Photovoltaic String Research Articles

Instantaneous Active and Reactive Power Control Using Direct Power Control Strategy for Multilevel Multistring Inverter Fed Photovoltaic System

This paper presents the Direct Power Control (DPC) strategy for Multilevel Multistring Inverter fed Photovoltaic (PV) system to control the instantaneous active and reactive power. The proposed system consists of PV strings, boost converter and three phase three level cascaded H-bridge (CHB) inverter. In multistring topology, each PV string is connected to the dc/dc converter and the distributed MPPT control algorithm is connected to the central inverter. The Space Vector Modulation (SVM) based DPC approach is used to obtain the constant switching frequency and reduced power ripple. The detailed model of the proposed system is developed in Matlab to evaluate the performance. A laboratory based prototype of the proposed system is developed to realize the system in real time. dSPACE DS1103 is used as the control interface to perform the real time implementation (RTI). Power Quality Analyser (PQ-Box 200) is used to analyse the system parameters like voltage, current and Total Harmonic Distortion (THD) of inverter. The effectiveness of the proposed system has been validated with the help of simulation and the experimental results.

Fault Detection, Classification and Localization Algorithm for Photovoltaic Array

In this study, analysis of photovoltaic (PV) string currents is performed to understand the behavior of the PV array under faults. The whole analysis is summed up in just two simple statements, and an algorithm is devised on the basis of these statements. The proposed algorithm can detect, classify and localize line-to-ground (L-G) & line-to-line (L-L) faults in the PV array. The proposed method can detect faults of any mismatch level and size of the PV array by using only one current sensor per string. It is also capable of working with finite fault-resistance as well as in the presence/absence of blocking diodes. To check the accuracy of the algorithm, it has been thoroughly tested through experimental setup.

A multi‐string fault‐tolerant multilevel inverter configuration for off‐grid photovoltaic applications

With the intensified demand for enhanced harmonic profile of output voltage, better power quality, reduced dv/dt stress of devices and minimal filter requirement, multilevel inverters (MLIs) have emerged as a viable solution in renewable energy applications. In the conventional MLI, the order of device count, as well as control complexity, will increase with respect to the number of voltage levels. Moreover, this higher order of semiconductor devices may increase the chance of failure rate, which degrades the reliability of the inverter. To address these issues, in this article a 1-Φ fault-tolerant MLI topology is proposed for multi-string photovoltaic (PV) applications with reduced device count as well as improved reliability. This topology required four isolated DC sources with a voltage magnitude of Vdc/4 (where Vdc is the voltage requirement for the conventional neutral point clamped MLI), which are realized with multiple PV strings. The proposed MLI enhances the reliability of the system in terms of switch open circuit failures as well as source failures. Moreover, as compared to other conventional MLIs, the proposed MLI topology has the energy balancing capability between sources during partial shading condition of PV panels. The mathematical analysis of energy balancing between the DC sources is also presented in this article. The proposed 1-Φ MLI has been analyzed for various source failures and/or switch open circuit failures using MATLAB/Simulink as well as by implementing the laboratory prototype.

Modeling and Resonance Damping for the CM Loop of the MLCL-Filtered Grid-Connected Inverter With Variable Inductors Under the Weak Grid Condition

The modified LCL (MLCL) is commonly applied in photovoltaic (PV) string inverters to suppress the leakage current and enhance the electromagnetic interference (EMI) performance of the grid current. However, the potential risk of resonance appears in the common mode (CM) loop, which was caused by the MLCL filter. In this article, the CM model of the inverter is first built considering the inductance saturation. It is found that the variable inductor brings phase margin decreasing problem and additional resonance points in the CM loop. To suppress the CM resonance (CMR), novel passive and active damping strategies are explored in this article. The improved passive damping (IPD) strategy is realized by introducing a paralleled <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> unit, which greatly reduces the damping resistor losses without affecting the damping performance. Besides, a novel active damping (NAD) strategy is proposed to compensate the time delay and to raise the phase margin. Moreover, considering the gradient mutation issue brought by the discontinuous pulsewidth modulation (DPWM) scheme, an antisaturation PWM (ASPWM) scheme is applied to eliminate the oscillation. Finally, experiments are conducted on an 80-kW PV string inverter. After applying the proposed CMR damping strategy with the ASPWM scheme, CMR is fully eliminated in the whole power range despite the grid condition.

