Photovoltaic String Research Articles

LLC Resonant Voltage Multiplier-Based Differential Power Processing Converter Using Voltage Divider with Reduced Voltage Stress for Series-Connected Photovoltaic Panels under Partial Shading

Partial shading on photovoltaic (PV) strings consisting of multiple panels connected in series is known to trigger severe issues, such as reduced energy yield and the occurrence of multiple power point maxima. Various kinds of differential power processing (DPP) converters have been proposed and developed to prevent partial shading issues. Voltage stresses of switches and capacitors in conventional DPP converters, however, are prone to soar with the number of panels connected in series, likely resulting in impaired converter performance and increased circuit volume. This paper proposes a DPP converter using an LLC resonant voltage multiplier (VM) with a voltage divider (VD) to reduce voltage stresses of switches and capacitors. The VD can be arbitrarily extended by adding switches and capacitors, and the voltage stresses can be further reduced by extending the VD. Experimental verification tests for four PV panels connected in series were performed emulating partial shading conditions in a laboratory and outdoor. The results demonstrated the proposed DPP converter successfully precluded the negative impacts of partial shading with mitigating the voltage stress issues.

Novel Open-Circuit Photovoltaic Bypass Diode Fault Detection Algorithm

In this article, a novel photovoltaic (PV) bypass diode fault detection algorithm is presented. The algorithm consists of three main steps. First, the threshold voltage of the current–voltage ( I–V ) curve is obtained using different failure bypass diode scenarios. Second, the theoretical prediction for the faulty regions of bypass diodes is calculated using the analysis of voltage drop in the I–V curve as well as the voltage at maximum power point. Finally, the actual I–V curve under any environmental condition is measured and compared with theoretical predictions. The proposed algorithm has been experimentally evaluated using a PV string that comprises three series-connected PV modules, and subtotal of nine bypass diodes. Various experiments have been conducted under diverse bypass diodes failure conditions. The achieved detection accuracy is always greater than 99.39% and 99.74% under slow and fast solar irradiance transition, respectively.

Increasing Energy Capture From Partially Shaded PV String Using Differential Power Processing

In photovoltaic (PV) strings based on series-connected PV modules, mismatches among the modules due to partial shading will cause power loss either due to series connection constraint or bypass diodes. This paper proposes a new distributed maximum power point tracking (DMPPT) scheme using multiwinding forward-based converter, which acts as a current balancing differential power processing converter. The converter is configured in the way that each converter port is connected in parallel with individual PV module to enable module-level maximum power extraction. The proposed architecture operates in two modes: local maximum power point tracking (MPPT) mode and current balancing mode. The new maximum power points (MPPs) at module levels are tracked in a local MPPT mode and then dispatched to start current balancing mode of operation. In the current balancing mode, this paper proposes a simple control approach to maximize the output power from partially shaded PV string by maintaining an effective MPPT at module levels and directly transferring the compensation currents toward the shaded modules from modules under normal conditions without any intermediary. The proposed DMPPT scheme is validated by the simulation and experimental results. The results show that using the proposed scheme, the system efficiency is significantly improved compared to other existing topologies.

Fast and precise global maximum power point tracking techniques for photovoltaic system

The multiple power peaks obtained in the power–voltage (P-V) curve of a photovoltaic string under partially shaded condition results in a complicated maximum power point tracking (MPPT) process. Under this condition, the conventional MPPT methods are not acknowledged as they result in false and slow tracking. In this study three novel global MPPT (GMPPT) methods have been proposed and validated. These are named as large and small duty step (LSDS), large and mutable duty step (LMDS) and fast and intelligent GMPPT (FI-GMPPT). The LSDS method sweeps almost the entire P-V curve using a combination of LSDS. Small duty steps are used in predefined areas near all local maximum power points of the P-V curve. LMDS is a further improved method, which uses a combination of LMDSs. The FI-GMPPT is an advance true GMPPT method which limits the area to be swept during the search process. This results in a further reduction in sweep time. In this method, the unnecessary area is skipped during the sweep process. The improved performance of the projected methods has been demonstrated and validated using MATLAB/SIMULINK and hardware implementation.

