基于注入电流扰动法的光伏直流模块的防孤岛控制

传统的交流型并网发电系统中,可以通过检测电网电压或频率,采用有功扰动、无功扰动或频率扰动等方法实现防孤岛控制。而在共直流母线系统中,母线直流电压是唯一可检测的变量,因此传统的防孤岛控制策略无法适用。以共直流母线分布式系统中光伏直流模块的防孤岛控制为研究对象,建立了模块正常运行和孤岛运行时的数学模型,推导得到了在两种工况下模块输出电压的时域表达式,并在此基础上提出了一种基于注入电流扰动法的新型防孤岛控制策略。该控制策略的核心思路是通过控制模块输出电流和扰动步长来改变在扰动时间内模块的输出电压特性,并根据检测判据来决策是否发生孤岛。最后,本文以Boost+FB-LLC的两级式结构作为光伏直流模块的工作拓扑,并设计了一台1000W的实验样机,样机实验结果验证了本文所提出的防孤岛控制策略的有效性。     

Investigation of anti-islanding protection of power converter based distributed generators using frequency domain analysis

The anti-islanding algorithm proposed by the Sandia national laboratories is analyzed in this study because this scheme, also known as the Sandia scheme, is considered to be effective in detecting islanding of distributed generation systems. Previously, other than heuristic approaches, there has not been any quantitative analysis for tuning the control gains of the algorithm based on the power rating and bandwidth of the distributed generation (DG) power converter. The paper interprets the components of the algorithm that affect the voltage magnitude and frequency into block diagrams that can be linearized and studied using continuous time approximations. This paper uses a frequency domain approach to analyze the range for the gains required by anti-islanding algorithm to effectively determine the disconnection of the mains grid within an acceptable time duration. The analysis provides guidelines for using Sandia's national laboratory schemes under different application conditions. The results are validated using detailed time domain DG and power system simulations.     

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     

一种新的频率偏移技术在光伏并网发电系统孤岛检测中的应用

目前通过频率偏移技术进行孤岛检测的方法有很多,但是这些方法一般都是在假设并网失败后,逆变器电压的频率一直朝某个方向变化(变大或变小)直到超出额定频率范围为止的前提下才成立的。而实际中,当负载的品质因数比较高时,逆变器频率将围绕某个值震荡而不会超出额定频率范围。本文提出一种新的频率偏移技术———自适应逻辑频率偏移技术。他以一般频率偏移技术为基本扰动工具,必要时再叠加一新的偏移量,从而可以快速检测到弧岛效应,并且正常并网时频率扰动量也较小。     

A Simplified Control Method for the Grid-Connected Inverter With the Function of Islanding Detection

In this paper, a novel method for the voltage-controlled grid-connected inverter with the function of islanding detection is proposed for the distribution power generation system. The salient features of the proposed voltage-controlled grid-connected inverter are simple control circuit and fast transient response. Besides, an islanding detection method is proposed and incorporated into the controller of a voltage-controlled grid-connected inverter. The proposed islanding detection method will cause variation in amplitude and frequency of the load voltage when the utility is interrupted so as to immediately detect the islanding operation. A prototype is developed and tested to demonstrate the performance of the proposed voltage-controlled grid-connected inverter. The experimental results show that the performance of the proposed voltage-controlled grid-connected inverter is as expected.      

Design, analysis, and real-time testing of a controller for multibus microgrid system

This paper concentrates on the design and analysis of a controller for multibus microgrid system. The controller proposed for use with each distributed generation (DG) system in the microgrid contains inner voltage and current loops for regulating the three-phase grid-interfacing inverter, and external power control loops for controlling real and reactive power flow and for facilitating power sharing between the paralleled DG systems when a utility fault occurs and the microgrid islands. The controller also incorporates synchronization algorithms for ensuring smooth and safe reconnection of the micro and utility grids when the fault is cleared. With the implementation of the unified controller, the multibus microgrid system is able to switch between islanding and grid-connected modes without disrupting the critical loads connected to it. The performance of this unified controller has been verified in simulation using a real-time digital simulator and experimentally using a scaled laboratory prototype.     

A $Q$– $f$ Droop Curve for Facilitating Islanding Detection of Inverter-Based Distributed Generation

This paper presents a new method for islanding detection of distributed generation (DG) inverter that relies on analyzing the reactive power versus frequency (Q-f) characteristic of the DG and the islanded load. The proposed method is based on equipping the DG interface with a Q-f droop curve that forces the DG to lose its stable operation once an islanding condition occurs. A simple passive islanding detection scheme that relies on frequency relays can then be used to detect the moment of islanding. The performance of the proposed method is evaluated under the IEEE 1547 and UL 1741 antiislanding test configuration. The studies reported in this paper are based on time-domain simulations in the power systems computer-aided design (PSCAD)/EMTDC environment. The results show that the proposed technique has negligible nondetection zone and is capable of accurately detecting islanding within the standard permissible detection times. In addition, the technique proves to be robust under multiple-DG operation.     

