Grid-connected Photovoltaic Array Research Articles

Current Compensation for Faulted Grid-Connected PV Arrays Using a Modified Voltage-Fed Quasi-Z-Source Inverter

Large-scale photovoltaic (PV) systems are being widely deployed to meet global environmental goals and renewable energy targets. Advances in PV technology have driven investment in the electric sector. However, as the size of PV arrays grows, more obstacles and challenges emerge. The primary obstacles are the occurrence of direct current (DC) faults and shading in a large array of PV panels, where any malfunction in a single panel can have a detrimental impact on the overall output power of the entire series-connected PV string and therefore the PV array. Due to the abrupt and frequent fluctuations in power, beside the low-PV systems’ moment of inertia, various technical problems may arise at the point of common coupling (PCC) of grid-connected PV generations, such as frequency and voltage stability, power efficiency, voltage sag, harmonic distortion, and other power quality factors. The majority of the suggested solutions were deficient in several crucial transient operating features and cost feasibility; therefore, this paper introduces a novel power electronic DC–DC converter that seeks to mitigate these effects by compensating for the decrease in current on the DC side of the system. The suggested solution was derived from the dual-source voltage-fed quasi-Z-source inverter (VF-qZSI), where the PV generation power can be supported by an energy storage element. This paper also presents the system architecture and the corresponding power switching control. The feasibility of the proposed method is investigated with real field data and the PSCAD simulation platform during all possible weather conditions and array faults. The results demonstrate the feasibility and capability of the proposed scheme, which contributes in suppressing the peak of the transient power-to-time variation (dP/dt) by 72% and reducing its normalized root-mean-square error by about 38%, with an AC current total harmonic distortion (THD) of only 1.04%.

Estimation of photovoltaic production using Spline fitting and TSCI

This study presents an alternative methodology for analyzing the difference in daily solar energy generation between two grid-connected photovoltaic arrays (PVAs) that are identical in capacity, technical characteristics, and have the same energy production measurement interval but are in two different municipalities of Mexico City (CDMX). Unlike conventional approaches, which often rely on environmental variables, this work proposes combining the cubic smoothing Spline technique with the separation of trend, seasonality, and cyclicality (TSCI) effects. The first technique reduces random variations in measurement, while the second separates the effects commonly found in time series data. The results obtained are useful for installers of this technology, as they allow them to determine in which borough a greater photovoltaic capacity is needed.

A review of passive cooling of photovoltaic devices

Our planet's ecosystems depend on the energy received from the sun to form a bubble of life. Technically, other sources of energy are converted from solar energy. An effective way to directly convert solar energy to electricity is through photovoltaic devices. They could be manufactured on small scales and used in pocket calculations up to large scales power plants. These power stations, known as a solar farm, is composed of grid-connected photovoltaic arrays. The conventional solar cells' efficiency, ceiled by the Shockley-Queisser limit, is in the range of 15%–25%. The rest of the solar energy is converted to heat which would have a detrimental influence on power production and the life span of photovoltaic devices. Hence, employing cooling systems to regulate their operative temperature is crucial. Passive cooling technologies without consuming additional power and with little maintenance cost could be a practical option. In this study, the extensive work of researchers applying passive cooling techniques is gathered and compared. Also, the study will shed light on finding an appropriate cooling technique concerning geographical or environmental conditions. Papers related to each specific passive method are discussed, and the challenges which have remained unsolved are noted. Finally, most of the papers with different environmental conditions are mentioned in an inventory which makes the comparison and evaluation easier. Among the six passive cooling methods, natural air ventilation is economically the most viable option. Hence, designing complex fin structures to help enhance air ventilation is recommended for most of the problems. In the case of water cooling, floatovoltaics are highly recommended because of supplementary advantages such as reducing the algae growth and evaporation rate. • Active cooling techniques would be a challenging task for a solar farm. • Passive cooling methods for photovoltaic modules/panels have been reviewed. • The passive cooling techniques are divided into six categories. • The possibility of combining multi-passive methods is discussed. • Floatovoltaics could solve both the water evaporation crisis and PV efficiency drop.

