Modeling and analysis of current harmonic distortion from grid connected PV inverters under different operating conditions

Due to the fast growth of photovoltaic (PV) installations, concerns are rising about the harmonic distortion generated from PV inverters. High current total harmonic distortion (THD) occurs when PV inverters operate under light load conditions due to low solar insolation. A general model modified from the conventional control structure diagram is introduced to analyze the harmonic formation process. Causes of the current harmonics are summarized and its relationship with output power levels is analyzed. The field measurement for current harmonics is carried out at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) microgrid to validate the analysis results. In particular for two-stage inverter, unlike previous papers that assume the DC-link voltage is constant, the DC-link voltage ripple is identified as the source of a series of odd harmonics. A mew periodic time-varying model is proposed by including the DC-link voltage ripple into the conventional current control loop model. This model is able to simulate the characteristics of the harmonic components and show their dependence on the DC-link voltage ripple.     

Daily wind speed harmonic analysis for Marmara region in Turkey

Presently exploited rates of fossil fuels are expected to deplete resources within the next 40–50 years, and consequently, human beings seek alternative energy resources that are clean, friendly to the environment and sustainable. Accumulation of carbon dioxide in the lower layers of the atmosphere may cause climate change and consequent occurrence of floods, intensive rainfalls and droughts. In order to reduce such dangerous effects all countries have to try to improve their energy resources quality and, if possible, to replace fossil fuels, such as coal, with the renewable alternatives of wind, solar and solar-hydrogen energies. Among these, wind power has a priori significance for Turkey. Wind time series depend very much on meteorological measurements of wind direction and velocity. Unfortunately, in many parts of the world, it is difficult to obtain such data for wind speed time series assessments.In this study, harmonic analysis is used to model the daily wind speed values recorded at ten stations in the Marmara region, Turkey, with distinct meteorological conditions from 1993 to 1997. The coefficients, amplitude, variance and phase angle, of each harmonic are calculated for the months of January, April, July and October, leading to total variance maps for spatial interpolations. It is seen that up to the 9th harmonic more than 80% of the total variance can be presented. The western and eastern parts of the Marmara region have different wind pattern characteristics. The contributions of each harmonic to the total variance are calculated, and then regional variance maps are evaluated.     

Differential Ångstrom model for predicting insolation from hours of sunshine

The Ångstrom model for predicting insolation is limited in scope beacuse it gives equal weighting to sunshine hours recorded at any time of the day. The differential Ångstrom model presented in this paper removes this limitation and relates insolation, q_j, in the j^th hour to the sunshine duration, n_j, of the same period by the equation: q_j = a_j + b_j. By regression analysis of monthly data, the set of constants a_j and b_j for each hour of each month of the year can be determined. Thus, using the appropriate set of a and b regression coefficients, any sunshine data can be transformed to insolation. The sum of the equation over a day gives the daily insolation from which monthly means can be calculated. The method has been applied to the 1986 and 1988 sunshine data recorded at the University of Papua New Guinea to predict the observed insolation to within 3.5%. The differential Ångstrom method has applications in places which have much recorded data on hours of sunshine but have limited observed insolation data.     

Economic Evaluation of a Residential Photovoltaic System based on a Probability Model using Actual Meteorological Data

To design a photovoltaic (PV) generation system economically, it is necessary to use data of the total insolation on a horizontal surface. However, such data is only the total daily value and does not represent the power variation caused by the cloud cover. This paper presents the probability method which represents not only the average but also the variance of the PV generation power, and shows simulated results using this methodology. This study's results indicate that the distribution of the PV power divided by the estimated value of the total insolation on a tilted surface is similar to a normal distribution and that a residential (privately-owned) system without storage, whose PV capacity is more than 2 kWp, has little effect upon the reduction of the energy of an average Japanese household.     

DAILY INSOLATION FORECASTING USING A MULTI-STAGE NEURAL NETWORK

So far a single-stage neural network has been proposed to forecast the insolation of the next day. The mean error of the forecast insolation by the single-stage neural network is about 30%. In this paper, a multi-stage neural network is developed for further reduction of the mean error. A first-stage neural network forecasts the average atmospheric pressure of the next day from atmospheric pressure data of the previous day. A second-stage neural network forecasts the insolation level of the next day from the average atmospheric pressure and weather data of the previous day. A third-stage neural network forecasts the insolation of the next day from the insolation level and weather data of the previous day. Meteorological data at Omaezaki, Japan in 1988–1993 are used as input data, and the insolations in 1994 are forecast. The insolations forecast by the multi-stage and the single-stage neural networks are compared with the measured ones. The results show that the mean error reduces from about 30% (by the single-stage) to about 20% (by the multi-stage).     

