Transformerless Photovoltaic Systems Research Articles

A Transformerless Photovoltaic System Employing a Cubic Cell Boost Converter for Low Voltage Application

A photovoltaic (PV) system is commonly used to satisfy the power demand of the utility. This paper investigates the utilization of a high voltage gain Cubic Cell Boost Converter (CCBC) in a transformerless photovoltaic (PV) system for low voltage applications. The proposed system is a combination of a CCBC and a three-phase inverter creating a two-stage three-phase PV system. The CCBC is linked with the low-scale PV sources to step up its input DC voltage and maintain a constant DC bus voltage by employing the Perturb and Observe (P&O) Maximum Power Point Tracking (MPPT) control algorithm. The DC bus is further connected to an LCL filter based three-phase inverter using a synchronous reference frame (dq) control scheme to power the three-phase utility. The proposed combination is validated by simulating in MATLAB/ SIMULINK environment for a 5.5 kW PV source and achieved an efficiency of 91% with 1.91% of inverter Total Harmonic Distortion (THD) at unity power factor. In the proposed system, CCBC with P&O MPPT control provides a high step-up DC bus voltage with less input current ripple and output voltage ripple and maintains a constant DC bus under variable solar irradiance. This work does not account for the common leakage current that is often produced in a transformerless PV system.

A five‐level common‐ground inverter with step‐up/step‐down dual‐mode operation for transformerless grid‐connected PV application

AbstractStep‐up multilevel inverters with common‐ground feature are attractive for transformerless photovoltaic systems. However, their performance deteriorates at step‐down voltage range. Considering a five‐level inverter with double voltage gain, the number of output voltage levels decreases from 5 to 3 for a modulation index smaller than 0.5, declining the quality of the output currents. This paper proposes a new dual‐mode five‐level common‐grounded inverter with a reduced number of switches. The proposed topology has the ability to operate as step‐up or step‐down making it well suited for application with wide input voltage range. The proposed inverter consists of five unidirectional switches, one bidirectional switch, two diodes, and two capacitors. In addition, the description of the topology, the design guidelines, and a comparison among the main topologies are given in detail. Experimental results are obtained to attest the practicability of the proposed solution.

Common-Mode Circuit Analysis of Current-Source Photovoltaic Inverter for Leakage Current and EMI

Leakage current and Electro-Magnetic Interference (EMI) are closely related to the common-mode (CM) circuit in transformer-less photovoltaic (PV) inverter systems. However, the correlation between them is elusive, as they are always studied independently because of the different frequency bands involved. This paper establishes the CM circuit models of the current source inverter (CSI), and the inherent relationship and the affecting factors between leakage current and CM EMI are revealed. Based on the established models, the leakage current and conducted EMI are accurately predicted with reduced computational burden. In addition, some suppression measures for leakage current and CM EMI are derived, including the effectiveness of connecting neutral wire. Finally, by simulations and experiments, the correctness of the theoretical analysis is verified.

Model-Free Predictive Current Control of a 3-φ Grid-Connected Neutral-Point-Clamped Transformerless Inverter

Utility grid-tied photovoltaic (PV) installations are becoming a typical component of the current electrical energy grid. The adoption of transformerless inverters has recently changed the topology of these systems. Despite being small, inexpensive, and effective, transformerless inverters have a recurring leakage current issue. Numerous studies are being conducted to improve its performance and bring the leakage current down to acceptable levels. The studies propose three tracks for addressing the leakage current problem of transformerless PV systems: the control technique, the inverter modulation, and the inverter topology. This study applies the model-free predictive control (MFPC) technique to a grid-connected NPC 3-φ transformerless converter powered by a PV panel. An LCL filter connects the transformerless inverter to the grid. The system model considers the grid filter components and the internal impedance of the utility grid. The proposed system’s discrete model is established before describing the MFPC controller’s algorithm. The suggested system is simulated in MATLAB using the MFPC and a standard PI current controller with SVPWM modulation. According to the simulation’s findings, the MFPC controller performs best regarding current spectrum, THD, and earth leakage current. Additionally, MFPC-based systems are more efficient than those that use PI controllers.

