直驱式风力发电系统的三相Z源并网逆变器建模与控制

提出将新型的三相Z源逆变器应用于直驱式风力并网发电系统中,并将传统三相PWM并网逆变器模型和等效的Z网络模型结合,建立了三相Z源并网逆变器的数学模型,分析了直通占空比改变对系统的影响,提出了在并网条件下获取稳定的Z网络电容电压和解耦的d、q轴网侧电流分量的控制策略,并给出了外环电压设定值的约束条件.实验结果证实了理论分析的正确性和控制策略的有效性.     

Novel Pulse-width Modulation Strategy to Minimize the Switching Losses of Z-Source Inverters

This article proposes a novel pulse-width modulation strategy to minimize switching losses of the Z-source inverter. The Z-source inverter has different pulse-width modulation patterns unlike the conventional voltage source inverter. Shoot-through states have been inserted within the zero states of the traditional pulse-width modulation patterns of a voltage source inverter to boost DC input voltage. Thus, the Z-source inverter has six active states, two zero states, and additional shoot-through states differentiating the Z-source and conventional voltage source inverters. The currents flowing through the switches of the Z-source inverter are larger than those of the conventional voltage source inverter, because Z-network currents must flow through the switches during the shoot-through states. Therefore, shoot-through currents increase the total switching losses of the Z-source inverter. In this article, switching losses of the Z-source inverter with the existing pulse-width modulation strategy are analyzed in detail. Then, new modulation signals of the Z-source inverter are introduced to produce unique pulse-width modulation patterns that minimize the switching losses of the Z-source inverter. The switching losses of the Z-source inverter with both pulse-width modulation strategies are simulated and compared. In addition, an experimental system has been built and tested to verify the effectiveness of the proposed strategy.     

一种单相双输入九电平逆变器及其调制策略

由于传统多电平逆变器存在电路结构复杂、功率器件多、控制复杂等缺点,提出一种单相双输入九电平逆变器,通过2个直流输入电源与电容的串并联实现九电平输出,拓扑结构简单,能够以较少的功率器件和电容数量产生更多的输出电平,谐波含量低,并且其多输入结构适用于分布式发电和微电网等领域。首先给出逆变器的电路结构和工作原理,然后设计逆变器调制策略,并分析电容充放电过程及其对电压波动的影响,实验结果验证了所提九电平逆变器及其调制策略的有效性和可行性。     

基于阻抗源的单相光伏并网系统研究

近年来太阳能并网发电系统得到广泛应用,并网逆变器是系统核心部件,系统中逆变器多采用多级式结构。采用阻抗源代替DC/DC升压装置,在升压的同时稳定逆变桥直流侧电压,保证后级逆变器的稳定工作,使并网电流相位严格跟踪电压相位,实现单位功率因数的并网。由于阻抗源不含开关管,允许桥臂直通短路状态的出现,消除了死区时间,降低成本的同时减小了并网电流的谐波畸变,改善了系统输出。采用文献[1]中的光伏阵列模型,利用改进的电导增量法实现最大功率点,解决跟踪速度及精度的矛盾,证明基于阻抗源的单相光伏并网系统的可靠性。     

Power generation maximization of distributed photovoltaic systems using dynamic topology reconfiguration

The ‘mismatch losses’ problem is commonly encountered in distributed photovoltaic (PV) power generation systems. It can directly reduce power generation. Hence, PV array reconfiguration techniques have become highly popular to minimize the mismatch losses. In this paper, a dynamical array reconfiguration method for Total-Cross-Ties (TCT) and Series–Parallel (SP) interconnected PV arrays is proposed. The method aims to improve the maximum power output generation of a distributed PV array in different mismatch conditions through a set of inverters and a switching matrix that is controlled by a dynamic and scalable reconfiguration optimization algorithm. The structures of the switching matrix for both TCT-based and SP-based PV arrays are designed to enable flexible alteration of the electrical connections between PV strings and inverters. Also, the proposed reconfiguration solution is scalable, because the size of the switching matrix deployed in the proposed solution is only determined by the numbers of the PV strings and the inverters, and is not related to the number of PV modules in a string. The performance of the proposed method is assessed for PV arrays with both TCT and SP interconnections in different mismatch conditions, including different partial shading and random PV module failure. The average optimization time for TCT and SP interconnected PV arrays is 0.02 and 3 s, respectively. The effectiveness of the proposed dynamical reconfiguration is confirmed, with the average maximum power generation improved by 8.56% for the TCT-based PV array and 6.43% for the SP-based PV array compared to a fixed topology scheme.     

