Two-stage Photovoltaic Research Articles

An Overview of Voltage Boosting Techniques and Step-Up DC-DC Converters Topologies for PV Applications

The development of technologies to improve the performance of photovoltaic (PV) module integrated converters (MICs) is fundamental to increase the use of distributed generation systems with photovoltaic power source in large urban centers, mainly for complex residential roofs. For two-stage PV MICs, high step-up DC-DC converters are required to boost the low PV module voltage to a higher voltage, in order to suit the DC bus voltage requirements of grid-tied inverters. Thus, to support researchers interested in developing DC-DC power conversion for PV microinverters, this paper classifies the DC-DC converters according to their operational and constructive characteristics and presents some elementary voltage-boosting techniques to aid in analyzing and understanding more complex topologies. Finally, high step-up DC-DC converters based on magnetic coupling and switched capacitor widely cited by important works related to PV applications are presented, with their principles of operation analysed in a simple and objective way, but sufficient to understand their capability to provide high voltage gain. The approach presented by this paper leads to insight into how to place the energy storage elements to create new topologies of DC-DC converters, so that high voltage gain is achieved, and how to analise the high voltage gain capability of complex topologies

Control of grid-connected residential solar-PV system using novel adaptive linear combiner filter for power quality improvement

ABSTRACT A grid-connected residential photovoltaic (PV) system has been proposed using novel adaptive linear combiner filter-based control for power quality (PQ) improvement. In the present work, novel adaptive linear combiner (ALC) filter-based control-based control has been proposed to generate reference current. MATLAB/Simulink software is used to develop the two-stage grid-connected residential PV. The proposed control is tested in MATLAB/Simulink2018(a) under nonlinear, linear load and variable input and validated experimentally on prototype hardware. The performance of the proposed ALC filter-based control has been analyzed and compared with that of conventional synchronous d-q control. It has been observed from results that THD in grid current using proposed control is 3.74% whereas using conventional d-q control THD is 4.75%. Proposed control provides more efficient performance by improving the PQ of the system by compensating the harmonics and reactive load demand. Harmonic distortion in the grid current is within the limits as per IEEE-1547. Furthermore, proposed control has better transient response viz. settling time, under shoot and overshoot in DC link voltage during grid connection of rooftop PV system and provides, reduced complexity due to absence of phase lock loop (PLL), thus less sampling time, better accuracy, ease of implementation, and adaptability.

Reliability-based trade-off analysis of reactive power capability in PV inverters under different sizing ratio

Due to the intermittent characteristic of solar irradiance, photovoltaic (PV) inverters usually operate below rated power conditions. In this scenario, commercial PV inverters can be used to provide ancillary services, such as reactive power compensation. On the other hand, over-sizing PV inverters by increasing the number of installed PV panels can increase the generated energy during low irradiance conditions. Despite the advantages, both approaches can increase the PV inverter wear-out, leading to early failures. The trade-off between reactive power compensation and lifetime consumption under different inverter sizing ratios (ISR) was not previously addressed in the literature. Hence, this paper proposes to evaluate the system-level reliability of a single-phase two-stage PV inverter performing reactive power compensation. The analysis is carried out from three different mission profiles (Aalborg, Goiânia, and Izaña). The thermal loading on the inverter components is translated to lifetime prediction. The results from Goiânia show that the PV inverter lifetime is reduced from 22.6 to 6.2 years when compensating reactive power. In addition, this reduction is observed in all three regions, going from 40.8 to 12.4 years and 10 to 4.4 years, for Aalborg and Izaña, respectively. To improve the lifetime, a reactive power capability factor QR is applied to the maximum reactive power condition. The unreliability map is presented as a tool to match the inverter sizing ratio with reactive power capability in agreement with lifetime requirements. An economic analysis based on Levelized Cost of Electricity (LCOE) showed that the reactive power compensation increases the costs of the system installed in Goiânia with ISR = 100 % by 46.6 % in comparison with the conventional operation. These additional costs can be reduced if the reactive power is partially compensated.