An Immune Firefly Algorithm for Tracking the Maximum Power Point of PV Array under Partial Shading Conditions

Photovoltaic (PV) string exhibits complex multiple-peak characteristics under various partial shading conditions (PSC). If the maximum power point tracking cannot be achieved quickly and accurately, it will lead to a large amount of energy loss. Therefore, it has become a hot topic to study a reliable maximum power tracking control algorithm to ensure the PV system can still output maximum power under PSC. This paper proposes an immune firefly algorithm (IFA), which utilizes vaccine data-base to shorten the convergence time, eliminates the influence of bad individuals in time by immune replenishment operation, and reduces the steady-state oscillation by the improving iteration formula. The simulations in static and dynamic environments verify that the immune firefly algorithm can track the maximum power point under various partial shading conditions. Compared with conventional firefly algorithm (FA), IFA has faster convergence speed, and can effectively restrain the oscillation of voltage and power.

Optimal design of the dual-axis tracking system used for a PV string platform

This work deals with the optimal design of the dual-axis solar tracker used for a PV (photovoltaic) string platform. The solution of the dual-axis mechanism was developed starting from the monoaxis version (for the diurnal movement), which was outlined in a previous paper. With reference to the initial monoaxis mechanism, the diurnal movement subsystem was simplified from a structural point of view; while adding the actuating and motion transmitting mechanisms for adjusting the elevation angle of the panels, the same type of linear actuator is being used for both the movements. The optimization study was carried out in a virtual prototyping environment, through the development of complex digital models (which integrate not only mechanical, actuating, and control components, but also algorithms for the solar radiation modeling), and two major optimization directions are being targeted: The geometric optimization of the mechanism, with the purpose of determining the optimal arrangement of two linear actuators while respecting several design constraints; the optimal design of the tracking program, with the purpose of maximizing the energy gain of the dual-axis system with reference to the equivalent fixed system (without tracking). The physical prototype of the tracking system was developed on the basis of the virtual prototype specifications, the experimental and simulation results, which are quite well correlated (especially in sunny days), proving the performance of the proposed design.

태양광 전력설비의 효율 향상을 위한 피드포워드 차동전력조절기(DPP)의 적용 검증

Differential power processing (DPP) is a power facility architecture that configures dc - dc converters in parallel with the PV string to improve its power yield. The parallel nature of the DPP architecture brings a number of benefits, such as low converter power rating and low power losses. In this paper, feedfoward DPP structure is applied to improve the efficiency of photovoltaic(PV) power facility. DPP structure is considered to a solution to problems were caused by solar irradiance mismatch. The effectiveness of DPP struture is confirmed by comparing module integrated converter (MIC) structure. Experiments and simulation with feedforward structure using a DSP(TMS320F28335) hardware design were conducted and the improvement in performance with DPP converters was experimentally confirmed.

Modularized Differential Power Processing Architecture Based on Switched Capacitor Converter to Virtually Unify Mismatched Photovoltaic Panel Characteristics

Photovoltaic (PV) strings consisting of multiple panels suffer from partial shading or characteristic mismatch issues, such as a significant reduction in power yield. Various kinds of differential power processing (DPP) converters have been developed to prevent the negative impacts of substring-level partial shading. For panel-level applications, however, conventional DPP converters face a variety of challenges, such as impaired extendibility and increased voltage stress of circuit elements. This paper proposes a switched capacitor converter (SCC)-based modular DPP architecture. Modules, each containing series-connected panels with a panel-level DPP converter, are connected through a switchless module-level DPP converter, and PV panel characteristics are unified at module- and panel-levels. The number of panels in each module is fixed, while the number of modules can be arbitrarily extended without redesigning DPP converters, hence offering good modularity. In addition, since voltage stresses of capacitors in the proposed architecture can be reduced lower than half the module voltages, the proposed DPP system realizes all-ceramic-capacitor SCC circuit by properly determining the module voltages. A prototype for eight panels, which were grouped into two modules, was built, and laboratory and field tests were performed. The experimental results demonstrated the enhanced power yield from the partially shaded PV string.

