Single-phase Grid-tied Inverter Research Articles

A single-phase grid-tied extendable-level inverter for renewable energy applications

The extraction of power from green energy sources (GES) has rapidly popularized owing to increasing energy demands, depletion of fossil fuels and growing concerns about climatic changes. Grid-tied multilevel inverters are critical components in the integration of green energy sources into the power grid and offer numerous benefits over standard two-level inverters. These benefits include superior power quality, diminished harmonic distortion, and the potential to utilize high power levels efficiently. A single-phase Grid-Tied Extendable-Level Inverter (GTELI) for green energy applications is demonstrated in this paper. The GTELI offers superior power quality and distortion-free output relative to existing grid-tied multilevel inverters and hence is an excellent choice for extracting power from sustainable energy sources. To ensure efficient utilization, the extracted power from GTELI is to be integrated into the supply grid. A new current control algorithm for the GTELI is also proposed in this paper. Using this current control technique a grid control technique is established to modulate the power flow to the grid. The proposed GTELI achieved sinusoidal grid voltage and injected current with minimised harmonic distortions of 1.6% and 2.7% respectively, which comply well with the tolerable range of the IEEE 519 standard.

A Novel Approach for Implementing and Optimizing Proportional-Resonant Controller and L-C-L Filter for Single-Phase Grid-tie Inverter

A Novel Approach for Implementing and Optimizing Proportional-Resonant Controller and L-C-L Filter for Single-Phase Grid-tie Inverter

Multifrequency Small-Signal Model for Single-Phase Grid-Tied Inverters Considering the Effect of PWM

Multifrequency Small-Signal Model for Single-Phase Grid-Tied Inverters Considering the Effect of PWM

Decoupled Unipolar Hysteresis Current Control for Single-Phase Grid-Tied Inverter Without Current Zero-Crossing Distortion

Decoupled Unipolar Hysteresis Current Control for Single-Phase Grid-Tied Inverter Without Current Zero-Crossing Distortion

Improvement of grid injected currents in single-phase inverters rejecting DC link voltage oscillations based on multi-resonant filtering

The operation of grid-tied single-phase inverters generates oscillations in its DC link voltage. If only active/reactive power is controlled by the inverter, this oscillation is at twice the grid frequency. In situations where the inverter output current and/or the grid voltage also has harmonic components, the DC link voltage becomes also polluted by oscillating components at even multiple frequencies of the grid voltage. Such oscillations are propagated through the DC link voltage control loop, increasing the THD index of grid current reference signal. To solve this drawback, this article proposes the insertion of a multi-resonant filter into the DC link voltage control loop, ensuring a harmonic free grid current reference signal. With the proposed filter and without any increase in the power circuit, the use of smaller capacitances in the DC link is allowed, since the DC link voltage oscillation does not interfere in the power quality indexes of the controlled current. A filter tuning methodology is presented to ensure that the design of each filter and of the DC link voltage controller is independent, relating the filter settling time and the system stability. At the same time, the use of the proposed filter allows the voltage controller to be tuned at higher frequencies, improving the DC link voltage dynamic response. The proposal is experimentally validated through a single-phase grid-tied double-stage photovoltaic (PV) system, that can also operate only as a parallel active power filter (PAPF) or, simultaneously as a multifunctional PAPF + PV system.

H9 and H10 Transformer-Less Solar Photovoltaic Inverters for Leakage Current Suppression and Harmonic Current Reduction

