Efstratios Batzelis
University of Southampton, ECS, Faculty Member
- Efstratios (Stratis) Batzelis is an “Engineering for Development” Research Fellow of the Royal Academy of Engineering... moreEfstratios (Stratis) Batzelis is an “Engineering for Development” Research Fellow of the Royal Academy of Engineering (RAEng) and has been a Lecturer with ECS at the University of Southampton since Apr 2021. Prior to that post, Stratis was a Research Fellow at Imperial College London since Nov 2019 and before that he held an EU Marie-Curie individual fellowship on photovoltaic control & integration from Nov 2017. He obtained his PhD degree from the National Technical University of Athens (NTUA), Greece in 2016.
Stratis is an experienced researcher with many IEEE publications and a funding track record of more than £1.6 million. He is a Senior IEEE member, a Fellow of the Higher Education Academy (FHEA) and an Associate Editor in IEEE Transactions on Sustainable Energy. He is an advocate of sustainable energy and the fight against climate change. His research interests involve renewable energy technologies and distributed energy resources, mainly solar photovoltaics and inverter-based generation, power electronics control and power system stability.edit
This paper presents a battery integrated Power Flow Controller (PFC) which is found effective for the interconnection of several dc microgrids. The configuration offers delicate control over load-flow and also provides a way for the... more
This paper presents a battery integrated Power Flow Controller (PFC) which is found effective for the interconnection of several dc microgrids. The configuration offers delicate control over load-flow and also provides a way for the integration of Common Energy Storage (CES) to the adjacent grids. The CES is more effective when both the grids have surplus or deficit of power compared to their individual storage capacity (if any). In this paper, a Universal Active Power Control Converter (UAPCC) is proposed (which is basically a three-port converter), where port-1 is connected in parallel with the line, port-2 is connected in series with the line, and port-3 is connected to the CES through a bidirectional dc-dc converter. Relevant control algorithms have been developed for the operation of such system satisfying various system requirements that are inevitable for the interconnection of dc microgrids. The proposed control methods allow decoupled operation of three ports to control power flow between dc grids and CES independently. The complete system along with control methods are initially verified through computer simulation using MATLAB/SIMULINK. Thereafter, a prototype is developed in the laboratory at 380 V level to experimentally validate the concept. The results show effectiveness of the UAPCC for interconnection of dc microgrids with CES. INDEX TERMS Common Energy Storage (CES), DC microgrids, DC power flow controller, DC-DC converters , renewable energy sources (RES).
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Research Interests: Batteries and Microgrids
In this paper, a module-level photovoltaic (PV) architecture in parallel configuration is introduced for maximum power extraction, under partial shading (PS) conditions. For the first time, a non-regulated switched capacitor (SC) nX... more
In this paper, a module-level photovoltaic (PV) architecture in parallel configuration is introduced for maximum power extraction, under partial shading (PS) conditions. For the first time, a non-regulated switched capacitor (SC) nX converter is a used at the PV-side conversion stage, whose purpose is just to multiply the PV voltage by a fixed ratio and accordingly reduce the input current. All the control functions, including the maximum power point tracking, are transferred to the grid-side inverter. The voltage-multiplied PV modules (VMPVs) are connected in parallel to a common DC-bus, which offers expandability to the system and eliminates the PS issues of a typical string architecture. The advantage of the proposed approach is that the PV-side converter is relieved of bulky capacitors, filters, controllers and voltage/current sensors, allowing for a more compact and efficient conversion stage, compared to conventional per-module systems, such as microinvert-ers. The proposed configuration was initially simulated in a 5 kW residential PV system and compared against conventional PV arrangements. For the experimental validation, a 10X Gallium Ni-tride (GaN) converter prototype was developed with a flat conversion efficiency of 96.3% throughout the power range. This is particularly advantageous, given the power production variability of PV generators. Subsequently, the VMPV architecture was tested on a two-module 500 WP prototype, exhibiting an excellent power extraction efficiency of over 99.7% under PS conditions and minimal DC-bus voltage variation of 3%, leading to a higher total system efficiency compared to most state-of-the-art configurations.