Dual-Mode Power Operation for Grid-Connected PV Systems with Adaptive DC-link Controller

Photovoltaic (PV) power systems are integrated with high penetration levels into the grid. This in turn encourages several modifications for grid codes to sustain grid stability and resilience. Recently, constant power management and regulation is a very common approach, which is used to limit the PV power production. Thus, this article proposes dual-mode power generation algorithm for grid-connected PV systems. The developed system considers the two-stage PV configuration for implementation, where the dual-mode power generation technique is executed within the DC–DC conversion (boost) stage. Most of the techniques adopted for dual-mode power operation employ the conventional perturb and observe method, which is known with unsatisfactory performance at fast-changing atmospheric conditions. Considering this issue, this study suggests a modified maximum power point tracker for power extraction. Furthermore, a new adaptive DC-link controller is developed to improve the DC-link voltage profile at different operating conditions. The adaptive DC-link controller is compared with the traditional PI controller for voltage regulation. The inverter control is accomplished using finite-set model predictive control with two control objectives, namely reference current tracking and switching frequency minimization. The overall control methodology is evaluated at different atmospheric and operating conditions using MATLAB/Simulink software.

Power production strategies for two-stage PV systems during grid faults

The paper comprehensively addresses the control algorithm for the photovoltaic (PV) system. The system’s operation during both nominal and faulty grid conditions, most notably voltage unbalances, is addressed. The control of the boost converters and the inverter that secures reliable and maximized power production is proposed. Specifically, the paper offers a novel DC-link voltage stabilization algorithm that is not based on conventional approaches, such as droop schemes. Rather, the DC-link voltage rate of change is used to realize safe primary energy sources’ power management during transients caused by grid disturbances. This offers improved robustness, reliability and scalability. Moreover, the improved grid-connected converter control is proposed. It enables the production of arbitrary currents and powers profiles, during different working regimes, respecting the converter’s ratings. Consequently, higher-layer control functions can be realized by utilizing the proposed scheme. Also, the flexible power production prioritization feature is integrated into the overall solution. Such an extensive set of features was not demonstrated for PV systems in the literature previously. Different hardware topologies are regarded – centralized, decentralized and distributed. The proposed control schemes were tested using the hardware-in-the-loop (HIL) environment and were validated through the analysis of the obtained results.

Maximum Power Point Tracking of PV Systems under Partial Shading Conditions Based on Opposition-Based Learning Firefly Algorithm

This work presents an alternative to the conventional photovoltaic maximum power point tracking (MPPT) methods, by using an opposition-based learning firefly algorithm (OFA) that improves the performance of the Photovoltaic (PV) system both in the uniform irradiance changes and in partial shading conditions. The firefly algorithm is based on fireflies’ search for food, according to which individuals emit progressively more intense glows as they approach the objective, attracting the other fireflies. Therefore, the simulation of this behavior can be conducted by solving the objective function that is directly proportional to the distance from the desired result. To implement this algorithm in case of partial shading conditions, it was necessary to adjust the Firefly Algorithm (FA) parameters to fit the MPPT application. These parameters have been extensively tested, converging satisfactorily and guaranteeing to extract the global maximum power point (GMPP) in the cases of normal and partial shading conditions analyzed. The precise adjustment of the coefficients was made possible by visualizing the movement of the particles during the convergence process, while opposition-based learning (OBL) was used with FA to accelerate the convergence process by allowing the particle to move in the opposite direction. The proposed algorithm was simulated in the closest possible way to authentic operating conditions, and variable irradiance and partial shading conditions were implemented experimentally for a 60 [W] PV system. A two-stage PV grid-connected system was designed and deployed to validate the proposed algorithm. In addition, a comparison between the performance of the Perturbation and Observation (P&O) method and the proposed method was carried out to prove the effectiveness of this method over the conventional methods in tracking the GMPP.

A 3-kW Two-Stage Transformerless PV Inverter With Resonant DC Link and ZVS-PWM Operation

Photovoltaic (PV) inverters play important roles in renewable energy integration. Reducing the switching loss is a main challenge in improving the efficiency and power density. This article presents the design, implementation, and field test of a 3-kW two-stage commercial-ready transformerless PV inverter with resonant dc link and zero-voltage-switching pulsewidth modulation (ZVS-PWM) operation. The ZVS-PWM operation periodically resonates the dc-link voltage of the inverter to zero for ZVS of the inverter switches, which enables higher switching frequency and significantly reduced magnetic component size. A 30-kW distributed PV system comprising ten ZVS-PWM PV inverters was built and tested for more than 100 days to evaluate the long-term performance of the PV inverter. The key figure-of-merits for grid-interface operation, such as efficiency, power density, total harmonic distortion, and leakage current, are measured, analyzed, and reported.

