A combined active anti-islanding method for photovoltaic systems

In a modern power system, photovoltaic as distributed generated source is growing larger and it can cause a variety of problems. The most important problem is that of the islanding phenomenon. In order to prevent islanding phenomenon, three kinds of active islanding detection methods have been studied. These are, respectively, to change magnitude, frequency, and the start phase of inverter output current. Among those, both a frequency variation method and a start-phase variation anti-islanding method make the islanding frequency drift away from the trip window of the frequency relay if islanding occurs. This paper presents a novel combined active anti-islanding method, which consists of a frequency variation method active frequency drift (AFD) and a start-phase variation method slip-mode frequency shift (SMS). Clearly, the proposed anti-islanding method shows the islanding detection ability to IEEE 1547 Standard. To validate the performance of the proposed method, simulations and experiments were performed. Possible islanding conditions are implemented with a unity quality factor by the IEEE Standard 1547. The methodology presented can be extended to the other active anti-islanding methods.     

A high power quality anti-islanding method using effective power variation

Islanding phenomenon is undesirable because it leads to a safety hazard to utility service personnel and may cause damage to power generation and power supply facilities as a result of unsynchronized re-closure. In order to prevent the phenomenon, various anti-islanding methods have been studied. Until now, frequency or start phase shift methods of inverter current have much attention as active anti-islanding methods, which cause reactive power variation to the utility. However, these methods deteriorate usually the power quality like power factor or harmonic performance. This paper proposes a high power quality active anti-islanding method using effective power variation, which is implemented by periodically increasing/decreasing variation of inverter current magnitude. If it causes the large variation of inverter output voltage after islanding, active frequency drift (AFD) method as a simple anti-islanding method will be injected into the inverter current for a designed period and islanding can be detected. In case of large voltage variation when the grid is connected, AFD method will be removed after the designed period. Unlike most active anti-islanding method deteriorating power quality, the proposed method will have high performance of islanding detection and good power quality. For the verification of the proposed method, simulated results and experimental results in addition to analysis are presented using a 3 kW PV inverter.     

Modeling and simulation of a grid-connected PV generation system for electromagnetic transient analysis

This paper addresses modeling and simulation of a grid-connected photovoltaic system (GCPS) to analyze its grid interface behavior and control performance in the system design. A simple circuit model of the solar array is used to easily simulate its inherent characteristics with the basic specification data. Detailed power and protection control of the GCPS as well as its electrical circuits have been represented by user-defined and built-in components to take into account transients in normal and fault conditions, which are dominated by its power electronic controller. The model has been described with the reference to and implemented in PSCAD/EMTDC, a power system transient software package. Extensive simulation results are presented and analyzed to validate that the proposed simulation model is effective for control and protection performance evaluation of the GCPS in terms of electromagnetic transient analysis.     

A voltage regulation method for dispersed grid-connected PV systems under high-density connection

This paper describes a voltage regulation method for dispersed grid-connected PV systems under high-density connection. In the near future, many PV systems will be connected to distribution systems. The reverse power flow of the grid-connected PV systems will raise line voltage to the upper limit of the legal voltage range. In the first part of this paper we examine the voltage characteristics of a distribution system under high-density connection, while the second part of the paper describes a voltage regulation method for high-density PV connections, using reactive power proportional to global irradiance. The results of tests clearly demonstrate a restraining effect on voltage during the daytime and a smoothing effect on voltage fluctuation.     

Development and performance test of smart power conditioner for value-added PV application

It is a disadvantage that PV power generation is only useful for clear daytime. However, a new 24 hours of application in an extended area can be added by active utilization of potential ability of PV power conditioner. For this purpose, a new multi-functional power conditioner was developed, which has a smoothing function to reduce PV output variation and load fluctuation, and also has additional function to compensate harmonics current and reactive power caused by customer's load.As a result of indoor testing, a reduction rate of around for harmonics current and reactive power were achieved. In addition, the reduction rate around for smoothing of PV output variation and load fluctuation were verified.The work was promoted by NEDO as part of the New Sunshine Project in Japan.     