Experimental study of PV strings affected by cracks

Crack is one critical factor that degrades the performance of photovoltaic (PV) panels. To gain a better understanding of the impacts of cracks appeared on PVs and also to mitigate it, its failure mechanism, detrimental effects, criticality, and potential risks on independent PV panels are firstly reviewed in this study. An experimental study which investigates the degree of series connected and parallel connected PV strings which are affected by cracked cells are presented. A comparison of impacts of the partially shaded PV panel string and cracked cells happened to the PV panel string is given to evaluate their criticality levels. The experimental results show that the series connected PV panel string is strongly affected once the cell is seriously cracked, as the current generation capability is clamped. Partial shading, however, shows better performance. In addition, though the overall power the parallel connected PV string is reduced, it is less affected by the cracked cells compared to the series connected one. Lastly, a bypass diode is added to a series connected PV panel string with cracked cells, and the experimental results show that it can be an effective way to minimise the negative impacts of cracks.

A Multi-Mode Flexible Power Point Tracking Algorithm for Photovoltaic Power Plants

Flexible power point tracking (FPPT) is the control of active power generated by grid-connected photovoltaic power plants (GCPVPPs) to provide grid-support functionality. An FPPT algorithm for the reduction of the extracted power from photovoltaic (PV) strings during voltage sags was previously proposed by the authors. An advantage of this algorithm, compared to conventional FPPT algorithms, was its fast dynamics facilitated by use of a simple PI controller that dynamically modifies the PV voltage reference. The previously proposed scheme could only be employed for the short duration in which the power system experiences a voltage sag. A novel modification to this algorithm with multi-mode operation is introduced in this letter, which provides FPPT capability for continuous operation of GCPVPPs. Unlike the previous algorithm, which was able to only move the operation point to the right-hand side of MPP, the proposed algorithm in this letter is able to move the operation point to both right- and left-hand sides of the MPP that provides the flexibility to operate in the optimum operation region for both single- and two-stage GCPVPPs. Experimental results are provided to demonstrate the performance of the proposed algorithm under dynamic irradiance conditions.

Review, Comparison, and Proposal for PWM Converters Integrating Differential Power Processing Converter for Small Exploration Rovers

Partial shading often appears on photovoltaic (PV) strings installed in small space probes, such as moon exploration rovers, due to their body structures, reducing the power yield of PV strings. Such partial shading issues can be precluded by differential power processing (DPP) converters. However, in addition to a main dc-dc converter, a DPP converter is required, increasing the system complexity and cost. In this paper, several kinds of integrated pulse width modulation (PWM) converters that can reduce the system complexity thanks to the integration are reviewed and are quantitatively compared in terms of the switch and magnetic component counts, and voltage conversion ratio. Based on the comparison and consideration from the perspectives of reliability, circuit volume, and voltage conversion requirement, a single-switch single-magnetic integrated PWM converter with a resonant voltage multiplier (RVM) was selected as the best suitable topology for small exploration rovers. Furthermore, the improved version with a non-resonant voltage multiplier (NRVM) was also proposed to achieve further circuit miniaturization and improved reliability. The experimental verification tests using the proposed integrated PWM converter were performed emulating a partial-shading condition. The maximum power yield from the PV modules was improved by 11.1% thanks to the DPP function of the proposed integrated converter.

