Transformerless Photovoltaic Grid-connected Inverters Research Articles

Ground Current Suppression for Grid Connected Transformerless Photovoltaic Inverter with Filter Capacitor

The full-bridge inverter with unipolar PWM (UPWM) or hybrid PWM (HPWM) has higher efficiency and lower filtering requirement. However, when it is used for grid-connected transformerless photovoltaic system, the ground current may be high due to the parasitic capacitors and high frequency common-mode voltage. In this paper, the output filter capacitor is proposed to suppress the ground current. In this method, the output filter capacitor is parallelled with the parasitic capacitors. The analytical method is established to calculate the ground current. The effect of the novel method under different modulation technique is studied. Efficiency estimation is done for comparison of different topologies. Finally, the validity of the proposed method is verified by simulation and experimental results.

Overview of Transformerless Photovoltaic Grid-Connected Inverters

Transformerless grid-connected inverters (TLI) feature high efficiency, low cost, low volume, and weight due to using neither line-frequency transformers nor high-frequency transformers. Therefore, TLIs have been extensively investigated in the academic community and popularly installed in distributed photovoltaic grid-connected systems during the past decade. This article analyzes and summarizes the state of the art of TLI techniques, three rules of maintaining constant common-mode voltage (CMV) of TLIs at switching frequency have been concluded from a generic CMV analysis model at the beginning. Second, suppression methods of leakage current (LC) and dc current injection (DCCI), so-called two key challenges for first-generation TLIs, have been classified and discussed in detail, respectively. Finally, future trends and some challenges of TLIs based on wide bandgap devices named second-generation TLIs have been presented in this article.

Transformerless Inverter Topologies for Single-Phase Photovoltaic Systems: A Comparative Review

In photovoltaic (PV) applications, a transformer is often used to provide galvanic isolation and voltage ratio transformations between input and output. However, these conventional iron- and copper-based transformers increase the weight/size and cost of the inverter while reducing the efficiency and power density. It is therefore desirable to avoid using transformers in the inverter. However, additional care must be taken to avoid safety hazards such as ground fault currents and leakage currents, e.g., via the parasitic capacitor between the PV panel and ground. Consequently, the grid connected transformerless PV inverters must comply with strict safety standards such as IEEE 1547.1, VDE0126-1-1, EN 50106, IEC61727, and $\text{A}S/N$ ZS 5033. Various transformerless inverters have been proposed recently to eliminate the leakage current using different techniques such as decoupling the dc from the ac side and/or clamping the common mode (CM) voltage (CMV) during the freewheeling period, or using common ground configurations. The permutations and combinations of various decoupling techniques with integrated voltage buck–boost for maximum power point tracking (MPPT) allow numerous new topologies and configurations which are often confusing and difficult to follow when seeking to select the right topology. Therefore, to present a clear picture on the development of transformerless inverters for the next-generation grid-connected PV systems, this paper aims to comprehensively review and classify various transformerless inverters with detailed analytical comparisons. To reinforce the findings and comparisons as well as to give more insight on the CM characteristics and leakage current, computer simulations of major transformerless inverter topologies have been performed in PLECS software. Moreover, the cost and size are analyzed properly and summarized in a table. Finally, efficiency and thermal analysis are provided with a general summary as well as a technology roadmap.

Integrated Common and Differential Mode Filter With Capacitor-Voltage Feedforward Active Damping for Single-Phase Transformerless PV Inverters

Recently, several publications have discussed the design of LCL filters and current controllers for grid-connected converters. However, the focus was the LCL filter used as a differential mode (DM) filter. Besides its conventional application as DM filter, a modified LCL filter configuration can be used as an integrated common mode (CM) and DM filter, which is particularly interesting for transformerless photovoltaic (PV) inverters, whereby the CM leakage current must be limited to comply with standards. This article proposes the joint design procedure for an integrated CM and DM filter plus the current controller used in single-phase grid-connected transformerless PV inverters. The increased number of components and resonance frequencies, compared to the conventional LCL filter, requires a careful analysis of the location of resonance and anti-resonance frequencies, taking into account parameters variation. The design procedure considers the extended region of stability for the DM resonance frequency allocation provided by the filter capacitor-voltage feedforward active damping with converter-side current feedback. It also considers continuous and discontinuous unipolar pulsewidth modulation. Design examples and simulation results for 1.5 and 10 kVA are presented. The effectiveness of the proposed design procedure is verified by experimental results obtained from a 1.5 kVA inverter.