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The extraction of the photovoltaic (PV) model parameters remains to this day a long-standing and popular research topic. Numerous methods are available in the literature, widely differing in accuracy, complexity, applicability, and their... more
The extraction of the photovoltaic (PV) model parameters remains to this day a long-standing and popular research topic. Numerous methods are available in the literature, widely differing in accuracy, complexity, applicability, and their very nature. This paper focuses on the class of non-iterative parameter extraction methods and is limited to the single-diode PV model. These approaches consist of a few straightforward calculation steps that do not involve iterations; they are generally simple and easy to implement but exhibit moderate accuracy. Seventeen such methods are reviewed, implemented, and evaluated on a dataset of more than one million measured I-V curves of six different PV technologies provided by the National Renewable Energy Laboratories (NREL). A comprehensive comparative assessment takes place to evaluate these alternatives in terms of accuracy, robustness, calculation cost, and applicability to different PV technologies. For the first time, the irregularities found in the extracted parameters (negative or complex values) and the execution failures of these methods are recorded and are used as an assessment criterion. This comprehensive and up-to-date literature review will serve as a useful tool for researchers and engineers in selecting the appropriate parameter extraction method for their application.
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Large-scale photovoltaic (PV) integration to the network necessitates accurate modeling of PV system dynamics under solar irradiance changes and disturbances in the power system. Most of the available PV dynamic models in the literature... more
Large-scale photovoltaic (PV) integration to the network necessitates accurate modeling of PV system dynamics under solar irradiance changes and disturbances in the power system. Most of the available PV dynamic models in the literature are scope-specific, neglecting some control functions and employing simplifications. In this paper, a complete dynamic model for two-stage PV systems is presented, given in entirely state-space form and explicit equations that takes into account all power circuit dynamics and modern control functions. This is a holistic approach that considers a full range of ancillary services required by modern grid codes, supports both balanced and unbalanced grid operation, and accounts for the discontinuous conduction mode of the dc/dc converter of the system. The proposed dynamic model is evaluated and compared to other approaches based on the literature, against scenarios of irradiance variation, voltage sags, and frequency distortion. Simulation results in MATLAB/Simulink indicate high accuracy at low computational cost and complexity.
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Due to the intrinsic intermittency and stochastic nature of solar power, accurate forecasting of the photovoltaic (PV) generation is crucial for the operation and planning of PV-intensive power systems. Several PV forecasting methods... more
Due to the intrinsic intermittency and stochastic nature of solar power, accurate forecasting of the photovoltaic (PV) generation is crucial for the operation and planning of PV-intensive power systems. Several PV forecasting methods based on machine learning algorithms have recently emerged, but a complete assessment of their performance on a common framework is still missing from the literature. In this paper, a comprehensive comparative analysis is performed, evaluating ten recent neural networks and intelligent algorithms of the literature in short-term PV forecasting. All methods are properly fine-tuned and assessed on a one-year dataset of a 406 MWp PV plant in the UK. Furthermore, a new hybrid prediction strategy is proposed and evaluated, derived as an aggregation of the most well-performing forecasting models. Simulation results in MATLAB show that the season of the year affects the accuracy of all methods, the proposed hybrid one performing most favorably overall.
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In electric grids with large photovoltaic (PV) integration, the PV system dynamics triggered by irradiance variation is an important factor for the power system stability. Although there are models in the literature that describe these... more
In electric grids with large photovoltaic (PV) integration, the PV system dynamics triggered by irradiance variation is an important factor for the power system stability. Although there are models in the literature that describe these dynamics, they are usually formulated as block diagrams or flowcharts and employ implicit equations for the PV generator, thus requiring application-specific software and iterative solution algorithms. Alternatively, to provide a rigorous mathematical formulation, a state-space representation of the PV system dynamics driven by irradiance variation is presented in this paper. This is the first PV dynamic model in entirely state-space form that incorporates the maximum power point tracking (MPPT) function. To this end, the Lambert W function is used to express the PV generator's equations in explicit form. Simulations are performed in MATLAB/Simulink to evaluate and compare the proposed dynamic model over the detailed switching modeling approach in terms of accuracy and computational performance.
