An evaluation method of the fluctuation characteristics of photovoltaic systems by using frequency analysis

Short-time fluctuations in solar irradiance will become an important issue with regard to future embedded photovoltaic (PV) systems. However, when PV systems are intensively installed, fluctuation of total output in clustered PV systems is not remarkable because there is the smoothing effect of irradiance in certain areas. In this paper, a new estimation method of irradiance fluctuation, which is based on the combination of the Fourier transform and the wavelet transform methods, is described.     

Study on areal solar irradiance for analyzing areally-totalized PV systems

This paper describes the methodology for quantification of solar irradiance fluctuation in order to analyze the areally totalized PV systems, which consist of a large number of PV systems connected to each other in the optimum size of a certain community area. Two factors quantifying the fluctuation, fluctuation factor and power spectral density, and some examples are presented. Also cross-correlation analysis between nine stations' measurements were operated to estimate the extent of the power fluctuation in a certain area size and to determine optimum size of the area of areally localized PV systems.     

Validation of the Sandia model with indoor and outdoor measurements for semi-transparent amorphous silicon PV modules

This paper presents a set of indoor and outdoor measurement methods and procedures to determine the empirical coefficients of the Sandia Array Performance Model (SAPM) for a semi-transparent amorphous silicon (a-Si) PV module. After determining and inputting the total 39 parameters into the SAPM, the dynamic power output of the a-Si PV module was predicted. In order to validate the accuracy of using SAPM for simulating the energy output of the a-Si PV module, a long-term outdoor testing campaign was conducted. The results indicated that the SAPM with indoor and outdoor measured coefficients could accurately simulate the energy output of the a-Si PV module on sunny days, but it didn't work well on overcast days due to the inappropriate spectral correction as well as the equipment measuring error caused by the intense fluctuation of solar irradiance on overcast days. Specifically, all the errors between the simulated daily energy output and the measured one were less than 4% on sunny days. In order to achieve a better prediction performance for a-Si PV technologies, the SAPM was suggested to incorporate a more comprehensive spectral correction function to correct the impact of solar spectrum on overcast days in future.     

Voltage fluctuations on distribution level introduced by photovoltaic systems

In moderate climates, short fluctuations in solar irradiance and their impact on the distribution grid will become an important issue with regard to the future large-scale application of embedded photovoltaic systems. Several related studies from the past are recalled. The approach that is presented here applies a localized spectral analysis to the solar irradiance and derived quantities in order to determine the power content of fluctuations, depending on their characteristic persistence. Pseudorandom time series of solar irradiance, based on measured values of the instantaneous clearness index, are applied as input data. Power-flow calculations are carried out in order to assess the impact of fluctuating solar irradiance on the grid voltage. The "fluctuation power index" is defined as a measure for the mean-square value of fluctuations of a specific persistence. A typical scenario is simulated, and the results are interpreted.     

Influence of photovoltaic generation model and time resolution on the reliability evaluation of distribution systems

This paper aims to investigate the influence of photovoltaic (PV) generation on reliability evaluation of distribution systems. Two PV generation models are used to predict the output power injected into the grid, taking into account the main relevant environmental variables, the irradiance and ambient temperature. Issues that directly affect the output power, such as the spatial smoothing effect due to the plant size and the influence of the irradiance and temperature measurement interval are taken into consideration. Using measurement time series of irradiance and local temperature, the models are used to generate power series in 4‐minute and hourly resolutions. The generated power series are used in a reliability assessment model, with the objective of evaluating the impact of solar resource variability on the reliability indices of the system. Case studies on the IEEE RBTS‐Bus 2 and on the real distribution system of Fernando de Noronha in Brazil are presented and discussed, for power plants of different capacities, considering the effect of the PV generation models, the temporal resolution of the time series and the spatial smoothing of the power output fluctuations. The results show that the power time series in hourly resolution significantly underestimates the frequency of interruptions. For the real system, this index is underestimated at the system level (up to 43%) and at the load points (up to 72%). On the other hand, for the interruption duration index, the temporal aggregation results in a small underestimation (just 4%). The results also indicates that the smoothing effect is irrelevant for typical PV system sizes of distribution systems with discretization equal to or above 4 minutes.     