Design of Fuzzy-PI and Fuzzy-Sliding Mode Controllers for Single-Phase Two-Stages Grid-Connected Transformerless Photovoltaic Inverter

Conventional Energy Resources (CER) are being rapidly replaced by Renewable Energy Resources (RER) due to their abundant, environmentally friendly, clean, and inexhaustible nature. In recent years, Solar Photovoltaic (SPV) energy installation is booming at a rapid rate among various RER. Grid-Connected PVS required advance DC-link controllers to overcome second harmonic ripple and current controllers to feed-in high-quality current to the grid. This paper successfully presents the design of a Fuzzy-Logic Based PI (F-PI) and Fuzzy-Logic based Sliding Mode Controller (F-SMC) for the DC-link voltage controller and Proportional Resonant (PR) with Resonant Harmonic Compensator (RHC) as a current controller for a Single-Phase Two-Stages Grid-connected Transformerless (STGT) Photovoltaic (PV) Inverter. The current controller is designed with and without a feedforward PV power loop to improve dynamics and control. A Second Order General Integral (SOGI)-based Phase Lock Loop (PLL) is also designed that has a fast-dynamic response, fast-tracking accuracy, and harmonic immunity. A 3 kW STGT-PV system is used for simulation in Matlab/Simulink. A comparative assessment of designed controllers is carried out with a conventionally well-tuned PI controller. The designed controllers improve the steady-state and dynamic performance of the grid-connected PV system. In addition, the results, performance measure analysis, and harmonics contents authenticate the robustness, fastness, and effectiveness of the designed controllers, related to former works.

An Improved H5 Topology With Low Common-Mode Current for Transformerless PV Grid-Connected Inverter

Transformerless photovoltaic (PV) grid-connected inverters have become more and more popular in distributed PV power generation systems due to the advantages on high efficiency, low cost, and small size. However, common-mode (CM) current in the transformerless PV inverters can result in serious electromagnetic interference and insecurity, which will further reduce the reliability of the PV inverter systems. In this paper, an improved H5 topology, namely H5-D topology, is proposed, in which a clamping diode is added on the basis of H5 topology to eliminate the CM voltage fluctuation in H5 topology. Further, the PSIM simulation results of the H5-D topology and H5 topology are given and compared, especially, the performance of H5-D topology for CM current suppression is presented and analyzed concretely. Finally, the experimental prototypes of the H5-D topology and H5 topology are built and tested, and the experimental results validate the advantages of the H5-D topology. The proposed H5-D topology provides a new practical topology for distributed PV grid-connected power generation systems.

Notice of Violation of IEEE Publication Principles: Single-Phase Common-Ground-Type Transformerless PV Grid-Connected Inverters

Notice of Violation of IEEE Publication Principles “Single-Phase Common-Ground-Type Transformerless PV Grid-Connected Inverters” By Zhiling Liao, Chenchen Cao , Diancheng Qiu, and Changbo Xu In IEEE Access, Vol 7, 2019 After careful and considered review of the content and authorship of this paper by a duly constituted expert committee, this paper has been found to be in violation of IEEE’s Publication Principles. The content of this paper contains significant similarities with the paper cited below that were presented with insufficient attribution. “Common-Ground-Type Transformerless Inverters for Single-Phase Solar Photovoltaic Systems” by Yam P. Siwakoti and Frede Blaabjerg in IEEE Transactions on Industrial Electronics, Vol. 65, No. 3, March 2018 This paper presents a family of novel common-ground-type transformerless photovoltaic (PV) grid-connected inverters, which requires only five power switches, one capacitor, and one filter. A simple dual-closed led-loop control is used to improve control stabilization and accuracy. The main advantages of proposed inverters are: 1) the leakage current is completely eliminated (unlike traditional topologies, which can only suppress leakage current); 2) the devices used are a few and the cost is low; 3) low loss and high efficiency; 4) the ability of realizing reactive power; and 5) there is no need for high DC input voltage compared with half-bridge-type topologies. The operating principle, modulation mode, and control strategy are introduced in detail. The performance of the proposed topology is compared with that of several traditional topologies. The leakage current suppression ability and efficiency of the proposed topology are superior to those of the traditional topologies. The model predictive control (MPC) is applied in the proposed topology, which is easy to realize and can accelerate the dynamic response. Finally, the simulation and experimental results of a 1-kVA prototype are given, which proves the validity of the proposed topology in PV grid-connected system.