Power system transient stability assessment based on the multiple paralleled convolutional neural network and gated recurrent unit

In order to accurately evaluate power system stability in a timely manner after faults, and further improve the feature extraction ability of the model, this paper presents an improved transient stability assessment (TSA) method of CNN?+?GRU. This comprises a convolutional neural network (CNN) and gated recurrent unit (GRU). CNN has the feature extraction capability for a micro short-term time sequence, while GRU can extract characteristics contained in a macro long-term time sequence. The two are integrated to comprehensively extract the high-order features that are contained in a transient process. To overcome the difficulty of sample misclassification, a multiple parallel (MP) CNN?+?GRU, with multiple CNN?+?GRU connected in parallel, is created. Additionally, an improved focal loss (FL) function which can implement self-adaptive adjustment according to the neural network training is introduced to guide model training. Finally, the proposed methods are verified on the IEEE 39 and 145-bus systems. The simulation results indicate that the proposed methods have better TSA performance than other existing methods.     

基于滑模控制的Z源逆变器在单相光伏系统中的应用

将Z源逆变器应用到单级单相光伏并网系统中。该逆变器集成了最大功率跟踪(maximum-power-point-tracking,MPPT)、升/降直流侧电压和并网发电三个功能。根据系统要求以及Z源网络在直流侧表现出的非最小相位的特性,该文提出使用两个具有积分补偿的固定频率滑模控制器以保证系统的鲁棒性,以此消除非最小相位特性给系统带来的不良影响,保证Z源电容电压不产生超调和振荡,从而并网电流不发生畸变。该文利用小信号法分析了该闭环系统的稳定性。仿真和实验结果均验证文中提出的逆变器和控制方法的正确性和有效性。     

简化型Z源并网逆变器

针对传统Z源逆变器电路体积庞大、启动冲击大、输入电流断续造成直流电压利用率低,提出了一种简化型Z源逆变器拓扑.该拓扑在基本保持传统Z源逆变器升压能力的前提下,减少阻抗网络中高压电容的数量,用二极管为直通状态下的阻抗网络电感电流提供续流电路,减小了电路的体积及设计成本,同时该拓扑不存在启动冲击问题,保证了输入电流的连续.将该拓扑应用于三相并网系统,基于dq坐标系电流解耦及阻抗网络电容电压恒定的原则设计了控制系统.对其稳态输出、并网功率突变、输入电压扰动等情况进行了仿真分析,验证了拓扑的可行性及性能.     

Z-源逆变器直流链升压电路控制器的研究

本文提出了一种PID控制器控制Z-源逆变器Z-源网络电容两端的电压,得到较为恒定的直流链电压。基于Z-源网络小信号模型和直通占空比调制信号传递函数的基础上,得到整个环路的开环增益,设计了PID控制器。该控制器具有抑制诸如电源电压波动、负载电流扰动等的功能。同时,由于增加了微分(D)调节,系统的动态性能和稳定裕度都得到了很大程度的改善;能够克服由于Z-源网络电容和直通占空比之间的非线性关系给系统带来的闭环控制器设计困难、系统的动态性能差等缺点,使控制器参数设计非常准确、简单,动态、静态有了明显的提高。通过仿真和实验结果验证了理论分析的正确性。     

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).     

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.     

基于Z源电容电压变化的并网电流控制策略

Z源逆变器属于单级系统,具有结构简单,允许逆变桥同一桥臂上下功率器件直通,输出波形畸变小等优点,在光伏发电等输入电压变换范围比较大的场合具有很好的应用前景。在光伏并网系统中,并网电流的频繁变化将会对电网的电能质量带来负面影响。为此,本文结合单相Z源光伏并网逆变器的特点,提出了通过Z源电容电压变化来调节并网电流幅值的控制方法。该方法能够减小并网电流的波动,从而改善并网电流的波形质量。实验结果证明了该控制策略的有效性。     

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.     

采用Z源变换器的小型风力并网逆变系统

基于Z源变换器的小型风力发电并网系统的工作特点及控制结构,不同于传统的电压型逆变器,Z源变换器引入特定的阻抗源网络将变换器主电路和电源耦合在一起,并且通过新增直通零矢量调制实现独特的直流升压功能,从而得到大范围变化的交流输出电压.经过对系统电路和要求的分析,文章对控制系统进行简单的设计.通过原有正弦波调制因子的控制,实现内环并网电流的单位功率因数运行和外环Z源电容电压的稳定;通过直通零矢量调制对发电机输出整流电流进行闭环控制,实现风力发电机的电能输出控制.最后,以2.5kW Z源变换器装置进行实验,实验结果验证了分析的正确性.     

改进型Z源逆变器

针对传统Z源逆变器的直流升压因子较小、启动冲击严重、直流侧的电压利用率较低等缺点,提出了一种改进型的Z源逆变器拓扑.在传统拓扑中加入一个电感和一个全控开关,提高了直流升压因子,利用该电感上的电流不能突变的特性,使得启动时阻抗网络上的电容的冲击电流大幅减小,从而不需要采用软启动策略;同时该拓扑还保证了输入电流连续,减小了...     