Dynamic evaluation of maximum power point tracking operation with PV array simulator

This paper presents an innovative method to measure the dynamic control ability of maximum power point tracking for PV inverters under the condition of irradiance fluctuation. The PV array I–V curve simulator is a kind of indoor testing facility and easy to be adopted by industries. Basic functions are given by a specially designed PV array I–V curve simulator composed of the active power load. Most of the parameters are controllable by sophisticated software with capability of treating a lot of 1-s data for a very long period of time. In this paper, detailed structure of the equipment is described and test examples are also given by using a commercial PV inverter.     

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.     

基于光伏组件的太阳能资源观测

鉴于太阳能资源的测量和评价是太阳能开发利用的重要基础,按照光伏电池两种主要安装方式(倾斜固定和太阳跟踪),利用单晶、多晶和非晶三种典型的光伏组件设计进行了太阳能光伏资源观测试验,获得了各季节典型晴天条件下各类型光伏组件辐照度的日变化特征和倾斜面光伏组件一年中月均每日可发电量的极大值、极小值及其月份。通过对比各类型光伏组件在太阳跟踪器上和纬度倾斜面上光伏辐照度变化,得出跟踪光伏组件日均光伏曝辐量与倾斜光伏组件日均光伏曝辐量的相比较优势。根据光伏组件的观测结果推算出各类型光伏组件的光伏反演辐照度,与气象辐射观测用总辐射表的总辐射辐照度趋势非常一致,在太阳能光伏主要利用时段相对误差基本在10%以内。     

多时间尺度下光伏功率波动特性概率分布研究

基于武汉地区分布式光伏电站大量实测数据,运用广义高斯分布和有限学生t混合模型等多种概率模型对不同时间尺度下光伏功率波动特性建模,发现在10～15min时间尺度下广义高斯分布最适用于描述分布式光伏功率变化的概率分布,而在30～60min时间尺度下高斯混合模型拟合效果最好。在此基础上,建立了逐时光伏出力波动与辐射量波动模型,用于定量分析光伏电站能量输出波动,可有效降低光伏功率波动随机性和不确定性对电力系统运行造成的影响,有利于提高光伏并网渗透率。     

A new approach in MPPT for photovoltaic array based on Extremum Seeking Control under uniform and non-uniform irradiances

This work presents a Maximum Power Point Tracking (MPPT) based on analyzing the output characteristics of PV array under uniform irradiance and partial shading conditions. In order to carry out MPPT in PV panels, under partial shading conditions a method based on Extremum Seeking Control (ESC) is introduced. In contrast with classic ESC, in this method the double of dithering signal frequency is not used, consequently PV output power has a ripple of a lower frequency. Also the drop which occurs when MPPT system starts to operate in classic ESC method is minimized in this paper. The ESC approach for MPPT in this paper uses a series combination of a Low Pass Filter (LPF) and a High Pass Filter (HPF). These two filters act as a Band Pass Filter (BPF) and let a specific frequency of input power which includes the derivative of PV with respect to its voltage pass through. Finally, the system does not operate in local optimal points for efficient point will be global. The algorithm adds partial shadow judging conditions in ESC method. The system runs the variable step ESC method to realize MPPT when photovoltaic array is under uniform irradiance. Under Partial Shading Condition (PSC), the control method can eliminate the interference of local maximum power point (MPP) to make 23 the PV array running at global MPP. In addition, unlike other methods, the proposed MPPT operates on the global MPPs. The proposed MPP tracker does not add any extra complexity compared to the classical ones. However, it increases significantly the efficiency of the PV installation under PSC. We will show that under uniform irradiance, the proposed MPPT leads to faster performances than classical approaches.     

