High Reliability and Efficiency Single-Phase Transformerless Inverter for Grid-Connected Photovoltaic Systems
TL;DR: In this article, a high-reliability single-phase transformerless grid-connected inverter that utilizes superjunction MOSFETs to achieve high efficiency for photovoltaic applications is presented.
Abstract: This paper presents a high-reliability single-phase transformerless grid-connected inverter that utilizes superjunction MOSFETs to achieve high efficiency for photovoltaic applications. The proposed converter utilizes two split ac-coupled inductors that operate separately for positive and negative half grid cycles. This eliminates the shoot-through issue that is encountered by traditional voltage source inverters, leading to enhanced system reliability. Dead time is not required at both the high-frequency pulsewidth modulation switching commutation and the grid zero-crossing instants, improving the quality of the output ac-current and increasing the converter efficiency. The split structure of the proposed inverter does not lead itself to the reverse-recovery issues for the main power switches and as such superjunction MOSFETs can be utilized without any reliability or efficiency penalties. Since MOSFETs are utilized in the proposed converter high efficiency can be achieved even at light load operations achieving a high California energy commission (CEC) or European union efficiency of the converter system. It also has the ability to operate at higher switching frequencies while maintaining high efficiency. The higher operating frequencies with high efficiency enables reduced cooling requirements and results in system cost savings by shrinking passive components. With two additional ac-side switches conducting the currents during the freewheeling phases, the photovoltaic array is decoupled from the grid. This reduces the high-frequency common-mode voltage leading to minimized ground loop leakage current. The operation principle, common-mode characteristic and design considerations of the proposed transformerless inverter are illustrated. The total losses of the power semiconductor devices of several existing transformerless inverters which utilize MOSFETs as main switches are evaluated and compared. The experimental results with a 5 kW prototype circuit show 99.0% CEC efficiency and 99.3% peak efficiency with a 20 kHz switching frequency. The high reliability and efficiency of the proposed converter makes it very attractive for single-phase transformerless photovoltaic inverter applications.
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TL;DR: In this article, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults is explored in order to map future challenges, and control strategies with reactive power injection are also discussed.
Abstract: Transformerless photovoltaic (PV) inverters are going to be more widely adopted in order to achieve high efficiency, as the penetration level of PV systems is continuously booming. However, problems may arise in highly PV-integrated distribution systems. For example, a sudden stoppage of all PV systems due to anti-islanding protection may contribute to grid disturbances. Thus, standards featuring with ancillary services for the next-generation PV systems are under a revision in some countries. The future PV systems have to provide a full range of services as what the conventional power plants do, e.g., low-voltage ride-through (LVRT) under grid faults and grid support service. In order to map future challenges, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults is explored in this paper. Control strategies with reactive power injection are also discussed. The selected inverters are the full-bridge (FB) inverter with bipolar modulation, the FB inverter with dc bypass, and the Highly Efficient and Reliable Inverter Concept (HERIC). A 1-kW single-phase grid-connected PV system is analyzed to verify the discussions. The tests confirmed that, although the HERIC inverter is the best candidate in terms of efficiency, it is not very particularly feasible in case of a voltage sag. The other two topologies are capable of providing reactive current during LVRT. A benchmarking of those inverters is also provided in this paper, which offers the possibility to select appropriate devices and to further optimize the transformerless system.
307 citations
Cites background from "High Reliability and Efficiency Sin..."
...Some are based on the multi-level topologies [26]-[28], and some are derived by optimizing traditional transformerless inverters [29], [30]....
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TL;DR: In this paper, a review of the existing and future standards that address the technical challenges associated with the growing number of Grid-Connected Photovoltaic Systems (GCPVS) is presented.
