Transformerless Inverter Research Articles

Comparative Analysis of Si- and GaN-Based Single-Phase Transformer-Less PV Grid-Tied Inverter

Recently, the interest in grid-tied PV transformer-less inverters has increased rapidly, because of their higher efficiency and lower cost compared to traditional line transformer inverters. This paper presents a new modified transformer-less topology derived from H5 inverter, and provides a detailed comparison between the use of GaN and Si devices for the proposed topology. Detailed operation modes, inverter structure and switching strategy of the proposed topology are presented. Datasheet information, conduction losses, switching losses, and heat sink requirements are studied and analyzed to provide an accurate comparison between GaN and Si power devices for the proposed topology operating at unity power factor. The results show that, GaN power devices significantly reduce the power losses in the system, which consequently allow a significant increase in either inverter power rating or switching frequency. Thus, the use of GaN power devices for the proposed inverter can be more appealing and cost-effective approach.

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.

Common-Ground-Type Transformerless Inverters for Single-Phase Solar Photovoltaic Systems

This paper proposes a family of novel flying capacitor transformerless inverters for single-phase photovoltaic (PV) systems. Each of the new topologies proposed is based on a flying capacitor principle and requires only four power switches and/or diodes, one capacitor, and a small filter at the output stage. A simple unipolar sinusoidal pulse width modulation technique is used to modulate the inverter to minimize the switching loss, output current ripple, and the filter requirements. In general, the main advantages of the new inverter topologies are: 1) the negative polarity of the PV is directly connected to the grid, and therefore, no leakage current; 2) reactive power compensation capability; and 3) the output ac voltage peak is equal to the input dc voltage (unlike neutral-point-clamped and derivative topologies, which requires twice the magnitude of the peak ac voltage). A complete description of the operating principle with modulation techniques, design guidelines, and comprehensive comparisons is presented to reveal the properties and limitations of each topology in detail. Finally, experimental results of 1-kVA prototypes are presented to prove the concept and theoretical analysis of the proposed inverter family for practical applications.

Improved modulation strategy for single phase grid connected transformerless PV inverter topologies with reactive power generation capability

In this study, several recently published topologies are analyzed for reactive power generation capability. It has been found that some of the topologies with their conventional modulation strategies are not suitable for reactive power applications. These are not able to generate zero voltage states during negative power flow. As a result, current is distorted and PV inverter injects current with high total harmonic distortion (THD) into the grid. In order to overcome these problems, modified modulation technique is proposed to achieve reactive power capability without having distortion in injected grid current. It provides path for current to flow in order to generate zero voltage states during negative power flow. With improved modulation strategies, reactive power generation is achieved in inverter topologies without any alteration on the converter structures. The controller software of these topologies is only required to be updated for reactive power generation. The implementation and generation of the proposed improved modulation techniques are straightforward and it can easily be realized. Moreover, single phase transformerless inverter topologies are classified for first time on the basis of reactive power handling capability. In order to verify the theoretical explanations, simulations have been carried out in Matlab/Simulink environment and validated through an experimental setup.

Design and implementation of single‐phase inverter without transformer for PV applications

For safety purposes, many photovoltaic (PV) systems are designed using galvanic isolation and transformers. The main problem in the existing topologies is that transformers are expensive, heavy and large. Another problem is that at conversion stage, the overall frequency is reduced. The efficiency of a PV inverter which is equipped with a transformer is usually between 91 and 94%. To tackle this issue, a transformerless (TL) PV system is proposed which has high efficiency and is lighter and cheaper. Due to stray capacitance, harmful leakage current will flow to the grid and PV array. H5, HERIC, H6 and oH5 are different types of existing TL inverter, and may be used to solve this problem. Each configuration has its drawbacks like shoot-through issues of switches, high conduction losses, metal-oxide-semiconductor field-effect transistor reverse recovery issues, or to avoid shoot-through fault when dead time requirements and grid voltage are nearly zero. The efficiency of the proposed TL inverter presented in this study is compared with other existing configurations. It shows that the proposed inverter efficiency is better than the conventional hard switching inverters. Moreover, to increase reliability, the inverters do not contain any shoot-through issues.

High‐efficiency neutral‐point‐clamped transformerless MOSFET inverter for photovoltaic applications

Here, a highly efficient MOSFET neutral-point-clamped (M-NPC) transformerless inverter is proposed for photovoltaic (PV) applications. By employing super-junction metal–oxide–semiconductor field-effect transistor (SJ-MOSFET) as well as silicon carbide (SiC) diodes, high efficiency is achieved. Furthermore, the common-mode voltage (CMV) is completely clamped at half of the input voltage with clamping branch. Therefore, leakage current is eliminated. The performances of different topologies, in terms of CMV, leakage current, total harmonic distortion (THD), losses, and efficiency, are compared with the proposed M-NPC topology. The analyses are carried out theoretically via MATLAB/Simulink, and further validated with experimental tests. The experimental results show maximum efficiency of 98.5% and European efficiency of 97.65%.

