Transformerless Topology Research Articles

Traditional and Hybrid Topologies for Single-/Three-Phase Transformerless Multilevel Inverters

With increasing interest in integrating solar power into the utility grid, multilevel inverters are gaining much more attention for medium- and high-power applications due to their high-quality waveform, low voltage stress across active components, and low total harmonic distortion in output voltage. However, to achieve these benefits, a large number of active and passive components are required. A transformer is also required to provide galvanic isolation, which increases its size and weight and reduces its power density and efficiency. In order to overcome the disadvantages posed by transformer-based inverters, research is being conducted on the transformerless topology of multilevel inverters. The first aim of this review article is to summarize traditional transformerless multilevel inverters (TMLIs) considering both single- and three-phase topologies. Secondly, the main aim of this article is to provide a detailed overview of the hybrid topologies of TMLIs that employ fewer components for photovoltaic applications. In addition, this study compares traditional and hybrid single-/three-phase topologies in terms of component count and performance factors, which will be useful to researchers.

Feedback Linearization Control for a Class of Transformerless Common-Ground VSI

As is well known in practice, conventional transformerless inverters may suffer the risk of high-frequency leakage currents between the photovoltaic (PV) array or battery energy storage systems (BESS) and the ground due to the stray capacitances. This operational condition can generate several electrical problems. Recently, we proposed in [1] a new method for designing transformerless topologies of single-phase voltage source inverters (VSI) with common ground derived from four classical DC-DC converters. As a sequence of that previous work, a novel control technique based on a feedback linearization approach is proposed to control that class of transformerless VSI with common ground in this paper. Unlike the classical local linearization techniques commonly used in the technical literature, this approach allows to obtain large-signal linear equations, by means of the feedback linearization of the non-linear terms. Real-time simulation results using Hardware-in-the-Loop allow validation of the proposed discrete control technique applied to this class of VSI topologies.

Fuel Cell Based Ultra-Voltage Gain Boost Converter for Electric Vehicle Applications

The proposed fuel cell-based ultra-voltage gain boost converter offers an alternative to transformer-based topologies for achieving high voltage gain. While transformers can achieve high voltage gains, they come with drawbacks such as high cost, design complexity, and increased weight. In this article, a switched inductor circuit is introduced as an alternative solution to achieve high voltage gain. This circuit enhances overall system performance by reducing size, weight, and cost. Operating in a transformer-less topology, the converter boosts voltage levels while ensuring low voltage stress on the switching devices. The suggested design enables larger voltage gain values even at low duty ratios. The output of the fundamental boost converter serves as the input for the switched inductor circuit, effectively boosting the voltage level and supplying more voltage to the output side. This converter is particularly suitable for applications requiring ultra-voltage gain in electric vehicles, which offer reduced pollution compared to internal combustion engines. Moreover, the utilization of this topology reduces space requirements. The article presents a thorough investigation of the steady state operation in continuous conduction mode, and theoretical verification and MATLAB simulations demonstrate the performance and operation of the proposed converter.

Sliding Mode Controller with Disturbance Rejection for Bidirectional Transformerless AC/DC Converter in EV Onboard Charger

Transformerless power converters are trending in automotive sector as the demand for onboard chargers rises. The high efficiency and leakage current mitigating property of highly efficient and reliable inverter concept (HERIC) converter make it an excellent choice among transformerless topologies. By using appropriate modulation technique, the HERIC converter can transmit bidirectional power. This article presents design of sliding mode controller (SMC) for bidirectional HERIC converter to enhance the tracking performance. The SMC parameters are selected utilizing Harris hawks optimization (HHO) algorithm. During vehicle-to-grid (V2G) operation, sliding mode controller is developed for reactive power regulation of the grid connected HERIC converter and for grid-to-vehicle (G2V) operation, the converter is operated with very low voltage ripples in the DC side. The stability analysis is done by considering model uncertainties to guarantee sinusoidal grid current, low THD, and unity power factor even in the presence of grid disturbances and parametric variations. The Typhoon Hardware in the Loop (HIL) 402 device is used to execute the real-time testing. The results confirmed the efficacy and robustness of the designed controller under different scenarios for both modes of operation.

