Transformerless Grid-connected Inverters Research Articles

A High-Voltage Gain Transformerless Grid-Connected Inverter

A High-Voltage Gain Transformerless Grid-Connected Inverter

A Zero-Voltage-Transition H5-Type Transformerless Photovoltaic Grid-Connected Inverter

Efficiency and leakage current are two major issues for transformerless grid-connected inverters (TLI). Soft-switching technologies can reduce switching losses and be applied in TLIs. Herein, a zero-voltage-transition H5 type (ZVT-H5) inverter with soft turn-on and turn-off transitions of high-frequency main switches is derived from basic resonant tanks. Compared with the hard-switching H5 (HS-H5) inverter, the conversion efficiency of ZVT-H5 is significantly improved. Moreover, the reverse recovery problem of freewheeling diodes is alleviated under the operation of the auxiliary resonant network. Meanwhile, a diode clamping branch is employed to achieve constant common-mode characteristic for the ZVT-H5. The construction process of the proposed topology is described, and the operation principle of the auxiliary resonant network is analyzed. Moreover, the resonant parameter design of ZVT-H5 and its circuit performance are discussed in detail. Finally, the experimental results of a 3-kW prototype at 100 kHz switching frequency are provided to verify the effectiveness of the ZVT-H5.

Transformer-Less Grid-Connected Photovoltaic Inverter

Transformer-Less Grid-Connected Photovoltaic Inverter

Transformerless Grid-Connected Inverters: Advancements, Challenges, and Future Prospects

Transformerless Grid-Connected Inverters: Advancements, Challenges, and Future Prospects

Transformer-less grid-connected 7-level inverter with model predictive control method suitable for PV application

Transformer-less grid-connected 7-level inverter with model predictive control method suitable for PV application

A Five-Level Switched-Capacitor Based Transformerless Inverter With Boosting Capability for Grid-Tied PV Applications

Transformerless grid-connected inverters have attained a lot of research interest in renewable energy interface applications, due to certain promising properties like greater efficiency, light weight, affordable price, and tolerable power density. Among various types of transformerless grid-tied photovoltaic (PV) inverters, multilevel inverters (MLIs) are mostly popular due to their ability to transmit reactive power, small filter size for reducing total harmonic distortion (THD) and their common-ground (CG) configuration to mitigate the detrimental leakage current due to the parasitic capacitances of the PV array. Again, among different types of MLIs for PV systems, switched-capacitor (SC) based multilevel inverter topologies are the burning topic in current decades due to their single source requirements for producing multilevel output voltage. However, for mostly used single-phase five-level inverters, most of the existing SC based topology requires at least two SCs for power conversion. In this paper, a five-level transformerless inverter based on a single SC is proposed, requiring only seven switches, no diode, a single capacitor, and one dc voltage source. The proposed transformerless MLI also has auto-boosting capability. Notably, the number of power switches operating at high frequency is limited to three, which lowers down the switching losses of the inverter. Rather than a new single SC based five-level transformerless inverter topology, a control scheme is also presented to inject a precisely regulated current into the grid that can govern both the active and reactive power support modes. In-depth comparisons between the proposed and cutting-edge MLIs are also provided. All these claims are validated through MATLAB/Simulink and PLECS computer simulation environments. A laboratory-scaled prototype is also built and tested to support the simulated claims further and validate the effectiveness and feasibility of the proposed five-level transformerless inverter topology.