Wavelet-Based Islanding Detection in Grid-Connected PV Systems

Distributed power generation systems (DPGSs) based on inverters require reliable islanding detection algorithms (passive or active) in order to determine the electrical grid status and operate the grid-connected inverter properly. These methods are based on the analysis of the DPGS voltage, current, and power in time or frequency domain. This paper proposes a time-frequency detection algorithm based on monitoring the DPGS output power considering the influence of the pulsewidth modulation, the output LCL filter, and the employed current controller. Wavelet analysis is applied to obtain time localization of the islanding condition. Simulation and experimental results show the performance of the proposed detection algorithm in comparison with existing methods.     

光伏并网逆变器孤岛测试技术研究

国内的光伏发电技术中,孤岛检测及孤岛保护常常被忽视,而孤岛的发生可能会带来巨大危害,因此,研究孤岛检测及其保护具有重要的现实意义。文章结合光伏并网逆变器的并网控制过程提出了一种正反馈频率漂移反孤岛检测方法,并详细介绍了该方法的原理和实现过程,并结合IEEE.Std.2000-929标准中的技术规范对仿真模型进行了分析,验证并比较了此类方法的有效性和适用性。     

基于功率扰动单相光伏并网系统的孤岛检测方法

由于光伏发电系统是通过逆变器作为桥梁与电力系统实现并网,为了保障电网安全稳定,抗孤岛检测成为光伏并网的关键技术之一。在研究现有传统被动式和主动式孤岛检测方法的基础上,根据电网稳定电压边界条件,通过设定阈值提出了基于光伏逆变输出功率扰动的复合式孤岛检测方法。结合IEEE.Std.2000-929和UL1741技术规范,阐述了该方法的工作机理、阈值整定与实现过程。仿真分析表明,该方法在逆变器输出功率与负载消耗功率匹配度较高时,能够实现快速的孤岛检测,无检测盲区和谐波污染,验证了理论方法的正确性,实现了光伏系统友好型并网。     

Analysis of Common-Mode Voltage in Utility-Interactive Fuel Cell Power Conditioners

Fuel cell stacks produce a dc output with a 2:1 voltage variation from no load to full load. It is customary for a utility-scale fuel cell stack to consist of several hundreds of series-connected cells, each producing 0.6 V at full load. A power conditioner consisting of dc-dc and dc-ac converters is required for utility interface, which are operated in high frequency, employing pulsewidth-modulation control for voltage and current regulation. Due to their switch-mode nature, a common-mode voltage with respect to ground is generated. The common-mode voltage, in turn, contributes to the circulating ground current, which can interfere with the ground fault protection system. In addition, it also contributes to the neutral shift and electromagnetic interference. Moreover, the electrostatic potential to ground within the fuel cell stack needs to be limited for safe operation. This paper presents an analysis of common-mode voltage in several fuel-cell-powered converter topologies connected to the electric utility and discusses several mitigation methods suitable for utility-scale generation.     

Islanding detection technique using wavelet energy in grid-connected PV system

This paper proposes a new islanding detection method using wavelet energy in a grid-connected photovoltaic system. The method detects spectral changes in the higher-frequency components of the point of common coupling voltage and obtains wavelet coefficients by multilevel wavelet analysis. The autocorrelation of the wavelet coefficients can clearly identify islanding detection, even in the variations of the grid voltage harmonics during normal operating conditions. The advantage of the proposed method is that it can detect islanding condition the conventional under voltage/over voltage/under frequency/over frequency methods fail to detect. The theoretical method to obtain wavelet energies is evolved and verified by the experimental result.     

500W光伏并网逆变器设计

光伏并网发电系统是光伏系统发展的趋势.根据光伏并网发电系统的特点,设计了一套额定功率为500W的光伏并网逆变器,该并网逆变器能实现最大功率跟踪和反孤岛效应控制功能,控制部分采用基于TMS320F240型DSP的电流跟踪控制策略,实现了与网压同步的正弦电流输出.     

Multi-cell uplink power allocation game for user minimum performance guarantee in OFDMA systems

The multi-cell uplink power allocation problem for orthogonal frequency division multiplexing access (OFDMA) cellular networks is investigated with the uplink transmission power allocation on each co-frequency subchannel being defined as a multi-cell non-cooperative power allocation game (MNPG). The principle of the design oftbe utility function is given and a novel utility function is proposed for MNPG. By using this utility function, the minimum signal to interference plus noise ratio (SINR) requirement of a user can be guaranteed. It can be shown that MNPG will converge to the Nash equilibrium and that this Nash equilibrium is unique. In considering the simulation results, the effect of the algorithm parameters on the system performance is discussed, and the convergence of the MNPG is verified. The performance of MNPG is compared with that of traditional power allocation schemes, the simulation results showing that the proposed algorithm increases the cell-edge user throughput greatly with only a small decrease in cell total throughput; this gives a good tradeoff between the throughput of cell-edge users and the system spectrum efficiency.     