On the estimation of global solar insolation

A curve suitable for estimation of global solar energy insolation in equatorial regions has been generated from humidity considerations using an empirical “universal” formula and experimental solar insolation data recorded for over 20 yr at specific locations in Kenya, East Africa. The curve has then been used to estimate global insolation in other locations in Kenya and the neighbouring countries with longstanding experimental data. The agreement between the experimental and theoretical insolation values is found to be satisfactory. This curve, we believe, can serve a useful purpose in estimating global solar insolation in other tropical locations with similar climatic conditions as those in Kenya which lack proper global solar insolation acquisition equipment.     

Steady-state performance of DC motors supplied from photovoltaicgenerators with step-up converter

A simple step-up converter circuit consisting of a single power transistor and an inductor is used as an interface between a PV (photovoltaic) generator and a shunt DC motor driving a centrifugal water pump. The step-up converter allows maximum power output from the PV generator to the motor at all insolation levels. Steady-state performance of the motor is vastly improved as its input voltage and current are stabilized by the regenerative action of the converter. The PV generator operates at maximum power regardless of insolation variations. The converter duty ratio can be set at a fixed optimal value which is valid for all insolation levels. This remarkable property makes this device economically attractive since it is easy to build and does not require any insolation-dependent control as compared to other peak-power tracking devices     

Characterization of insolation data for use in photovoltaic system analysis models

A statistical technique to characterize insolation data for use in photovoltaic (PV) systems is presented. We start by examining the frequency distribution of long-term insolation data. The histogram is generated for observed insolation for a particular hour over a month for a number of years. It is fitted to three distributions (Weibull, β and log normal). Four goodness-of-fit criteria are employed in checking the best fit. These are Chi-square, Kolmogorov-Smirnov, Cramer-Von Mises-Smirnov, and log-likelihood. SOLMET data from Sterling, Va, Raleigh-Durham, N.C. and Miami, Fla are analyzed. It is found that the β distribution fits the long-term hourly global horizontal insolation data best for these three southeastern U.S. locations.     

Evaluation of models for predicting insolation on slopes within the Colorado alpine tundra

This empirical study evaluates insolation predictions for the Colorado tundra from models based upon isotropic and anisotropic distribution approximations for diffuse sky and reflected solar radiation. The data set of hourly insolation values was obtained from 40 locations on an alpine ridge by simultaneous measurement of direct beam irradiance and total insolation to the horizontal ridge crest and two nearby sloping surfaces. Six models are used to predict insolation, two based on the isotropic distribution of diffuse solar radiation and four on anisotropic diffuse distribution fields. Three models (one isotropic and two anisotropic) employ measurements of normal direct beam irradiance while the other 3 models incorporate the correlation between the “clearness index” and the ratio of diffuse sky to total insolation. The precision of insolation estimates from models using the correlation method is only slightly less than from other models. Accounting for the increase of diffuse radiation in the circumsolar sky improves insolation predictions. However, inclusion of additional regions of diffuse radiation anisotropy decreases model accuracy. Errors of insolation estimates for the alpine tundra from all models vary in a systematic manner as a function of relative azimuth and ground slope angles.     

The frequency distribution of daily insolation values

Frequency distributions of insolation values are needed in order to derive simple correlations for predicting the performance of solar energy systems. In this paper the frequency distribution of daily total hemispherical solar irradiation values on the horizontal surface is derived from measured data for 90 locations in the U.S. The results can be approximated by generalized distribution curves which depend only on the clearness index, defined as ratio of terrestrial over extraterrestrial insolation. The results agree well with the curves originally derived by Liu and Jordan, except for a correction at high insolation values. The deviation of individual locations from the generalized curves is examined. A breakdown according to time of year reveals some seasonal variation. The theoretical distribution corresponding to random insolation sequences is also derived; it agrees very well with the generalized frequency distribution curves.     

A fuzzy based method for leveling output power fluctuations of photovoltaic-diesel hybrid power system

A Photovoltaic system’s output power fluctuates as insolation varies with weather condition. Fluctuating PV power causes frequency deviations when large PV power is penetrated in the isolated utility. In this paper, a fuzzy based method for leveling the fluctuations of PV power in a PV-diesel hybrid power system is proposed. By means of the proposed method, output power control of PV system becomes possible considering power utility conditions and the conflicting objective of output power leveling and maximizing energy capture is achieved. Here, fuzzy control is used to generate the output leveling power command. The fuzzy control has three inputs of average insolation, variance of insolation, and absolute average of frequency deviation. First, the proposed method is compared with the method where captured maximum power is given to the utility without leveling. Second, the proposed method is compared with a conventional method where captured maximum power is leveled by using an energy storage system and is given to the isolated utility. Simulation results show that the proposed method is effective in leveling PV power fluctuations and is feasible to reduce the frequency deviations of the isolated power utility.     