HERIC-Clamped and PN-NPC Inverters With Five-Level Output Voltage and Reduced Grid-Interfaced Filter Size

Transformerless grid-connected inverters with highly efficient and reliable inverter concept (HERIC) and positive-negative neutral point-clamped (PN-NPC) circuit configurations exhibit excellent performance in terms of overall efficiency, common-mode voltage, and alleviated leakage current concern. These inverters are designed to generate only a three-level (3 L) output voltage waveform, thereby reducing the power density of a conversion system. On the other hand, it is well-known that increasing the inverter output voltage levels entails the reduction of the output filter size. In this regard, this article aims to develop a five-level (5 L) inverter output voltage for the improved versions of HERIC-clamped and original PN-NPC inverters. This is achieved with either phase-shifted or level-shifted pulse width modulation technique leading to further quality improvement of ac voltage and current waveforms through increasing the number of output voltage levels. Therefore, a much smaller output filter size can be utilized, while the overall efficiency of the entire system is enhanced. Two laboratory-built SiC-based prototypes for both the proposed HERIC-clamped (1.8 kW) and PN-NPC-5 L (2 kW) inverters have been fabricated to show the feasibility and effectiveness of the proposed solution via experiments.

Resonance Network Structuring Method for Zero-Voltage-Transition Transformerless Inverters

In this article, the soft-switching technique is an important way to achieve high frequency and high conversion efficiency for transformerless grid-connected inverters (TLIs) with distributed photovoltaic generation. Meanwhile, new technical standards require TLIs to have a wider power factor operation range to support grid voltage, which means that there are positive power regions and negative power regions in a grid period. In order to fulfill soft-switching for high-frequency working switches during all power regions, a pair of zero-voltage-transition (ZVT) resonant cells with different capacitor connections are first derived in this article, and two equivalent two-quadrant resonance networks are originated by using the pair of ZVT resonant cells alternatively. Furthermore, defined connection points of the obtained two-quadrant resonance networks in TLIs are explored, and a guideline to invent new ZVT-TLIs is summarized. As a result, two families of ZVT-TLIs with full power factor range (ZVT-TLIs-FPF) are obtained. In order further to verify the correctness of the proposed guideline, the obtained ZVT-HERIC-FPF is taken as an example to analyze its operation principle. Finally, experimental results from a 1 kW prototype at 100 kHz are provided to verify the effectiveness of the proposed structuring method.

A New Six-Level Transformer-Less Grid-Connected Solar Photovoltaic Inverter With Less Leakage Current

This paper presents a novel structure of the transformer-less grid-connected inverters. The proposed inverter is combined with six power switches and two power diodes which can generate six voltage levels at the output. Furthermore, the proposed inverter can overcome the leakage current issue in the photovoltaic (PV) system, which is the major problem in grid-tied PV applications. Additional significant features include-reduced filter size, lower total harmonic distortion (THD) of the injected current to the grid, and voltage boosting ability. Moreover, the proposed topology provides full reactive power support to the grid. A control strategy is designed and implemented to provide a voltage boost ability without using any additional dc-dc boost converter. Finally, the performance of the proposed inverter is validated by the 770 <i>W</i> laboratory prototype.

Control of Transformerless Inverter-Based Two-Stage Grid-Connected Photovoltaic System Using Adaptive-PI and Adaptive Sliding Mode Controllers

To enhance the move towards a sustainable society, the solar Photovoltaic (PV) industry and its applications are progressing at a rapid rate. However, the associated issues need to be addressed when connecting PV to the grid. Advanced and efficient controllers are required for the DC link to control the second harmonic ripple and current controllers to inject quality active and reactive power to the grid in the grid-connected PV system. In this paper, DC-link voltage, active power, and reactive power are successfully controlled in stationary reference using Adaptive-PI (A-PI) and Adaptive-Sliding Mode Controller (A-SMC) for a 3 kW single-phase two-stage transformerless grid-connected inverter. A Resonant Harmonic Compensator (RHC)-based Proportional Resonant (PR) controller is employed in the current-controlled loop. The magnitude, phase, and frequency information of the grid voltage are provided by Second-Order General Integral (SOGI)-based PLL that has harmonic immunity, fast-tracking accuracy, and a rapid-dynamic response. MATLAB®/Simulink®/Simscape R2017b were used for the test bench implementation. Two scenarios were considered: in the first case, the input PV power feedforward loop was avoided, while in second case, it was included. The feedforward loop of input PV power improved the overall system dynamics. The results show that the designed controller improves both the steady-state and dynamic performance as compared with a proper-regulated PI-controller. The proposed controllers are insensitive to active and reactive power variations, and are robust, stable, faster, and fault tolerant, as compared to controllers from prior studies.