Advanced Control Strategy With Voltage Sag Classification for Single-Phase Grid-Connected Photovoltaic System

This article develops a fault ride-through control strategy for achieving low-voltage ride through in single-phase grid-connected photovoltaic (PV) systems (GCPVS). The proposed control system adapts a neural network classifier for islanding classification and model predictive control for achieving the control of the two-stage PV system. This control scheme takes advantage of the nonlinear nature of the power converters and develops a cost function-based approach to achieve fast and efficient control. In addition, the proposed controller provides voltage support to the grid during voltage sags by injecting minimum reactive current within the threshold. The operation of the proposed control strategy is verified by performing simulation tests on a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4\ {\text{kW}}$</tex-math></inline-formula> GCPVS by creating a sag type of fault in the utility. Further, laboratory experiments were carried out with the developed controller. The results ensure that the proposed control system adheres to the grid requirements by enabling voltage support during grid faults.

An Improved Lyapunov Function Based Control Strategy for Multitasking Distributed PV Inverter

The ELF (Extended Lyapunov Function) based control approach for single phase two-stage multitasking PV (photovoltaic) inverter is demonstrated in this paper. The introduced PV inverter can operate under the uncertainty of non-linear loads and PV characteristic beyond giving the closed feedback system stability analysis. The two stage PV inverter is introduced, in initial step a boost circuit is used to harnessing of crest power at PV panel side. Here time ratio of the boost circuit is planed with adaptive INC (Incremental Conductance Method). In the secondary step, the control of grid connected VSI (Voltage Source Inverter) is managed by ELF (Extended Lyapunov Function) strategy. To get best dynamic response, difference of PV feed forward to the loss component of DC bus capacitor. Other side, control of VSI and maintain stability of PV inverter, the ELF technique is reduces error in-between input and output, with there is no disturbance of equilibrium point in PV inverter. The PV inverter performance observed at changing of insolation and load variations using MATLAB /simulation. Various simulation results are presented to validate the harmonics compensation strategy.

Second Harmonic Current Reduction for Flying Capacitor Clamped Boost Three-Level Converter in Photovoltaic Grid-Connected Inverter

In a two-stage single-phase photovoltaic (PV) grid-connected inverter, the second harmonic current (SHC) in the PV panel will affect the maximum power point tracking operation and degrade the energy harvesting efficiency. In order to solve this problem, a flying capacitor clamped (FCC) boost three-level (TL) converter is adopted in this article, and the flying capacitor is employed to compensate the SHC for removing a large electrolytic capacitor. The propagation mechanism of the SHC is analyzed from the perspective of impedance, and a virtual impedance in parallel with the intermediate dc bus is introduced, achieving a perfect SHC compensation performance. The implementation of the virtual parallel impedance is illustrated, and the key parameters design of the FCC boost TL converter is given. A 3-kW two-stage single-phase PV grid-connected inverter is fabricated and tested, and the experimental results verify the validity of the theoretical analysis and the proposed control scheme.

Highly Efficient and Robust Grid Connected Photovoltaic System Based Model Predictive Control with Kalman Filtering Capability

Renewable energy sources, especially photovoltaic (PV) ones, are gaining more and more interest due to the predicted lack of conventional sources over the coming years. That shortage is not the only concern, as environmental issues add to this concern also. Thus, this study proposes two-stage PV grid connected system, which is supported with extended Kalman filter (EKF) for parameter estimation. In the first stage, maximum power point tracking (MPPT) for the boost converter is accomplished using new MPPT method in which the switching state of the converter is directly generated after the measurement stage, so it is called direct switching MPPT technique. This technique is compared with the conventional finite control set model predictive control (FCS-MPC) method, where the design of the cost function is based on minimizing the error between the reference and the actual current. The reference current is obtained by employing perturb and observe (P&amp;O) method. In the second stage, the two-level inverter is controlled by means of model predictive control (MPC) with reduced computation burden. Further, to overcome the parameter variations, which is a very common problem in MPC applications, an extended Kalman filter is utilized to eliminate the control algorithm’s dependency on the parameters by providing an efficient estimation. After the inverter, an RL filter is inserted to guarantee the quality of the currents injected into the grid. Finally, the system is validated using Matlab under different operating conditions of atmospheric variation and parameter changes.

