Common-mode Model Research Articles

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.

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.

Leakage-Ground Currents Compensation in a Transformerless HB-NPC Topology Using a DC-Link-Tied LC Filter for Photovoltaic Applications

In this article, a solution to mitigate the leakage-ground currents (LGCs) in a grid-tied H-bridge neutral-point-clamped inverter is presented. This solution consists in splitting the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter capacitor and connecting each capacitor to a dc-link terminal. The proposal is referred to as the dc-link-tied <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> output filter. Out of this hardware modification, it is not necessary to increase the number of switches nor design a new modulation scheme. Furthermore, a formal analysis is provided based on the common-mode (CM) model describing the role of this particular connection of the output <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter on the minimization of the LGC. In addition, a closed-loop control strategy has been implemented to allow testing the prototype connected to the electrical grid. The mitigation of the LGCs has been evaluated under this condition as well. To validate the proposed circuit modification, numerical simulations and experimental tests have been performed in a 1-kW laboratory prototype of the H-bridge neutral-point-clamped inverter with the dc-link-tied <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LC$ </tex-math></inline-formula> filter solution.

DC-Link Capacitors’ Voltage Balance in an HB-NPC Five-Level Grid-Tied Inverter via the Common-Mode Control Component

This paper presents a control method to balance, in average, the DC-link capacitor voltages while injecting power to the grid using an h-bridge neutral-point clamped five-level inverter grid-tied through an LCL filter. The method exploits the structure of a particular model representation of the system comprising a differential-mode part and a common-mode part, which turn out to be decoupled from one another. These decoupled representation enormously facilitates the control design as it can be performed as two independent processes as well. The first design process involves the differential-mode model part and focuses on the grid current injection, while the second design process focuses on the balance of the DC-link capacitors voltages, and is based on the common-mode model part. Out of this, two control components are derived, namely differential-mode and common-mode control components, which are later recombined to form the two control signals, one per inverter branch, required as references in a conventional modulation algorithm to generate the switching sequence. Numerical and experimental results are performed to evaluate the performance of the proposed controller for the h-bridge neutral-point clamped inverter subject to those interconnections.

A 5LCHB Inverter for PV Transformerless Applications With Reduced Leakage Ground Current

The transformerless inverters for photovoltaic systems are widely used as it features low cost, volume, and weight. Thus, in recent years, its study has been of great interest to the research community. In this paper a transformerless cascade multilevel inverter for photovoltaic applications with leakage ground current compensation capability is presented. The proposed solution involves a second-order LC output filter with a particular connection, which is referred to as the DC-link-tied LC output filter. This solution is aimed to deal with the leakage-ground current issue, regardless of the considered PWM strategy. The mathematical model of the system involving such a particular LC output passive filter configuration is presented, out of which, both the differential-mode and the common-mode models are obtained. These models are used to explain the leakage-ground current improvement of the proposed DC-link-tied LC output filter. This hardware solution is evaluated under different modulation schemes to contrast the converter output response and the leakage-ground current performance. Finally, simulation and experimental results are performed using a 1 kW academic prototype to assess the performance of the proposed DC-link-tied LC output filter used in a transformerless inverter application.

Building Common-Mode Analytical Model for Dual Active Bridge Incorporating With Different Modulation Strategies

The modulation strategies of dual active bridge (DAB) including single phase-shift (SPS), dual phase-shift (DPS), and triple phase-shift (TPS) have been proposed to optimize the efficiency, eliminate the reactive power, and release the current stress. While the previous literature focuses on the accurate loss model in steady states or small-signal analysis for dynamic response, the common-mode (CM) model has been largely ignored. The main contribution of this article then is to propose an analytical CM model for DAB, which considers the CM parasitic capacitors including those of grounded heatsink and transformer windings. Based on such model, the CM performance is compared among different modulation strategies. The impact of two H-bridge on the input and output CM voltage has been addressed. A prototype based on SiC devices is used to verify the proposed analytical mode under SPS, DPS, and TPS. It is found that while TPS improves the light-load efficiency it introduces the worst CM noise. Influence of the ZVS current setting on the CM performance is also discussed.

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 Fast Transient Model for Bearing Fault Analysis in Induction Machine Drives

In this paper, a common-mode and medium-range frequency model for an electrical drive, which includes an inverter, an electrical machine, and bearings, is proposed. The bearing model is a piecewise linear model that can model simultaneously most types of bearing current mechanisms. This common-mode model can be used in the stage of designing an electrical machine to evaluate the machine against the maximum permissible level of bearing voltage, which is stated in the standards, such as NEMA-MG1. Instead of using spectrum analysers, an impulse generator circuit, with voltage which is high enough, is presented for finding the impulse response of the drive in common mode. Then, an effective and low-cost method is suggested to identify bearing failures. In this method, the conditions of the bearing of an inverter-driven electrical machine can be monitored, and failures can be detected by analyzing the measured time waveforms of the voltages of bearing ends or the magnitude of the fast Fourier transform of ground currents near the frame, while neither the installation of complex mechanical measurement devices nor sophisticated spectral analysis techniques for finding correct frequencies is required. Finally, to validate the failure detection method, the numerical results of the proposed model with a bearing failure are compared with the experimental measurements.

