Three-phase Grid-connected Photovoltaic Inverter Research Articles

Performance enhancement of a three-phase grid-connected PV inverter system using fractional-order integral sliding mode controls

Grid-integrated photovoltaic (PV) systems hold significant promise for sustainable energy production. However, these systems often struggle with maintaining energy quality and stability in the face of fluctuating conditions. To address these challenges, this study proposes the use of fractional-order integral sliding mode control (FO-ISMC) for grid-connected PV systems. The system comprises solar panel arrays, a DC/DC boost converter with its controller, and a three-phase inverter integrated into the utility grid. The primary goals of this study are to maximize power extraction from the PV system and to regulate system currents, thereby improving effectiveness and power quality. The proposed method is further refined using the particle swarm optimization (PSO) algorithm to fine-tune parameter settings. By optimizing the command gains of the FOI-SMC, the PSO algorithm enhances system performance and stability. Leveraging fractional-order and integral terms provides the control signal with additional degrees of freedom, enhancing system performance in the presence of internal and external disturbances. Simulation results demonstrate the effectiveness of the proposed controller compared to the classical controller, where in the case of Irradiation taking the value of 500 W/m2, the proposed strategy reduced the values of constant voltage, active power, and reactive power by percentages estimated at 88.18%, 90%, and 70%, respectively, compared to the traditional strategy. Also, the overshoot value of active power was reduced in all completed cases compared to the traditional strategy by percentages estimated at 53.57%, 66.66%, and 65%, respectively. The proposed strategy reduced the response time of reactive power in all cases compared to the traditional strategy by percentages estimated at 58.59%, 28.80%, and 75%, respectively. These ratios show the high performance of the proposed control in improving the characteristics of the studied system compared to traditional control based on PI control.

Aliasing Suppression Method for a Three-Phase Grid-Connected Photovoltaic Inverter Based on Multi-Sampling and Mean Filtering

In order to reduce the sampling delay and improve bandwidth, sability margin, and the robustness of the active damping in LCL-filtered grid-connected inverters, real-time sampling provides a convenient method. However, aliasing is easily introduced in the control loop because of high-frequency switching harmonics, resulting in a rise in low-order harmonics. To address the challenge of aliasing under real-time sampling, a new method based on multisampling and mean filtering is proposed by combining the proposed harmonic detection and control methods in this paper. This method works by sampling and controlling the fundamental current and selective subharmonic currents separately. The fundamental current control loop without any additional filters maintains real-time sampling, while multisampling and mean filters are applied to the harmonic current control loop. The proposed control method can not only improve the dynamic response performance and control stability of the system but also effectively suppress the selective low-order current harmonics and the aliasing of sampling. Finally, the correctness of the proposed sampling scheme and control strategy is verified by the simulation based on R2018b, MathWorks, Natick, MA, USA and an experimental prototype of a three-phase grid-connected photovoltaic inverter rated at 230 V/50 Hz/40 kW.

A Method Based on NGO-HKELM for the Autonomous Diagnosis of Semiconductor Power Switch Open-Circuit Faults in Three-Phase Grid-Connected Photovoltaic Inverters

With accelerating grid decarbonization and technological breakthroughs, grid-connected photovoltaic (PV) systems are continuously connected to distribution networks at all voltage levels. As the grid interaction interfaces between PV panels and the distribution network, PV inverters must operate flawlessly to avoid energy and financial losses. As the failure of semiconductor switches is the leading cause of abnormal operation of PV inverters and typically cannot be detected by internal protection circuits, this paper aims to develop a method for the autonomous diagnosis of semiconductor power switch open-circuit faults in three-phase grid-connected PV inverters. In this study, a ReliefF-mRMR-based multi-domain feature selection method is designed to ensure the completeness of the fault characteristics. An NGO-HKELM-based classification method is proposed to guarantee the desired balance between generalization and exploration capability. The proposed method overcomes the common problems of poor training efficiency and imbalances between generalization and exploration capabilities. The performance of the proposed method is verified with the detection of switch OC faults in a three-phase H-bridge inverter and neutral-point-clamped inverter, with diagnostic accuracy of 100% and 99.46% respectively.

Research on the control strategy of LCL-type PV grid-connected inverter

This paper examines a three-phase grid-connected photovoltaic inverter using LCL technology. Circuit for a full-bridge inverter with three phases and a filter of type LCL are used, and the control strategy consisting of two closed-loop loops is used to remove the effects caused by harmonics on the system. Finally, the simulation and analysis are carried out by MATLAB software to express the possibility of inverter system operation in a more intuitive way, such as a waveform diagram, to verify the rationality of the paper.