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—The challenges of modern power systems will inevitably impose increased ancillary service requirements to pho-tovoltaic (PV) plants in the future, including operating reserves. Recent studies investigate methods to maintain active power... more
—The challenges of modern power systems will inevitably impose increased ancillary service requirements to pho-tovoltaic (PV) plants in the future, including operating reserves. Recent studies investigate methods to maintain active power reserves without energy storage in the standard case of uniform illumination. In this paper, this functionality is extended to partial shading conditions, often encountered in PV systems. A new control scheme is proposed that permits operation at a reduced power level, estimating at the same time the shading conditions and maximum available power. This is achieved by applying a least squares curve fitting algorithm on voltage and current measurements, without relying on any irradiance or temperature sensors. To the best of our knowledge, this is the first power reserves control scheme for PV systems under partial shading presented in the literature. The robustness and effectiveness of the proposed method is validated under rapidly changing shading conditions through simulations and experimental tests on a 2 kW PV system prototype. Index Terms—Active power control, maximum power point tracking (MPPT), partial shading, power reserves, photovoltaic (PV) system.
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There are several photovoltaic (PV) performance models in the literature, but most of them either employ complex and tedious calculations or require additional measurements apart from datasheet information. In this paper, a new set of... more
There are several photovoltaic (PV) performance models in the literature, but most of them either employ complex and tedious calculations or require additional measurements apart from datasheet information. In this paper, a new set of performance equations to evaluate the short circuit (SC) current, open circuit (OC) voltage and maximum power point (MPP) at any operating conditions is introduced. The proposed expressions are simple functions of the irradiance and temperature, while they are generally applicable to any crystalline PV module and require only datasheet information as input data. This is achieved by introducing new formulas to determine the irradiance and temperature coefficients that are not provided in the datasheet, thus avoiding empirical constants or additional measurements. The novelty of the performance equations is their solid theoretical background, as they are in excellent agreement with the single-diode PV model, combined with simple and easy application. The proposed PV model is validated and compared to other methods found in the literature through simulations in MATLAB and outdoor measurements on commercial PV modules.
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In order for a PV system to provide a full range of ancillary services to the gird, including frequency response, it has to maintain active power reserves. In this paper, a new control scheme for the dc/dc converter of a two-stage PV... more
In order for a PV system to provide a full range of ancillary services to the gird, including frequency response, it has to maintain active power reserves. In this paper, a new control scheme for the dc/dc converter of a two-stage PV system is introduced, which permits operation at a reduced power level, estimating the available power (maximum power point-MPP) at the same time. This control scheme is capable of regulating the output power to any given reference, from near-zero to 100% of the available power. The proposed MPP estimation algorithm applies curve fitting on voltage and current measurements obtained during operation to determine the MPP in real time. This is the first method in the literature to use the non-simplified single-diode model for the determination of the MPP and the five model parameters while operating at a curtailed power level. The developed estimation technique exhibits very good accuracy and robustness in presence of noise and rapidly changing environmental conditions. The effectiveness of the control scheme is validated through simulation and experimental tests using a 2 kW PV array and a dc/dc converter prototype at constant and varying irradiance conditions. Index Terms — Active power control, curve fitting, linearized converter model, maximum power point tracking (MPPT), photovoltaic (PV), power reserves, single-diode model.
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Standard maximum power point tracking (MPPT) algorithms often fail to locate the global maximum of a photovoltaic (PV) system under partial shading conditions, while other more sophisticated approaches usually involve extra perturbation... more
Standard maximum power point tracking (MPPT) algorithms often fail to locate the global maximum of a photovoltaic (PV) system under partial shading conditions, while other more sophisticated approaches usually involve extra perturbation of the operating point, which entails undesired output power fluctuation. In this paper, a new MPPT method is introduced, which continuously detects the shading parameters and estimates all power peaks (MPPs) on the P-V curve, guaranteeing continuous operation at the global maximum. The algorithm applies least squares (LSQ) curve fitting (CF) to measurements at the current MPP, utilizing the inherent ripple, without the need for additional perturbation on the operating point. The calculations performed are entirely mathematical and no extra measurement equipment is required, such as irradiance or temperature sensors. The method is designed for PV strings illuminated at two irradiance levels.
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In this paper, a PV inverter control scheme is presented suitable for operation under distorted and unbalanced grid voltage, as well as when the grid frequency varies, as is the case in isolated systems such as those in non-interconnected... more
In this paper, a PV inverter control scheme is presented suitable for operation under distorted and unbalanced grid voltage, as well as when the grid frequency varies, as is the case in isolated systems such as those in non-interconnected islands. The analysis is performed both in the frequency and time domains, using a suitable linearized model, as well as the full non-linear electrical model of the system. The objective is to determine the main factors affecting the system dynamic behavior as well as the power quality characteristics of the PV output current in presence of distorted grid conditions. To this end, a comparative assessment of the proposed PV inverter controller versus a conventional one is performed.