Optimal PID Tuning of PLL for PV Inverter Based on Aquila Optimizer

Phase-locked loop (PLL) is a fundamental and crucial component of a photovoltaic (PV) connected inverter, which plays a significant role in high-quality grid connection by fast and precise phase detection and lock. Several novel critical structure improvements and proportional-integral (PI) parameter optimization techniques of PLL were proposed to reduce shock current and promote the quality of grid connection at present. However, the present techniques ignored the differential element of PLL and did not acquire ideal results. Thus, this paper adopts Aquila optimizer algorithm to regulate the proportional-integral-differential (PID) parameters of PLL for smoothing power fluctuation and improving grid connection quality. Three regulation strategies (i.e., PLL regulation, global regulation, and step regulation) are carefully designed to systematically and comprehensively evaluate the performance of the proposed method based on a simulation model in MATLAB/Simulink, namely, “250-kW Grid-Connected PV Array”. Simulation results indicate that PLL regulation strategy can effectively decrease power fluctuation and overshoot with a short response time, low complexity, and time cost. Particularly, the Error(P) and the maximum deviation of output power under optimal parameters obtained by PLL strategy are decreased by 418 W and 12.5 kW compared with those under initial parameters, respectively.

A new passive islanding detection technique for different zones in utility grid

Distributed generation (DG) has reformed the meaning of traditional generation of power from large-scale to small-scale generation. The main issue of connecting the DG to the utility grid is the detection of unintended islanding. This paper shows the impact of the islanding phenomenon in the case of grid-connected photovoltaic arrays and how to develop a convenient technique to detect this phenomenon. A passive islanding detection algorithm is proposed for all types of DGs by varying and analyzing the DC-link voltage for voltage source converter in the photovoltaic inverter. The proposed algorithm is applied on the low and medium voltage scales. Furthermore, a comparison for applying the proposed technique with resistance load on the two scales is presented. In addition, the proposed technique for anti-islanding protection is performed and compared with a lot of techniques such as underfrequency, overfrequency, and rate of change of frequency according to the detection time of islanding. The simulation results using MATLAB/ SIMULINK platform illustrate the effectiveness of the proposed method.

Implementation of Incremental Conductance MPPT Algorithm with Integral Regulator by Using Boost Converter in Grid-Connected PV Array

ABSTRACT This paper presents a rapid, effective, and linear incremental conductance (IC) algorithm for chasing the maximum power point (MPP) of the grid-tied photovoltaic (PV) array. A technique has been proposed to find the optimum values of the design parameters of the PV array. This methodology enables the prime matching of the PV system with the DC/AC converter configuration by calculating the optimal values of the inductor, capacitor, and the duty cycle of the boost converter. Analysis of different techniques for maximum power point tracking (MPPT) is presented and the comprehensive design of the PV converter system is also incorporated. The number of switches has been reduced for optimal power generation. Similarly, a technical inter-comparison has been made with the existing topologies. MATLAB/Simulink is used to characterize solar PV characteristics at various irradiance and temperature profiles. Furthermore, the simulation results agree well with the designed version in terms of PV panel efficiency.

Shading fault detection in a grid-connected PV system using vertices principal component analysis

Partial shading severely impacts the performance of the photovoltaic (PV) system by causing power losses and creating hotspots across the shaded cells or modules. Proper detection of shading faults serves not only in harvesting the desired power from the PV system, which helps to make solar power a reliable renewable source, but also helps promote solar versus other fossil fuel electricity-generation options that prevent making climate change targets (e.g. 2015’s Paris Agreement) achievable. This work focuses primarily on detecting partial shading faults using the vertices principal component analysis (VPCA), a data-driven method that combines the simplicity of its linear model and the ability to consider the uncertainties of the different measurements of a PV system in an interval format. Data from a grid-connected monocrystalline PV array, installed on the rooftop of the Power Electronics and Renewable Energy Research Laboratory (PEARL), University of Malaya, Malaysia, have been used to train the VPCA model. To prove the effectiveness of this VPCA method, four partial shading patterns have been created. The obtained performance has, then, been tested against a regular PCA. In addition to its ability to acknowledge the uncertainty of a PV system, the VPCA method has shown an enhanced performance of detecting partial shading fault in comparison with the standard PCA. Also, included in the article is an extension of the contribution plot diagnosis-based method, of the Q-statistic, to the interval-valued case aiming to pinpoint the out-of-control variables.