Analysis on topology derivation of single-phase transformerless photovoltaic grid-connect inverters

Aiming at the leakage current problem of single-phase transformerless photovoltaic(PV) grid-connected inverters, the recent proposed topologies are classified and reviewed. These topologies are divided into half-bridge type and full-bridge type. The topology characteristics of transformerless PV inverter are analyzed. This paper presents some of transformerless PV inverter topologies and discusses the evolution laws among various topologies to clarify the interrelationship between them. Furthermore, according to the evolution laws, a new topology is proposed. Finally, Simulations and experiments compare leakage current suppression capability of the proposed topology, H5 topology and HERIC topology.

A New Soft-Switching Configuration and Its Application in Transformerless Photovoltaic Grid-Connected Inverters

Soft-switching techniques of transformerless photovoltaic grid-connected inverters (TLIs) can significantly reduce switching losses, as well as soften switching processes. Conventional dc–ac soft-switching configurations proposed by Dr. Deepakraj M. Divan are invalid in TLIs because of the leakage current problem. In order to develop soft-switching techniques in TLIs, this paper proposes a new soft-switching configuration and a procedure to guide the invention of soft-switching TLIs. First, this paper proposes two basic resonance tanks related to dc bus polarities, and then, uses these basic tanks to elevate four popular full-bridge-type TLIs according to the proposed guideline. As a result, four soft-switching TLIs are gained. Second, this paper picks obtained soft-switching highly efficient and reliable inverter concept (HERIC) as example to analyze its soft-switching operation principle and performance. As a consequence, all active switches of the gained soft-switching HERIC circuit are switched under both of zero-current turn- on and turn- off conditions; the reverse recovery problem of freewheeling diodes is alleviated owing to the zero-current turn- off of diodes; meanwhile, the common-mode voltage at the switching frequency scale is still constant. Finally, some experimental results from a 3-kW universal prototype at 50-kHz switching frequency are provided to verify the effectiveness of the main contributions of this paper.

Integrated DC–DC Converter Based Grid-Connected Transformerless Photovoltaic Inverter With Extended Input Voltage Range

Owing to low cost, small size, and low weight, transformerless inverters became prominent in single-phase grid connected photovoltaic (PV) systems. Key issues pertaining to these inverters include suppression of common mode (CM) leakage current and improvement of conversion efficiency. Achieving higher efficiency in single-phase grid-connected photovoltaic systems depends on the number of stages involved in feeding power to the grid, predominantly, if the PV array voltage is less than the peak value of the grid voltage. In this paper, an integrated dc–dc converter based grid-connected transformerless PV inverter is proposed which is aimed at maintaining high efficiency, even if the PV array voltage falls below the peak value of grid voltage (efficient operation at an extended input voltage range). A modulation strategy is discussed in order to minimize the flow of CM leakage current. Further, the efficiencies of certain transformerless inverter topologies are analyzed and compared with that of the proposed topology. Detailed simulation studies are carried out in MATLAB/Simulink environment to verify the analysis. Experimental results for a scaled down laboratory prototype are included as a proof-of-concept to validate the claims.

Comparative Analysis of Grid Connected Transformerless Photovoltaic Inverters for Leakage Current Minimization

Objectives: To make a comparative analysis of four transformerless topologies namely H5, H6, oH5 and H-Bridge Zero Voltage State Rectifier (HBZVR) in terms of leakage current and THD. Methods/Statistical Analysis: H5, H6, oH5 and HBZVR topologies have been simulated. Then these topologies are compared in terms of leakage current and THD. Based on the comparative analysis, the topology with less leakage current and THD has been chosen for photovoltaic interface. The interfacing circuit is simulated and realized as prototype to validate the results practically. Findings: It is found from the review that HBZVR topology has less leakage current and THD. This topology employs both galvanic isolation and Common Mode Voltage (CMV) clamping. It is found that the common mode voltage in HBZVR is almost eliminated. The conclusion derived from this paper may be helpful in selecting proper topology for PV interface. Application/ Improvements: Generally, in grid connected PV inverters, transformers provide galvanic isolation between the two electrical circuits, thus preventing the flow of leakage current between the stray capacitance of PV and ground. But, ransformers reduce the overall efficiency of the system, which lead to the development of transformerless PV system. Galvanic connection exists between the PV and grid in transformerless inverter, which leads to the presence of leakage current. So, to reduce leakage current, many topologies are introduced which employs either dc-decoupling or ac-decoupling to provide galvanic isolation. The HBZVR topology provides proper isolation for PV interface where the capacitance of the panel with respect to ground is large. Keywords: Comparison, Leakage Current, Isolation, Transformerless PV Inverters, THD