Small-Signal Modeling and Analysis of Virtual Inertia-Based PV Systems

Renewable energy sources normally provide low inertia, which allows a little time for controlling the system. The inertia emulation has recently been introduced for photovoltaic (PV) generation to fight against the small inertia and improve the energy efficiency. The main purpose of this article is to analyze the stability of a virtual inertia-based three-phase two-stage PV system for smart grid applications. Dynamics of DC/DC converter, DC/AC converter, AC side filter, virtual inertia controller, and controllers of the DC/DC and DC/AC converters are considered in the modeling procedure. Eigenvalue analysis is performed to evaluate the sensitivity of the design parameters on stability of a grid-connected virtual inertia PV system. Time-domain simulations using MATLAB/Simscape Power System toolbox are used to validate the eigenvalue analysis results.

Simulation Models For Three-Phase Grid Connected PV Inverters Enabling Current Limitation Under Unbalanced Faults

Normally unbalanced grid voltage dips may lead to unbalanced non-sinusoidal current injections, DC-link voltage oscillations, and active and/or reactive power oscillations with twice the grid fundamental frequency in three-phase grid-connected Photovoltaic (PV) systems. Double grid frequency oscillations at the DC-link of the conventional two-stage PV inverters can further deteriorate the DC-link capacitor, which is one of the most important limiting components in the system. Proper control of these converters may efficiently address this problem. In such solutions, Current Reference Calculation (CRC) is one of the most important issues that should be coped with the reliable operation of grid-connected converters under unbalanced grid faults. Therefore, this paper proposes and simulates CRC methods and presents the results in order to improve the quality of the grid-connected PV system under unbalanced grid voltage fault.

Structure-Preservation Model Aggregation for Two-Stage Inverters Based Large-Scale Photovoltaic System

With the increasing penetration level of large-scale photovoltaic (PV) generator connected to the grid, an accurate simulation model is required for the dynamic analysis of the PV system. However, the detailed electromagnetic simulation of the large-scale system is complex and the dynamic response capability is estimated with obstacle caused by large computational burdens. Therefore, a precise dynamic aggregated model is indispensable for the displacement of the large-scale PV system. The structure-preservation based aggregated model with comprehensive equivalent parameters for large-scale PV system is proposed in this paper. A complete two-stage PV system model is established to analyze the dynamics of the system. Then, the aggregation method is obtained by comparing the dynamic equations of the detailed model with the aggregated model, which is based on the energy relationship in the PV system. Furthermore, four different case studies are considered including the aggregation of identical and different ten parallel-connected PV units both under the same irradiance condition, and the aggregation of different ten parallel-connected PV units under different irradiance and weak grid scenarios, where the aggregation models are obtained through the proposed equivalent modeling method. Finally, the effectiveness of the proposed aggregation method is verified by the simulation results from PSCAD/EMTDC platform, and the consistency between the aggregated model and the detailed model is confirmed under different disturbances of irradiance variation, and continuous symmetric and asymmetric grid faults.

Harmonic Analysis and Control of Grid-Connected Solar PV Inverter under Normal and LVRT Operating Modes

Environmental factors and active involvement in grid-connected solar PV inverter ancillary operations may impact the quality of the current injected into the grid. The future grid-connected solar PV system with ancillary facilities (e.g., low voltage ride-through (LVRT)) will be more active and intelligent, which will degrade grid current reliability. The grid current distortions are specifically caused by the dc-link voltage variations and the modulation of pulse width (PWM) control applied to the PV inverter. This article analyzes the current harmonic distortion under the two-stage grid-connected PV system's regular (MPPT) and fault (LVRT) condition. Furthermore, a dc-link voltage variation control system for the two-stage photovoltaic (PV) inverter is presented during low voltage ride-through (LVRT) operation mode..The dc-link voltage differences are regulated under the fault condition to preserve the high modulation ratio in order to considerably mitigate the distortion rate of the grid current. Besides, the proposed system of control is designed to protect the PV inverter from the overcurrent failure under the faults to meet the modern LVRT grid codes. The conducted simulation tests have confirmed that the proposed control scheme leads to reduce a grid currents harmonics level by controlling the dc-link voltage variations.