A simplified method for fault detection and identification of mismatch modules and strings in a grid-tied solar photovoltaic system

AbstractThe foremost problem facing by the photovoltaic (PV) system is to identify the faults and partial shade conditions. Further, the power loss can be avoided by knowing the number of faulty modules and strings. Hence, to attend these problems, a new method is proposed to differentiate the faults and partially shaded conditions along with the number of mismatch modules and strings for a dynamic change in irradiation. The proposed method has developed in two main steps based on a simple observation from the Current versus Voltage (I-V) characteristic curve of PV array at Line-Line (LL) fault. First, the type of fault is detected using defined variables, which are continuously updated from PV array voltage, current, and irradiation. Second, it gives the number of mismatch modules (or short-circuited bypass diodes) and mismatch strings (or open-circuited blocking diodes) by comparing with the theoretical predictions from the I-V characteristic curve of PV array. The proposed algorithm has been validated both on experimentation using small scale grid-connected PV array developed in the laboratory as well as MATLAB/Simulink simulations. Further, the comparative assessment with existing methods is presented with various performance indices to show the effectiveness of the proposed algorithm.

Leakage current alleviation in solar energy conversion system enabling power quality improvement

This work presents a generalised integrator-based control algorithm for power quality (PQ) amelioration of the grid in the presence of non-linear load enabling leakage current suppression feature. Owing to the presence of stray capacitance between solar photovoltaic (PV) array and the ground, the variation in common-mode voltage across stray capacitance leads to potential safety issues, electromagnetic interference, and distortion into the injected grid currents. Furthermore, the PQ of the grid is highly deteriorated under the existence of non-linear loads. To solve the aforementioned issues, the harmonic compensation controller is presented herein to ensure the unity power factor operation, harmonic compensation, leakage current suppression using the grid-connected solar PV array system. Simulated results show the effectiveness of the control strategy under various operating scenario. In contrast to the state-of-the-art techniques, the grid currents are attained sinusoidal and balanced even in the presence of unbalanced non-linear load. Furthermore, the leakage current is suppressed within 300 mA as per the VDE-00126-01 standard. The comparative performance explicitly validates the effectiveness of the presented harmonic compensating strategy over the conventional schemes. Test results manifest the response of the solar PV array system for various operating conditions and follows the revised IEEE-519–2014 sand VDE-00126-01 standards.

Reactive-Active Power Control for Grid-Connected PV Arrays to Enlarge the Hosting Capacity in A Low Voltage Distribution System

The stochastic behavior of solar radiation is one of the challenges faced by increasing the hosting capacity of the photovoltaic (PV) power. At times of exceptional high PV power output, unacceptable voltage rise could happen at PV system terminals. Violating the voltage threshold adversely affect the power quality and security at the customer sides. In order to avoid this, the PV system is normally disconnected during high PV power output, thus causing losses in power generated. Local control approaches have been introduced as a measure to overcome this issue by using only the inverter. One of these approaches is to control the reactive power output of a PV unit in order to suppress the voltage rise without ceasing the PV system. The results from this paper have shown that the existing reactive power control (RPC) approach is unable to suppress this voltage rise effectively in case of low X/R ratio. Active power curtailment (APC) is another approach that results in remarkable power losses. Hence, this paper proposes a methodology that combines both RPC and APC approaches to limit the voltage rise in a low voltage distribution feeder. The effectiveness of the proposed methodology have been examined and demonstrated in this paper. The obtained results show the superiority of the proposed methodology over the conventional approaches, which enlarges the hosting capacity for PV power penetration in a low voltage distribution system.

Single-Input Space Vector Based Control System for Ripple Mitigation on Single-Phase Converters1

Applications such as renewable energy generation, electric vehicles, and low-power UPS require single-phase ac/dc conversion. However, this conversion introduces a considerable amount of second-harmonic ripple on the dc link. If not filtered, this distortion hinders the converter's performance as well as the energy quality on both the ac and dc side. To mitigate this problem, a large electrolytic capacitor is usually the solution of choice, which mitigates the voltage ripple, but has drawbacks of its own, including the increased size and cost associated with the large capacitance and the limited lifespan of electrolytic capacitors. Alternative solutions to the problem include integration of an active filter circuit to the converter, which can utilize a storage element with the objective of mitigating power ripple. Such solutions have often been proposed alongside a control system, which is either highly complex or relies on open-loop feedforward techniques. This paper presents a control system, which adapts the single-input space vector concept for a single-phase application and leverages its simplicity and closed-loop architecture, allowing the controller to perform well in the presence of disturbances and parameter uncertainty. Experimental results are provided to elucidate the controller's performance on a single-phase grid-connected photovoltaic array application.