Simulation and Performance Comparison of Si and SiC Based on a Proposed H6 Inverter for PV Grid-tied Applications

Multilevel transformerless inverters are widely employed in PV grid-tied applications because of their significant benefits in reducing switching losses and enhancing system efficiency. Wide bandgap (WBG) semiconductor devices such as SiC MOSFETs can increase the system performance and reduce the total losses due to their superior features over Si switching devices. This paper investigates a three level transformerless inverter based on H6 topology. The proposed topology is implemented with SiC MOSFETs and a modulation approach is adopted to reduce the number of conducting switches. Also, a detailed comparison of the use of SiC and Si switching devices in terms of switching and conduction losses, high switching frequency operation, filter size reduction, and thermal analysis and heat sink volume is presented. Simulation results show that total conduction and switching losses are reduced by about 50%, which allow a significant increase in either the switching frequency or the inverter power rating level for the same switching device losses. Additionally, operating at higher switching frequencies using SiC MOSFET leads to a significant reduction in volume and weight of the inductor filter. Furthermore, a thermal analysis is performed using COMSOL software to investigate the heat sink requirement.

A Grid-Connected Single-Phase Transformerless Inverter Controlling Two Solar PV Arrays Operating Under Different Atmospheric Conditions

A grid-connected single-phase transformerless inverter that can operate two serially connected solar photovoltaic (PV) subarrays at their respective maximum power points while each one of them is exposed to different atmospheric conditions is proposed in this paper. As two subarrays are connected in series, the number of serially connected modules within a subarray is reduced to half. Reduction in the number of serially connected PV modules within a subarray leads to an overall improvement in the magnitude of power that can be abstracted from a subarray while the modules of the subarray are exposed to varied atmospheric conditions. The topological structure of the inverter ensures that the common mode voltage does not contain high-frequency components, thereby reducing the magnitude of leakage current involved with the solar panels well within the acceptable limit. An in-depth analysis of the scheme along with the derivation of its small signal model has been carried out. Detailed simulation studies are performed to verify its effectiveness. A 1-kW laboratory prototype of the scheme has been fabricated. Detailed experimental validations have been carried out utilizing the prototype to confirm the viability of the proposed scheme.

Combined Reactive Power Injection Modulation and Grid Current Distortion Improvement Approach for H6 Transformer-Less Photovoltaic Inverter

In this paper, a combined reactive power modulation and grid current distortion improvement approach is proposed for an H6 transformer-less full-bridge single-phase photovoltaic (PV) grid-connected inverter. H6 transformer-less inverters with traditional modulation and control strategies may not satisfy the requirement of reactive power compensation or may result in more severe zero-crossing current distortion. Therefore, contrary to the traditional modulation, a novel reactive power injection space vector pulse width modulation (SVPWM) is proposed, which consists of two operation stages—inverter modulation and reactive power modulation. The implementation of SVPWM for reactive power modulation using a digital signal processor is also investigated. Furthermore, to suppress the current zero-crossing distortion in the reactive power injection mode, a global sliding mode function based on the proportion-integration-resonance current controller is designed, and the control law of the global sliding mode is derived. Using the segment modulation and grid current distortion improvement approach, the function of reactive current injection is implemented in commercial PV inverters, and the total harmonic distortion of the grid current is decreased significantly by more than 5% in the low-power segment, under the operating conditions of a lagging or leading power factor of 0.95. The effectiveness and feasibility of the proposed approach are verified through simulation and experiment using a 5-kVA prototype.

Improved Transformerless Inverter with Common-mode Leakage Current Elimination for a Photovoltaic Energy Conversion System

Improved Transformerless Inverter with Common-mode Leakage Current Elimination for a Photovoltaic Energy Conversion System

Design of an inverter-side current reference and controller for a single-phase LCL-based grid-connected inverter

Summary This paper presents the design of a current reference used in an inverter-side current controller for a grid-connected single-phase transformerless inverter with an Inductive-Capacitive-Inductive of LCL filter. The idea is to indirectly control the inverter-side current, by directly acting on the grid-side current. Furthermore, it can be shown that the simple proportional term guarantees damping injection. However, the design of the current reference for such a controller is a more involved process, which turns out to be even more challenging in the case where the grid voltage signal is subject to harmonic distortion. In this case, the distortion propagates into the converter through the LCL filter. Therefore, an inverter-side current reference must be proposed to be as distorted as necessary to assure indirect tracking of the grid-side current towards its corresponding reference, which is built as a sinusoidal with a given phase displacement to allow reactive current injection. The inverter-side controller must then include a harmonic compensation mechanism, to assure perfect tracking of the inverter-side current towards its distorted reference, and thus, achieve an adequate performance under harmonic distortion. Experiments are performed in a prototype to evaluate the performance of the proposed controller.