Highly reliable inverter topology with a novel soft computing technique to eliminate leakage current in grid-connected transformerless photovoltaic systems

Grid-connected transformerless photovoltaic inverters are widely accepted in the renewable energy market, owing to their high power density, low cost, and high efficiency. However, the leakage current is the main issue in these inverters, which is to be investigated carefully. In this study, leakage current analysis of both transformer and transformerless bridge inverter topologies are widely investigated. Based on that, a new topology and modulation technique is proposed to eliminate the leakage current in the system. The mechanism of a creating high-impedance path between the photovoltaic module and the system, by properly isolating them in the freewheeling state and maintaining a constant common mode voltage in all the switching states, is elaborately discussed in this paper. The experimental results are finally presented to validate the proposed topology with respect to other conventional topologies.

Analytical study and simulation of a transformer-less photovoltaic grid-connected inverter with a delta-type tri-direction clamping cell for leakage current elimination

PurposeThis paper aims to propose a new transformer-less inverter structure to reduce the common-mode leakage current in grid-connected photovoltaic (PV) systems.Design/methodology/approachThe proposed circuit structure is the same as the conventional full-bridge inverter with three additional power switches in a triangular structure. These three power switches are between the bridge and the output filter, and they mitigate the common-mode leakage current flowing toward the PV panels’ capacitors. The common-mode leakage current mitigation is done through the three-direction clamping cell (TDCC) concept. By clamping the common-mode voltage to the middle voltage of the DC-link capacitors, the leakage current and the total harmonic distortion (THD) of the injected current to the grid is effectively reduced. Therefore, the efficiency is improved.FindingsThe switching modes and the control method are introduced. A comparison is carried out between the proposed structure and other solutions in the literature. The proposed topology and its respective control method are simulated by PSCAD/EMTDC software. The simulation results validate the advantages of the presented structure such as clamping the common-mode voltage and reducing leakage current and THD of injected current to the grid.Originality/valuePresenting a single phase-improved inverter structure with low-leakage current for grid-connected PV power systems represents a significant original contribution to this work. The proposed structure can inject a sinusoidal current with low THD to the AC grid, and the power factor is unity on the AC side. In the half positive cycle, one of the switches in the TDCC is turned off under zero current. Besides, one of the other switches in TDCC is turned on with zero voltage and, therefore, its turn-on switching losses are zero. The efficiency of the proposed topology is high because of the reduction of leakage current and power losses. Accordingly, the presented topology can be a good solution to the leakage current elimination.

A Novel Neutral Point Clamped Full-Bridge Topology for Transformerless Photovoltaic Grid-Connected Inverters

This paper presents a novel neutral point clamped full-bridge topology for transformerless photovoltaic grid-tied inverters. Transformerless grid-connected inverters have been used widely in recent years since they offer higher efficiency and lower costs. Ground leakage current suppression is the main issue which should be considered carefully in transformerless photovoltaic grid-connected inverters. Among different methods used to decline ground leakage current, neutral point clamped (NPC) topologies are considered more useful and effective. In NPC topologies, the short-circuited output voltage at the freewheeling period is clamped to the middle of the DC bus voltage. Therefore, the common-mode voltage (CM) will be constant at the whole switching period. Various NPC topologies such as H6 [1], HB-ZVR [2], oH5 [3], and PN-NPC [4] have been proposed. In this paper, a novel NPC topology is proposed which has lower power losses and higher efficiency over previous topologies. Furthermore, the proposed NPC topology exhibits a similar ground leakage current with the PN-NPC topology. The proposed NPC topology is analyzed theoretically using simulation studies and an experimental prototype is provided to verify theoretical analysis and simulation studies.