Now-a-days, transformer-less inverters integrating renewable energy resources as solar photovoltaic systems are commonly employed in many grid-connected distributed energy systems in both industrial and residential settings. Transformer-less photovoltaic (TLPV) inverters have attracted increased attention due to their unique advantages, such as increased efficiency, low cost, and ease of installation. Converting fluctuating solar dc power to consistent ac power has been a difficulty in the recent decade to avoid security risks like ground fault currents and leakage currents. Leakage current minimization is one of the most crucial factors in the TLPV inverters. Different TLPV inverter topologies have been presented in the past, with leakage current being reduced by galvanic isolation on the dc or ac side of the inverter, common mode voltage (CMV) clamping, and modulation techniques. This paper proposed two novel single-phase grid-tied TLPV inverter topologies with control blocks, which can handle a wide variety of grid voltages. The proposed TLPV inverters offer very small leakage current, reduced total harmonic distortion (THD), and improved power quality. Based on the number of semiconductor devices, the proposed inverters are termed as H9 and H10 inverters. The switches are controlled using the unipolar sinusoidal pulse width modulation, which results in constant common mode voltage and reduction in leakage currents to 11.09 and 11.73 mA for the proposed H9 and H10 inverters, respectively. The proposed H9 and H10 inverters minimize the THDs to 1.62 and 2.09%, respectively. Besides, the proposed H9 and H10 inverters exhibit 98.19 and 98.12% European efficiency. The simulations are carried out on MATLAB/Simulink and the results are experimentally validated using downscale laboratory test platforms

Synchronization and Control of a Single-Phase Grid-Tied Inverter under Harmonic Distortion

Grid-connected inverters in renewable energy systems must provide high-quality power to the grid according to regulatory standards such as the IEEE 1547. To provide high-quality current control when the inverter is connected to a distorted grid, the frequency and phase information of the fundamental harmonic of the grid should be accurately obtained. This paper examines controller design for a single-phase inverter when there is distortion in the grid voltage. The control structure is designed to enhance the quality of the injected current into the grid. To this end, a frequency-locked loop (FLL) sinusoidal tracking controller which is able to reject the grid harmonics is proposed. Thus, the contribution of this paper is a new frequency-locked loop structure with adaptive notch filters that can provide accurate estimation of grid phase and frequency and improve the performance of single-phase inverters working under harmonic distortion. We also present an explanation of how the proposed adaptive nonlinear scheme can be discretized for digital implementation on a microcontroller. Experimental and simulation results are presented to demonstrate the performance of the proposed controller in eliminating distortion and enhancing the quality of the produced power.

Adaptive super-twisting sliding mode for DC-AC converters in very weak grids

ABSTRACT This paper presents the design of a direct-type adaptive control structure for current regulation of a single-phase grid-tied Voltage-Source Inverter (VSI) with an LCL filter under very weak grid condition. The initial content of this work was previously published, where the controller was presented and its discrete-time stability proof was provided. Power converters connected to weak grids bring challenges to the control system, since the grid can change abruptly from a strong to a very weak condition , degrading system power quality. This study evaluates the feasibility of a RMRAC-based robust adaptive Super-Twisting Sliding Mode controller for current control of VSI when the grid inductance changes substantially, from 0 mH (strong grid) to 10 mH (very weak grid). To show the benefits of the control strategy and its robustness, hardware-in-the-loop (HIL) simulation results are presented, comparing its performance with two other controllers..

Frequency Coupling Suppression Control Strategy for Single-Phase Grid-Tied Inverters in Weak Grid

In single-phase voltage source inverters (VSI) under a weak grid, the frequency coupling, caused by the asymmetrical system structure, poses a challenge to system modeling and controller design. In this article, a frequency coupling suppression control strategy is presented, where all the frequency coupling components in the current reference are eliminated. As a result, the single-phase grid-tied inverter is directly modeled as a simple single-input single-output (SISO) admittance rather than a complicated multiple-input multiple-output (MIMO) admittance matrix. The SISO admittance model is simple, compact, and accurate, which facilitates design-oriented analysis. The admittance characteristic analysis shows that the Sym-PLL has a critical impact on the system stability under weak grid conditions. To improve the system stability, a prefilter-based impedance-shaping control strategy is proposed by mitigating the negative resistor behavior of the Sym-PLL. Finally, simulations and experimental results validate the effectiveness of the frequency-coupling suppression control strategy.