主动频移孤岛检测的抗干扰测频算法

针对并网光伏发电系统主动频移孤岛检测中,并网侧待测信号在过零点附近存在的噪声干扰,对过零测频法和锁相环测频法精度带来的不良影响,提出了一种具有噪声抑制功能的三角变换测频法。通过对最优检测点间隔进行估计和少数频率奇异点的剔除,即使并网光伏发电系统主动频移孤岛检测中并网侧待测信号受噪声干扰,带噪声抑制的三角变换测频法也能避免孤岛效应误判现象,提高了系统运行的可靠性。在MATLAB/Simulink下进行了建模和仿真,验证了该三角变换测频算法的正确性。     

Modeling and simulation of three phase multilevel inverter for grid connected photovoltaic systems

This paper presents a control for a three phase five-level neutral clamped inverter (NPC) for grid connected PV system. The maximum power point tracking (MPPT) is capable of extracting maximum power from the PV array connected to each DC link voltage level. The MPPT algorithm is solved by fuzzy logic controller. The fuzzy MPPT is integrated with the inverter so that a DC–DC converter is not needed and the output shows accurate and fast response. A digital PI current control algorithm is used to remain the current injected into the grid sinusoidal and to achieve high dynamic performance with low total harmonic distortion (THD). The validity of the system is verified through MATLAB/Simulink and the results are compared with three phase three-level grid connected NPC inverter in terms of THD.     

Modeling and simulation of water-gas shift in a heat exchange integrated microchannel converter

The aim of this study is to analyze the operation of a heat exchange integrated, Pt-CeO₂/Al₂O₃ washcoated microchannel water-gas shift (WGS) reactor under fuel processing conditions by mathematical modeling techniques. In this context, operation of a single microchannel is modeled, whose outcomes are compared with experimental data obtained from the literature. Simulations show good agreement with the experimental data, with an error below 4%. Upon its validation, single channel model is used to simulate a heat exchange integrated microchannel reactor, which involves periodically located groups of reaction and air-fed cooling channels. The integrated reactor is modeled by 2D Navier-Stokes equations together with reactive transport of heat and mass. Incorporation of heat exchange function minimizes the impact of thermodynamic limitations on WGS by precise regulation of reaction temperature and gives 77.6% CO conversion, which is 67.4% in the absence of cooling. Improvement in conversion from 69.2% to 77.6% is observed upon increasing feed temperature of the reaction stream from 565 to 595 K, above which the reaction is controlled by equilibrium. Coolant feed temperature, however, changes conversion only by <1%. Isothermal conditions are obtained upon feeding reaction and coolant channels at 595 K and 587 K, respectively. Changes in the thickness and material of the wall between the channels give limited deviations in conversion. An integrated reactor with 2.37 L volume is sufficient for supplying H₂ necessary to drive a 1 kW PEMFC unit.     

Modeling and maximum power point tracking (MPPT) method for PV array under partial shade conditions

Influenced by partial shade, PV module aging or fault, there are multiple peaks on PV array's output power–voltage (P–V) characteristic curve. Conventional maximum power point tracking (MPPT) methods are effective for single peak P–V characteristic under uniform solar irradiation, but they may fail in global MPP tracking under multi-peak P–V characteristics. Existing methods in literature for this problem are still unsatisfactory in terms of effectiveness, complexity and speed. In this paper, we first analyze the mathematical model of PV array that is suitable for simulation of complex partial shade situation. Then an adaptive MPPT (AMPPT) method is proposed, which can find real global maximum power point (MPP) for different partial shade conditions. When output characteristic of PV array varies, AMPPT will adjust tracking strategies to search for global peak area (GPA). Then it is easy for conventional MPPT to track the global MPP in GPA. Simulation and experimental results verify that the proposed AMPPT method is able to find real global MPP accurately, quickly and smoothly for complex multi-peak P–V characteristics. Comparison analysis results demonstrate that AMPPT is more effective for most shade types.     