A Novel Neutral Point Clamped Full-Bridge Topology for Transformerless Photovoltaic Grid-Connected Inverters

A Novel Neutral Point Clamped Full-Bridge Topology for Transformerless Photovoltaic Grid-Connected Inverters

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 Family of Zero-Current-Transition Transformerless Photovoltaic Grid-Connected Inverter

Low leakage current and high efficiency are two key indexes for transformerless PV grid-connected inverter. The transformerless inverter topologies have superior efficiency thanks to saving transformer, but their semiconductor devices are still on hard-switching state at present. First and foremost, a novel zero-current-transition (ZCT) concept for the single-phase full-bridge transformerless PV grid-connected inverters is presented in this paper. Second, the zero-current turn-off for high-frequency main switches of the inverters and the zero-current turn-on for auxiliary switches added are achieved by introducing two resonant tanks. Furthermore, a family of ZCT transformerless grid-connected inverters with sinusoidal pulse width modulation is deduced. Especially, taking zero-current-transition six-switch full-bridge topology (ZCT-H6-I) as an example, its operation principle, soft-switching conditions, duty cycle constraints, and parameter design procedure of the resonant tank are analyzed in detail. Finally, the validity of the ZCT concept is verified by a ZCT-H6-I prototype rated at 50 kHz, 1 kW.

Classification of Grid Connected Transformerless PV Inverters with a Focus on the Leakage Current Characteristics and Extension of Topology Families

Grid-connected transformerless photovoltaic (PV) inverters (TPVIs) are increasingly dominating the market due to their higher efficiency, lower cost, lighter weight, and reduced size when compared to their transformer based counterparts. However, due to the lack of galvanic isolation in the low voltage grid interconnections of these inverters, the PV systems become vulnerable to leakage currents flowing through the grounded star point of the distribution transformer, the earth, and the distributed parasitic capacitance of the PV modules. These leakage currents are prohibitive, since they constitute an issue for safety, reliability, protection coordination, electromagnetic compatibility, and module lifetime. This paper investigates a wide range of multi-kW range power rating TPVI topologies and classifies them in terms of their leakage current attributes. This systematic classification places most topologies under a small number of classes with basic leakage current attributes. Thus, understanding and evaluating these topologies becomes an easy task. In addition, based on these observations, new topologies with reduced leakage current characteristics are proposed in this paper. Furthermore, the important efficiency and cost determining characteristics of converters are studied to allow design engineers to include cost and efficiency as deciding factors in selecting a converter topology for PV applications.

Zero-Voltage-Transition Full-Bridge Topologies for Transformerless Photovoltaic Grid-Connected Inverter

Low leakage current and high efficiency are two key indexes for the transformerless photovoltaic (PV) grid-connected inverter. The transformerless inverter topologies have superior efficiency owing to saving the transformer, but their switches are still on a hard-switching state at present. This paper presents a novel zero-voltage transition (ZVT) concept for the full-bridge transformerless PV grid-connected inverter topologies. The zero-voltage turn-on and turnoff of the high-frequency main switches and the zero-current turn-on of the auxiliary switches, as well as the zero-voltage turn-on and turnoff of the antiparallel diodes of the line-frequency switches, are realized by introducing two resonant tanks. Take ZVT-H6-I for example. The operation principle, soft-switching conditions, duty cycle restriction, and parameter design rule of the resonant tank are analyzed in detail. Finally, the validity of the ZVT concept is verified by a ZVT-H6-I prototype rated at 100 kHz and 1 kW.

Optimised full‐bridge transformerless photovoltaic grid‐connected inverter with low conduction loss and low leakage current

The unipolar sinusoidal pulse width modulation (SPWM) full-bridge inverter brings high-frequency common-mode voltage, which restricts its application in transformerless photovoltaic grid-connected inverter. The freewheeling path added in ac side is adopted to restrict leakage current, but the common-mode voltage is variable at switching frequency scale. To solve this problem, an improved full-bridge structure with two switches and a capacitor divider has been proposed, which guarantees that freewheel path is clamped to half of input voltage in freewheel period. Sequentially, the high-frequency common-mode voltage has been avoided in unipolar SPWM full-bridge inverter. The operation mode and the clamping mode are analysed, and several related topologies are discussed. Finally, the differential-mode and common-mode performance of the proposed topology are verified by a universal prototype inverter rated at 1 kW.