Research on three-phase photovoltaic grid-connected inverter based on fuzzy PI control

With the vigorous development of photovoltaic industry, the research on three-phase photovoltaic grid-connected inverter is deepening. For the problem, in this article, a combination of fuzzy control methods and traditional PI control is proposed to improve the adaptive capability of the system by constructing a composite fuzzy PI control pattern. At the same time, The LCL-type three-phase grid-connected inverter is simultaneously studied and the fuzzy PI control is validated through demonstration experiments. It is shown that the fuzzy PI control method is somewhat feasible in this system.

Optimization of Grid-Connected Photovoltaic Power Generation Technology Based on Nonlinear Back-Stepping Controller

To address the issue of energy scarcity and to use solar photovoltaic energy as a renewable source, a three-phase grid-connected photovoltaic inverter system with uncertain system model parameters is investigated, which converts DC power into AC power, feeds it into the grid, and maintains the grid-connected part’s quality. An enhanced back-stepping approach is proposed to explore the control strategy of three-phase solar grid-connected inverter, which includes adaptive control, dissipative theory, sliding mode control, and rapid terminal sliding mode control, respectively. It is verified that the system is subjected to external interference F = 5 during 0.5 s–0.8 s. As per the simulation results, the improved adaptive terminal sliding mode control method can suppress the interference to a certain extent and has strong robustness to the external interference. When compared with the adaptive sliding mode control approach, the anti-interference ability is better, and the system stability is improved. Then, to improve the above approach, a high speed terminal sliding mode control method is proposed, which can swiftly converge in finite time and the estimated value of uncertain parameters is closer to the actual value, thereby boosting the system’s robustness.

Research on LCL-type three-phase photovoltaic grid-connected inverter based on passive damping

The traditional LCL filter has resonance phenomenon in the working process of three-phase photovoltaic grid-connected inverter system. Based on the analysis of the frequency characteristics of LCL filter equivalent circuit before and after the introduction of passive damping resistor, it is concluded that the resonance of the system can be suppressed after the introduction of passive damping resistor. In the meantime, the current double closed-loop control strategy used in the system is introduced in detail. Finally, the simulation model is built by Matlab/Simulink simulation platform to verify the feasibility of the research method of LCL-type three-phase photovoltaic grid-connected inverter based on passive damping.

Enhanced Control Scheme for a Three-Phase Grid-Connected PV Inverter under Unbalanced Fault Conditions

This paper presents an improved control strategy to cancel the double grid frequency oscillations in the active power, reactive power, and DC-link voltage of a three-phase grid-connected photovoltaic (PV) system under unbalanced grid condition. To achieve these goals, an enhanced positive–negative-sequence control (PNSC) to remove oscillations of active power and an instantaneous active–reactive control (IARC) to mitigate the fluctuations of active and reactive power, simultaneously, are suggested. These methods are also effective to reduce the oscillations of the DC-link voltage. To track the desired unbalanced or harmonic reference currents, improved proportional resonant (PR) current controllers have been designed using the Bode frequency analysis. Simulation studies are carried out via Matlab/Simulink® software to verify the effectiveness of the suggested control strategies.

Three-Phase Filter-Clamped Transformerless Inverter for Grid-Connected Photovoltaic Systems with Low Leakage Current

In this paper, a Filter-Clamped (FC) inverter is employed as a three-phase grid-connected Transformerless Photovoltaic (TLPV) inverter. TLPV inverters are more efficient and more cost-effective compared to the alternatives based on high and low-frequency isolation transformers. On the other hand, TLPV inverters generate high leakage current due to the removal of the isolation transformer. Prior-art TLPV topologies have been developed to overcome this issue by either adding more semiconductor switches to block potential leakage current paths or modifying the control method. In contrast, the FC inverter addresses the leakage current issue without adding any extra component or improving the control technique. Thus, it is a promising solution to TLPV systems. Having decreased the leakage current, the FC inverter improves the THD of grid injected current and the efficiency compared to the conventional three-phase Full-Bridge (FB) inverter. Simulations and experiments are provided to validate the effectiveness of the FC inverter in terms of reduced leakage current, improved grid current THD, and improved overall efficiency.

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.

Modified Discontinuous PWM Strategy for Three-Phase Grid-Connected PV Inverters With Hybrid Active–Passive Damping Scheme

This article proposes a modified discontinuous pulsewidth modulation (MDPWM) strategy for transformerless three-phase grid-connected photovoltaic (PV) inverters. MDPWM reduces the leakage current from PV inverter by reducing the magnitude of low-frequency harmonics presented in the common-mode voltage in comparison to classical discontinuous PWM technique. To enable the application of the MDPWM, a new hybrid active–passive damping scheme is proposed where common-mode and differential-mode dynamics are damped by passive and active schemes, respectively. Theoretical and experimental results show that the proposed scheme results in higher system efficiency in comparison to classic continuous PWM scheme keeping the leakage current reduced.