Modeling and simulation of a stand-alone photovoltaic system using an adaptive artificial neural network: Proposition for a new sizing procedure

This paper presents an adaptive artificial neural network (ANN) for modeling and simulation of a Stand-Alone photovoltaic (SAPV) system operating under variable climatic conditions. The ANN combines the Levenberg–Marquardt algorithm (LM) with an infinite impulse response (IIR) filter in order to accelerate the convergence of the network. SAPV systems are widely used in renewable energy source (RES) applications and it is important to be able to evaluate the performance of installed systems. The modeling of the complete SAPV system is achieved by combining the models of the different components of the system (PV-generator, battery and regulator). A global model can identify the SAPV characteristics by knowing only the climatological conditions. In addition, a new procedure proposed for SAPV system sizing is presented in this work. Different measured signals of solar radiation sequences and electrical parameters (photovoltaic voltage and current) from a SAPV system installed at the south of Algeria have been recorded during a period of 5-years. These signals have been used for the training and testing the developed models, one for each component of the system and a global model of the complete system. The ANN model predictions allow the users of SAPV systems to predict the different signals for each model and identify the output current of the system for different climatological conditions. The comparison between simulated and experimental signals of the SAPV gave good results. The correlation coefficient obtained varies from 90% to 96% for each estimated signals, which is considered satisfactory. A comparison between multilayer perceptron (MLP), radial basis function (RBF) network and the proposed LM–IIR model is presented in order to confirm the advantage of this model.     

IGBT型8—15kHz／250kW双频电源的开发和应用

介绍了ＩＧＢＴ型２５０ｋＷ、８－１５ｋＨｚ双频感应电源。文中较详细地了其原理组成及工艺方式，同时也介绍了该电源的控制电路及其设计原则及应用效果。     

SMC-DPC based active and reactive power control of grid-tied three phase inverter for PV systems

In this paper, sliding mode control (SMC) – direct power controller (DPC) based active and reactive power controller for three-phase grid-tied photovoltaic (PV) system is proposed. The proposed system consists of two main controllers: the DC/DC boost converter to track the possible maximum power from the PV panels and the grid-tied three-phase inverter. The Perturb and Observe (P&O) algorithm is used to transfer the maximum power from the PV panels. Control of the active and reactive powers is performed using the SMC-DPC strategy without any rotating coordinate transformations or phase angle tracking of the grid voltage. In addition, extra current control cycles are not used to simplify the system design and to increase transient performance. The fixed switching frequency is obtained by using space vector modulation (SVM). The proposed system provides very good results both in transient and steady states with the simple algorithms of P&O and SMC-DPC methods. Moreover, the results are evaluated by comparing the SMC-DPC method developed for MPPT and the traditional PI control method. The proposed controller method is achieved with TMS320F28335 DSP processor and the system is experimentally tested for 12 kW PV generation systems.     

分布式电源与电网同步的研究

在分布式电源并网系统中,通过获取电网侧的瞬时相位信息,使逆变器输出与电网保持同频、同相。文章分析了单同步坐标系软件锁相环算法和双二阶广义积分器锁相环算法的性能特点,根据优势互补的方法,通过检测电路检测三相电路的状态,正确地对锁相环进行切换,以快速精确的检测出正序电压和相位,保证系统中的逆变器正常工作。同时通过利用单纯形法对其锁相环的控制器参数进行优化。研究结果表明,根据电网侧的工作状态,合理地切换锁相环的工作模式,可以快速精确地追踪电网相位和频率信息。     

Numerical analysis and parameters optimization of shell-and-tube heat storage unit using three phase change materials

