Control Design Guidelines for Single-Phase Grid-Connected Photovoltaic Inverters With Damped Resonant Harmonic Compensators

The injection of low-harmonic current to the electrical grid is nowadays regulated by international standards. In order to accomplish these standards, selective harmonic compensation is carried out in grid-connected photovoltaic inverters by means of resonant harmonic compensators. This paper gives a systematic design procedure for selecting the gains and parameters of these harmonic compensators. Other factors considered in the design process include frequency deviation, grid synchronization, and transient response. Both a design example meeting the requirements of grid interconnection and selected experimental results from a digital-signal-processor-based laboratory prototype are also reported.     

Linear Current Control Scheme With Series Resonant Harmonic Compensator for Single-Phase Grid-Connected Photovoltaic Inverters

This paper deals with an important aspect in the operation of single-phase grid-connected photovoltaic inverters, i.e., injecting low harmonic current into the electrical grid. Specifically, we propose a linear current control scheme with a resonant harmonic compensator connected in series with a tracking regulator (the standard harmonic compensator location is in parallel with the tracking regulator). The series connection provides an efficient attenuation of the grid voltage background distortion, an accurate synchronization with the grid voltage, and a low computational time in relation to the standard control scheme. Experimental results from a digital signal processor-based laboratory prototype validate the features of the proposed control scheme.     

双级式单相光伏并网系统的研究与仿真

介绍了双级式单相光伏并网系统,分析了其拓扑结构。升压采用模糊推理进行最大功率点跟踪控制;双闭环控制实现了对电网电压的同频同相控制,达到了并网要求。通过MatlabR2010对系统进行了仿真,仿真结果表明逆变输出电流和电网电压同频同相,并有效地跟踪了光伏阵列的最大功率点。     

单相UPS逆变控制技术研究

为了进一步提高UPS(Uninterrupted Power Supply)输出的动态和稳态性能，介绍了一种采用电流环做内环的双环控制技术，实现了对负载电流扰动的解耦。同时为了补偿输入电压的不稳定，采用输入电压前馈控制。文中就提到的控制方法分别进行了时域和频域仿真分析，并以DSP(TMS320LF2407)为控制芯片进行了实验验证，结果显示逆变器输出电压在恶劣的负载条件下波形畸变很小。     

A Robust Predictive Current Control for Three-Phase Grid-Connected Inverters

This paper presents a new predictive control algorithm for grid-connected current-controlled inverters. The control combines a two-sample deadbeat control law with a Luenberger observer to estimate the future value of the grid currents. The resulting control offers robustness against the computational delay inherent in the digital implementation and considerably enhances the gain and phase margins of the previous predictive controls while maintaining the high-speed response typical of the deadbeat controllers.      

Intelligent PV Module for Grid-Connected PV Systems

Most issues carried out about building integrated photovoltaic (PV) system performance show average losses of about 20%–25% in electricity production. The causes are varied, e.g., mismatching losses, partial shadows, variations in current–voltage$(I$–$V)$characteristics of PV modules due to manufacturing processes, differences in the orientations and inclinations of solar surfaces, and temperature effects. These losses can be decreased by means of suitable electronics. This paper presents the intelligent PV module concept, a low-cost high-efficiency dc–dc converter with maximum power point tracking (MPPT) functions, control, and power line communications (PLC). In addition, this paper analyses the alternatives for the architecture of grid-connected PV systems: centralized, string, and modular topologies. The proposed system, i.e., the intelligent PV module, fits within this last group. Its principles of operation, as well as the topology of boost dc–dc converter, are analyzed. Besides, a comparison of MPPT methods is performed, which shows the best results for the incremental conductance method. Regarding communications, PLC in every PV module and its feasibility for grid-connected PV plants are considered and analyzed in this paper. After developing an intelligent PV module (with dc–dc converter) prototype, its optimal performance has been experimentally confirmed by means of the PV system test platform. This paper describes this powerful tool especially designed to evaluate all kinds of PV systems.     