ESI: A Novel Three-Phase Inverter With Leakage Current Attenuation for Transformerless PV Systems

Adding the auxiliary switches to the conventional H-bridge inverter is an effective way to eliminate the leakage current for transformerless PV systems, such as H5, H6, etc. Inspired by the newly developed embedded-switch H6 topology, a novel embedded-switch inverter (ESI) is proposed in this paper for three-phase transformerless PV systems. First, the operation principle and characteristics of the proposed ESI are analyzed. Second, the common-mode model of the three-phase ESI is established, based on which the main factors that affect the leakage current are discussed. The finding reveals that the switching states with conventional modulation strategy result in high-frequency common-mode voltage, which is the source of leakage current. In order to solve the problem, a new modulation strategy is presented for ESI to eliminate the high-frequency leakage current. Finally, the time-domain simulation and the experimental tests are carried out. The results verify the effectiveness of the proposed solution.

Leakage Current Suppression of Three-Phase Flying Capacitor PV Inverter With New Carrier Modulation and Logic Function

Flying capacitor photovoltaic (PV) inverters have been widely discussed in the literature. However, the relevant leakage current issues have not received much attention. In this paper, the modulation techniques for a transformerless three-phase flying capacitor PV inverter are investigated for the leakage current suppression. First, the theoretical analysis of the system common mode model and leakage current is presented. Second, three kinds of conventional carrier-based modulation techniques are evaluated, and results indicate that they fail to effectively suppress the leakage current. Third, a new carrier modulation and logic function is presented. It is able to maintain the constant common mode voltage, and consequently, the leakage current can be significantly reduced. Finally, the experimental tests are conducted on a transformerless three-phase flying capacitor PV inverter to verify the effectiveness of the proposed method.

Efficient Single Phase Transformerless Inverter for Grid-Tied PVG System With Reactive Power Control

There has been an increasing interest in transformerless inverter for grid-tied photovoltaic (PV) system due to low cost, high efficiency, light weight, etc. Therefore, many transformerless topologies have been proposed and verified with real power injection only. Recently, almost every international regulation has imposed that a definite amount of reactive power should be handled by the grid-tied PV inverter. According to the standard VDE-AR-N 4105, grid-tied PV inverter of power rating below 3.68KVA, should attain power factor (PF) from 0.95 leading to 0.95 lagging. In this paper, a new high efficiency transformerless topology is proposed for grid-tied PV system with reactive power control. The new topology structure and detail operation principle with reactive power flow is described. The high frequency common-mode (CM) model and the control of the proposed topology are analyzed. The inherent circuit structure of the proposed topology does not lead itself to the reverse recovery issues even when inject reactive power which allow utilizing MOSFET switches to boost the overall efficiency. The CM voltage is kept constant at mid-point of dc input voltage, results low leakage current. Finally, to validate the proposed topology, a 1 kW laboratory prototype is built and tested. The experimental results show that the proposed topology can inject reactive power into the utility grid without any additional current distortion and leakage current. The maximum efficiency and European efficiency of the proposed topology are measured and found to be 98.54% and 98.29%, respectively.

Common-Mode Filter Design for a Transformerless ZVS Full-Bridge Inverter

The zero voltage switching (ZVS) full-bridge inverter can achieve high efficiency using ZVS sinusoidal pulsewidth modulation (ZVS SPWM) and superjunction MOSFET. For further application in a photovoltaic (PV) system, the leakage current problem of a ZVS full-bridge inverter is investigated. First, the common-mode model of the ZVS full-bridge inverter is presented. The common-mode voltage with the ZVS SPWM is found changing with high frequency and leakage current with such frequency is generated. In order to suppress the leakage current below safety standards, the common-mode filter composed of common-mode inductor and bypass capacitor is applied. The impedance of the common-mode path is analyzed, and the parameters of a common-mode filter are designed based on the common-mode model. With a small common-mode inductor and bypass capacitors, the leakage current of the ZVS full-bridge inverter is remarkably reduced, and the method is verified by the experiment.

A Novel Inverter Topology for Single-Phase Transformerless PV System

Transformerless photovoltaic (PV) power system is very promising due to its low cost, small size, and high efficiency. One of its most important issues is how to prevent the common mode leakage current. In order to solve the problem, a new inverter is proposed in this paper. The system common mode model is established, and the four operation modes of the inverter are analyzed. It reveals that the common mode voltage can be kept constant, and consequently the leakage current can be suppressed. Finally, the experimental tests are conducted. The experimental results verify the effectiveness of the proposed solution.

Evaluation of Three-Phase Transformerless Photovoltaic Inverter Topologies

This paper analyzes and compares three transformerless photovoltaic inverter topologies for three-phase grid connection with the main focus on the safety issues that result from the lack of galvanic isolation. A common-mode model, valid at frequencies lower than 50 kHz, is adopted to study the leakage current paths. The model is validated by both simulation and experimental results. These will be used to compare the selected topologies, and to explain the influence of system unbalance and the neutral conductor inductance on the leakage current. It will be demonstrated that the later has a crucial influence. Finally, a comparison of the selected topologies is carried out, based on the adopted modulation, connection of the neutral and its inductance, effects of unbalance conditions, component ratings, output voltage levels, and filter size.

Schreiber approach for GPS carrier phase ambiguity resolution

Differenced GPS carrier phase observations are usually used in GPS positioning to eliminate the various common GPS errors. A different approach is proposed in this paper where no differencing of the observations is required. In this method, the common GPS errors are modelled explicidy and eliminated by introducing the Schreiber method, which is used in traditional geodesy to eliminate the azimuth unknowns in observation equations. The paper first introduces the Schreiber method, and then discusses its application to the various GPS common-mode error models. It is shown that if only the real ambiguity solution is concerned, the estimate of the position parameters from the Schreiber approach is the same as that from the differencing approach. However, the variance of the ambiguity estimate from the Schreiber approach is smaller than that from the differencing approach. Numerical examples are given to show that the results are reliable.