Abstract: This paper presents a literature review of the recent developments and trends pertaining to Grid-Connected Photovoltaic Systems (GCPVS). In countries with high penetration of Distributed Generation (DG) resources, GCPVS have been shown to cause inadvertent stress on the electrical grid. A review of the existing and future standards that addresses the technical challenges associated with the growing number of GCPVS is presented. Maximum Power Point Tracking (MPPT), Solar Tracking (ST) and the use of transformless inverters can all lead to high efficiency gains of Photovoltaic (PV) systems while ensuring minimal interference with the grid. Inverters that support ancillary services like reactive power control, frequency regulation and energy storage are critical for mitigating the challenges caused by the growing adoption of GCPVS.
264 citations
TL;DR: In this paper, a simple modified H-bridge zero-voltage state rectifier is proposed, to combine the benefits of the low-loss ac-decoupling method and the complete leakage current elimination of the CMV clamping method.
Abstract: 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.
250 citations
Cites background from "High Reliability and Efficiency Sin..."
...because of the slow reverse-recovery of the body diode [22], [27]....
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...Therefore, topologies employing ac-decoupling techniques [23] are found to be higher in efficiency as compared to dc-decoupling topologies [20]–[22], [27]....
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...duced via proper choice of power electronics switches, such as using MOSFETs instead of IGBTs [22], [27]....
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TL;DR: In this paper, the leakage current issues of transformerless inverter, which depends on the topology structure and modulation scheme, have to be addressed very carefully, and a performance comparison in MATLAB/Simulink environment is done among different topologies.
Abstract: Grid-tied inverters are the key components of distributed generation system because of their function as an effective interface between renewable energy sources and utility. Recently, there has been an increasing interest in the use of transformerless inverter for low-voltage single-phase grid-tied photovoltaic (PV) system due to higher efficiency, lower cost, smaller size and weight when compared to the ones with transformer. However, the leakage current issues of transformerless inverter, which depends on the topology structure and modulation scheme, have to be addressed very carefully. This review focuses on the transformerless topologies, which are classified into three basic groups based on the decoupling method and leakage current characteristics. Different topologies under the three classes are presented, compared and evaluated based on leakage current, component ratings, advantages, and disadvantages. An examination of demand for the inverter, the utility grid, and the PV module are presented. A performance comparison in MATLAB/Simulink environment is done among different topologies. Also an analysis has been presented to select a better topology. Finally, based on the analysis and simulation results, a comparison table has been presented. Furthermore, some important experimental parameters have been summarized.
216 citations
TL;DR: In this paper, the authors presented the outcome of a new system architecture and control algorithm that can use both battery storage and manage the temperature of thermal appliances, which is an important part of the smart grid that enables residential customers to execute demand response programs autonomously.
195 citations
Cites background from "High Reliability and Efficiency Sin..."
...charging of most of the lead-acid batteries are more than 85% [24, 25] and the efficiency of most inverters are greater than 96% [26], the energy conversion efficiency of the battery is greater than 85% × 96% = 81....
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References
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TL;DR: In this article, the authors focus on inverter technologies for connecting photovoltaic (PV) modules to a single-phase grid and categorize the inverters into four classifications: 1) the number of power processing stages in cascade; 2) the type of power decoupling between the PV module(s) and the single phase grid; 3) whether they utilizes a transformer (either line or high frequency) or not; and 4) the kind of grid-connected power stage.
Abstract: This review focuses on inverter technologies for connecting photovoltaic (PV) modules to a single-phase grid. The inverters are categorized into four classifications: 1) the number of power processing stages in cascade; 2) the type of power decoupling between the PV module(s) and the single-phase grid; 3) whether they utilizes a transformer (either line or high frequency) or not; and 4) the type of grid-connected power stage. Various inverter topologies are presented, compared, and evaluated against demands, lifetime, component ratings, and cost. Finally, some of the topologies are pointed out as the best candidates for either single PV module or multiple PV module applications.
3,530 citations
TL;DR: In this paper, a topology study of the PV MICs in the power range below 500 W and covers most topologies recently proposed for MIC applications is presented, where the MIC topologies are classified into three different arrangements based on the dc link configurations.
