Transformerless Grid-connected Photovoltaic Inverter Research Articles

Transformer-Less Grid-Connected Photovoltaic Inverter

Transformer-Less Grid-Connected Photovoltaic Inverter

A New Transformer-Less Single-Phase Photovoltaic Inverter to Improve the Performance of Grid-Connected Solar Photovoltaic Systems

Photovoltaic (PV) energy systems have found diverse applications in fulfilling the increasing energy demand worldwide. Transformer-less PV inverters convert the DC energy from PV systems to AC energy and deliver it to the grid through a non-isolated connection. This paper proposes a new transformer-less grid-connected PV inverter. A closed-loop control scheme is presented for the proposed transformer-less inverter to connect it with the power grid. The proposed transformer-less inverter reduces extra leakage current and holds the common-mode voltage at a constant point. To eliminate extra leakage current, as well as achieve constant common-mode voltage, a midpoint clamping method is utilized to operate the inverter. The proposed transformer-less inverter is formed of seven insulated gate bipolar transistors (IGBTs) employing a unipolar sinusoidal pulse width modulation (SPWM) technique for switching purposes. An LCL filter is employed to reshape the two-level inverter output voltage and current to obtain closer sinusoidal waveforms. The output voltage and current total harmonic distortion (THD) of the proposed transformer-less inverter were found to be 1.25% and 0.94%, respectively, in the grid-connected mode. The leakage current elimination mechanism with the proposed transformer-less inverter is deeply analyzed in this paper. The performances of the proposed transformer-less inverter were evaluated with MATLAB/Simulink simulation and validated in a laboratory scale experiment.

A Novel Structure for Transformerless Grid-Connected PV Inverter to Eliminate Common-Mode Current under Mismatch Condition

Common-mode current is one of the major challenges in transformerless grid-connected photovoltaic (PV) inverters. This current is affected when the PV arrays are exposed to different environmental conditions and its value increases. This paper attempts to investigate the effect of mismatched condition on voltage balance of PV arrays, which leads to the increment of common-mode current. A new circuit structure is presented for compensating for voltage drop caused by partial shading and ambient temperature and removing the effect on the common-mode current. To validate the proposed structure, a laboratory prototype of this circuit is implemented.

Transformerless Six-Switch (H6)-Based Single-Phase Inverter for Grid-Connected Photovoltaic System With Reduced Leakage Current

Due to the advancements of power electronic devices and photovoltaic (PV) power systems, there has been an increasing interest in transformerless grid-connected PV inverters, which offer lower cost, smaller volume, and higher efficiency than the one with the transformer. However, transformerless inverters with conventional topologies suffer from leakage current problem, which needs to be handled carefully. To overcome the conduction loss and leakage current problem associated with conventional transformerless inverters, a new transformerless H6 (six-switch-based) inverter topology is proposed in this article that can maintain constant common-mode (CM) voltage. At the same time, reduce or eliminate the leakage current problem. The proposed inverter topology is simulated using MATLAB/Simulink software to validate the proposed system's accuracy. The proposed topology's effectiveness is verified through the comparative results of CM voltage, CM leakage current, and conduction loss of the proposed topology with the conventional H6-I, H6-II, and H5 topologies. Finally, to validate the proposed topology, a laboratory prototype is also built and tested. The experimental results validate the theoretical analysis and simulation results.

Electromagnetic interference‐based comparative study between transformerless H5 and optimised H5 grid‐connected photovoltaic inverters

In this study, a comparative study between two single-phase transformerless grid-connected photovoltaic (PV) inverters, namely H5 and optimised H5 (oH5), is carried out based on conducted electromagnetic interference (EMI) performance. For this purpose, a high-frequency (HF) model of the grid-connected inverters is proposed and simulated using Simulation Program with Integrated Circuit Emphasis (SPICE)-based circuit simulation software. Moreover, an experimental setup is developed to measure the conducted emissions. The obtained results show an EMI advantage of the oH5 inverter at low frequencies but not at HF frequencies. Therefore, a mitigation method is proposed to optimise the oH5 topology in the HF range. It consists in using only one Resistor–Capacitor (RC) snubber circuit connected in parallel with the inverter's extra switch. The obtained results show that the oH5 inverter with only one RC snubber circuit provides lower conducted EMI than the H5 in a wider frequency range without worsening the efficiency. The proposed EMI mitigation method can therefore be considered as an adequate and preferred solution for the transformerless grid-connected oH5 PV inverter.