A new topology and power control of grid-connected photovoltaic array

Under the partial shading, the series photovoltaic modules will generate additional power loss and present a multi-peak power-voltage curve which brings difficulties to the maximum power point tracking. By using a novel grid-connected topology and power control method presented in this paper, the photovoltaic array can be connected through a full parallel structure and thus the shortcomings mentioned above can be overcome. A higher voltage, required for grid connection, also can be obtained through the topology without the need for any step-up transformer or boost circuit. Furthermore, the output power of the photovoltaic array can be adjusted by controlling the phase of the modulated wave. As result, the maximum power point tracking can be achieved with a simple method. The simulation and experiment results verified the validity of the proposed topology and control method.

A new topology and power control of grid-connected photovoltaic array

A new topology and power control of grid-connected photovoltaic array

Impact of solar radiation variation on the optimal tilted angle for fixed grid-connected PV array—case study in Beijing

A key design parameter for fixed grid-connected photovoltaic (PV) arrays, the optimal tilt angle, does not only depend on the geographic location but is also directly affected by atmospheric conditions. In this paper, long-term variations of solar radiation (i.e. global solar irradiance, direct horizontal irradiance, diffuse irradiance, and ratios of direct and diffuse irradiance) in Beijing are considered to determine their effect on the optimal tilt angle for a fixed grid-connected PV array. We found that there is a declining trend in global solar irradiance over the past 55 years, mainly caused by the decreased direct horizontal irradiance. In contrast, the decline of diffuse irradiance is not obvious, leading to a considerable decrease in the direct irradiance ratio and consequent increase in the diffuse irradiation ratio. Likewise, the long-term optimal tilt angle shows a downward trend. Compared with the optimum in the 1960s, the optimal tilt angle has decreased by 2° in 2011–2015. These results suggest that the declining trend in the optimal tilt angle is mainly caused by the decrease in direct irradiance ratio, which is highly related to atmospheric conditions. Therefore, the design and construction of PV power stations must consider the variations of atmospheric conditions and solar irradiance to determine the optimal tilt angle.

Voltage-Based Protection Scheme for Faults Within Utility-Scale Photovoltaic Arrays

Permanent power loss and major catastrophic failures are among the potential consequences of undetected faults within photovoltaic (PV) arrays. This paper proposes a voltage-based protection scheme that detects, classifies, and locates string-to-ground, string-to-string, and open-circuit faults in utility-scale PV arrays. The protection scheme operates based on the magnitude and wave-shape properties of voltage signals. Some salient features of this method include its capability to distinguish between fault and partial shading conditions, and its independence from meteorological data. The performance of the proposed approach is assessed on two utility-scale grounded PV arrays under various conditions. Moreover, the proposed method is tested on an experimental setup of a small grid-connected PV array. Simulation and experimental results verify the scheme’s accuracy, high speed, and scalability.

High-Speed Monitoring of Multiple Grid-Connected Photovoltaic Array Configurations and Supplementary Weather Station.

Three grid-connected monocrystalline silicon photovoltaic arrays have been instrumented with research-grade sensors on the Gaithersburg, MD campus of the National Institute of Standards and Technology (NIST). These arrays range from 73 kW to 271 kW and have different tilts, orientations, and configurations. Irradiance, temperature, wind, and electrical measurements at the arrays are recorded, and images are taken of the arrays to monitor shading and capture any anomalies. A weather station has also been constructed that includes research-grade instrumentation to measure all standard meteorological quantities plus additional solar irradiance spectral bands, full spectrum curves, and directional components using multiple irradiance sensor technologies. Reference photovoltaic (PV) modules are also monitored to provide comprehensive baseline measurements for the PV arrays. Images of the whole sky are captured, along with images of the instrumentation and reference modules to document any obstructions or anomalies. Nearly, all measurements at the arrays and weather station are sampled and saved every 1s, with monitoring having started on Aug. 1, 2014. This report describes the instrumentation approach used to monitor the performance of these photovoltaic systems, measure the meteorological quantities, and acquire the images for use in PV performance and weather monitoring and computer model validation.