Implementation of new high efficient input parallel output series partial power DC-DC converter for solar photovoltaic partial shading conditions

The proposed research suggests an improvised performance which is efficient and a reliable converter as a reasonable solar photo Voltaic (PV) approach. The proposed research helps in elevating the performance levels so that power extraction is maximized from the respective PV arrays specifically during the case of limited shading. In order to track the maximum amount of power from every solar PV string, connection of DC-DC converters is done amid the solar PV strings. By doing so, maximum power output can be ensured by the operation of each string irrespective of the occurrence of variation in the solar radiation amongst diverse strings. Usually, the DC-DC converter that would be employed for this purpose would process the string’s power completely. This Full Power Processing (FPP) architecture gives rise to extreme losses of conversion. Hence, the suggested work propound a Partial Power Processing (PPP) that processed a part of the total system power, while the rest of the power is straight away delivered to the output side, thus provides high conversion efficiency when compared to the existing DC-DC FPP converters. An input parallel output series structure having a novel Non-Isolated Partial Power Processing topology (NIPPP) is recommended and the operation’s details are elucidated depending on the operating principle. For the purpose of evaluating the performance of the converter, Matlab Simulink environmental platform are carried out. The closed loop prototype experimental approach is also carried out with the improvement of efficiency compared to conventional it appears in efficient manner.

Arm Power Control of the Modular Multilevel Converter in Photovoltaic Applications

In this paper, a control method is proposed that allows the extraction of maximum power from each individual photovoltaic string connected to the Modular Multilevel Converter (MMC) and inject balanced power to the AC grid. The MMC solution used does not need additional DC–DC converters for the maximum power point tracking. In the MMC, the photovoltaic strings are connected directly to the sub-modules. It is shown that the proposed inverter solution can provide balanced three-phase output power despite an unbalanced power generation. The maximum power of the photovoltaic string is effectively harnessed due to the increased granularity of the maximum power point tracking. An algorithm that tracks the sub-module capacitor voltages to their respective voltage references is proposed. A detailed modeling and control method for balanced operation of the proposed topology is discussed. The operation of the MMC under unbalanced power generation is discussed. Simulation results are provided that show the effectiveness of the proposed control under unequal irradiance.

Hybrid heuristic for the optimal design of photovoltaic installations considering mismatch loss effects

We consider the Photovoltaic Installation Design Problem (PIDP). In this problem, photovoltaic modules must be connected in “strings” and wired to a set of electronic components. The aim is to minimize installation costs and maximize power production, which is affected by “mismatch losses” caused by non-uniform irradiation (shading) and is also directly related to design decisions. We relate the problem to the known class of location routing problems and we design a route-first/cluster-second heuristic. We propose an efficient machine learning approach to evaluate Photovoltaic (PV) string performances accounting for mismatch losses. We prove that our approach is effective in real-world instances provided by our industrial partner.

Discussion on module-based hot-spot suppression in a PV generation system

This paper presents a novel control system for the hot-spot detection and suppression in a photovoltaic (PV) generation system. In this novel system, a distributed structure of a PV string is employed by installing a buck-boost converter to each module. Following this structure, a systematic control strategy is proposed for the module-based hot-spot detection. Moreover, the safe operation is adopted to suppress the prolonged high temperature when the hot-spot is detected. On the other hand, the normal modules can work at their maximum power point tracking (MPPT), respectively. On this occasion, the PV string could be smart because every module could function in their optimal state which is determined to their respective conditions. In order to validate this novel control system, the experiments have been carried out in a laboratory prototype. The verified results show that the PV generation system can operate safely at module level by using this novel control system.

A new model-based technique for fast and accurate tracking of global maximum power point in photovoltaic arrays under partial shading conditions

This paper presents a new model-based method to extract the global maximum power point (GMPP) of photovoltaic (PV) arrays under partial shading conditions (PSCs). In the proposed method, it is shown that independent of the pattern and intensity of the solar radiation, the position of the PV array GMPP voltage is always in the neighboring of one of the strings GMPP voltages in the PV array. Therefore, the number of local peak points tested in the proposed method is independent of the solar radiation pattern and is equal to the number of parallel strings in the PV array. As a result, for tracking of PV array GMPP in the PSCs, at first, the GMPP of each of PV strings is estimated. Next, by the numerical solution of the PV string nonlinear equations, the PV array output current and power are calculated corresponding to each of PV string GMPP voltages, and the voltage that results in the highest PV array output power is detected as the PV array GMPP voltage. Numerical results and comparative studies with similar methods confirm the satisfactory operation of the proposed technique.