Suppressing DC Current Injection in Transformerless Grid-Connected Inverter Using a Customized Current Sensor

Transformerless grid-connected inverters are widely applied due to their low cost and small volume. But the dc current injection (DCI) is one of the main problems that need to be solved. It is difficult to measure milliamperes dc current mixed in large (≥10 A peak) ac current, which causes it difficult to suppress the DCI using the ordinary current sensor (CS). To solve this problem, a novel CS is proposed. It uses a current transformer (CT), a power amplifier, and an additional winding to cancel the ac magnetic field in a commercial current sensor (CCS). Under this circumstance, the CCS is used to detect the remaining small dc magnetic fields. As the range of CCS does not need to be large, the dc current measurement error is significantly reduced. Meanwhile, the CT can measure the ac current and provide feedback to control the inverter. In this way, the proposed CS can replace the original CS instead of adding a new CS to the inverter, which relatively reduces the cost. The volume of CT is relatively small; hence, the CT can be integrated with the CCS, which achieves a relatively small volume of the sensor. A dc current suppression experiment using a 1.5-kVA transformerless grid-connected inverter verified the effectiveness of the proposed technique.

An Improved Zero-Voltage-Transition H6-Type Transformerless Grid-Connected Inverter with Reactive Power Capability

Soft-switching technique contributes to higher conversion efficiency and switching frequency for transformerless grid-connected inverters (TLIs), which is beneficial to improve power density and reduce cost. However, existing soft-switching TLIs can only operate with unity power factor; as a consequence, they could not supply reactive power required by newly technical standards. In order to solve this issue, a novel zero-voltage-transition (ZVT) scheme has been proposed in this article, and two resonant tanks with a compact structure are applied in H6-type topology. The derived ZVT inverter enables high-frequency switches of both positive and negative power regions to be turned on and off under zero-voltage conditions, as well as all auxiliary switches can be turned on with zero-current switching. Meanwhile, unipolar differential-mode voltage and constant common-mode voltage characteristics are also maintained. Finally, comprehensive experiment results from a 100-kHz/1-kW prototype are provided to verify the correctness of the proposed inverter.

Grid Connected Three-Phase Boost-Inverter for Solar PV Systems

This paper presents a transformerless grid-connected three-phase boost-type inverter derived from the Swiss Rectifier (SR) and can be used in solar systems. The proposed boost-inverter retains the great advantages of the Swiss rectifier. Since the number of switches that are instantly switched with high frequency is only two, the switching losses are very low, therefore the proposed boost-inverter is very advantageous in terms of efficiency. Without having large transformer and electrolytic bus capacitors, the boost-inverter has an advantage in terms of size and weight. The switching states of the topology are given in detail. The operation principle of the boost-inverter is described with the help of equivalent circuits. A cascaded control method including current and voltage loops is proposed. Simulation results are presented to validate the operation of the boost-inverter and show quality performance of the proposed control approach.

Single-Stage Doubly Grounded Transformerless PV Grid-Connected Inverter With Boost Function

Doubly grounded transformerless grid-connected inverters are widely used in the PV application because of no common mode leakage current. A boost converter should be added into the doubly grounded PV inverters under shading condition to have boost function. However, the inverter with boost capability is a two-stage system and all switches are operated at high frequency, which reduce efficiency of the system. In addition, processing power of the boost converter is high. Therefore, a single-stage doubly grounded transformerless PV grid-connected inverter with boost function is proposed. The proposed inverter consists of two boost and one buck converters. A cooperative control method among different periods is proposed to reduce processing power of the boost converter and simplify the two-stage system to a single-stage system in each period. In addition, only one switch is operated at high frequency in each period, so efficiency of the system can be improved. Operating principle of the proposed inverter is illustrated. Control strategy is designed. Filter design guidelines and example are given. Loss calculation of the proposed inverter is given. Simulation and experimental results of the proposed inverter verify the theoretical analysis. Finally, comparison among other inverters and the proposed inverter is provided.