Improved Model Predictive Control for Single-Phase Grid-Tied Inverter With Virtual Vectors in the Compacted Solution-Space

In this letter, an improved model predictive control (MPC) has been proposed for single-phase grid-tied converter with virtual vectors in a reshaped and compacted solution space, while improving ac power quality, without increase of the virtual vectors number. Combining linear solution space reconstruction method, the global optimization solution space can be shrunken and reshaped in real-time, only relying on grid angle information. Therefore, MPC can select the optimal virtual-vector in a finer and closer solution subspace, then, much finer control effect can be obtained. Implemented by finite-control set MPC with an integrated difference modulator, equivalent fixed double switching frequency at the output can be achieved. Experimental results validate the effectiveness of the proposed methods.

A Harmonic Mitigation Technique for Multi-Parallel Grid-Connected Inverters in Distribution Networks

Different harmonic mitigation techniques have been utilized in grid-connected inverters to suppress the effect of grid voltage distortion on the output current of these inverters. In practice, to scale up the injected current into the grid, a set of parallel grid-connected inverters are often utilized rather than a single high-power inverter in solar Photovoltaic or motor drive systems. Conventional Point of Common Coupling (PCC) feedforward approaches have been used for harmonic rejection of multi-parallel grid-connected inverters. However, these methods need to implement a separate feedforward scheme for each individual inverter. In this paper, a novel full PCC voltage feedforward strategy is proposed to reduce the cost investment, which is implemented only in one of the grid-connected inverters. The proposed strategy is based on cancelling the total parallel admittance of the system by adding a virtual negative admittance to the target inverter. It is shown that the proposed scheme causes no instability issue on the system since it does not affect the system phase margin. To verify the efficiency of the proposed strategy against emitted harmonics from the grid, simulations and experiments on a system with two parallel single-phase grid-tied inverters are performed.

Nonlinear Modeling and Global Stability Condition of Single-Phase Grid-Tied Inverter Considering SRF-PLL and Duty-Cycle Saturation

Considering that the traditional linearization method cannot accurately analyze the grid-tied inverter (GTI) system with nonlinear nature, in the single-phase case, a novel nonlinear model for the GTI system considering nonlinear factors, including synchronous reference frame phase-locked loop (PLL) and duty-cycle saturation in pulsewidth modulation, is proposed, and the corresponding global asymptotic stability condition is given. Specifically, some system nonlinear factors, e.g., duty-cycle saturation phenomenon and Park ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> ) transformation and cosine operation in second-order generalized integrator-based PLL (SOGI-PLL), are investigated, and their sector-bounded conditions are derived. In order to seek less conservative results, the system state of the SOGI-PLL is restructured such that the linear part of the PLL system can be stabilized. Then, the nonlinear state-space model with sector bounded for the whole GTI system is formulated. Furthermore, the deduced sector conditions are introduced into the derivation of the Lyapunov function by using the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S-procedure</i> lemma, and the global asymptotic stability condition of the inverter system is, thus, obtained. The system stability regions under different grid impedances, current references, and control parameters are further analyzed. Finally, the effectiveness of the proposed theories is fully supported by transient experimental results.

A novel sliding mode observer-based compound sliding mode current control with active damping for single phase grid-tied inverter system in weak grid

This paper proposes a novel sliding mode observer (SMO)-based compound sliding mode current control (CSMCC) strategy with active damping for the single-phase grid-tied inverter system in weak grid. To decrease the number of measurement sensors and thus reduce system cost, the SMO with rapid response and low chattering level is designed for estimating the inverter-side current and capacitor voltage of LCL-type filter, which is used for feedback control. To enhance robustness against the parameter variation of grid-tied inverter system, the CSMCC is put forward, which is composed of two parts. One part is the equivalent control for grid-tied inverter system with nominal parameters, and the other part is the reaching law-based second-order nonsingular terminal sliding mode control (NTSMC) for compensating the disturbance due to the parameter variations. Since an adaptive gain of switching function is employed and there is integral operation for this discontinuous switching function, the proposed NTSMC strategy not only speed up system response but also attenuate the system chattering level significantly. Moreover, for inhibition of inherent resonance resulting from LCL-type filter, the weighted average current-based active damping is used to provide feedback current for CSMCC. Comprehensive simulation and experimental results show that the proposed strategy can enable the grid-tied inverter system to not only have strong robustness against the filter parameters & grid impedance variations but also improve the quality of grid current, and have satisfactory suppression effect of background grid voltage harmonics on grid current.