非晶硅与晶硅太阳电池在太阳能光伏发电应用中热性能的研究

对在北京地区屋面上固定角度安装(目前光伏发电应用中最常见的安装形式)的非晶硅和多晶硅太阳电池组件进行了近二年的数据采集,纪录了北京地区温度数据和太阳电池阵列的实际发电量,分析了它们各自的特点,为用户更为关心的户外使用情况提供了参考依据;认为如果仅从温度特性考虑,是否采用非晶硅替代晶体硅电池在不同地区应有不同考虑,如果再考虑到人们普遍认为的非晶硅电池没有解决的稳定性问题,表面玻璃的非钢化、效率低等其它问题,非晶硅的使用应慎重,不应盲从.同时在使用中不论何种电池都不应忽视组件的通风问题.     

Wavelet cross-correlation analysis of wind speed series generated by ANN based models

To obtain, over medium term periods, wind speed time series on a site, located in the southern part of the Paris region (France), where long recording are not available, but where nearby meteorological stations provide large series of data, use was made of ANN based models. The performance of these models have been evaluated by using several commonly used statistics such as average absolute error, root mean square error, normalized mean square error, and correlation coefficient. Such global criteria are good indicators of the “robustness” of the models but are unable to provide useful information about their “effectiveness” in accurately generating wind speed fluctuations over a wide range of scales. Therefore a complementary wavelet cross coherence analysis has been performed. Wavelet cross coherence, wavelet cross-correlation and spectral wavelet cross-correlation coefficients, have been calculated and displayed as functions of the equivalent Fourier period. These coefficients provide quantitative measures of the scale-dependence of the model performance. In particular the spectral wavelet cross coherence coefficient can be used to have a rapid and efficient identification of the validity range of the models. The results show that the ANN models employed in this study are only effective in computing large-scale fluctuations of large amplitude. To obtain a more representative time series, with much higher resolution, small-scale fluctuations have to be simulated by a superimposed statistical model. By combining ANN and statistical models, both the high and the low-frequency segments of the wind velocity spectra can be simulated, over a range of several hours, at the target site.     

Design of a sun tracker for the automatic measurement of spectral irradiance and construction of an irradiance database in the 330–1100 nm range

An automatic global and direct solar spectral irradiance system has been designed based on two LICOR spectroradiometers equipped with fibre optics and remote cosine sensors. To measure direct irradiance a sun tracker based on step motors has been developed. The whole system is autonomous and works continuously. From the measurements provided by this system a spectral irradiance database in the 330–1100 nm range has been created. This database contains normal direct and global horizontal irradiances as well as diffuse irradiance on a horizontal plane, together with total atmospheric optical thickness and aerosol optical depth.     

Monitoring and remote failure detection of grid-connected PV systems based on satellite observations

Small grid-connected photovoltaic systems up to 5 kW_p are often not monitored because advanced surveillance systems are not economical. Hence, some system failures which lead to partial energy losses stay unnoticed for a long time. Even a failure that results in a larger energy deficit can be difficult to detect by PV laymen due to the fluctuating energy yields.Within the EU project PVSAT-2, a fully automated performance check has been developed to assure maximum energy yields and to optimize system maintenance for small grid-connected PV systems. The aim is the early detection of system malfunctions and changing operating conditions to prevent energy and subsequent financial losses for the operator. The developed procedure is based on satellite-derived solar irradiance information that replaces on-site measurements. In conjunction with a simulation model the expected energy yield of a PV system is calculated. In case of the occurrence of a defined difference between the simulated and actual energy yield, an automated failure detection routine searches for the most probable failure sources and notifies the operator.This paper describes the individual components of the developed procedure—the satellite-derived irradiance, the used PV simulation model, and the principles of the automated failure detection routine. Moreover, it presents results of an 8-months test phase with 100 PV systems in three European countries.     