A New Topology and Modulation Strategy to Suppress the Leakage Current in Transformerless Cascaded H-Bridge Inverters in PV Application

The use of transformerless inverters is common due to the lower cost, lighter weight, and higher efficiency. The lack of the transformer leads to the leakage current circulation through the parasitic capacitors, which increases the power losses, electromagnetic interferences, and safety issues. In this article, first, the proposed cascaded H-bridge (CHB) -based inverter’s equivalent common-mode (CM) model is determined. Then, a new multicarrier pulsewidth modulation strategy is proposed to suppress the leakage current. In the presented method, new redundancy states are provided due to the use of just one blocking switch. These extra redundancy states and the proposed modulation strategy help to keep the CM voltage (CMV) constant and suppress the leakage current. The validity of the proposed solution is verified by simulation in the MATLAB/Simulink environment. In addition, a universal prototype with a power rating of 600 W is built and tested, and the experimental results are presented to confirm the theoretical and simulation results. The experimental results show a maximum efficiency of 98.5% and a European efficiency of 98.3%. The proposed topology is also compared to the existing transformerless topologies to show its effectiveness.

Bidirectional Synchronous H6 Inverter for Hybrid AC/DC Distribution System With Improved Light Load Efficiency

The overall efficiency of DC distribution systems may be impacted by the operation of the bidirectional inverter connected to the grid; one of its most critical components. When compared to high-demand industries, the typical load profile of such system in residential buildings is highly variable, including several periods of light-load. Therefore, this paper presents a method to overcome the reduced efficiency at light-load of bidirectional inverters integrated into a DC distribution system powered by renewable energy. Existing topologies are evaluated for obtaining optimal efficiency as well as an investigation of loss breakdown at light-load using analytic models and PSIM simulation. A novel synchronous H6 transformer-less topology is proposed. Simulation results show that the proposed topology can achieve an efficiency of 96.5% under full load and 98.3% under light-load. Efficiency improvements over a standard H6 topology are 4.5% under light-load and 1.5 % at full load. This is mostly due to reduced conduction and switching ON loss of MOSFETs used to replace diodes. When using the rectifier mode, similar levels of improvement are observed, with a 4% increase under light-load and a 1.2% improvement under full load conditions, respectively. Experimental results confirm the effectiveness of the proposed topology, showing nearly the same trend in improvement vs. the standard H6.

Analysis of a Photovoltaic System Based on a Highly Efficient Single-Phase Transformerless Inverter

The essential requirement for a cleaner environment, along with rising consumption, puts a strain on the distribution system and power plants, reducing electricity availability, quality, and security. Grid-connected photovoltaic systems are one of the solutions for overcoming this. The examination and verification of transformerless topologies and control techniques was a significant goal of this study. The transformerless concept is advantageous for its high efficiency; the transformerless converter has added advantages of reduced price, complexity, weight, and size. This study presents a novel high-efficiency transformerless architecture that does not create common-mode currents and does not inject DC current into the grid. A single-phase transformerless inverter circuit with two step-down converters was constructed in this study. Low-frequency switches determine the polarity of the grid connection. In order to control the gate pulses of switching devices, which each regulate a half-wave of the output current, a PIC 16F877 was employed. Because there were fewer semiconductors and they were simpler to operate, it was possible to achieve a high degree of efficiency and reliability. A prototype model with input 12 V, 2 A was fabricated, test results were obtained with reduced common-mode current and DC current, and high efficiency was obtained with reduced switching losses. Further investigation for the improvement of efficiency with the elimination of ground current and leakage current has been analysed through simulation.

Topology Review of Three-Phase Two-Level Transformerless Photovoltaic Inverters for Common-Mode Voltage Reduction

In grid-connected photovoltaic (PV) systems, a transformer is needed to achieve the galvanic isolation and voltage ratio transformations. Nevertheless, these traditional configurations of transformers increase the weight, size, and cost of the inverter while decreasing the efficiency and power density. The transformerless topologies have become a good solution. However, the problem is that commode-mode voltage and leakage current can occur via the stray capacitors between the PV array and the ground of the inverter. Various transformerless inverters have been introduced with different techniques, such as reducing the common-mode voltage or eliminating the leakage current. Furthermore, to introduce the development of transformerless PV inverters, especially in three-phase two-level inverter systems, this paper provides a comprehensive review of various common-mode voltage reduction three-phase two-level inverters.