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In this paper, an enhanced Maximum Power Point Tracking (MPPT) strategy for a two-stage gridconnected PV system is proposed, which enables accurate tracking of the maximum power point irrespective of the rate of change in solar irradiance... more
In this paper, an enhanced Maximum Power Point Tracking (MPPT) strategy for a two-stage gridconnected PV system is proposed, which enables accurate tracking of the maximum power point irrespective of the rate of change in solar irradiance levels. The analysis is performed both in the frequency and time domains, using a suitable linearized model of the system. A comparative assessment of the proposed MPPT strategy versus a conventional Perturbation and Observation (P&O) method is carried out for operation under trapezoidal irradiance profiles.
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In this paper a complete model of a-Si:H single junction module operating in partial shading conditions is presented. It is based on the typical single-diode model, properly adjusted for reversed operation and enhanced by a term... more
In this paper a complete model of a-Si:H single junction module operating in partial shading conditions is presented. It is based on the typical single-diode model, properly adjusted for reversed operation and enhanced by a term describing the recombination phenomenon. Both single and multi-string modules are studied at partial shading operation and compared on power production and shaded cells voltage level. The simulation results are experimentally validated by measurements on two modules at various shading patterns.
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In this paper, an active power control strategy is presented in order for a PV plant to operate at a specified suboptimal output power setpoint and maintain power reserves, expressed as a fraction of the maximum available power. Given... more
In this paper, an active power control strategy is presented in order for a PV plant to operate at a specified suboptimal output power setpoint and maintain power reserves, expressed as a fraction of the maximum available power. Given that the latter is not known a priori, a quadratic curve fitting algorithm is applied to recent past measurements in order to estimate the P-V curve and calculate the operating voltage to achieve the desired power reserve levels. The proposed method offers sufficient accuracy, increased robustness and computational efficiency, combined with excellent dynamic response at irradiance variations and reserve command changes. The effectiveness of the control strategy and the P-V curve estimation method is validated by simulations in MATLAB/Simulink.
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In this paper, the maintenance records of a major service center in Greece are presented, regarding inverter faults for autonomous PV systems. A great variety of commercial inverters have been examined, while the fault diagnosis, apparent... more
In this paper, the maintenance records of a major service center in Greece are presented, regarding inverter faults for autonomous PV systems. A great variety of commercial inverters have been examined, while the fault diagnosis, apparent symptoms and estimated cause of failure are recorded. Analyzing these data, useful conclusions are derived regarding the robustness and common failures of off-grid inverters.
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In this paper, a simple algorithmic enhancement for MPPT methods is introduced, which mathematically determines if the PV system is shaded, thus avoiding unnecessary curve scanning to locate the global maximum if it is unshaded. The... more
In this paper, a simple algorithmic enhancement for MPPT methods is introduced, which mathematically determines if the PV system is shaded, thus avoiding unnecessary curve scanning to locate the global maximum if it is unshaded. The proposed technique improves the overall efficiency and applies to any PV system at any irradiance distribution, using only a common temperature sensor.
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Under partial shading conditions, several power peaks (maximum power points-MPPs) are presented on the P-V curve of a photovoltaic system, hindering the effectiveness of typical maximum power point tracking (MPPT) algorithms, due to... more
Under partial shading conditions, several power peaks (maximum power points-MPPs) are presented on the P-V curve of a photovoltaic system, hindering the effectiveness of typical maximum power point tracking (MPPT) algorithms, due to possible convergence to a local suboptimal MPP. In this paper, a global MPPT (GMPPT) method for PV strings is proposed, which exploits the theoretical MPP characterization to detect the shading conditions and estimate all MPPs on the P-V curve. The calculations performed do not involve unnecessary operating point variations and output power fluctuations. The proposed method is designed for PV strings illuminated at two irradiance levels and only needs the standard voltage and current sensors of the DC/DC converter.