Certain investigations of ANFIS assisted CPHO algorithm tuned MPPT controller for PV arrays under partial shading conditions

This paper conceptualizes development of solar energy harvesting in partial shading situations. Two methods, namely Photovoltaic (PV) system topology design and Maximum Power Point Tracking (MPPT) techniques, can well reduce partial shading effects. An active hybrid MPPT controller suggested harvesting PV power called the Adaptive Neuro-Fuzzy Inference System (ANFIS) assisted Crowded Plant Height Optimisation (CPHO) algorithm. In the multi-string PV system configuration, the DC–DC boost converter is operated between the PV string and inverter to maximise the power yield. The duty ratio of converter must be adjusted and kept at an optimal level to obtain maximum power from the PV array. During uniform irradiance condition, the CPHO tuned MPPT controller itself gives the optimal duty cycle. Under partial shading circumstances, the PV array power versus voltage characteristics has several peaks. Hence optimal duty cycle can be obtained in two stages. In the first stage, ANFIS performs estimation of Global Maximum Power Point (GMPP), among multiple Local Maximum Power Point (LMPP) peaks and tracks close to a peak power point. The second stage CPHO algorithm fine-tunes and attains the exact point of GMPP. Hence, the suggested hybrid MPPT controller detects the GMPP more accurately and settles down the oscillations of the duty cycle in a fast manner. The performance enlivened by getting optimal duty cycle, and this proposed approach fortified through the MATLAB/Simulink platform.

Modified Partial-Shading-Tolerant Multi-Input-Single-Output Photovoltaic String Converter

Photovoltaic (PV) energy systems rapidly penetrate global renewable energy market recently due to their direct energy conversion, environmental friendly and modularity features. Partial shading and irradiance mismatch create one of the major challenges that face commercial PV converters. Software based maximum power point tracking (MPPT) algorithms show limited performance as only the global peak power of the attached PV string can be captured. On the contrary, modified PV string converters can achieve true MPP acquisition, hence maximizing the system power yield and enhance the overall performance. In this article, a partial-shading-tolerant PV string system is proposed that feature (i) a multi-input multi-output (MISO) PV string converter and (ii) true MPP seeking event-driven based MPPT technique as well. The presented converter feature a single inductor buck configuration with multi-input ports attached to N number of PV panels offering reduced passive components and extended modularity feature in addition to single digital controller implementation. The presented MPPT technique exhibits decoupled tracking of the true power of each connected PV panel independently irrespective of the other attached panels even under extreme inhomogeneous irradiance distribution. Rigorous simulation results accompanying matched experimental validation assure the proposed system applicability. Dynamic modelling, in addition to various operating conditions assessments, verify the claimed effectiveness of the proposed system.

Photovoltaic Failure Detection Based on String-Inverter Voltage and Current Signals

The existence of failures in photovoltaic systems causes energy losses, security problems, and damage to its components. Therefore, it is necessary to develop monitoring systems to improve their productivity, reliability, efficiency, and safety. This work proposes a method for detecting and indicating short-circuit failure and partial shading present in grid-connected photovoltaic modules. The novelty of this proposal is the processing of voltage and current signals generated (ripple signals) by the electrical interaction between the photovoltaic string, the photovoltaic inverter, the condition of the modules, temperature, and irradiance. The magnitudes of specific frequency components are obtained from these electrical signals using DFT, and Cartesian coordinates are formed in a three-dimensional plane. Coordinates belonging to each of the string conditions are mostly located in different octants from this plane. This distribution allows an assessment of the modules state. Certain scattering is solved with a K-nearest neighbors algorithm. The effectiveness of the methodology was experimentally validated in two PVG with different characteristics. In each test case, the method was adaptable to the real conditions of the photovoltaic system. The results show effectiveness greater than 90% in the first evaluation levels, there is a slightly lower detection certainty in anomalous conditions with coordinates that are very close to each other. However, the system has 100% certainty detecting the presence of an abnormal condition. The method allows adaptation to different conditions, and takes advantage of the electrical signals derived from the actual performance of the used devices.