Dual-mode operation based second-order sliding mode control for grid-connected solar photovoltaic energy system

This paper proposes a second-order sliding mode control technique for the grid-connected solar photovoltaic system. The proposed technique is tested under grid faults to meet the requirements of low-voltage ride through capability. A dual-mode control technique of the converter and inverter is designed to maintain the required dc-link voltage regulation and the active-reactive power during grid fault. The sliding mode control technique is developed from the model of the grid connected to the converter and the dc-link. The proposed dual-mode control technique is validated using real-time simulator OPAL-RT and experimental three-phase grid-connected photovoltaic inverter system. Simulation and experimental results show that the control technique is quite effective in maintaining the dc-link voltage regulation and the power control. As it shows smooth transitions between the two modes of operation (normal and grid fault). Moreover, the sliding mode control approach is robust against unknown disturbances and uncertainties in the system.

Active Damping Scheme for Leakage Current Reduction in Transformerless Three-Phase Grid-Connected PV Inverters

This paper analyzes the benefits of the active damping applied to transformerless three-phase grid-connected photovoltaic (PV) inverters using modified LCL (MLCL) filter for leakage current reduction. In comparison with passive damping techniques, the active damping can reduce the MLCL filter resonance peaks, improving the dynamic performance of the control system without addition of power losses. Besides these classical advantages, this paper demonstrates that the active damping also contributes for PV leakage current reduction, preserving the high-frequency attenuation of the MLCL filter. Additionally, an active damping for the inverter common-mode circuit is also proposed, reducing the low-frequency leakage current caused by the common-mode signal used to extend the inverter modulation index. Experimental results are given to demonstrate the leakage current reduction and the grid current control for a 10 kW PV inverter.

Control Strategy for Three-Phase Grid-Connected PV Inverters Enabling Current Limitation Under Unbalanced Faults

Power quality and voltage control are among the most important aspects of the grid-connected power converter operation under faults. Nonsinusoidal current may be injected during unbalanced voltage sag, and active or/and reactive power may include double frequency content. This paper introduces a novel control strategy to mitigate the double grid frequency oscillations in the active power and dc-link voltage of the two-stage three-phase grid-connected photovoltaic (PV) inverters during unbalanced faults. With the proposed control method, PV inverter injects sinusoidal currents under unbalanced grid faults. In addition, an efficient and easy-to-implement current limitation method is introduced, which can effectively limit the injected currents to the rated value during faults. In this case, the fault-ride-through operation is ensured, and it will not trigger the overcurrent protection. A non-maximum power point tracking (non-MPPT) operation mode is proposed for the dc–dc converter. The mode is enabled under severe faults when the converter cannot handle the maximum PV power. Finally, experimental validation is provided by implementing a method in an experimental setup, including a 2 kW PV inverter.

Robust Feedback Linearizing Control With Sliding Mode Compensation for a Grid-Connected Photovoltaic Inverter System Under Unbalanced Grid Voltages

A robust feedback linearizing control strategy, based on sliding mode compensation, is proposed for the operation of a grid-connected photovoltaic inverter system under grid faults, characterized by unbalanced voltages, to meet low-voltage ride through requirements. Under normal grid condition, the control system is developed for maximum power transfer from the photovoltaic source to the grid by maximum power point tracking operation of the dc–dc converter, and regulation of the dc-link voltage and the current at the inverter-grid side. Under grid fault, which is unbalanced grid voltage due to voltage dips, the active power is regulated to reduce the current excess and the reactive power is injected to avoid the inverter damage or disconnection, while the dc-link voltage is controlled via the dc–dc converter. A sliding mode compensator is injected into the control system to enhance its robustness to uncertainties. The feedback linearizing control schemes are developed from the grid model at the inverter side and the dc-link model at the dc–dc converter side. The proposed control strategies are experimentally validated on a three-phase grid-connected photovoltaic inverter system and experimental results show that the control system is effective in terms of voltage and power control with smooth transitions between the modes.

An improved synchronous reference frame current control strategy for a photovoltaic grid-connected inverter under unbalanced and nonlinear load conditions.