A New Off-Board Electrical Vehicle Battery Charger: Topology, Analysis and Design

The extensive use of electric vehicles (EVs) can reduce concerns about climate change and fossil fuel shortages. One of the main obstacles to accepting EVs is the limitation of charging stations, which consists of high-charge batteries and high-energy charging infrastructure. A new transformer-less topology for boost dc-dc converters with higher power density and lower switch stress is proposed in this paper, which may be a suitable candidate for high-power fast-charging battery chargers of EVs. Throughout this paper, two operating modes of the proposed converter, continuous current mode (CCM) and discontinuous current mode (DCM), are analyzed in detail. Additionally, critical inductances and design considerations for the proposed converter are calculated. Finally, real-time verifications based on hardware-in-loop (HiL) simulation are carried out to assess the correctness of the proposed theoretical concepts.

Neutral Point Clamped Transformer-Less Multilevel Converter for Grid-Connected Photovoltaic System

Transformer-less (TL) inverter topologies have elicited further special treatment in photo-voltaic (PV) power system as they provide high efficiency and low cost. Neutral point clamped (NPC) multilevel-inverter (MLI) topologies-based transformer-less are being immensely used in grid-connected medium-voltage high-power claims. Unfortunately, these topologies such as NPC-MLI, full-bridge inverter with DC bypass (FB-DCBP) suffer from the shoot-through problem on the bridge legs, which affect the reliability of the implementation. Based on the previous above credits, a T type neutral point clamped (TNP)—MLI (TNP-MLI) with transformer-less topology called TL-TNP-MLI is presented to be an alternate which can be suitable in the grid-connected PV power generation systems. The suggested TL-TNP-MLI topologies free from inverter bridge legs shoot-through burden, switching frequency common-mode current (CMC), and leakage current. The control system of the grid interface with hysteresis current control (HCC) strategy is proposed. The effectiveness of the proposed PV connected transformer-less TNP-MLI topology with different grid and PV scenario has been verified through the MATLAB/Simulink simulation model and field-programmable gate area (FPGA)-based experimental results for a 1.5 kW system.

Analysis and Implementation of a Single-Stage Transformer-Less Converter With High Step-Down Voltage Gain for Voltage Regulator Modules

A novel transformer-less topology with high step-down voltage gain is proposed for low-voltage high-current point-of-load voltage regulator modules (VRMs). The proposed topology consists of three capacitors, two inductors, three active switches, and two synchronous rectifiers. It triples the effective duty-ratio and lowers the voltage stresses across active switches compared with conventional buck topologies. Converters based on the topology have a potential benefit of achieving high power density and can be capable of delivering high current to the output by two interleaving phases. Small inductance and capacitance are only needed by using interleaving technology, which results in very fast dynamic response to large load changes. A 60 W, 48-1.8 V prototype is built to verify the performance of the proposed topology.

An Efficient H7 Single-Phase Photovoltaic Grid Connected Inverter for CMC Conceptualization and Mitigation Method

Transformerless inverters are the economic choice as power interfaces between photovoltaic (PV) renewable sources and the power grid. Without galvanic isolation and adequate power convert design, single-phase grid connected inverters may have limited performance due to the presence of a significant common mode ground current by creating safety issues and enhancing the negative impact of harmonics in the grid current. This paper proposes an extended H6 transformerless inverter that uses an additional power switch (H7) to improve common mode leakage current mitigation in a single-phase utility grid. The switch with a diode in series connection aims to make an effective clamp of common mode voltage at the DC link midpoint. The principles of operation of the proposed structure with bipolar sinusoidal pulse width modulation (SPWM) is presented and formulated. Laboratory tests’ performance is detailed and evaluated in comparison with well-known single-phase transformer-less topologies in terms of power conversion efficiency, total harmonic distortion (THD) level, and circuit components number. The studied topology performance evaluation is completed with the inclusion of reactive power compensation functionality verified by a low-power laboratory implementation with 98.02% efficiency and 30.3 mA for the leakage current.

An Improved Multilevel Inverter for Single-Phase Transformerless PV System

A single-phase Three-Level Split-Inductor Neutral Point Clamped Inverter-Improved (3L-SI-NPCI2) for transformerless photovoltaic (PV) application is proposed in this article. The proposed inverter utilizes two split inductors, which eliminate the shoot-through issue. Therefore, it is more reliable than conventional converters. Not needing dead time, achieving a high efficiency with lower current THD and low leakage current are other advantages of the proposed inverter. The operation principle, dynamic performance, output filter design and impacts of asymmetric filter inductors on leakage current are illustrated in the 3L-SI-NPCI2. The proposed topology is compared to seven existing transformerless topologies by simulation studies and results. Finally, an experimental prototype has been developed to show the benefits of the proposed inverter.