A Generic Robust Model-Based Estimator for Active Damping of Single-Phase GridTied Inverters

This paper presents the design and analysis of a signal estimator with improved robustness applied in the active damping (AD) of a single-phase grid-tied inverter with LCL low-pass filter. The proposed estimator is based on the mathematical model of the LCL filter, with resistors in series with the converter-side inductor and capacitor for improving robustness. The implementation method is also capable of estimate multiple filter signals. The control strategy presented uses a virtual resistor based AD, that suppresses the resonance ensuring stability and robustness, helping comply with power quality standards, such as IEEE 1547 and IEC 61727. The signal necessary for achieving the AD is obtained through the estimator, dismissing the use of additional sensors. The current control structure uses grid-side current feedback with proportional-resonant controller and harmonic compensation. The system is designed by the root locus approach to achieve stability despite grid parameter variance. The estimator and overall system is analyzed through mathematical modeling, computational simulation and experimental setup. The results show that the estimator presented is capable of providing the necessary AD feedback signal, even with grid inductance variation.

State-and-Disturbance-Observer-Based Current Control Scheme for LCL-Filtered Single-Phase Grid-Tied Inverters Under Nonideal Conditions

This paper presents a current control methodology equipped with a state and disturbance observer (SDO) to address the uncertain disturbance problem existing in an LCL-type single-phase grid-tied inverter system. In this methodology, disturbance feedforward compensation and full-state feedback of the combined proportional-integral (PI) regulator are utilized to provide more degrees of freedom for a robust design of the controller. In order to realize this technology without additional sensors, a reduced-order SDO is designed by decoupling the real inverter plant into a nominal model with lumped uncertainty and disturbance (LUD), making the estimation results more robust adaption for system uncertainty. Furthermore, the upper bound of the LUD is concerned and innovatively used to evaluate the size of the uncertainty region of the LUD estimated by the observer, that may be important to anti-disturbance control. Other system performances, such as grid current distortion suppression and robust stability, are also analyzed. Finally, the effectiveness of the proposed control strategy is fully supported by simulation and experimental results.

Damping techniques for grid-connected converters with LCL filter: an overview

Distributed generation systems, based on renewable energy sources, are typically connected to the main grid by a voltage-source inverter with a low-pass filter. The need for improved efficiency led to the use of third order low-pass filters, such as the LCL configuration, which has resonant behavior. In order to meet energy quality requirements and ensure the systems stability it is necessary to suppress the LCL filters resonance through damping techniques. Therefore, this paper presents an overview of some damping strategies found in literature and its design procedure, applied to a simulated single-phase grid-tied inverter. The comparison of each presented damping methodology characteristics is described, with analysis of advantages and drawbacks for each case.

A Novel Design of PR Controller With Antiwindup Scheme for Single-Phase Interconnected PV Systems

This article presents the design methodology of an alternative approach for implementing proportional-resonant (PR) controller for single-phase grid-tied inverter. The developed PR controller is designed to regulate the grid current with the aim of controlling either the dc-link voltage or the active/reactive power injected into the grid. The control design is based on combining a state feedback controller with a disturbance observer. It turns out that the composite controller can be expressed as a PR controller plus an antiwindup scheme. The latter is required by the resonant portion of the PR controller to attenuate the effect of the control input saturation during transients. On the other hand, when the voltage harmonics are taken into account in the observer design, the composite controller reduces to multiple resonant controllers with an antiwindup scheme. The performance of the existing PR controller was investigated under control saturation to highlight the poor transient performances associated with modulating signal limitation. Experimental tests have been conducted to verify the the performance of the proposed controller. The obtained results demonstrated good transient performances compared with the existing PR controller, particularly during control saturation. Furthermore, performance results under voltage harmonics showed that the proposed controller can achieve accurate control of the current with low total harmonic distortion values.