Filter-Clamped Two-Level Three-Phase Transformerless Grid-Connected Photovoltaic Inverter for Leakage Current Reduction

Transformerless Photovoltaic (PV) grid-connected systems benefit from improved cost, size, weight, and efficiency compared to the isolated alternatives. A drawback of the trasnsformerless PV inverters is the leakage current that flows through the parasitic capacitance formed between the PV cells and panel metal frame connected to the earth. The leakage current increases the current harmonics injected into the utility grid, the radiated and conducted electromagnetic interference (EMI), and losses. In this paper, a two-stage three-phase Filter-Clamped (FC) transformerless PV system is employed that reduces the leakage current and output current THD compared to the conventional three-phase inverters. Unlike other transformerless PV inverters, the FC inverter does not require any additional component. The FC transformerless PV inverter is analyzed mathematically and its excellence is compared to the conventional three-phase inverter through simulation results.

An intelligent dc current minimization method for transformerless grid-connected photovoltaic inverters

Due to the scaling and zero-drift of current sensor errors, unbalanced grid voltages, tolerance of power switching devices, and asymmetry of PWM gate driving pulses, transformerless grid-connected inverters usually have certain amount of dc components injected to the ac grid. Therefore, power quality of the grid is degraded. Many efforts, such as using blocking capacitors, a dc current feedback control method, and a voltage dc component feedback control method, etc., have been introduced to minimize the dc injection. This paper proposes an intelligent control strategy of dc current injection suppression to the grid by utilizing adaptive-back-propagation (ABP) neural network PID controller. The performance of the proposed scheme is evaluated and compared with the traditional and existing method. Finally, the control scheme is verified on a 2-kW three-phase grid-connected inverter in the laboratory.

Comparison and Analysis of Single-Phase Transformerless Grid-Connected PV Inverters

Leakage current minimization is one of the most important considerations in transformerless photovoltaic (PV) inverters. In the past, various transformerless PV inverter topologies have been introduced, with leakage current minimized by the means of galvanic isolation and common-mode voltage (CMV) clamping. The galvanic isolation can be achieved via dc-decoupling or ac-decoupling, for isolation on the dc- or ac-side of the inverter, respectively. It has been shown that the latter provides lower losses due to the reduced switch count in conduction path. Nevertheless, leakage current cannot be simply eliminated by galvanic isolation and modulation techniques, due to the presence of switches' junction capacitances and resonant circuit effects. Hence, CMV clamping is used in some topologies to completely eliminate the leakage current. In this paper, several recently proposed transformerless PV inverters with different galvanic isolation methods and CMV clamping technique are analyzed and compared. A simple modified H-bridge zero-voltage state rectifier is also proposed, to combine the benefits of the low-loss ac-decoupling method and the complete leakage current elimination of the CMV clamping method. The performances of different topologies, in terms of CMV, leakage current, total harmonic distortion, losses and efficiencies are compared. The analyses are done theoretically and via simulation studies, and further validated with experimental results. This paper is helpful for the researchers to choose the appropriate topology for transformerless PV applications and to provide the design principles in terms of common-mode behavior and efficiency.

Design Optimization of Transformerless Grid-Connected PV Inverters Including Reliability

This paper presents a new methodology for optimal design of transformerless photovoltaic (PV) inverters targeting a cost-effective deployment of grid-connected PV systems. The optimal switching frequency as well as the optimal values and types of the PV inverter components is calculated such that the PV inverter LCOE generated during the PV system lifetime period is minimized. The LCOE is also calculated considering the failure rates of the components, which affect the reliability performance and lifetime maintenance cost of the PV inverter. A design example is presented, demonstrating that compared to the nonoptimized PV inverter structures, the PV inverters designed using the proposed optimization methodology exhibit lower total manufacturing and lifetime maintenance cost and inject more energy into the electric-grid and by that minimizing LCOE.

Methodology for the optimal design of transformerless grid-connected PV inverters

The transformerless photovoltaic (PV) inverters are the major functional units of modern grid-connected PV energy production systems. In this study, a new optimisation technique is presented for the calculation of the optimal types and values of the components comprising a transformerless PV inverter, such that the PV inverter levelised cost of the generated electricity is minimised. The proposed method constitutes a systematic design process, which is capable to explore the impact of the PV inverter configuration on the trade-off between the PV inverter manufacturing cost and the power losses affecting the corresponding energy production. The design optimisation results demonstrate that the optimal values of the PV inverter design variables depend on the inverter specifications, the technical and economical characteristics of the components used to build the PV inverter and the meteorological conditions prevailing at the installation area. Compared with non-optimised transformerless inverters, the PV inverter structures derived using the proposed design optimisation methodology exhibit lower manufacturing cost and simultaneously are capable of producing more energy into the electric grid system.