In this paper, a mathematical model of shell-and-tube latent heat thermal energy storage (LHTES) unit of two-dimension of three phase change materials (PCMs) named PCM1, PCM2 and PCM3 with different high melting temperatures (983 K, 823 K and 670 K, respectively) and heat transfer fluid (HTF: air) with flowing resistance and viscous dissipation based on the enthalpy method has been developed. Instantaneous solid–liquid interface positions and liquid fractions of PCMs as well as the effects of inlet temperatures of the air and lengths of the shell-and-tube LHTES unit on melting times of PCMs were numerically analyzed. The results show that melting rates of PCM3 are the fastest and that of PCM1 are the slowest both x, r directions. It is also found that the melting times of PCM1, PCM2 and PCM3 decrease with increase in inlet temperatures of the air. Moreover, with increase in inlet temperatures of the air, decreasing degree of their melting times are different, decreasing degree of the melting time of PCM1 is the biggest and that of PCM3 is the smallest. Considering actual application of solar thermal power, we suggest that the optimum lengths are L₁ = 250 mm, L₂ = 400 mm, L₃ = 550 mm (L = 1200 mm) which corresponds to the same melting times of PCM1, PCM2 and PCM3 are about 3230 s and inlet temperature of the air is about 1200 K. The present analysis provides theoretical guidance for designing optimization of the shell-and-tube LHTES unit with three PCMs for solar thermal power.     

Synthesis of PdV/C nanoparticles using phase transfer method for oxygen reduction in alkaline electrolytes

Bimetallic PdV/C nanoparticles have been synthesized by phase-transfer method for catalyzing the oxygen reduction reaction (ORR) in alkaline electrolytes. PdV/C nanoparticles with different V contents are spherical with a mean diameter of 3–4 nm, and the addition of V expands the lattice parameter of Pd as shown by XRD. XPS analysis indicated the binding energy of Pd⁰ 3d peak increased by ca. 1 eV. Density functional theory (DFT) calculations results shows d-band center of Pd down-shift after V-doping. Based on the electrocatalytic results, introducing V can improve the catalytic activity for ORR, methanol crossover tolerance and stability. Especially, Pd₄V/C shows higher initial potential (E_onset = 1.027 V), excellent methanol crossover tolerance (98.03% retained) and long-term stability (81.30% retained), which are comparable with Pt/C_JM. This work provides a new method of synthesizing PdV/C nanoparticles, which have the potential to be used as the cathode electrocatalysts for fuel cells.     

Experimental determination of the effective thermal capacity function and other thermal properties for various phase change materials using the thermal delay method

The recently presented thermal delay method is an improved version of the well-known T-history method, which is widely used for thermal properties measurement of phase change materials (PCM). The most important difference between the thermal delay and the T-history methods is that the former is based on the use of thermal delay (i.e. temperature difference) between PCM and a reference fluid at any specified time while the latter makes use of their time delay at any specified temperature. Although the thermal delay method has been documented in our previous publication, measurements are performed of the known and indisputable values of ethyl alcohol thermal capacity and the latent heat of the double distilled water (WFI), which confirm the accuracy of the method. Additional comparisons with values provided by PCM producing companies show disagreements lower than 1.7%. The main volume of measurements of the present study includes the following thermal properties of various practically interesting PCM: (a) the temperatures at the ends of the two-phase region; (b) the liquid and solid PCM thermal capacities; (c) the phase change heat; (d) the heat storage capacity during any specified temperature range; and (e) the effective thermal capacity function, which is a very important and useful property for practical applications. The above function is provided for each one of the PCM examined in the form of diagrams, as well as in the form of analytical expressions derived by curve fitting to the processed experimental values. All measurements were repeated 20 times and the results were averaged in order to minimize errors from accidental incidents.     

Modeling and control design of hydrogen production process for an active hydrogen/wind hybrid power system

This paper gives a control oriented modeling of an electrolyzer, as well as the ancillary system for the hydrogen production process. A Causal Ordering Graph of all necessary equations has been used to illustrate the global scheme for an easy understanding. The model is capable of characterizing the relations among the different physical quantities and can be used to determine the control system ensuring efficient and reliable operation of the electrolyzer. The proposed control method can manage the power flow and the hydrogen flow. The simulation results have highlighted the variation domains and the relations among the different physical quantities. The model has also been experimentally tested in real time with a Hardware-In-the-Loop Simulation before being integrated in the test bench of the active wind energy conversion system.     