The performance analysis of photovoltaic generation using the improved incremental conductance method under varying solar irradiance

Photovoltaic (PV) is an important clean and free-pollution renewable energy. The performance of a photovoltaic module is mostly affected by array configuration and solar irradiance. Due to the intermittent feature of weather, it becomes an unreliable and intermitted source. The variations of solar irradiance are optimally tracked by the proposed incremental conductance (IC) technique with proportional integral regulator is maintain the maximum power point tracking (MPPT) of a three-phase grid-connected PV array. Simulations results show the proposed IC-MPPT method has achieved as fast response performance under sudden change of solar irradiation. The proposed MATLAB/Simulink® model implemented as a PV inverter can be achieved with 95.0% peak MPPT efficiency.

A new backstepping finite time sliding mode control of grid connected PV system using multivariable dynamic VSC model

This paper presents independent active and reactive power management of a three-phase grid-connected photovoltaic (PV) generation system using a new nonlinear control approach for the voltage source converter (VSC). Instead of controlling the direct and quadrature-axis currents of the VSC, the instantaneous active and reactive powers are used as error estimation parameters. This mode of control dispenses the unmodelled dynamics of the VSC phase-locked loop system and produces a robust control for the active–reactive power, and dc voltage excursions. This approach reduces computational time as well as complexity by avoiding unnecessary PLL phase calculation in the beginning. However, the PLL is used only to obtain the frequency component needed to generate the PWM signal. Further to improve the stability and robust tracking of the grid connected PV array, backstepping finite time fast sliding mode (BFTSM) control strategy is presented in this paper. The proposed controller offers invariant stability to modeling uncertainties due to converter parameter changes, changes in system frequency and exogenous inputs. Also the finite time sliding mode control offers an important tool for designing continuous finite time control laws. Comprehensive computer simulations are carried out in MATLAB/Simulink to verify the proposed control scheme under several system disturbances like changes in solar insolation, changes in local load, converter parametric changes, and faults on the converter and inverter buses, and partial shading condition of PV array. EMTDC/PSCAD model is established as confirmative study.

Finite-time sliding mode power control of grid-connected three-phase photovoltaic array

A new direct power control formulation for the voltage source converter (VSC) in a grid-connected photovoltaic (PV) array is discussed in this paper. Instead of controlling the direct- and quadrature-axis currents of the VSC through the use of a phase-locked loop system (PLL), the instantaneous real- and reactive-power components are used as control variables. This approach, therefore, does not take into account the unmodelled dynamics of the PLL resulting in a robust control of the active, reactive powers while maintaining the photovoltaic (PV) array voltage at a constant value. Further, the stability of the grid-connected PV array is enhanced by designing a Lyapunov’s function-based finite-time non-linear reactive power and dc-link voltage controller, which essentially uses the terminal sliding mode approach. To prove the robustness of the proposed approach, several system disturbances like changes in solar insolation and active and reactive power references, islanding conditions, converter parametric changes, and faults on the converter and inverter buses are considered. In all these cases, the new controller is found to enhance overall system stability in comparison to the inferior performance by the proportional-integral controller

Novel method for the improvement in the evaluation of outdoor performance loss rate in different PV technologies and comparison with two other methods

The interest in the assessment of performance loss rate (PLR) of Photovoltaic (PV) modules and arrays has been increasing as long as the global installed power expands and ages. Reliable performance metrics, statistical methods and filtering techniques exploiting continuous outdoor measurements are therefore needed, in order to foster solar bankability of PV systems. This work presents an improved estimation method to decrease the uncertainty associated to PLR assessment by (a) using the array generated power metric corrected to Standard Test Conditions (STC), namely Pmax,STC, to minimize seasonal oscillations, (b) applying a filtering technique to eliminate outliers and (c) performing linear interpolation on Pmax,STC monthly averages series. Estimated PLR and its uncertainty are assessed using three-years data from twenty-four grid-connected PV arrays representing nine different PV technologies and results are compared with two other widely-recognized performance metrics, namely: the Array Performance Ratio (PRa) and the Array Photovoltaic for Utility Systems Applications (PVUSAa). Results show (a) that adding spectral correction to irradiance and temperature correction reduces the uncertainty of 25% on average and (b) that the uncertainty associated to Pmax,STC metric is reduced to more than 60% on average with respect to the other investigated metrics for crystalline silicon-based technologies, while it is comparable in the case of thin-film technologies. Finally, two procedures estimating the first year PLR and the the first five months PLR are presented and discussed.