光伏并网发电模拟装置研制

文中研制了一套模拟并网发电系统,实现了频率跟踪、最大功率跟踪、相位跟踪、输入欠压保护、输出过流保护、反孤岛效应等功能;采用Atmega16高速单片机,实现了内部集成定时、计数器功能;利用定时器T/C2的快速PWM功能,实现SPWM信号的产生;采用T/C1的输入捕获功能,实现了频率相位监测和跟踪以及对失真度、输入电压、输...     

Frequency-Shift Acceleration Control for Anti-Islanding of a Distributed-Generation Inverter

This paper proposes frequency-shift acceleration control for anti-islanding of an inverter-based distributed generator. The proposed control is based on frequency positive feedback in the synchronous $dq$ frame. Overall, the scheme of the proposed control is described. The design methodology of the frequency-shift acceleration gain is presented based on small-signal stability and step input response. The proposed control is intended for zero nondetection zone, noncompromised power quality, and easy implementation without additional devices required. Simulation and experimental results verify that the proposed anti-islanding scheme and the gain design approach are effective and useful in real applications.      

A new hybrid active power filter (APF) topology

In this paper, a new hybrid active power filter topology is presented. A higher-voltage, low-switching frequency insulated gate bipolar transistor (IGBT) inverter and a lower-voltage high-switching frequency metal oxide semiconductor field effect transistor (MOSFET) inverter are used in combination to achieve harmonic current compensation. The function of the IGBT inverter is to support utility fundamental voltage and to compensate for the fundamental reactive power. The MOSFET inverter fulfills the function of harmonic current compensation. To further reduce cost and to simplify control, the IGBT and MOSFET inverters share the same DC-link via a split capacitor bank. With this approach harmonics can be cancelled over a wide frequency range. Compared to the conventional APF topology, the proposed approach employs lower dc-link voltage and generates less noise. Simulation and experimental results show that the proposed active power filter topology is capable of compensating for the load harmonics     

A Wide-Range Active and Reactive Power Flow Controller for a Solid Oxide Fuel Cell Power Conditioning System

A wide-range active and reactive power flow controller is designed to operate the inverter in pure leading, pure lagging, and the mix with active and reactive power conditions. The key to achieving lagging power flow control is to ensure sufficiently high enough dc bus voltage to avoid duty cycle saturation. The key to achieving precision power flow control for a wide range of power level is to adopt the quasi-proportional resonant controller for the current loop and the admittance compensator to cancel the grid-voltage-induced negative power flow. In this paper, the current loop transfer function has been systematically derived for the controller design purpose. Phasor analysis was adopted to explain the need of dc bus voltage requirement. A 5-kVA grid-tie fuel cell inverter was used as the platform to show current loop controller design and admittance compensation. The proposed controller has been simulated, and the same parameters have been used for a DSP-based controller. Both simulation and hardware experimental results agree well with the theoretical analysis.      

High-efficiency power conversion for low power fuel cell generation system

This study presents a newly designed topology for a fuel cell energy source conversion in order to supply a highly reliable utility power. Because the fuel cell has the power quality of low voltage as well as high current due to the electrochemical reaction, a high step-up dc-dc converter is utilized for boosting the fuel cell voltage up to a constant dc-bus voltage for the utilization of later inverter. Moreover, a current-source sine-wave voltage inverter is designed in the sense of voltage-clamping and soft-switching techniques to enable the use of a smaller inductor in the current source circuit and the compression of the voltage stress across switches about two times of the dc-bus voltage. In this power conversion scheme, the output voltage has the salient features of lower distortion, fast dynamic regulating speed and insensitivity to load variation, even under nonlinear loads. In addition, experimental results via an example of a proton exchange membrane fuel cell generation system with 250-W nominal power rating are given to demonstrate the effectiveness of the proposed power conversion strategy. According to the experimental measure, the maximum power inverter efficiency is over 95% and the total harmonic distortions for various load conditions are all within 1.1%.     

Loss-Minimizing Control of PMSM With the Use of Polynomial Approximations

Normally, lookup-table-based methods are being utilized for loss-minimizing control of permanent magnet synchronous motors (PMSMs). But numerous repetitive experiments are required to make a lookup table, and the program size becomes bulky. In this paper, analytic methods for finding the loss-minimizing solution are studied. Since the solution lies either in the interior or on the voltage limit boundary, two different cases are dealt with separately. In both cases, fourth-order polynomials are derived. To obtain approximate solutions, methods of order reduction and linear approximation are utilized. The accuracies are good enough for practical use. These approximate solutions are fused into a proposed loss-minimizing algorithm and implemented in an inverter digital signal processor. Experiments were done with a real PMSM developed for a sport utility fuel cell electric vehicle. The analytically derived minima were justified by experimental evidences, and the dynamic performances over a wide range of speed were shown to be satisfactory.