光伏并网系统反孤岛控制策略研究

随着分布式供电系统的逐步发展,反孤岛控制已成为并网供电中不可缺少的环节。文中结合光伏并网逆变器的控制过程介绍了几种传统的反孤岛方法,分析了这些方法中的检测盲区,详细介绍了正反馈频率法的控制原理和控制过程,并给出程序控制流程图,运用Matlab对该方法进行了仿真。仿真结果表明该方法能够快速地检测出孤岛的发生并断开逆变器与电网的连接,无检测盲区,对孤岛效应能够做到有效的防范。     

Robust Model-Following Control of Parallel UPS Single-Phase Inverters

This paper presents a robust control technique applied to modular uninterruptible power-supply (UPS) inverters operating in parallel. When compared to conventional proportional–integral (PI) control, the proposed technique improves the response of the output voltage to load steps and to high distorted output currents, reducing the distortion of the output voltage. Furthermore, an excellent distribution of currents between modules is achieved, resulting in fine power equalization between the inverters on stream. The crossover frequency of the different loop gains involved is moderate, so that robustness to variations of the operation point and to modeling uncertainties is achieved. A comparative study with a two-loop conventional PI control scheme is presented. Experimental results on a 1-kVA modular online UPS system confirm the viability of the proposed scheme.      

A DC Voltage Monitoring and Control Method for Three-Phase Grid-Connected Wind Turbine Inverters

Space vector pulsewidth modulation (SVPWM) based three-phase voltage source inverters provide a widely used interface between electric grids and wind turbine systems. PI controllers, predictive algorithms and real-time sampling techniques are often used to overcome the shortcomings of SVPWM. These techniques depend highly on accurate measurements of inverter voltages and currents, thus making sensors the key elements in the control process. Among these sensors, the dc link voltage (Vdc) sensor is critical: if it sends out a signal with a significant error, the output current will be distorted. The Authors have developed a combination of PI and predictive methods, using them simultaneously to control a three-phase grid-connected inverter. Under this new control scheme, the PI controller is given a new task of monitoring and controlling Vdc. As a result, the output current of the inverter is of high quality, and more importantly, Vdc can be double checked for its correctness of measurements. When the Vdc sensor fails or its signals are corrupted, the Vdc PI controller will become a Vdc controller, adding an extra protective function for the reliable operation of wind turbine inverters.     

Control of a Single-Phase Cascaded H-Bridge Multilevel Inverter for Grid-Connected Photovoltaic Systems

This paper presents a single-phase cascaded H-bridge converter for a grid-connected photovoltaic (PV) application. The multilevel topology consists of several H-bridge cells connected in series, each one connected to a string of PV modules. The adopted control scheme permits the independent control of each dc-link voltage, enabling, in this way, the tracking of the maximum power point for each string of PV panels. Additionally, low-ripple sinusoidal-current waveforms are generated with almost unity power factor. The topology offers other advantages such as the operation at lower switching frequency or lower current ripple compared to standard two-level topologies. Simulation and experimental results are presented for different operating conditions.     

基于重复控制算法的光伏并网逆变器的研究

本文介绍了一种基于重复控制算法的光伏并网逆变器系统。该控制算法对死区影响及其它周期性扰动有明显的抑制作用,而且能消除跟踪误差,使系统输出达到无静差。同时,在控制中还将前馈控制与重复控制相结合,克服了重复控制固有的滞后调节特性,改善了光伏并网逆变器的输出并网电流波形和稳态误差。     

大功率光伏并网逆变器控制系统分析与实现

文章深入分析了大功率光伏并网逆变器的控制系统。介绍了光伏并网逆变器的分类,以及大功率光伏并网逆变器的电路拓扑;重点分析了最大功率跟踪MPPT、功率因数调节、锁相、孤岛检测、低电压穿越等关键技术。应用这些关键技术设计的500 kW光伏并网逆变器,具有变换效率高、MPPT范围宽、并网电流失真度小、功率因数高、体积重量小、成本低等优点。     

An Anti-Islanding Method for Single-Phase Inverters Based on a Grid Voltage Sensorless Control

The detection of the islanding condition of a distributed generation (DG) system is crucial for safety reasons, as discussed in the IEEE standards and specifically required by some national codes. Several anti-islanding methods that are resident in the inverter have been investigated and classified as passive (measurement of the natural effects of islanding) or active (based on the measurement of the effects due to transients or harmonics deliberately introduced in the system). In case the power drained by the load matches the power generated by the DG inverter, the effect of islanding is small, and the passive methods fail. However, the active methods, which have been developed to overcome these limits, create disturbances that can interact with those generated by other DG systems. In this paper, a new anti-islanding method is proposed. It exploits the natural sensitivity of a grid-voltage sensorless control to disturbances to highlight the islanding condition. The adopted grid-voltage sensorless control is adapted to a single-phase system with the use of resonant controllers based on the internal model control law: resonant-controller-based observer results. Then, a Kalman-filter-based algorithm is used to detect the islanding condition based on the energy mismatch between the estimated third and fifth harmonics and the real ones. Experimental results support the analysis     