In recent years, renewable energy sources have been considered the most encouraging resources for grid and off-grid power generation. This paper presents an improved current control strategy for a three-phase photovoltaic grid-connected inverter (GCI) under unbalanced and nonlinear load conditions. It is challenging to suppress the harmonic content in the output current below a pre-set value in the GCI. It is also difficult to compensate for unbalanced loads even when the grid is under disruption due to total harmonic distortion (THD) and unbalanced loads. The primary advantage and objective of this method is to effectively compensate for the harmonic current content of the grid current and microgrid without the use of any compensation devices, such as active and passive filters. This method leads to a very low THD in both the GCI currents and the current exchanged with the grid. The control approach is designed to control the active and reactive power and harmonic current compensation, and it also corrects the system unbalance. The proposed control method features the synchronous reference frame (SRF) method. Simulation results are presented to demonstrate the effective performance of the proposed method.

An AC MPPT with Active/Reactive Power Control Feature for Single-Stage Three-Phase Grid-Connected Photovoltaic VSIs

This article introduces a new AC MPPT (maximum power point tracking) controller for single-stage three-phase grid-connected photovoltaic (PV) voltage source inverters (VSIs) with active/reactive power control capability. Novelty lies in employing the theory of power transfer in short AC transmission lines to achieve MPPT, where two independent control variables exist: load angle and inverter modulation index. Accordingly, the proposed controller maximizes the capabilities of the single-stage VSI and enables it to control either/both active power or/and reactive power, besides achieving MPPT function. Such contribution opens up avenues in MPPTs world in terms of flexibility, fast performance, and low hardware count/cost. AC refers to Alternate Current since the MPPT is managed in AC terms. Theoretical framework of the proposed controller has been addressed. Moreover, the overall system is simulated in MATLAB (The MathWorks, Natick, Massachusetts, USA) environment, and results have been introduced. Design and implementation of proposed algorithms have been carried out using a fixed-point digital signal processor (DSP)TMS320F2812. Furthermore, a breadboard circuit is built up for experimentally investigating the controller's performance using 3-phase VSI that connects PV with the grid. In addition to that, a comprehensive discussion is conducted for demonstrating results. Moreover, a detailed comparison of proposed work with a number of previously published papers in literature is carried out and addressed in this manuscript. Results reveal that introduced controller achieves satisfactory performance.

Direct power control of a grid-connected photovoltaic system using a fuzzy-logic based controller

In this paper a fuzzy logic-based controller is proposed for direct power control of three-phase grid-connected photovoltaic PV inverters. To demonstrate the effectiveness of the control method, a typical grid-connected PV inverter is implemented and the performance of the proposed technique is compared to a traditional proportional-integral PI controller. Numerical simulation results illustrate that the proposed fuzzy controller performs well in independently regulating the active and reactive output powers. Moreover, comparisons of simulation results between fuzzy and PI controllers show advantages of the fuzzy-based technique in terms of transient response characteristics, such as fast response, small overshoot, acceptable steady-state fluctuation, and high robustness.

Impact of Common Mode Signal Injection on the Leakage Current of a Grid-connected Transformerless Pv Inverter

Leakage current is one of the most relevant problems in grid-connected transformerless photovoltaic (PV) inverters due to parasitic capacitance of the PV modules. This problem is directly related to the common mode voltage generated by inverter modulation. Therefore, this paper presents an analysis of the impact of the common mode signal usually injected in carrier based modulation approaches, aiming the reduction of the leakage current in three-phase grid-connected transformerless PV inverter. The resulting leakage current is analyzed for an inverter topology with a modified LCL filter, which is utilized to attenuate the high frequency components of the voltage on the PV parasitic capacitance. Experimental results are given to evaluate the performance of a 10 kW transformerless PV inverter.

A ripple-based maximum power point tracking method for three-phase grid-connected photovoltaic inverter

As a prevailing solar energy utilization equipment, the three-phase grid-connected photovoltaic (PV) inverter is widely operated in partially shaded conditions and thus tends to generate multiple local maximum power points on its power-to-voltage and current-to-voltage characteristic curves. In order to identify the global maximum power point (GMPP) quickly and precisely, this paper proposes a ripple-based maximum power point tracking method. It aims to perform the optimization of tracking using the segmented scanning of DC-side voltage. An improved adaptive perturb and observe (AP&O) method is introduced to maximize the solar conversion and to increase working stability. This method applies a hybrid model of fixed and variable step-size perturbation to restrain the fluctuation of PV-side voltage. It belongs to a two-stage GMPP tracking method. That is, when environmental factors such as irradiance and temperature change quickly PV power fluctuates sharply. Correspondingly, the AP&O method tracks the latest maximum power point (MPP) with a large fixed-step voltage reference command. When the PV power fluctuates smoothly under a slow environmental change rate, the algorithm applies multiple small and variable step-size voltage perturbations to vibrate round the location of GMPP. Simulation and experimental results show that this method improves the efficiency of the PV inverter tracking performance. In addition, the stability of DC bus voltage is guaranteed, and the operational stability of the photovoltaic power generation system is improved.