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In order for a PV system to offer ancillary services to the grid, it should be capable of maintaining active power reserves and controlling its output power. In this paper, such a power regulation technique is proposed, which is based on... more
In order for a PV system to offer ancillary services to the grid, it should be capable of maintaining active power reserves and controlling its output power. In this paper, such a power regulation technique is proposed, which is based on an improved algorithm to estimate the maximum available power when operating suboptimally. The method introduced employs the fundamental equation of the single-diode PV model and applies linear least squares curve fitting to provide the model parameters in an analytical and computationally efficient way. The effectiveness of the control strategy is validated through simulations in MATLAB/Simulink, recording the system's response in irradiance/temperature variations and reserve command step changes, in both noiseless and noisy environment. Results show excellent dynamic response and increased accuracy and reliability compared to previous approaches.
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In this paper, new expressions are introduced for the determination of the maximum power point (MPP) of photovoltaic (PV) systems as explicit functions of the five parameters of the single-diode model employing the Lambert W function.... more
In this paper, new expressions are introduced for the determination of the maximum power point (MPP) of photovoltaic (PV) systems as explicit functions of the five parameters of the single-diode model employing the Lambert W function. These equations provide the voltage and current at MPP in a direct and straightforward manner, thus dispensing with any need for iterative solution. They are initially derived for a PV system operating under uniform conditions, and subsequently extended for mismatched conditions at the PV string level. The novelty of these formulae lies in their solid theoretical foundation, which supports their validity in the general case and offers a well-founded symbolic formulation for the MPP evaluation problem. Extended simulations and experimental validation are performed to verify the accuracy and computational efficiency of the proposed equations compared with other methods available in the literature.
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The partial shading phenomenon and its implications on the electrical response and energy yield of photovoltaic (PV) systems have received increased attention in the last years. In order to study, foresee and mitigate such effects,... more
The partial shading phenomenon and its implications on the electrical response and energy yield of photovoltaic (PV) systems have received increased attention in the last years. In order to study, foresee and mitigate such effects, several energy models are proposed in the bibliography, presenting different degrees of complexity, accuracy and applicability. This study presents an overview of the state of the art in the development of models for PV systems under partial shading conditions. Alternative modelling approaches are analysed, highlighting their advantages and shortcomings and models available in the literature are reviewed and classified according to important attributes, related to their accuracy and implementability. Current research trends, as well as topics that warrant further investigation, are identified and discussed.
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In this paper, the electrical response of a partially shaded photovoltaic (PV) array, comprising several strings connected in parallel, is investigated. The PV array is simulated by employing an enhanced version of the widely used... more
In this paper, the electrical response of a partially shaded photovoltaic (PV) array, comprising several strings connected in parallel, is investigated. The PV array is simulated by employing an enhanced version of the widely used single-diode model, reformulated in an explicit manner employing the Lambert W function. The multiple maximum power points (MPPs) that appear on the P-V characteristic of the array in partial shading conditions are analyzed, in terms of their number and properties. Simplified empirical expressions are then derived to calculate the voltage, current, and power for each local MPP, at any irradiance level and temperature, using only datasheet information, in a most simple and straightforward manner, without resorting to detailed modeling and simulations. The derived formulae are validated using both simulation and experimental results.
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In this paper, a reformulation of the widely used one-diode model of the photovoltaic (PV) cell is introduced, employing the Lambert W function. This leads to an efficient PV string model, where the terminal voltage is expressed as an... more
In this paper, a reformulation of the widely used one-diode model of the photovoltaic (PV) cell is introduced, employing the Lambert W function. This leads to an efficient PV string model, where the terminal voltage is expressed as an explicit function of the current, resulting in significantly reduced calculation times and improved robustness of simulation. The model is experimentally validated and then used for studying the operation of PV strings under partial shading conditions. Various shading patterns are investigated to outline the effect on the string I-V and P-V characteristics. Simplified formulae are then derived to calculate the maximum power points of a PV string operating under any number of irradiance levels, without resorting to detailed modeling and simulation. Both the explicit model and the simplified expressions are intended for application in shading loss and energy yield calculations.
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In this paper, a comprehensive analysis on the local maxima developed in a partially shaded PV string, experiencing multiple irradiance levels, is presented. The number and the trends of the local and global maximum power points (MPPs)... more
In this paper, a comprehensive analysis on the local maxima developed in a partially shaded PV string, experiencing multiple irradiance levels, is presented. The number and the trends of the local and global maximum power points (MPPs) are studied, initially considering two irradiance levels and subsequently extending to the more complicated three-irradiance levels scenario. The shading patterns at which each MPP appears are investigated, and the conditions under which they constitute the global MPP (GMPP) are identified, while the effect of the operating temperature and the intensity of irradiance is studied.