Study of diffuse and global radiation characteristics in India

Recently, the Indian Meteorological Department has made available, for 13 locations in India, pyranometric data for total and diffuse radiation on an hourly and daily basis. The period of observation is from 1957 to 1975. This data is analysed to reexamine the correlations between monthly-average daily values of diffuse and total insolation and between hourly and daily insolation. The relationship between monthly diffuse/total and total/extraterrestrial ratios is found to be linear. The present correlation implies that the diffuse component is significantly larger than that predicted by other correlations. No noticeable effect of location or seasonal variation was found. Comparison of the present data with earlier studies for India indicates a trend of increasing diffuse radiation with the lapse of time. The ratio of hourly to daily insolation agrees with the Liu and Jordan correlation for total insolation but differs significantly for diffuse radiation. The need for refinement of the Liu and Jordan correlation between hourly and daily diffuse radiation is pointed out.     

Transient multidimensional second law analysis of solar collectors subjected to time-varying insolation with diffuse components

This paper presents an unsteady two-dimensional analysis of a flat-plate solar collector subjected to time varying insolation with considerable diffuse components as a better characterization of practical solar collectors. The analysis considers the thermal masses of the absorber, tube, glazing, and the working fluid in the system, as well as the different optical and thermodynamic properties of beam and diffuse radiation. Using two sets of insolation data, one for a near clear day and the other for an overcast day, an exergetic optimization of the system was carried out and flow rates which maximize the total exergy output (extracted and accumulated) were determined for flow update periods of once every 30 min, 1 h, and the entire daylight period. The instantaneous optimum flow rates were found to follow the insolation pattern. On a daily basis, the optimum exergetic efficiencies and optimum flow rates were almost independent of the choice of the interval of fluid update, but were about 30% and 10% respectively, higher for the clear day than for the overcast day.     

An operational system for mapping insolation from goes satellite data

A system for mapping the mesoscale distribution of insolation from GOES satellite data has become operational at the University of Wisconsin Space Science and Engineering Center. An energy balance model of the earth-atmosphere system is used in conjunction with radiometrically calibrated GOES measurements to estimate surface insolation from satellite images taken at hourly intervals. Integration of these hourly estimates into daily insolation totals has produced results within 9 per cent (standard error of the mean) of pyranometer measurements on a much finer space scale than is available from the surface pyranometer network.The model has been applied, using satellite data display facilities and software designed for large satellite data sets, to produce monthly maps of insolation over large geographical areas at a resolution of approx. 12 × 12 km. Although the density of the surface pyranometer network is inadequate to directly verify the mapping results, the quality of the daily insolation estimates compared with the measurements of single pyranometers suggests that the insolation maps will have a similar accuracy.     

Report on the Photovoltaic R&D Program in Hawaii

Photovoltaic systems provide one of the best options to generate energy in Hawaii, a state that is more than 90 percent dependent on imported oil for its energy supply. Hawaii's excellent year-round insolation rates will contribute to the success of solar energy projects. The Hawaii Natural Energy Institute's program to accelerate utilization of PV power in Hawaii has thus far consisted of a continuation of a 50-year data base of solar insolation; an experimental program with grid-connected residential PV systems; and public information dissemination on PV technology and performance. HNEI is now developing a program that includes: (1) compiling a solar data base that includes a full solar resource assessment, with spectral analysis and measurement of direct insolation, (2) tests of various PV devices relative to insolation; (3) test and evaluation of contemporary PV powered systems (e.g. water pumping, refrigeration, communication and stand-alone residential applications) under various tropical island conditions; and (4) a technology transfer effort aimed at Hawaii and other Pacific Basin islands, including cost-benefit and market analyses.     

The utilizability method with hourly vs daily insolation data

The significant difference between using hourly vs daily insolation data in the utilizability method is demonstrated for both the specific case of Bet Dagan, Israel and the more general case of utilizability curves that are generated from hourly and daily insolation correlations. For the annual utilizability curves for flat-plate collectors, the curves based on daily insolation data are shown to be significantly different from the corresponding curves based on hourly data, with the differences between the curves increasing with increasing threshold. A simple explanation for this observation is presented and the importance of the need for accurate utilizability curves at high thresholds is discussed.     

Estimating insolation based on PV-module currents in a cluster of stand-alone solar systems: Introduction of a new method

In order to evaluate the performance of solar home systems (SHS), data on local insolation is a prerequisite. We present the outline of a new method to estimate insolation if direct measurements are unavailable. This method comprises estimation of daily irradiation by correlating photovoltaic (PV)-module currents from a number of solar home systems, located a few kilometres apart. The objective is to obtain reliable daily and monthly insolation figures that are representative for an area of a few square kilometres.     

Estimating direct, diffuse and global solar radiation on an arbitrarily inclined plane in greece

A simple algorithm for the calculation of the direct, diffuse and global insolation, on a surface of arbitrary orientation, based on a model for insolation on clear days proposed by Bird and Hulstrom, is developed which takes into account cloudiness variation from month to month and hour to hour. The algorithm utilizes only routine meteorological data as input and requires very limited computational resources. Its results for monthly average daily and hourly values are accurate to better than 10% in the worse case examined for global irradiation and 15% for the diffuse irradiation.