Second Harmonic Current Reduction in Front-End DC−DC Converter for Two-Stage Single-Phase Photovoltaic Grid-Connected Inverter

The instantaneous output power of the two-stage single-phase grid-connected photovoltaic (PV) inverter pulsates at twice the line frequency ( $2f_{{\text{o}}}$ ), generating second harmonic current (SHC) in the front-end dc–dc converter and PV panel, which will affect the maximum power point tracking operation and deteriorate the overall conversion efficiency. The generating mechanism of the SHC is analyzed in this paper and it is pointed that in order to eliminate the SHC in the front-end dc–dc converter and PV panel, the voltage loop gain of the front-end dc–dc converter should be high enough at $2f_{{\text{o}}}$ . Since there is a −180° phase abrupt at the resonant frequency of the input side filter capacitor and the inductor, the system may be unstable. To cope with this problem, the inductor current feedback active-damping scheme (ADS) is adopted. For further improving the loop gain at $2f_{{\text{o}}}$ , proportional–integral–resonant regulator with ADS (PIR + ADS) is proposed in this paper. Besides, a step-by-step closed-loop parameters design method is presented. Finally, a 3-kW two-stage single-phase grid-connected PV inverter has been fabricated and tested, and the experimental results verify the feasibility of the proposed control schemes.

A State-Space Dynamic Model for Photovoltaic Systems With Full Ancillary Services Support

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.

Study on a Simplified Structure of a Two-Stage Grid-Connected Photovoltaic System for Parameter Design Optimization

Conventional parameter designs of two-stage grid-connected photovoltaic (PV) system relied on its mathematical model of the cascade structure (CS), but the procedure is excessively cumbersome to implement. Besides, for a two-stage converter system, the coupling interaction between the power converters can directly lead to a poor parameter design. To overcome this drawback, this paper uses a simplified structure (SS) of single-phase two-stage grid-connected PV system to better design the parameters of the front-stage dc-dc converter. After establishing the small-signal model for SS and CS in the PV system, the relative eigenvalue sensitivity is used as the criterion for judging the influence of some parameters on the stability of the two structures. The stable boundary of MPPT control parameters is compared and discussed in SS and CS, respectively. In addition, the relationship between the front-stage dc-dc converter and the rear-stage dc-ac inverter is analyzed by the modal participation factor calculated in CS. An experiment is also performed at the end of this paper to further verify the feasibility of using SS to design the parameters of the dc-dc converter in the PV system.

A Single-Stage Buck-Boost Three-Level Neutral-Point-Clamped Inverter with Two Input Sources for the Grid-Tied Photovoltaic Power Generation

This paper proposes a novel single-stage buck-boost three-Level neutral-point-clamped (NPC) inverter with two independent dc sources coupled for the grid-tied photovoltaic (PV) application, which can effectively solve the unbalanced operational conditions generally appearing between two independent PV sources. The proposed control scheme can simultaneously guarantee the maximum power point (MPP) operation of both PV sources and maintain the output waveform quality. Compared to the traditional two-stage PV inverter, the proposed NPC inverter could reduce the PV array voltage requirement and the voltage rating of dc-link capacitors; also it shows advantages in operational efficiency. MATLAB simulations and experimental results are presented to examine the performance of the proposed three-level NPC inverter.

A comprehensive state-space model of two-stage grid-connected PV systems in transient network analysis

The large-scale deployment of distributed photovoltaic (PV) systems on the low voltage distribution network (LVDN) poses significant challenges to network operators related to the dynamic interactions of the PV systems and the network. To address these challenges and implement grid support services (GSS’s), accurate and efficient PV system models are required that allow large networks with integrated PV to be simulated in an economical time frame. For this purpose, an accurate, computationally inexpensive, linearised, state-space model of a two-stage PV system in the dq0 rotating reference frame is presented in this paper. The model includes both the DC/DC and DC/AC converters, all system controllers, parasitic components, system losses, and an efficient PV array model. The performance of the model is validated with measured experimental data from a power hardware in the loop (PHIL) testbed at various simulated step changes in irradiance. The developed model is applied to a modified radial European LVDN benchmark with multiple distributed PV systems to demonstrate some of the functions, applications and advantages. A combined active and reactive power voltage-dependent droop control characteristic for GSS’s is demonstrated with solar irradiation step changes and measured solar irradiation data applied to each PV system. A full electromagnetic transient (EMT) PV system model developed in MATLAB is presented and compared to the linear state-space (LSS) model. The results demonstrate a computational burden reduction of 116:1 without loss of accuracy providing significant benefits and functionality for the study of large-scale grid integration of PV systems.