Modeling and control for new LLCL filter based grid-tied PV inverters with active power decoupling and active resonance damping capabilities

LLCL filters utilization decreases single-phase transformerless inverters’ AC-Side volume significantly. Conversely, the inherent 2nd order power harmonic ripples burden inverters’ DC-Side. In fact, the passive solution to buffer these ripples imposes a threat to inverter’s reliability and power density. Hence, varieties of active power decoupling methods were introduced in the literature to improve inverter’s DC-Side volume and system’s reliability. Yet, most existing techniques require auxiliary power electronics and energy storage elements. This contradicts the goal of optimizing system’s overall power density. Therefore, a novel LLCL filter for grid-connected applications is introduced that merges AC-Side and DC-Side volume minimization methods without additional power electronics devices. Precisely, the common-mode (CM) operation is harnessed for active power decoupling and the differential-mode (DM) is utilized for active power injection. Besides, the stability analysis of the proposed system revealed that the CM and DM are resonating. Thus, an active resonance damping control scheme based on decoupled CM and DM capacitor currents feedbacks was developed. With the proposed topology, the DC-link capacitor was reduced 40 times compared to the passive solution. The robustness of the proposed solution to the grid-side inductance variation was also verified and validated on 750W prototype system.

A Multilevel Transformerless Inverter Employing Ground Connection Between PV Negative Terminal and Grid Neutral Point

For the safe operation of transformerless grid connected photovoltaic (PV) inverters, the issue of common mode (CM) leakage current needs to be addressed carefully. In this paper, a novel multilevel transformerless inverter topology is proposed, which completely eliminates CM leakage current by connecting grid neutral point directly to the PV negative terminal, thereby bypassing the PV stray capacitance. It provides a low-cost solution consisting of only four power switches, two capacitors, and a single filter inductor. As compared to half-bridge topologies, with this inverter a minimum of 27% and maximum of 100% more output voltage is obtained for the same dc-link voltage. The proposed inverter is analyzed in detail and its switching pattern to generate multilevel output while maintaining the capacitor voltage is discussed. Simulations and experiments results confirm the feasibility and good performance of the proposed inverter.

비절연인버터 방식 BIPV시스템에서의 감전 사고 위험성 분석

Recently, As a result of government policy to promote renewable energy sources, BIPV(Building Integrated Photo Voltaic) System replacing building finishing material with photo voltaic module has been increasing. Building integrated photo voltaic system is installed in a building by inserting a solar cell in the building finishing material. Thus BIPV systems have possibility of an electric shock accident due to decreased insulation performance because it is easy to contact human body from conductor. Especially, in case of a system using transformerless inverter system, it is structurally vulnerable to safety, so that the risk of accidents may increase. Therefore, the risk of human shock due to accidents should be studied through actual accident experiments. In this paper, as a study for the development of standards for securing human body electrical safety in BIPV system, we considered the hazard of electrical safety in the BIPV systems applied transformerless inverter and conducted analysis risk of electric shock through verification experiment.

Single phase transformerless inverter topology with reduced leakage current for grid connected photovoltaic system

Leakage current is the main concern of the grid connected transformerless photovoltaic (PV) inverters. Many single phase transformerless inverter topologies with reduced leakage current have been introduced in the past few years. These are mainly classified on the basis of leakage current reduction methods Galvanic isolation without- common mode voltage (CMV) clamping and with-CMV clamping. It has been shown that leakage current generation is highly dependent on CMV. CMV of the topologies without- CMV clamping oscillates and oscillation amplitude depends on switches’ junction capacitances and parasitic parameters of the topology. In order to eliminate the leakage current completely, CMV must be constant throughout the inverter operation. Moreover, inverter should also be capable to inject definite amount of reactive power into the grid, as demanded by the international regulations. In this study, reduced leakage current CMV clamped topology is proposed which can eliminate leakage current and capable of injecting reactive power into the grid. Total harmonic distortions (THD) of injected grid current at various solar irradiance levels are also analyzed. In order to verify the theoretical explanations, the proposed topologies are simulated in Matlab/Simulink environment. Finally, the simulated results are validated experimentally.

Modelling and Control of Parallel-Connected Transformerless Inverters for Large Photovoltaic Farms

This paper presents a control structure for transformerless photovoltaic inverters connected in parallel to manage photovoltaic fields in the MW range. Large photovoltaic farms are usually divided into several photovoltaic fields, each one of them managed by a centralized high power inverter. The current tendency to build up centralized inverters in the MW range is the use of several transformerless inverters connected in parallel, a topology that provokes the appearance of significant zero-sequence circulating currents among inverters. To eliminate this inconvenience, this paper proposes a control structure that avoids the appearance of circulating currents by controlling the zero-sequence component of the inverters. A second contribution of the paper is the development of a model of n parallel-connected inverters. To validate the concept, the proposed control structure has been applied to a photovoltaic field of 2 MW managed by four 500 kW photovoltaic inverters connected in parallel.