Proposal for an Active PV Array to Improve System Efficiency During Partial Shading

Owing to time-varying external environmental influences, undesirable string mismatches frequently occur in photovoltaic (PV) power generation systems, which will cause conventional PV systems to experience extremely severe power losses. Under partial shading conditions (PSCs) caused by passing clouds, neighboring buildings, and animal excrement, the characteristic curve of the PV string becomes complicated with multiple peaks, which increases the difficulty of determining the maximum power point (MPP) of the array. However, it is essential to ascertain the global maximum power point (GMPP) to extract the full power of each PV module. Therefore, we propose a new type of PV generation module constituting an active PV array (APV) system as well as its control strategy. It can ensure that the system continuously operates in the optimal state when the solar irradiation intensity drops drastically due to unanticipated environmental variables. Depending on the experiment of two PV arrays under identical conditions, the efficiency and reliability of the APV system are substantiated. Meanwhile, in the case where the external environmental conditions are deliberately changed, the experimental results fully illustrate the adaptability of the APV system to diverse harsh conditions.

Optimized Global Maximum Power Point Tracking of Photovoltaic Systems Based on Rectangular Power Comparison

Global maximum power point tracking (GMPPT) refers to the extraction of the maximum power from photovoltaic (PV) modules in real time under changing ambient conditions. Due to the installation of PV systems in densely built-up areas, partial shading scenarios are commonplace. Commercially established GMPPTs suffer from low tracking speeds and inefficiency. A novel GMPPT algorithm is proposed here based on the rectangular power comparison (RPC), which exploits the fundamental relationship between the shading factor, the bypass diode voltage and the global maximum power point. The entire theoretical formulation of RPC is presented systematically for the first time. This method boasts of increased conversion speeds owing to the precomputation of the module voltage versus the shading factor correlations using the regression of diode model from the experimentally obtained bypass diode characteristics. The proposed method is simple to implement with the computational complexity of order $n$ , which represents the number of uniquely shaded PV modules in a series string. The proposed approach addresses the much-needed intersection problem between the distributed and centralized PV systems and therefore targets PV strings which are most common in residential and small to medium scale commercial PV installations world over. The proposed approach is validated with the in-house developed prototype hardware set-up and software control implementation giving a 99% tracking efficiency with a recorded tracking time of 10 ms. The experimental results show 50 times improvement in speed and 95% increase in power gain as compared to the other popular existing methods namely scanning based GMPPT and local MPPT methods respectively, with negligible computational burden and less than 0.5% added cost to the conventional PV energy conversion system.

Visualization and Diagnosis Method of Defective Photovoltaic String Based on the Temporal Variation Characteristics of String Currents in Generation Mode

Visualization and Diagnosis Method of Defective Photovoltaic String Based on the Temporal Variation Characteristics of String Currents in Generation Mode

A Novel Differential Power Processing Architecture for a Partially Shaded PV String Using Distributed Control

This article proposes a differential power processing (DPP) configuration applied to a series photovoltaic (PV) string. The proposed configuration operates each PV element at its local maximum power point (MPP) while processing only a small portion of its total generated power through the modular-integrated converters. The current processed through each converter is the difference between the local PV element MPP current and the local PV string current. This leads to higher energy capture at an increased conversion efficiency while overcoming the difficulties associated with unmatched MPPs that result from partial shading, temperature gradients, manufacturing defects, and nonuniform cells aging. A state-space model of the proposed system is driven, and a comparative analysis is carried out with respect to the conventional DPP architectures. Additionally, modular and compact design is proposed for a large number of PV panels in a series PV string. The authors designed and built a hardware prototype for three PV panels connected in series to validate the proposed configuration effectiveness and experimentally demonstrate its robustness.

Dual-Input Photovoltaic System Based on Parallel Z-Source Inverters

This paper aims to present a new structure of the parallel Z-source inverters (ZSIs) for dual-input single-phase grid-connected photovoltaic (PV) systems. The ZSI is a single-stage buck-boost converter that uses an inductor-capacitor network between the inverter bridge and the PV string and follows the maximum power point by applying the shoot-through vector. Therefore, a DC/DC converter is no longer needed to track the maximum power point, and the cost and complexity of the power conditioning system (PCS) are reduced. For controlling the proposed PCS, a cascade control structure is employed in this paper. The inner current loop injects the maximum active power with unity power factor sinusoidal current to the grid. The outer capacitor voltage loop is applied to control capacitors voltages in the Z-source networks. Additionally, an enhanced dual-string maximum power point tracking (eDS-MPPT) method is proposed to find MPPs with minimum burden competitional. The eDS-MPPT does not need the PVs voltages measurements compared to other MPPT methods. The simulation results confirm the accuracy of the performance of the system.