Facile synthesis of M-CuFe2O4 (M= Al2O3, CeO2, La2O3, ZrO2, Mn2O3) catalysts using a one-pot method and their applications in high-temperature water gas shift reaction

Water gas shift reaction is an essential process of hydrogen production and carbon monoxide removal from syngas. In this study, the promotional effect of ZrO₂, CeO₂, La₂O₃, Al₂O₃, and Mn₂O₃ was investigated on the CO conversion and thermal stability of the copper ferrite in high-temperature water gas shift reaction (HTSR) and hydrogen purification. The powders were synthesized by a simple solid-state route and characterized by XRD, H2-TPR, SEM, FT-IR, TG-DTA, and BET analyses. Promoters (ZrO₂, CeO₂, La₂O₃, Al₂O3, and Mn₂O₃) could affect the WGSR performance in activity and stability. In the M-CuFe2O4 catalyst, alumina acts as a texture promoter and aids in the fine dispersion of copper ferrite. The results indicated that the surface area of the Al₂O₃–CuFe₂O₄ (210 m²/g) catalyst was higher than the other samples. This catalyst presented higher CO conversion in HTSR and had higher stability at 1000 min on stream. It was found that the incorporation of different contents of alumina had a significant influence on the textural and catalytic properties of the CuFe₂O₄-based catalysts. The 30%Al₂O₃–70%CuFe₂O₄ catalyst exhibited the highest CO conversion of 65% at 350 °C, uniform pore size distribution, and intense interaction between copper ferrite and alumina, causing the effective stabilization of the active phase in the catalyst structure. The findings of this study represent that the solid-state method, due to its simplicity and creation of a mesoporous structure, can also be applied for the preparation of many heterogeneous metal oxide catalysts.     

Modeling and design of a 25 MW osmotic power plant (PRO) on Bahmanshir River of Iran

Osmotic energy is one of the renewable energies. This osmotic energy source is easily obtainable all over the world where river water flows into the ocean or sea. In this article, Bahmanshir River in Iran which falls into the Persian Gulf was considered as selected place for modeling and design of 25 MW osmotic power plant in PRO method. Results indicates that for a 15 year return on investment, annual increase in purchase price of electricity 10% and interest rate 6%, sale price of electricity should be 0.4117 €/KWh which is highly expensive compared to state's suggested price of electricity from renewable source (i.e. 0.09 €/KWh).In order to balance purchase and sale price of electricity, unit price of intake and outfall, unit price of micro-filtration and interest rate should be reduced 9.1%, 15% and 25% respectively and Δλ should be increase 4.3%.     

Probability strength design of steam turbine blade and sensitivity analysis with respect to random parameters based on response surface method

Many stochastic parameters have an effect on the reliability of a steam turbine blade during practical operation. To improve the reliability of blade design, it is necessary to take these stochastic parameters into account. An equal cross-section blade is investigated and a finite element model is built parametrically. Geometrical parameters, material parameters and load parameters of the blade are considered as input random variables while the maximum deflection and maximum equivalent stress are output random variables. Analysis file of the blade is compiled by deterministic finite element method and applied to be loop file to create sample points. A quadratic polynomial with cross terms is chosen to regress these samples by step-forward regression method and employed as a surrogate of numerical solver to drastically reduce the number of solvers call. Then, Monte Carlo method is used to obtain the statistical characteristics and cumulative distribution function of the maximum deflection and maximum equivalent stress of the blade. Probability sensitivity analysis, which combines the slope of the gradient and the width of the scatter range of the random input variables, is applied to evaluate how much the output parameters are influenced by the random input parameters. The scatter plots of structural responses with respect to the random input variables are illustrated to analyze how to change the input random variables to improve the reliability of the blade. The results show that combination of the finite element method, the response surface method and Monte Carlo method is an ideal way for the reliability analysis and probability strength design of the blade. __________ Translated from Proceedings of the CSEE, 2007, 27(20): 12–17 [译自: 中国电机工程学报]