Using CAD software to simulate PV energy yield - The case of product integrated photovoltaic operated under indoor solar irradiation

In this paper, we show that photovoltaic (PV) energy yields can be simulated using standard rendering and ray-tracing features of Computer Aided Design (CAD) software. To this end, three-dimensional (3-D) sceneries are ray-traced in CAD. The PV power output is then modeled by translating irradiance intensity data of rendered images back into numerical data. To ensure accurate results, the solar irradiation data used as input is compared to numerical data obtained from rendered images, showing excellent agreement. As expected, also ray-tracing precision in the CAD software proves to be very high. To demonstrate PV energy yield simulations using this innovative concept, solar radiation time course data of a few days was modeled in 3-D to simulate distributions of irradiance incident on flat, single- and double-bend shapes and a PV powered computer mouse located on a window sill. Comparisons of measured to simulated PV output of the mouse show that also in practice, simulation accuracies can be very high. Theoretically, this concept has great potential, as it can be adapted to suit a wide range of solar energy applications, such as sun-tracking and concentrator systems, Building Integrated PV (BIPV) or Product Integrated PV (PIPV). However, graphical user interfaces of ‘CAD-PV’ software tools are not yet available.     

基于数据预处理与特征表示的多核SVM短期光伏发电预测

针对气温、降雨等气象数据有时难以获得且仅有的辐照度、光伏发电功率属于多源数据、毛刺多、具有不同特征的问题,提出了基于数据预处理与特征表示的多核支持向量机光伏预测方法,利用小波阈值分析法对光伏发电功率与辐照度进行去噪处理,通过对辐照度特征表示参数提取,进行SOM与k-means结合的二次聚类选取相似日,并采用多核支持向量机进行预测。结果表明,小波阈值去噪后能大幅提高预测精度,仅用辐照度与功率数据进行预测也能取得较高的预测精度,且多核预测精度高于单核预测精度。     

A parametric study of a photovoltaic panel with cylindrical fins under still and moving air conditions in Iraq

Photovoltaic (PV) panel temperature plays a vital role in the performance of PV panels. When PV temperature increases, the electrical power and efficiency decrease. In the present study, a simple simulated model is derived and implemented for a 70‐W finned PV panel and the results are compared with those for a reference (unfinned) PV panel. The effect of various parameters including fin diameter, fin height, ambient temperature, fin coverage‐to‐area ratio, and solar irradiance level on PV performance are examined. It is concluded that more electrical power is generated under moving air than under still air conditions as solar irradiance increases. The optimum values for fin diameter, fin height, and fin coverage‐to‐area ratio are 0.005 m, 0.25 m, and 50%, respectively. The maximum simulated power difference between the finned and unfinned PV panels is 5 W under realistic weather conditions. The amount of power generated during the summer season would be about 3.32 kW·h greater than that by the reference PV panel over the same period. The root square percent deviation values between the results of the proposed model and previously published experimental data for panel temperature, electrical power, and electrical efficiency are 3.36%, 5.26%, and 5.48%, respectively.     

一种并网型风光互补发电系统的建模与仿真

分析了风光互补发电系统的技术优势,设计了基于固态变压器结构的并网型风光互补发电系统。分别建立了光伏系统,风力发电系统,超级电容和蓄电池的模型,并分析各环节的控制策略,提出了基于平均功率的储能设备容量配置方法。仿真结果表明,该系统能模拟风光互补系统在不同模式下的运行特性,可以有效降低功率波动和维持电压稳定,并能在低光照强度、低风速等情况下为系统提供短时能量支撑。     

聚光型光伏系统光电耦合性能研究

针对复合抛物面聚光型光伏（PV-CPC）系统,建立光电耦合模型,模拟不同聚光比下光伏阵列表面太阳辐照度分布规律,并比较3种不同网格电路模型的适用性和准确性。结果表明：采用离散网格照度方法模拟精度最高,同时聚光型光伏系统在提升发电功率的同时也导致一定效率损失。当聚光比增大时,光伏阵列表面辐照度值及不均匀性增大,最佳聚光面处的照度分布均匀性优于CPC出口处;光伏阵列的发电功率和效率与太阳总辐照度呈正相关、散射比呈负相关关系。     

光伏电站中光伏组件串联数的优化设计

目前在光伏电站设计中,光伏组件串联数的计算广泛使用GB 50797-2012或IEC 62548-2016中的计算方式,假定在极端环境低温的情况下对光伏组串的开路电压、最大功率点电压等进行计算,进而确定光伏组件成串数量,即光伏组件串联数.但随着对光伏电站度电成本的要求日益严苛,上述对于计算光伏电站的光伏组件串联数的要求...