Abstract: The annual world photovoltaic (PV) cell/module production is growing at almost an exponential rate and has reached 1727 MW in 2005. Building integrated PV (BIPV) projects are emerging as the strongest part of the PV market and grid interactive inverters are a key component in determining the total system cost. Module integrated converter (MIC) technology has become a global trend in grid interactive PV applications and may assist in driving down the balance of system costs to secure an improved total system cost. This paper concentrates on the topology study of the PV MICs in the power range below 500 W and covers most topologies recently proposed for MIC applications. The MIC topologies are classified into three different arrangements based on the dc link configurations. A systematic discussion is also provided at the end of the paper that focuses on the major advantages and disadvantages of each MIC arrangement. These are considered in detail and will provide a useful framework and point of reference for the next generation MIC designs and applications.
1,158 citations
TL;DR: In this paper, an overview of single-phase inverters developed for small distributed power generators is presented, compared, and evaluated against the requirements of power decoupling and dual-grounding, the capabilities for grid-connected or/and stand-alone operations, and specific DG applications.
Abstract: This paper presents an overview of single-phase inverters developed for small distributed power generators. The functions of inverters in distributed power generation (DG) systems include dc-ac conversion, output power quality assurance, various protection mechanisms, and system controls. Unique requirements for small distributed power generation systems include low cost, high efficiency and tolerance for an extremely wide range of input voltage variations. These requirements have driven the inverter development toward simpler topologies and structures, lower component counts, and tighter modular design. Both single-stage and multiple-stage inverters have been developed for power conversion in DG systems. Single-stage inverters offer simple structure and low cost, but suffer from a limited range of input voltage variations and are often characterized by compromised system performance. On the other hand, multiple-stage inverters accept a wide range of input voltage variations, but suffer from high cost, complicated structure and low efficiency. Various circuit topologies are presented, compared, and evaluated against the requirements of power decoupling and dual-grounding, the capabilities for grid-connected or/and stand-alone operations, and specific DG applications in this paper, along with the identification of recent development trends of single-phase inverters for distributed power generators.
899 citations
TL;DR: This paper proposes a new topology, based on the H-bridge with a new ac bypass circuit consisting of a diode rectifier and a switch with clamping to the dc midpoint, which achieves high conversion efficiency and low leakage current.
Abstract: There is a strong trend in the photovoltaic inverter technology to use transformerless topologies in order to acquire higher efficiencies combining with very low ground leakage current. In this paper, a new topology, based on the H-bridge with a new ac bypass circuit consisting of a diode rectifier and a switch with clamping to the dc midpoint, is proposed. The topology is simulated and experimentally validated, and a comparison with other existing topologies is performed. High conversion efficiency and low leakage current are demonstrated.
743 citations
"High Reliability and Efficiency Sin..." refers background in this paper
...Highfrequency common-mode (CM) voltages must be avoided for a transformerless PV grid-connected inverter because it will lead to a large charge/discharge current partially flowing through the inverter to the ground....
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TL;DR: In this article, the authors proposed a new high-efficiency topology that generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full bridge.
Abstract: When no transformer is used in a grid-connected photovoltaic (PV) system, a galvanic connection between the grid and the PV array exists. In these conditions, dangerous leakage currents (common-mode currents) can appear through the stray capacitance between the PV array and the ground. In order to avoid these leakage currents, different inverter topologies that generate no varying common-mode voltages, such as the half-bridge and the bipolar pulsewidth modulation (PWM) full-bridge topologies, have been proposed. The need of a high-input voltage represents an important drawback of the half-bridge. The bipolar PWM full bridge requires a lower input voltage but exhibits a low efficiency. This letter proposes a new high-efficiency topology that generates no varying common-mode voltage and requires the same low-input voltage as the bipolar PWM full bridge. The proposed topology has been verified in a 5-kW prototype with satisfactory results
650 citations
"High Reliability and Efficiency Sin..." refers background in this paper
...Digital Object Identifier 10.1109/TPEL.2012.2214237 I. INTRODUCTION TRANSFORMERLESS photovoltaic (PV) grid-connectedinverters have the advantages of higher efficiency, lower cost, less complexity, and smaller volume compared to their counterparts with transformer galvanic isolation [1]–[27]....
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