An improved single phase transformerless inverter topology for grid connected PV system with reduce leakage current and reactive power capability

Abstract Single phase transformerless inverters are widely being adopted for small scale grid connected PV systems due to their high efficiency, lower cost and high power density. However, leakage current is the main concern in these inverters which needs to be addressed carefully. Moreover, PV inverters should also be capable of injecting a definite amount of reactive power into the grid as prescribed by the international regulations. In this paper, an improved H6 common mode voltage (CMV) clamped topology with modified modulation strategy is proposed. The proposed topology offers low loss ac side decoupling, complete elimination of leakage current via CMV clamping and reactive power generation capability. The proposed modulation technique facilitates the flow of current in order to generate zero voltage state during negative power flow. Total harmonic distortion (THD) of injected grid current at various solar irradiance levels is also analyzed. Moreover, implementation of inverter control algorithm in real time digital simulator (OP-5600) has been discussed in detail. In order to justify the suitability of the proposed topology for grid-tied PV system, simulations have been carried out in Matlab/Simulink environment and validated through an experimental setup.

Leakage Current Energy Harvesting Application in a Photovoltaic (PV) Panel Transformerless Inverter System

Present-day solar panels incorporate inverters as their core components. Switching devices driven by specialized power controllers are operated in a transformerless inverter topology. However, some challenges associated with this configuration include the absence of isolation, causing leakage currents to flow through various components toward ground. This inevitably causes power losses, often being also the primary reason for the power inverters’ analog equipment failure. In this paper, various aspects of the leakage currents are studied using different circuit analysis methods. The primary objective is to convert the leakage current energy into a usable DC voltage source. The research is focused on harvesting the leakage currents for producing circa 1.1 V, derived from recently developed rectifier circuits, and driving a 200 Ω load with a power in the milliwatt range. Even though the output voltage level is low, the harvested power could be used for charging small batteries or capacitors, even driving light loads.

Capacitive Emulation for LCL-Filtered Grid-Connected Converters

This paper proposes an effective capacitive emulation (CE) technique to reduce the grid current distortion when the converter currents are controlled (converter current feedback (CCF)) in grid-connected converters with an LCL filter. Although the CCF scheme is preferable to the grid current feedback (GCF) scheme in terms of stability and inherent current limitation of the power semiconductors, the former presents a problem of additional grid-current harmonic distortion due to the LCL capacitor, which generates a distorted current by line voltage differentiation. To solve this problem, the distorting capacitive current is reproduced on the converter output, so that both cancel out each other on the grid. That is, the converter emulates a negative capacitance while delivering the active and reactive power at the fundamental frequency. This is achieved by adding an estimation of the distorting capacitive current to the converter current reference. Moreover, an effective CE technique requires a current control capable of tracking all harmonics added to the current reference and compensating for any control delay. To compensate this delay, the paper proposes a buffer-based method to advance and filter the current reference. The effectiveness of the CE method has been tested on a 10-kVA transformerless inverter with CCF and a simple proportional-integral (PI) control with grid-voltage feedforward cancellation. Experimental results prove that the proposed solution reduces the line current THD (total harmonic distortion) compared with the GCF strategy with proportional-resonant (PR) control.

A Single-Phase Transformerless Inverter With Charge Pump Circuit Concept for Grid-Tied PV Applications

This paper proposes a new single-phase transformerless photovoltaic (PV) inverter for grid-tied PV systems. The topology is derived from the concept of a charge pump circuit in order to eliminate the leakage current. It is composed of four power switches, two diodes, two capacitors, and an LCL output filter. The neutral of the grid is directly connected to the negative polarity of the PV panel that creates a constant common mode voltage and zero leakage current. The charge pump circuit generates the negative output voltage of the proposed inverter during the negative cycle. A proportional resonant control strategy is used to control the injected current. The main benefits of the proposed inverter are: 1) the neutral of the grid is directly connected to the negative terminal of the PV panel, so the leakage current is eliminated; 2) its compact size; 3) low cost; 4) the used dc voltage of the proposed inverter is the same as the full-bridge inverter (unlike neutral point clamped (NPC), active NPC, and half-bridge inverters); 5) flexible grounding configuration; 6) capability of reactive power flow; and 7) high efficiency. A complete description of the operating principle and analysis of the proposed inverter are presented. Experimental results are presented to confirm both the theoretical analysis and the concept of the proposed inverter. The obtained results clearly validate the performance of the proposed inverter and its practical application in grid-tied PV systems.