current for controlling the real and reactive power to the grid to minimize the total harmonic distortion (THD) and to achieve unity power factor (UPF) operation at the grid side. The stated technique makes the grid current sinusoidal even under unbalanced grid voltages and the harmonic distortion factors are well within the IEEE limits. The system is simulated under changes in the real power fed from RES to the grid for both balanced and unbalanced grid conditions. The simulation results are validated by the experimental results.     

Analysis and comparison of current control methods on bridgeless converter to improve power quality

In this paper, the performance analysis of power factor correction (PFC) current control methods is presented for a bridgeless converter operating under continuous conduction mode (CCM). The bridgeless converter has been proposed using proportional-integral control (PIC), average sliding control (ASC) and predictive current control (PCC) methods to obtain unity power factor (PF) and lower total harmonic distortion (THD) of input current. Several PFC methods have been developed to satisfy the international standards such as IEC 61000-3-2 and IEEE 519-1992. The detailed steady-state theoretical analysis of the bridgeless converter is presented, which is verified by simulations and experiments carried out on 600 W and 50 kHz. The performance of the current control methods for the bridgeless converter is investigated by a Matlab/Simulink program. The experiments performed in the laboratory under input voltage and load variation ranges verify the theoretical and simulation studies. The control methods are programmed by the TMS320F2812 DSP microprocessor.     

故障时双馈风力发电系统的控制策略研究

基于变速恒频双馈风力发电系统的有功、无功的解耦控制,提出了电网故障时双馈电机的紧急控制策略,在Mtalab/Simulink里建立了模型,并验证了故障控制策略的正确性。在常规情况下,双馈电机转子侧的有功采用最大功率点跟踪控制,无功采用额定功率因数控制,可以获得最大的风能利用效率和较好的功率因数;但若电网故障时仍采用此常规控制策略,电网不但获取不到最大的风能利用效率和较好的功率因数,而且电网的频率和电压波动影响较大。所以在故障期间把最大功率点追踪控制和额定功率因数控制切换到频率控制和电压控制策略,通过控制有功电流分量和无功电流分量来减小或者增加有功和无功的输出,减小电网频率和电压的波动。建立了故障控制的模型,针对系统单相接地故障和两相短路故障这两种短路故障类型进行了仿真,仿真结果验证了故障控制策略的正确性,提高了风力发电系统在故障时的稳定性。     

大型风电场对电力系统暂态稳定性的影响

对大型风电场接入后的电力系统暂态稳定性进行了研究.从理论上分析了基于双馈感应电机的风电机组的暂态稳定机理;基于双馈风电机组的动态数学模型,通过仿真计算研究了大型风电场并网对电力系统暂态稳定性的影响,分析比较了在同一接入点接入双馈风电机组与接入同步发电机组对电力系统稳定性的影响,仿真计算进一步验证了理论分析的结果;最后得出结论:基于双馈风电机组的风电场对电力系统暂态稳定性的影响要好于在同一接入点接入相同容量的同步发电机组.     

基于双馈风力发电机变流器的谐波分析

由于引入了变流器等电力电子装置,变速恒频双馈风力发电系统输出的电流中含有大量谐波,分析双馈风力发电机的谐波特性并减少其谐波含量,对风电并网具有重要意义.介绍了双馈电机的基波和谐波等效电路,在此基础上提出了双馈风力发电机转子侧变流器谐波分析方法,并用一台i.5 MW双馈电机进行了仿真验证.对电压空间矢量脉宽调制( SVPWM)的三种调制方式进行了研究,在1.5 MW双馈风力发电机满载和半载两种工况的仿真下,分析比较了SVPWM三种调制方式下的谐波情况,仿真结果表明采用七段式调制方式合成SVPWM时,双馈电机谐波含量最小.     

Evaluating the integration of wind power into distribution networks by using Monte Carlo simulation

This paper provides a probabilistic method to assess the impact of wind turbines (WTs) integration into distribution networks within a market environment. Combined Monte Carlo simulation (MCS) technique and market-based optimal power flow (OPF) are used to maximize the social welfare by integrating demand side management (DSM) scheme considering different combinations of wind generation and load demand over a year. MCS is used to model the uncertainties related to the stochastic variations of wind power generation and load demand. The market-based OPF is solved by using step-controlled primal dual interior point method considering network constraints. The method is conceived for distribution network operators (DNOs) in order to evaluate the effect of WTs integration into the network. The effectiveness of the proposed method is demonstrated with an 84-bus 11.4 kV radial distribution system.     

Integrated DFIG‐based wind turbine model with direct power control

This paper is focused on the model development of doubly fed induction generator (DFIG)‐based wind turbines. The model integrates mechanical and electrical systems. In the mechanical system, the aerodynamic properties are calculated in every section and every angle of attack by using a finite element analysis software, and the dynamic structure analysis is performed by using the Fatigue, Aerodynamics, Structures, and Turbulence (FAST) code. On the other hand, the electrical system includes DFIG modeling, the grid side converter (GSC) with DC bus voltage control, and the rotor side inverter (RSI) with direct power control (DPC). By using the direct power control strategy, the active power, reactive power, and power factor from the DFIG can be controlled to meet the requirements of the grid code. Moreover, the actual measured wind speed data are used in this work to simulate the outputs from the DFIG in order to confirm the feasibility of the wind turbine modeling. According to the simulation results, the proposed model is useful and can provide a reference for further studies about wind turbine technology. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

双馈风力发电系统的建模与运行特性研究

双馈风力机是一种绕线式异步发电机,可以实现变速恒频运行,提高风能利用率,并减少对电网稳定性的影响。本文对双馈风力系统的结构进行了研究,分析了其实现最大风能捕获的工作原理,建立了风力发电机的数学模型,并分析了基于定子交轴电流和直轴电流的有功功率、无功功率解耦控制的基本原理,然后讨论了双馈风力发电机在两种运行状态的能量流动过程,最后运用Mtlab/Simulink仿真分析了双馈风力发电机的运行特性,验证了理论分析的正确性。     

A SCR crowbar commutated with power converter for DFIG-based wind turbines

This paper presents an active crowbar, constructed with silicon controlled rectifiers (SCRs), for doubly fed induction generator (DFIG)-based wind turbines to fulfill low-voltage ride-through (LVRT) requirements demanded by grid codes. By this design, not only the active deactivation of the costly IGBT crowbar, replacing the passive crowbar for this ability, can be achieved with the more cost-effective SCRs, but also the reliability of the circuit is strengthened thanking to the high surge capability of SCR. In this topology, three back-to-back SCR switches are connected in delta form and further engaged with the rotor circuit of DFIG through power dissipation resistors, working as the main circuit of crowbar, while the rotor side converter (RSC) is used to commutate the SCRs at the desired deactivation moment to disconnect the crowbar, allowing for resumption of the feedback control to DFIG. By this topology the harmonics introduced into the rotor circuit by the diode bride in the IGBT crowbar are got rid of and by this commutation method the overvoltage risk at turning off the IGBT is eliminated. With the simulative results on a 2 MW DFIG, the comparison with the IGBT crowbar is made. The feasibility of the proposed crowbar technique is further demonstrated with experiments on a laboratory-scale test rig.