基于DSP的光伏逆变并网的同步控制

文章介绍了光伏并网逆变器的基本原理,通过无差拍PWM控制方式来实现逆变电流快速跟踪电网电流。给出了基于DSP的三相光伏并网逆变器的软硬件设计方案和实验结果。实验结果说明了DSP在光伏发电方面具有广阔的应用前景。     

Grid-Connected Photovoltaic Multistring PCS With PV Current Variation Reduction Control

In this paper, a grid-connected photovoltaic (PV) multistring power conditioning system with PV input current reduction control is proposed. An improved maximum power point tracking (MPPT) method for the multistring converter is suggested. The suggested MPPT algorithm tracks the maximum power point even though measurement errors exist. To reduce the PV current variation introduced by the inverter, a PV current variation reduction control is suggested. This PV current variation reduction control reduces the PV current variation without additional components. The low current variation reduces the filter size and improves the MPPT efficiency. All algorithms and controllers are implemented on a single-chip microcontroller. Experimental results obtained on a 3-kW prototype show high performance such as a MPPT efficiency of 99.7%, an almost unity power factor, a power efficiency of 96.7%, and a total harmonic distortion of 2.0%.     

A Three-Level Hysteresis Function Approach to the Sliding-Mode Control of Single-Phase UPS Inverters

A new approach to the sliding-mode control ofsingle-phase uninterruptible-power-supply inverters is introduced in continuous time. A three-level hysteresis sliding function is used to directly control the inverter switches, with the result that a transistor is switched during a half-cycle while it remains either on or off during the other cycle. An expression is derived for the predicted switching frequency. The performance of the proposed control method has been tested through computer simulations and experiments under different loads (a resistive load and a diode bridge rectifier load). Simulation and experimental results verify the correct operation of the proposed control strategy. In addition, the switching frequency prediction is seen to be quite accurate.      

Electrical PV Array Reconfiguration Strategy for Energy Extraction Improvement in Grid-Connected PV Systems

This paper applies a dynamical electrical array reconfiguration (EAR) strategy on the photovoltaic (PV) generator of a grid-connected PV system based on a plant-oriented configuration, in order to improve its energy production when the operating conditions of the solar panels are different. The EAR strategy is carried out by inserting a controllable switching matrix between the PV generator and the central inverter, which allows the electrical reconnection of the available PV modules. As a result, the PV system exhibits a self-capacity for real-time adaptation to the PV generator external operating conditions and improves the energy extraction of the system. Experimental results are provided to validate the proposed approach.     

A New Multilevel Conversion Structure for Grid-Connected PV Applications

A novel scheme for three-phase grid-connected photovoltaic (PV) generation systems is presented in this paper. The scheme is based on two insulated strings of PV panels, each one feeding the dc bus of a standard two-level three-phase voltage-source inverter (VSI). The inverters are connected to the grid by a three-phase transformer having open-end windings on the inverter side. The resulting conversion structure performs as a multilevel power active filter (equivalent to a three-level inverter), doubling the power capability of a single VSI with given voltage and current ratings. The multilevel voltage waveforms are generated by an improved space-vector-modulation algorithm, suitable for the implementation in industrial digital signal processors. An original control method has been introduced to regulate the dc-link voltages of each VSI, according to the voltage reference given by a single maximum power point tracking controller. The proposed regulation system has been verified by numerical simulations and experimental tests with reference to different operating conditions.     

基于Buck-Boost逆变器的离散滑模控制仿真研究

为了获得比较理想的正弦输出电压，优化逆变器的动态性能，文中基于Buck—Boost逆变器．采用了离散滑模变结构的控制策略。Buck—Boost逆变器可以获得比直流输入电源高或低的交流输出电压，文中阐述了其工作原理．并结合状态方程，推导出滑模面的存在条件、到达条件和稳定条件，然后对电路参数、控制系数以及控制算法进行了设计。仿真结果表明采用离散滑模控制的Buck—Boost逆变器对系统扰动和负载变化具有很强的鲁棒性，系统具有良好的动态响应。     

Switching Flow-Graph Modeling Technique for Three-Phase Inverters

In this paper, a unified graphical modeling technique is developed for investigating the behavior of pulsewidth- modulated three-phase inverters. By introducing the virtual- switch concept together with its virtual switching function, it is seen that the existing attractive switching flow-graph modeling technique for dc-to-dc converters can be extended to three-phase inverters without losing its merits. The corresponding large- signal, steady-state, and small-signal models can then be also obtained easily. Some examples, implemented easily with MATLAB/ Simulink, are also given to demonstrate the feasibility of the proposed technique.