Quadratic boost converter with low‐output‐voltage ripple

This study proposes a non-isolated quadratic boost converter (QBC) that features a low-output-voltage ripple with respect to traditional QBCs. This advantage is in contrast with other topologies that require a higher amount of stored energy by capacitors to achieve the same output-voltage ripple specification. This benefit permits to design a compact converter, since the size of capacitors is proportional to their energy storage rating. Moreover, the proposed transformerless topology is suitable for applications that require high-voltage gains as in the case of renewable energy applications. The main properties of the converter are corroborated as well as its advantages by providing mathematical models, analytical waveforms and experiments.

Leakage Current Reduction in Single-Phase Grid-Connected Inverters—A Review

The rise in renewable energy has increased the use of DC/AC converters, which transform the direct current to alternating current. These devices, generally called inverters, are mainly used as an interface between clean energy and the grid. It is estimated that 21% of the global electricity generation capacity from renewable sources is supplied by photovoltaic systems. In these systems, a transformer to ensure grid isolation is used. Nevertheless, the transformer makes the system expensive, heavy, bulky and reduces its efficiency. Therefore, transformerless schemes are used to eliminate the mentioned disadvantages. One of the main drawbacks of transformerless topologies is the presence of a leakage current between the physical earth of the grid and the parasitic capacitances of the photovoltaic module terminals. The leakage current depends on the value of the parasitic capacitances of the panel and the common-mode voltage. At the same time, the common-mode voltage depends on the modulation strategy used. Therefore, by the manipulation of the modulation technique, is accomplished a decrease in the leakage current. However, the connection standards for photovoltaic inverters establish a maximum total harmonic distortion of 5%. In this paper an analysis of the common-mode voltage and its influence on the value of the leakage current is described. The main topologies and strategies used to reduce the leakage current in transformerless schemes are summarized, highlighting advantages and disadvantages and establishing points of comparison with similar topologies. A comparative table with the most important aspects of each converter is shown based on number of components, modes of operation, type of modulation strategy used, and the leakage current value obtained. It is important to mention that analyzed topologies present a variation of the leakage current between 0 to 180 mA. Finally, the trends, problems, and researches on transformerless grid-connected PV systems are discussed.

A Topology Synthetization Method for Single-Phase, Full-Bridge, Transformerless Inverter with Leakage Current Suppression Part I

Single-phase full-bridge transformerless topologies, such as the H5, H6, or the highly efficient and reliable inverter concept (HERIC) topologies, are commonly used for leakage current suppression for photovoltaic (PV) applications. The main derivation methodology of full-bridge topologies has been used based on both a DC-based decoupling model and an AC-based decoupling model. However, this methodology is not suited to the search for all possible topologies, and cannot verify whether they are inclusive. Part I of this paper will propose a new topology derivation methodology based on unipolar sinusoidal pulse width modulation (USPWM) to search all possible full-bridge topologies for leakage current suppression. First of all, a unified circuit model is proposed, instead of the DC- and AC-based models. Secondly, a mathematic method called the MN principle is then proposed to search for all possible topologies, and a derivation procedure is provided. It was verified that all existing topologies could be found using the proposed method; furthermore, seven new topologies were derived. The proposed topology derivation methodology is extended to search topologies under Double-Frequency USPWM (DFUSPWM). Twenty topologies under USPWM and four topologies under DFUSPWM have been derived.

МОДЕЛИРОВАНИЕ ПЕРЕХОДНЫХ ПРОЦЕССОВ ДИЗЕЛЬ-ГЕНЕРАТОРНОЙ УСТАНОВКИ ПЕРЕМЕННОЙ ЧАСТОТЫ ВРАЩЕНИЯ НА БАЗЕ АКТИВНОГО ВЫПРЯМИТЕЛЯ НАПРЯЖЕНИЯ