A complete framework for the simulation of photovoltaic arrays under mismatch conditions

The mismatch is a phenomenon intrinsically related to photovoltaic (PV) arrays, either because they are eventually subject to non-uniform irradiance and temperature conditions, or because of inner variations on the components, ageing or occurrence of failures. Despite mismatch losses being so present in the life of a PV system, the available tools for the study of PV arrays response through simulation today present limitations for the handling of mismatch conditions. Most of them do not support non-uniform conditions, and the ones that do, solely explore irradiance and temperature variations, ignoring other causes for mismatches, such as defects on PV cells. This paper proposes a simulation framework that allows manipulation of each parameter in each cell individually to reproduce mismatch conditions, including failures. Each PV cell is represented by the one diode equivalent circuit model, for which two different parameterisation methods were tested: California Energy Commission (CEC) and PVSyst. The proposed simulation framework was validated with laboratory measurements of a PV module under 15 different mismatch conditions of partial shading and failure (short circuit). The results showed a consistent similarity between calculated and measured I–V curves, and errors at the curves’ maximum power points stayed within the instruments’ accuracy range, roughly 1.0 V and 0.2 A. The proposed simulation framework brings great flexibility to the studies of mismatch through simulation, allowing the calculation of the I–V curve of PV strings and small arrays subjected to mismatch conditions due to non-uniform irradiance and temperature profiles, but most significantly failures in the cells.

Improvement in the Power of Shingled-Type Photovoltaic Module by Control of the Overlapped Width

Shingled string photovoltaic (PV) cells have a larger active area, and they significantly reduced damage from the interconnection process using electrically conductive adhesives. This new method using a shingled design provides increased PV module power compared with the conventional PV module for the same installed area. In this work, we study the optimization of shingled string PV cells for high-power shingled design modules (SDMs). To successfully fabricate shingled string PV cells and high-power SDMs, we adjusted the overlap width between solar cells and achieve conversion powers of 406.5 W for a 1100 mm × 1980 mm area. Finally, we perform a reliability tests to verify the durability of our SDMs.

Thermo-Mechanical Stress Comparison of a GaN and SiC MOSFET for Photovoltaic Applications

Integrating photovoltaic applications within urban environments creates the need for more compact and efficient power electronics that can guarantee long lifetimes. The upcoming wide-bandgap semiconductor devices show great promise in providing the first two properties, but their packaging requires further testing in order to optimize their reliability. This paper demonstrates one iteration of the design for reliability methodology used in order to compare the generated thermo-mechanical stress in the die attach and the bond wires of a GaN and SiC MOSFET. An electro-thermal model of a photovoltaic string inverter is used in order to translate a cloudy and a clear one-hour mission profile from Arizona into a junction losses profile. Subsequently, the finite element method models of both devices are constructed through reverse engineering in order to analyze the plastic energy. The results show that the plastic energy in the die attach caused by a cloudy mission-profile is much higher than that caused by a clear mission-profile. The GaN MOSFET, in spite of its reduced losses, endures around 5 times more plastic energy dissipation density in its die attach than the SiC MOSFET while the reverse is true for the bond wires. Potential design adaptations for both devices have been suggested to initiate a new iteration in the design for reliability methodology, which will ultimately lead to a more reliable design.

Switched-Capacitor-Based Multilevel Inverter for Grid-Connected Photovoltaic Application

In order to approach a voltage of high magnitude in photovoltaic (PV) application, several PV modules are connected in series to form a PV string, then the voltage of PV string is converted to an ac voltage through an H-bridge inverter. This system causes harmful harmonics, brings about leakage current, and increases the voltage stress of the components. To tackle the issue, this article proposes a new switched-capacitor-based multilevel inverter topology that uses series-connected dc sources as the input dc source and generates a staircase ac voltage of high quality and mitigates the leakage current. The proposed topology can be constructed as a three-phase inverter as well, in the three-phase configuration all the phases are connected to only one series-connected PV module or dc sources. Simulation and experimental results are provided to validate the proposed topology.