相同风速功率下两种风电机组响应电网短路故障的对比分析

介绍了双馈和直驱风力发电机组的结构区别。从二者数学模型入手,其交直交变频器采用相同的控制策略,机侧变频器采用解耦控制,网侧变频器采用恒功率因数控制。在EMTPWorks中分别搭建了具有相同风速功率曲线的直驱和双馈风电机组的仿真模型。并在相同条件分析了电网发生三相、单相短路故障时的二者的暂态特性差别,得出了PMSG在故障期间的无功变化幅值要比DFIG要大,但机端母线电压跌落、机组有功功率输出、变频器直流电容电压变化幅度以及频率波动均比DFIG要小,对风电机组的选型具有十分重要的意义。     

采用Boost的两级式光伏发电并网逆变系统

在光伏并网发电中,为了提高效率,必须实行最大功率点跟踪,而为了实现并网,直流侧电压必须高于电网电压幅值,这就限制了光伏电池电压的调节范围。对一种单相光伏发电并网逆变系统进行了研究,它由Boost DC/DC电路和逆变桥组成。前级Boost斩波电路则通过调节占空比而改变光伏阵列的输出电压,实现最大功率点跟踪;后级逆变电路采用电压外环,电流内环的双环控制方法,电压外环控制逆变侧电容电压的稳定,电流内环控制并网电流实现并网。在这种系统中,最大功率点跟踪和并网是相互独立的,互不干扰,使整个系统更加灵活可靠。主要研究了逆变系统各重要元件参数的选取方法以及逆变系统的控制方法。最后用MatlabR2007a/Simulink进行了仿真,证明了该逆变系统的可行性。     

具有改进最大功率跟踪算法的光伏并网控制系统及其实现

光伏并网控制系统输送到电网的功率随着光照强度、环境温度以及光伏阵列输出电压的不同而变化,控制光伏阵列的工作点使其连续稳定地向电网输出最大功率非常必要。该文提出了基于同步旋转坐标变换实现光伏阵列最大功率跟踪与电流控制的电压源型逆变器相结合的三相光伏并网控制系统,该系统主要包括光伏阵列、直流母排电容、电压源型逆变器、滤波电感、数字信号控制器与电网。提出的改进最大功率跟踪方法,根据光伏阵列dP/dU-U的特性曲线,利用Newton-Raphson方法快速计算光伏阵列输出功率对电压的微分值,由此进一步形成光伏阵列工作在最大功率点的参考电压值。整个控制系统为双环控制,外环为电压控制环,利用一个PI调节器使光伏阵列输出电压工作在最大功率工作点;内环为电流控制环,利用2个PI调节器分别对d-q轴电流进行解耦控制,使逆变器输出电流与参考电流一致。根据所提出的控制算法,研制了一台三相光伏并网系统原理性样机,仿真与实验结果一致,系统具有良好的动稳态性能,说明了所提出的控制方案是非常有效的。     

VRB储能系统对风电场LVRT特性影响分析

为满足电网规定的并网风电场必须具有低电压穿越能力（LVRT）要求,提出一种在风电场并网点加入直接功率控制的钒液流电池（VRB）储能系统的拓扑结构来提高风电场LVRT.根据目前风电机组发展趋势风电场采用基于全功率双脉宽调制AC/DC/AC控制策略的逆变器的永磁直驱风电机组（PMSG）,VRB储能系统逆变器采用DC/AC双向功率流动的控制策略.所提出的控制策略通过协调控制风电机组机侧整流器、网侧逆变器和VRB变换器,实现平抑风电场出力和电压跌落时PCC点电压稳定控制及向电网提供一定的无功补偿.仿真结果表明,风速波动和系统电压跌落时,提出的方案可以有效平抑风电场出力波动,提高风电场LVRT能力,减小对系统安全稳定运行的负面影响.     

Grid integration improvement for single-phase inverters of small wind turbines under distorted voltage conditions

In this work, an enhanced control method for the grid-tied single-phase inverter in small wind turbines based on PMSG (permanent-magnet synchronous generator) under grid voltage distortion is introduced. Composite observers are designed for decomposing harmonic and fundamental components of the grid current and voltage accurately, which facilitate current control action and synchronize the inverter with the grid. Grid voltage phase angle is detected by an advanced PLL (phase-locked loop) scheme based on the grid voltage fundamental component. The inverter current is adjusted by multi-PR (proportional-resonant) controllers, where its harmonic components are regulated to zero resulting in a sinusoidal grid current. In this scheme, the DC-link voltage is maintained at constant by a DC boost converter, while generator output power is controlled by the grid-tied inverter. An MPPT (maximum power-point tracking) operation of wind turbine is obtained, with which generator speed is estimated from the input voltage and current of the DC boost converter, thus avoiding the use of an expensive and complex mechanical sensor. The presented method is proved through experimental investigation, where experimental results show that the THD (total harmonic distortion) of grid current is about 3% under 12.25% THD of the grid voltage.