Maximum Power Point Estimation for Photovoltaic Strings Subjected to Partial Shading Scenarios

Partial shading is an unavoidable complication in the field of photovoltaic (PV) generation. Bypass diodes have become a standard feature of solar cell arrays to improve array performance under partial shading scenarios (PSS). However, the current–voltage and power–voltage characteristic data vary with different shading patterns. In this paper, a shading pattern detection algorithm is first proposed to estimate the number of shaded modules in a PV string. The result is then fed to a field-support vector regression (F-SVR) model, in which the measured features are transferred into a style-free high-dimension space prior to data training. The process of style-normalized transformation enables the features to be independent and identically distributed. Both simulations and experiments are conducted to evaluate the F-SVR model's ability to estimate the voltage at maximum power points. The results show that the proposed maximum power point estimation method can evidently reduce prediction errors.

PV-Battery Series Inverter Architecture: A Solar Inverter for Seamless Battery Integration With Partial-Power DC–DC Optimizer

This paper presents system architecture and control scheme of a photovoltaic (PV) string inverter allowing seamless battery integration with the dc-series integration method. The architecture uses the partial-power processing universal dc–dc optimizer to have flexible power control by regulating the T-node compensation current. The universal optimizer is configured with a dual active bridge dc–dc converter allowing bidirectional power flow and galvanic isolation. Three operation modes are proposed depending on the input sources connected. Based on the operation mode, the universal optimizer and a dc–ac inverter switch their control reference to deliver the maximum PV power and perform battery current control. The steady-state and transient operation of the closed-loop control scheme have been demonstrated through controller hardware-in-the-loop tests to verify feasibility and effectiveness of the PV-battery series inverter architecture.

A Multilevel DC to Three-Phase AC Architecture for Photovoltaic Power Plants

This paper presents a photovoltaic (PV) inverter architecture composed of stackable dc to three-phase ac converter blocks. Several such blocks, each containing a converter power stage and controls, are connected in series on their ac sides to obtain transformerless medium-voltage ac interfaces for PV power plants. The series-connected structure is made possible by a quadruple active bridge dc–dc converter that provides isolation between the PV input and each of the three ac-side phases within each block. Furthermore, since incoming PV power is transferred as constant balanced three-phase ac power, instantaneous input–output power balance bypasses the need for bulk energy storage. To streamline implementation and maximize system scalability and resilience, decentralized block-level controllers accomplish dc-link voltage regulation, maximum power point tracking, and ac-side power sharing without centralized means. The proposed architecture is validated by simulations of a PV string to medium-voltage ac system consisting of six blocks and on a proof-of-concept hardware prototype that consists of three cascaded converter blocks.

An intelligent flying system for automatic detection of faults in photovoltaic plants

For several years, fault diagnosis of photovoltaic (PV) plants has been manually performed by the human operator by a visual inspection or automatically, by evaluating electrical measures collected by sensors mounted on each PV module. In recent years, a notable interest of the scientific community has been devoted towards the definition of algorithms able to automatically analyse the sequence of images acquired by a thermal camera mounted on board of an unmanned aerial vehicle (UAV) for early PV anomaly detection. In this paper, we define a model-based approach for the detection of the panels, which uses the structural regularity of the PV string and a novel technique for local hot spot detection, based on the use of a fast and effective algorithm for finding local maxima in the PV panel region. Finally, we introduce the concept of global hot spot detection, namely a multi-frame recognition of PV faults which further improves the anomaly detection accuracy of the proposed method. The algorithm has been designed and optimized so as to run in real-time directly on an embedded system on board of the UAV. The accuracy of the proposed approach has been experimented on several video sequences with a standard protocol in terms of Precision, Recall and F-Score, so that our dataset and our quantitative results can be used for future comparisons and to evaluate the reliability of computer vision techniques designed for thermographic PV inspection.