HERIC-Based Cascaded H-Bridge Inverter for Leakage Current Suppression in PV Systems

Leakage current can flow through the transformerless photovoltaic (PV) systems because of no galvanic isolation. This undesirable current results in more losses, electromagnetic interferences, and total harmonic distortion. This article presents a hardware-based method to suppress the leakage current in transformerless cascaded H-bridge (CHB) based PV systems. The proposed scheme uses high-efficient and reliable inverter concept in CHB converters for the first time. Using this idea and proper utilization of ac-side filters leads to leakage current suppression in individual cells and, thus, the whole system. In contrast to the most existing solutions, the proposed technique can be easily extended to CHB inverters with an arbitrary number of voltage levels. Moreover, this solution is applicable in the system's conditions with unequal parasitic capacitances and unequal dc-link voltages of H-bridge cells. The validity of the proposed solution is verified by several simulations in the PSCAD/EMTDC environment. Then, the experimental results on a five-level CHB inverter are presented to confirm the theoretical and simulation results.

Analysis and Design of H5 Topology in Grid-Connected Single-Phase Transformerless Photovoltaic Inverter System

Objectives: The transformerless inverters are more efficient, less bulky and cost-effective. But the system faces the problem of leakage current on account of the elimination of isolation transformers. Thus, the problem needs to be mitigated through the inverter topology itself. The objective of this paper is to study and analyze a topology designed for reducing the leakage current. The H5 inverter significantly reduces the leakage current by checking the variation of common mode voltages. The topology uses only one extra switch apart from the conventional full bridge and is simple to design. Method: To reduce the leakage current H5 inverter topology has been designed which works on the basis of decoupling. This work is to be implemented in MATLAB/SIMULINK. The main parameters for analysis are efficiency, leakage current, common mode voltage and their variation with load. H4 and H5 topology will be simulated and results will be compared. Findings: The H5 topology has been found to be effective in suppressing the leakage current to a fairly acceptable magnitude. It has been observed that the H5 topology has a lower leakage current than the conventional H4 topology, i.e. 23mA as opposed to 402mA in H4 topology. But the leakage current increases to up to 5 times if the system is used at load less than the full load. Novelty: The study presents the H5 inverter topology and its SPWM modulation strategy. In order to verify the topology, simulation results are provided. The performance has been tested for different loads connected to grid. The leakage current has been found to be reduced to 23.37 mA but it rises to about 100mA if used at 10% of the full load. Keywords: PV Inverters; H5 Inverters; Leakage Current (Common Mode Current); Common-Mode Voltages; Transformerless Power Converters

A control design for a photovoltaic transformerless HB-NPC inverter with leakage-ground current reduction

A control design for an HB-NPC transformerless inverter is presented in this paper, the proposal includes a modified LCL passive output filter which is aimed to attenuate leakage-ground currents in a transformerless photovoltaic generation system. This modification consists of a simple connection of the filter capacitors to the positive and negative nodes of the DC-link. A theoretical analysis of the differential-mode and common-mode models is performed. Thus, the proposed control law is designed in these new variables of the system. Therefore, the proposed control law consists of two main loops, an inner control loop which is based on the differential-mode model, and a DC-link voltage balance control loop based on the common-mode model. The current tracking loop features a proportional gain plus a bank of resonant filters to enhance the tracking on the differential-mode current reference. The common-mode voltage loop features a PI controller aimed to guarantee the DC-link common-mode voltage regulation objective. To assess the performance of the proposed solution a 1 kW experimental setup of the H-bridge neutral-point clamped transformerless inverter is used with the proposed LCL filter and controller.