Detection and Localization of Damaged Photovoltaic Cells and Modules Using Spread Spectrum Time Domain Reflectometry

The operating efficiency of photovoltaic (PV) plants can be improved if damaged or degraded modules can be detected and identified. Currently, string-level power electronics can detect problems with modules or cabling but not locate them, which would facilitate addressing these issues. Here, we investigate the ability of spread spectrum time domain reflectometry (SSTDR) to both detect and locate/identify damaged cells and modules within a series-connected PV string. We tested the ability of SSTDR to detect and locate single-cell mini-modules and full-sized PV modules, which were intentionally damaged by impacts with a hammer (breaking the glass and damaging the silicon below) or by cutting through some or all busbars. Damage to the glass and silicon of cells was detected and located within a small string of minimodules. Busbar damage was detectable only if an open was created by cutting through all intercell busbars. Physical impact damage to the glass and silicon of a full-sized PV module could be detected, but further development of signal processing is needed to achieve localization of such damaged modules within a string.

Potential assessment of solar power plant: A case study of a small island in Eastern Indonesia

Indonesia as a tropical country has high solar energy potential to generate power. Solar power plant (SPP) using Photovoltaic (PV) is suitable for power generation in a small power system, such as in several small islands in Eastern Indonesia since solar energy is easy to obtain there. The implementation of SPP for power generation needs an examination of energy yield to observe the performance ratio of SPP. Those parameters are important to study to further analyze the economic potential and feasibility of SPP development. This study estimates the performance ratio (PR) and potential energy yield of SPP in a small island in Eastern Indonesia through a self-developed program using MATLAB based Graphical User Interface (GUI). Moreover, this program also provides the levelized cost of energy (LCOE) calculation and land use optimization by readjusting the PV array configuration. According to the simulation result, the implementation of SPP in Eastern Indonesia has 85.4% and 23.54% for annual PR and capacity factor (CF) respectively. The simulation also shows that the optimum distance between PV strings produces higher annual energy yield and CF.

An Enhanced 0.8$\text{V}_\text{OC}$-Model-Based Global Maximum Power Point Tracking Method for Photovoltaic Systems

Under partial shading conditions (PSC), the power-voltage ( P–V ) characteristic curve of a photovoltaic string exhibits multiple peaks, posing a big challenge to the problem of global maximum power point tracking (GMPPT). The traditional 0.8 $\text{V}_\text{OC}$ -model-based GMPPT method locates the global maximum power point (GMPP) locus by comparing the power at each local power peak. However, a considerable amount of time is required for iteratively scanning each 0.8 $\text{V}_\text{OC}$ vicinity. To address this problem, an improved 0.8 $\text{V}_\text{OC}$ -model-based GMPPT method is proposed in this article. A shading vector is introduced to characterize the PSC. The proposed GMPPT method estimates the 0.8 $\text{V}_\text{OC}$ region with the GMPP directly from the measured shading vector by the k -nearest neighbors approach and saves the time consumed in the comparison process used in the conventional method. Simulation and experimental results demonstrate that the proposed method is capable of tracking the GMPP efficiently and accurately under various shading patterns.

Analysis of Resistance According to Metal Ribbon Connection for Application to Interconnection of Shingled Photovoltaic Strings.

The Shingle Photovoltaic (PV) module is a new high power PV module technology manufactured by 'Dividing and ECA (Electrical Conductivity Adhesive) bonding' method for solar cell. In the case of a general PV module, a metal ribbon is soldered on the bus bar of the solar cell and connected to others. The dividing/ECA bonding technology connects the divided cells through bonding to manufacture a string. In order to make a module, the fabricated strings must be connected with Bus ribbon. The Shingled strings produced by the dividing and bonding method are not limited to the interconnection method by the metal ribbon. Also, it is not standardized for interconnections between strings. Therefore, we analyzed the characteristics of the shingled strings according to the soldering method. The characteristics of the string vary depending on the number of metal ribbons that contact the solar cell electrodes. Experimental results show that the series resistance increases significantly with fewer contacts. As a result, the efficiency of two-point contact decreased by 0.458%, four-point contact decreased by 0.048%, and eight-point contact decreased by 0.034%. This is because as the number of contacts increases, the resistance of the busbars becomes smaller and the contact resistance becomes smaller.