A novel control technique for a single-phase grid-tied inverter to extract peak power from PV-Based home energy systems

In this paper, a single-phase full-bridge grid-tied inverter is considered for home-based photovoltaic applications. The dc-dc converter is inevitable in boosting the voltage and tracking the maximum power from the photovoltaic source. As a result, the size and cost of the home-based photovoltaic grid-tied systems increases. A dc-dc converter is eliminated in this work, and the PV voltage is considered as the input voltage to a single-phase full-bridge inverter system. Also, to overcome the demerits of traditional peak power techniques, a fuzzy logic-based peak power controller is proposed. A fuzzy logic-based dc-link voltage controller is also proposed to overcome the conventional PI-based dc-link voltage controller's demerits. Primarily the theoretical concept is validated by using the MATLAB/SIMULINK tool for simulation analysis. The Spartan-6 FPGA control board is used to implement the controller program. A laboratory prototype is fabricated in the experimental laboratory to verify the theoretical and simulation analysis. Different case studies are comprehended in this work to present the robustness of the recommended control scheme.

An enhanced full‐feedforward strategy to mitigate output current harmonics in grid‐tied inverters

The grid-tied inverters are the most vital components in renewable energy-based power systems. Hence, maintaining the power quality of a grid-tied inverter output within the standard range is an ongoing challenge in the power system. The appeared individual harmonics at inverter output current caused by grid voltage harmonics depend on the inverter output equivalent admittance. This consists of a combination of admittance seen from the point of common coupling and the phase-locked loop path. Therefore, the precise calculation of the aforementioned admittance is an inevitable requirement to design a harmonic mitigation strategy. By adding a virtual admittance to the system through modifications in the inverter control loop, the output equivalent admittance can be removed. For this purpose, in this paper, a novel full-feedforward harmonic suppression scheme is proposed which can effectively eliminate individual harmonics injected from the grid. Consequently, the individual output current harmonics of the inverter will be effectively suppressed. Simulation and experimental results have been carried out for a typical single-phase grid-tied inverter to verify the efficiency of the proposed scheme against emitted harmonics from the grid.

A Novel Trapezoidal Wave Control Method for a Single-Phase Grid-Tied T-Type Inverter

Triangular conduction mode (TCM) is used in grid-tied inverters to achieve high efficiency by its soft switching feature. However, it suffers from a wide switching frequency variation over the whole power range, which impacts the weighted efficiency. To address this issue, this article proposes a novel trapezoidal wave control method for a single-phase grid-tied T-type inverter. By the proposed method, the inductor current is divided into three stages in a switching cycle, so it looks like trapezoidal wave rather than triangular wave. The proposed trapezoidal wave has two independent conduction times. In order to simplify the control, the proportion between the two conduction times is controlled as a variable. As a result, only one conduction time needs to be calculated, and the derived equation is easy to be calculated in digital controllers. Furthermore, this article proposes a design consideration for the value selection of the proportion between the two conduction times, which ensures proper operation of the circuit. The proposed scheme has three advantages. First, it realizes soft switching in every switching cycle. Second, it reduces the switching frequency and related losses compared to the full-bridge inverter in TCM operation and thus improves efficiency. Third, it can be applied over the whole line cycle as a uniform operation mode. Therefore, mode transitions are avoided, which improves the quality of the grid-injected current. A prototype has been built and the experimental results verify the feasibility of the proposed scheme.