High‐frequency common‐mode voltage mitigation for current‐source inverter in transformerless photovoltaic system using active zero‐state space vector modulation

Space vector modulation (SVM) schemes with leakage current mitigation capability for current-source inverters (CSIs) in transformerless photovoltaic (PV) systems have not been well explored yet. In this study, the advantage of a special SVM scheme, called active zero-state SVM (AZS-SVM), on suppressing high-order harmonics in the common-mode voltage (CMV) is revealed and theoretically analysed, since the high-frequency components in the CMV have decisive impacts on the generated leakage current. A five-segment sequence is proposed for AZS-SVM applied for CSIs. Both simulation and experiments are conducted on a (2.5 kW/208 V/6.94 A) grid-connected three-phase CSI, which validates the effectiveness of the designed AZS-SVM to mitigate the CMV high-order harmonics when compared to that by conventional SVM under different operating conditions.

Artificial neural network based DC-DC converter for grid connected transformerless PV system

The transformerless photo voltaic (PV) inverter system connected to grid has created a new trend in the energy market due to its reduced space requirement, low cost and increased efficiency when compared to its counterpart i.e with transformer. Transformerless inverter system suffers from common mode leakage currents due to parasitic capacitances between PV panels and ground. However, different new inverter topologies and state of the art modulation strategies are proposed in the literature to counter it. A dc-dc converter is of more significant to maintain the constant PV output voltage at string level and extract maximum power from PV. This paper presents Artificial neural network (ANN) algorithm-based dc-dc converter to track maximum power from PV module connected to grid without transformer. It also compares the performance of ANN based algorithm with conventional perturb and Observe maximum power point tracking (MPPT) technique. MATLAB/Simulink environment is used to pursue the simulation of ANN based algorithm and analyses its performance for variety of irradiance levels.

Enhanced Power Quality PV Inverter With Leakage Current Suppression for Three-Phase SECS

This article presents an enhanced power quality solar photovoltaic (PV) inverter enabling common-mode leakage current elimination. A three-phase transformerless solar energy conversion system is considered here, which, along with peak active-power production from PV array, ensures different power quality improvement capabilities such as grid current harmonics mitigation, grid currents balancing while also offering the grid reactive power support. Unlike conventional power quality inverters, this strategy is robust with respect to abnormalities in grid voltages at far radial ends, and does not compromise with the leakage currents caused by parasitic capacitance of PV array with ground. Common practice in the PV inverter power quality control is to neglect the PV leakage currents; however, they considerably affect the system performance by deteriorating the power quality and causing the safety issues of operating personnel. The standards VDE-00126 and NB/T-32004, therefore, compel the transformerless PV systems to operate with leakage current under 300 mA range. Various simulation and test results show the satisfactory performance of the presented strategy, even under various grid-side abnormalities. The comparative analysis with the state-of-the-art techniques shows the effectiveness of the strategy. Under all test conditions, the harmonics in grid currents are observed within limits as per the IEEE-519 and IEC-61727 standards, whereas the PV leakage currents are maintained well within the range recommended by VDE-00126 standard.

Coupled Inductor Single-Stage Boost Three-Phase Inverter is used to Create a Transformer Less Grid-Connected Photovoltaic System

This letter presents a modulation technique for the modified coupled-inductor single-stage boost inverter (CL-SSBI)- based grid-connected photovoltaic (PV) system. This technique can reduce the system leakage current in a great deal and can meet the VDE0126-1-1 standard. To maintain the advantages of the impedance network, only a diode is added in the front of the original topology, to block the leakage current loop during the active vectors and open-zero vectors. On the other hand, the near- state pulse width modulation (NSPWM) technique is applied with one-leg shoot-through zero vectors in order to reduce the leakage current through the conduction path in the duration of changing from and to open-zero vectors. Simultaneously, the leakage current caused by other transitions can also be reduced due to the fact that the magnitude of common-mode voltages is reduced. Simulation results of the transformerless PV system are presented in two cases: modified CL-SSBI modulated by maximum constant boost (MCB) control method and NSPWM. Experimental results for both CL- SSBI topology modulated by the MCB control method and modified CL-SSBI topology modulated by NSPWM are also obtained to verify the accurateness of theoretical and simulation models.