A New Switched-Capacitor Five-Level Inverter Suitable for Transformerless Grid-Connected Applications

Transformerless grid-connected inverters have been extensively popular in renewable energy-based applications owing to some interesting features such as higher efficiency, reasonable cost, and acceptable power density. The major concern of such converters is the leakage current problem and also the step-down feature of the output voltage, which causes a costly operation for a single-stage energy conversion system. A new five-level transformerless inverter topology is presented in this study that is able to boost the value of the input voltage and can remove the leakage current problem through a common-ground architecture. Here, providing the five-level of the output voltage with only six power switches is facilitated through the series-parallel switching of a switched-capacitor module. Regarding this switching conversion, the self-voltage balancing of the integrated capacitors over a full cycle of the grid's frequency can be acquired. Additionally, to inject a tightly controlled current to the local grid, a peak current controller-based technique is employed, which can regulate both the active and reactive power support modes. Theoretical analyses besides some experimental results are also given to corroborate the correct performance of the proposed topology.

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.

A novel photovoltaic inverter topology based on common mode current suppression

ABSTRACT Aiming at the problems of large leakage current and low efficiency of existing transformerless single-phase grid-connected photovoltaic inverters, a novel transformerless single-phase grid-connected photovoltaic inverter topology is proposed. In this topology, two freewheeling diodes are added to the single-phase full-bridge inverter, and the filter inductors are moved between the upper and lower bridge arms, so that the common mode voltage remains constant during the switching period and the common mode current is effectively suppressed. Finally, through simulation and the experimental platform of 3kW single-phase grid-connected photovoltaic inverter, it is verified that compared with the traditional full-bridge topology, the novel topology can effectively suppress the common mode current, while compared with the H5 and H6 topology, the novel topology reduces the system loss and improves the system efficiency.

A Genetic-Algorithm-Based DC Current Minimization Scheme for Transformless Grid-Connected Photovoltaic Inverters

Transformerless grid-connected inverters are of great industrial value in photovoltaic power generation. However, the direct current (DC) induced into the inverter’s output degrades the power quality of the grid. Recently, a back-propagation neural work proportional–integral–derivative (BP-PID) scheme has proven helpful in solving this problem. However, this scheme can be improved by reducing the suppressing time and overshoot. A genetic algorithm (GA)-based DC current minimization scheme, namely the genetic-algorithm-based BP-PID (GA-BP-PID) scheme, was established in this study. In this scheme, GA was used off-line to optimize the initial weights within the BP neural network. Subsequently, the optimal weight was applied to the online DC current suppression process. Compared with the BP-PID scheme, the proposed scheme can reduce the suppressing time by 59% and restrain the overshoot. A prototype of the proposed scheme was implemented and tested on experimental hardware as a proof of concept. The results of the scheme were verified using a three-phase inverter experiment. The novel GA-PB-PID scheme proposed in this study was proven efficient in reducing the suppressing time and overshoot.

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

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

Analysis and Validation for an Inverter-side Current Controller in LCL Grid-connected Power Systems

Transformerless grid-connected inverters offer greater efficiencies when transferring power from renewable energy sources to the electrical grid. If the grid-inverter connection is done with an LCL filter, high attenuation of switching harmonics is achieved while preserving a small-size output filter. However, damping must be included in the controller to assure closed-loop stability. This paper proposes a reference computation methodology for the inverter-side current feedback in a photovoltaic (PV) generation system connected to the grid through an LCL filter. Theoretical analysis of the closed-loop system stability and of the steady-state performance are presented as well as experimental validation of the closed-loop performance. The feedback controller includes active damping and relies on a resonant control structure which improves the ability of dealing with grid harmonic distortion. The controller uses a reduced set of measurements, which requires the inverter-side current and grid voltage only, and assures a power factor close to unity.