The variants of transformerless power topologies of ship diesel generator sets of variable rotational speed are considered. A power circuit of a diesel generator set of variable rotational speed on the basis of an active voltage rectifier and a buffer energy store, which has the highest efficiency in terms of weight and size and performance, is proposed. A block diagram of a control system for semiconductor converters as part of a diesel generator set of variable rotational speed is presented. The control system of the active voltage rectifier is implemented upon the principle of regulation according to the position of the generalized voltage vector of the network. In order to increase the energy efficiency of the inverter diesel generator set there has been developed the algorithm for smooth control of the rotational speed of the internal combustion engine using setting mechanism of suspend mode. The proposed algorithm (gradient descent method) is borrowed from the neural network technologies and allows calculating the rotation frequency of the internal combustion engine shaft corresponding to the minimum specific fuel consumption when operating at the current fractional load. In MatLab Simulink computer environment a simulation model of a variable-frequency diesel generator set based on active voltage rectifier was developed and transients were studied. The results of simulation modeling of the dynamic modes of the load switching have been presented, taking into account its size and nature; the indicators of the specific fuel consumption of the internal combustion engine as part of a diesel generator set of variable rotational speed are presented, as well. Using a variable-speed diesel generator set based on an active voltage rectifier has been proved to allow ensuring the required quality indicators of the generated electricity when the internal combustion engine is operating at lower energy-efficient rotation frequencies in the fractional load mode.

Common Mode Voltage Reduction in a Single-Phase Quasi Z-Source Inverter for Transformerless Grid-Connected Solar PV Applications

The quasi-Z-source inverter (qZSI) is becoming a popular inverter topology that can buck or boost input voltage without a dc–dc converter and hence can be used in transformerless configuration. Due to its single-stage conversion, the qZSI can be used as an efficient transformerless grid-tie inverter. However, the common mode current is a major problem in transformerless topologies due to the absence of galvanic isolation. This paper proposes a modified pulsewidth modulation (PWM) technique to control the qZSI, along with two extra semiconductor switches, to reduce the common mode current. The proposed method offers an efficient solution for grid integration of solar photovoltaic systems. The proposed topology has the following features: 1) uses phase-leg shoot-through for boosting the dc voltage to the required level which eliminates the additional dc–dc converter; 2) eliminates the PWM deadtime and provides freewheeling through additionally connected switches; 3) minimizes the common mode current by modifying the PWM and adding additional switches at the output side of inverter; and 4) avoids the conduction of body diode of H-bridge which has poor reverse recovery characteristics. The proposed topology can efficiently control the reactive power, and the suitable PWM scheme is also reported. Simulation results have been performed with the Standard Test conditions of the photovoltaic panel. Experimental results for a single-phase 500-W prototype are presented to validate the proposed PWM scheme for the qZSI topology.

Effects of modulation techniques on leakage ground currents in a grid‐tied transformerless HB‐NPC inverter

In this paper, a comparative study of modulation techniques used in a transformerless H-bridge neutral point clamped (HB-NPC) multilevel inverters is presented. Transformerless inverters, usually found in photovoltaic (PV) applications, are more efficient and economically affordable than isolated systems. Nevertheless, they may cause leakage ground currents. The latter are produced by the stray capacitances formed between the PV panel frame and the ground path. There are different ways to overcome the leakage ground current issue in transformerless topologies, for instance using topologies that generate almost zero leakage currents, by using adequate modulation techniques, by inserting additional filters, and by using adequate control schemes, among others. The comparative study, considers five modulation strategies, namely, level-shifted pulse-width modulation (PWM), phase-shifted PWM, three-level PWM, two sectors hybrid PWM and six sectors hybrid PWM. As it will be demonstrated through the common-mode model analysis, the three last modulations strategies are aimed at alleviating the leakage ground current issue. In this study, the two sectors hybrid PWM and the six sectors hybrid PWM are proposed. These modulation techniques lead to obtaining five output voltage levels keeping the RMS value of the leakage currents bounded for the transformerless HB-NPC inverter.

A Novel Three-Phase Transformerless H-8 Topology With Reduced Leakage Current for Grid-Tied Solar PV Applications

In this paper, a novel transformerless topology is proposed for three-phase grid-connected applications. The proposed topology has eight switches, and it combines the advantages of both dc-bypass and ac-bypass circuit configurations. The modulation strategy for the proposed topology is based on sinusoidal pulsewidth modulation followed by dedicated logic functions enabling control over the full range of modulation index. Logic functions are responsible for producing gate pulses for the eight switches, which are controlled in such a way that the variation in common-mode voltage (CMV) during inverter operation is low. Low variation in CMV throughout the operation results in reduced leakage current. The topology is capable of limiting the leakage current as per the German standard DIN VDE 0126-1-1. The output voltage of the proposed topology is unipolar, which results in low stress on the output filter inductors and low THD. There are two additional switches in the proposed topology compared to the conventional three-phase topology, but due to the special modulation strategy used , the drop in efficiency is insignificant. Theoretical analysis, simulation, and experimental results are presented to validate the concept of the proposed topology. Results show low variation in CMV and low THD in the output grid current.