Fault detection technique for modified cascaded half-bridge multi-level inverter with polarity changer in pv grid system

Multilevel Inverters (MLI) are a viable option for a filter-less and transformerless photovoltaic system for direct grid integration, reducing losses, space and cost provided the issues of control and reliability are resolved. A modified cascaded half-bridge MLI with polarity changer is proposed with a reduced number of switches, thereby reducing control complexity. The proposed converter is designed for both seven-level and nine-level topologies. Selective harmonic elimination has been adopted to switch converter and the transcendental equations are solved by the gravitational search algorithm. The THD for nine-level configuration is 7.94% and 5.86% with MPPT and DC source inputs, respectively. The analysis on the open-circuit fault of switches confirms the presence of only five critical switches irrespective of the number of increase in levels, thereby requiring only five redundant switches at the Polarity Changer. The output voltage waveform is subjected to multiresolution analysis for feature extraction of voltages under various irradiance and temperature conditions. A conditions based fault detection algorithm is developed based on the observations of energies of signals to detect the open circuit fault in switches. Based on the comparative analysis, the proposed converter has fewer controlled switches for nine levels or higher configurations than other topologies. Apart from this, the high reliability due to inherent fault isolation capability restricted to half-bridge compared to different MLI topologies shows its superiority. For a nine-level MLI proposed topology reduces the cost considering redundancies for FDI in range of 67–30% depending on the topology for comparision. The system is simulated using MATLAB –Simulink and further validated by experimental results.

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

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

Step‐Up seven‐level neutral‐point‐clamped inverter based topology for TL‐PVS

Single-phase transformerless inverters are widely installed in grid connected photovoltaic systems due to their outstanding advantages, namely, high efficiency, low cost and high power density. The lack of galvanic isolation leads to common-mode voltage (or current) issues, which are solved by means of modifications to topologies, control schemes and modulation strategies. This study proposes a novel multilevel inverter for single-phase transformerless photovoltaic systems. The topology is based on the neutral-point-clamped inverter, and is aimed to attenuate the leakage ground current. The latter is caused by the common-mode voltage fluctuations affecting the stray capacitances formed between the photovoltaic cells and the grounded frame. On the one hand, the proposed multilevel inverter can synthesise a seven levels output voltage, which reduces the total harmonic distortion. On the other hand, the magnitude of the output voltage can reach up to 1.5 times the DC input voltage, which eliminates the need of a voltage-boosting stage for grid interconnection. This study also presents three dedicated pulse-width modulation strategies, all of them based on the level-shifted multicarrier sinusoidal pulse-width modulation scheme. Numerical simulations and experimental tests in a 1 kW setup have been performed to evaluate the performance of the proposed topology

Neutral-Point Voltage Oscillation Mitigation Scheme for Transformerless Three-Level PV Inverter in LVRT Operation With Selective Space Vector Modulation

In transformerless photovoltaic (PV) generation system, the neutral-point (NP) voltage balancing and the ground current reduction of three-level inverter are carefully regulated using a space vector modulation (SVM) scheme. Nevertheless, in low-voltage ride-through (LVRT) operation, the simultaneous reduction of NP voltage oscillation and common-mode voltage (CMV) has not been researched yet. To overcome these issues, a selective SVM (SSVM) strategy is proposed in this article. First, the NP voltage oscillation of three-level inverter in LVRT operation is analyzed in detail, which reveals that the NP voltage is affected by phase currents and duty cycles. Based on the location of output voltage and the direction of grid currents, the correct small vectors are selected under different power factor angles. Then, the small vector force (SVF) including the grid currents and duty cycle on NP voltage is proposed for the first time. According to the value of SVF, the optimal small vector having the most effective impact on NP voltage among the correct vectors and switching sequence is determined. With the proposed method, the NP voltage oscillation and CMV are reduced simultaneously in LVRT operation. Finally, the effectiveness of the proposed method is verified with experimental results.

Three-Phase Split-Source Inverter-Fed PV Systems: Analysis and Mitigation of Common-Mode Voltage

Common-mode voltage (CMV), caused by standard high-frequency pulsewidth modulated (PWM) voltage source inverters (VSIs), is a major area of interest in various applications, such as electric motor drives, transformerless photovoltaic systems, and grid-tied inverters. Since VSIs have only the buck capability during the inversion mode, an additional boosting stage either in the dc side or in the ac side has to be utilized for low-voltage-fed applications. On the other hand, single-stage solutions, with buck and boost capabilities, represent an interesting alternative with reduced cost, complexity, and converter volume. Among the possible single-stage solutions, split-source inverter (SSI), which has been recently proposed, has several merits over its counterparts. One of the questionable aspects concerning this topology is the CMV, which has not been investigated yet. This article examines the induced CMV with the SSI. This is done by examining the CMV waveforms for VSI and SSI topologies, and then, comparing each other under different PWM schemes. Furthermore, this article proposes the use of the discontinuous space vector PWM scheme with the SSI to mitigate the CMV, where the analysis shows a significant reduction in the CMV compared to the other schemes. In order to validate the introduced analysis, simulation model has been designed using MATLAB/Simulink environment, and simulation results are presented. Moreover, a prototype has been built-up for testing the proposed concepts and the obtained experimental results are provided.

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.

Modulation Techniques for a Modified Three-Phase Quasi-Switched Boost Inverter With Common-Mode Voltage Reduction

In a single-stage buck-boost quasi-switched boost inverter (qSBI), the shoot-through state insertion causes high amplitude common-mode voltage. Consequently, the qSBI becomes less attractive in transformerless photovoltaic (PV) systems. In this paper, a novel space vector pulse-width modulation for a modified qSBI is introduced to reduce the magnitude of common-mode voltage and push the modulation index up to 1. By properly choosing the shoot-through interval time, shoot-through states are considered to be inserted for boosting voltage and also reducing the THD value of the output voltage. The mathematical analysis and operating principles of the converter are discussed and verified through PSIM simulations. Finally, an experimental prototype is validated based on a TMS320F28335 DSP microcontroller and a DE0-Nano FPGA digital control platform.

Leakage Current Mitigation in Transformer-less Photovoltaic Inverter Systems for Mining Equipment

Challenging task of leakage current in photovoltaic converter is dealt for the application of portable mining electrical equipment. Inductive load and earth leakage causes distortion in the performance of the equipment in the transformer-less photovoltaic inverter systems. This leakage current introduce electromagnetic interference with neighboring circuits. The basic six device B6 type voltage source inverter is widely used for photovoltaic inverter systems, but leakage current is not addressed. H7 and H8 with additional switching device topologies reduces the leakage current problem and control strategy detects zero voltage states for reduction in switching losses. The proposed concept minimizes the leakage current due to inductance of RL loads, in general and specifically for mining equipment. Simulations study was carried out using MATLAB simulation software. Experimental were also carried out in real time, results showed that, reduced leakage current and total harmonic distortion in the output voltage and current waveforms. The elimination of the transformer and loading effects were analyzed for mining equipment having 1 kW Photovoltaic inverter system.

Evaluation of Three-Phase Transformerless DC-Bypass PV Inverters for Leakage Current Reduction

In transformerless photovoltaic (PV) systems, the leakage current reduction is one of the most important issues. Many interesting single-phase dc-bypass transformerless PV inverters have been proposed for the leakage current reduction, and there is a well-known conclusion that the clamping topology is better than the unclamping one in terms of leakage current reduction. However, is this conclusion correct for three-phase dc-bypass ones? The main objective of this article is to investigate the impact of clamping and unclamping dc-bypass switches on the leakage current reduction capability of three-phase transformerless inverters. The findings reveal that the conclusion is different from the single-phase dc-bypass ones. The theoretical analysis and experimental results are provided to verify the new insights.