Giampaolo Buticchi

A Nine-Level Grid-Connected Converter Topology for Single-Phase Transformerless PV Systems

This paper presents a single-phase transformerless grid-connected photovoltaic converter based on two cascaded full bridges with different dc-link voltages. The converter can synthesize up to nine voltage levels with a single dc bus, since one of the full bridges is supplied by a flying capacitor. The multilevel output reduces harmonic distortion and electromagnetic interference. A suitable switching strategy is employed to regulate the flying-capacitor voltage, improve the efficiency (most devices switch at the grid frequency), and minimize the common-mode leakage current with the help of a novel dedicated circuit (transient circuit). Simulations and experiments confirm the feasibility and good performance of the proposed converter.

Digital Control of Actual Grid-Connected Converters for Ground Leakage Current Reduction in PV Transformerless Systems

The design of a photovoltaic (PV) grid-connected converter usually comprehends a galvanic isolation between the grid and the photovoltaic panels. Recently, in low power systems, the galvanic isolation has been removed with the aim to increase efficiency and reduce the cost of the converter. Due to the presence of a parasitic capacitance between the photovoltaic cells and the metal frame of the PV panel, usually connected to earth, a high value of common mode current (i.e., ground leakage current) can arise. In order to limit the ground leakage current (which deteriorates the power quality and generates EMI), new converter topologies have been proposed. Their effectiveness is based on the symmetrical (ideal) commutations of the power switches and some of them adopt a further voltage level derived from a capacitive divider of the DC bus voltage. Unfortunately, in actual implementations, asymmetrical power switches transients and variations of this added voltage lead to higher ground leakage current with respect to the ideal case. After a review of the state of the art this paper investigates these two issues and presents a particular solution (based on digital control and PWM strategy) that, in conjunction with a compensation strategy of power switches actual commutations, guarantees low ground leakage current regardless the parameters tolerance of the power circuit. Simulation and experimental results confirm the effectiveness of the proposed solution.

A Five-Level Single-Phase Grid-Connected Converter for Renewable Distributed Systems

In low-power renewable systems, a single-phase grid-connected converter is usually adopted. This paper deals with a novel five-level converter topology that follows this trend. A review of the state of the art of the five-level topologies and a theoretical power loss comparison with the proposed solution is realized. The proposed converter architecture is based on a full-bridge topology with two additional power switches and two diodes connected to the midpoint of the dc link. Since the two added levels are obtained by the discharge of the two capacitors of the dc link, the balancing of the midpoint voltage is obtained with a specific pulse width modulation (PWM) strategy. Simulation and experimental results show the effectiveness of the proposed solution.

The Smart Transformer: Impact on the Electric Grid and Technology Challenges

The increasing proliferation of renewable energy resources and new sizeable loads like electric vehicle (EV) charging stations has posed many technical and operational challenges to distribution grids [1]. Encouraged by attractive tax incentives and promotion policies, local grid end consumers are becoming not only consumers of electricity but, in many cases, also producers. The actual electric distribution system limits the use of renewable energy resources, offers poor EV infrastructure, and is based on a unidirectional information flow from sources to control centers.

Active Common-Mode Filter for Ground Leakage Current Reduction in Grid-Connected PV Converters Operating With Arbitrary Power Factor

This paper proposes a solution for reducing the ground leakage current in transformerless single-phase grid-connected photovoltaic converters. This is obtained with the introduction of an active common-mode filter able to compensate for variations of the output common-mode voltage of the power converter. The active common-mode filter is applied to a widespread and efficient full-bridge driven by a three-level pulse width modulation, allowing the power converter to operate with low ground leakage current and with an arbitrary power factor. After showing the desired voltage waveform for common-mode voltage compensation, this paper presents the design guidelines for the needed additional magnetic component together with the power loss considerations for all the devices added for the proposed solution. Experimental results show the performance of the proposed solution in terms of ground leakage current reduction, effectiveness of dead-time compensation, total harmonic distortion of the injected grid current, and power losses.

A DC Offset Current Compensation Strategy in Transformerless Grid-Connected Power Converters

A line frequency transformer is usually employed in grid-connected power converters, from both renewable and traditional energy sources, in order to suppress the DC current component and the ground leakage current. Solutions employing a high frequency transformer or employing no transformer at all have recently been investigated in order to reduce size, weight and cost. As a consequence, unless a suitable remedy is adopted, a DC current component exceeding the limits enforced by international standards may be injected into the grid. This paper proposes a simple and cheap solution to reduce the DC current component injected into the grid in the case of a full-bridge, single-phase, transformerless converter. The proposed strategy is intrinsically insensitive to offset measurement errors and can be utilized as a robust and dynamic offset compensator for the current transducer. The simulation results have confirmed the theoretical behavior of the proposed solution, while the experimental ones, performed for different values of output power and for different current control architectures, have shown its effectiveness.

Unipolar PWM Strategy for Transformerless PV Grid-Connected Converters

In domestic grid-connected PV applications, a single-phase converter is usually employed. In such low-power plants, it is possible to adopt converter topologies without galvanic isolation between the photovoltaic (PV) panels and the grid. The absence of a high- or line-frequency transformer permits us to reduce power losses, cost, and size of the converter. On the other side, in the presence of a galvanic connection, a large ground leakage current could arise due to parasitic PV panel capacitance. Leakage currents cause electric safety problems, electromagnetic interference increase and, consequently, a reduction of the converter power quality. This paper presents a converter topology able to minimize the ground leakage current also in the case of unipolar pulsewidth modulation without increasing inductive common mode filter size and preserving efficiency. Simulations and experimental results show the feasibility of the proposed solution.

Active Filter for the Removal of the DC Current Component for Single-Phase Power Lines

Nonlinear loads and grid-connected converters can cause, in addition to the generation of several current harmonics in the grid current, a dc current component injection. A dc current component can cause the magnetic core saturation of distribution power transformers. Transformers operating under saturation conditions present increased power losses, overheating, and distorted current waveforms. Since a dc current component causes a small dc voltage component drop across the parasitic resistance of the distribution grid conductors, canceling the dc voltage component at the point of common coupling (PCC) implies the compensation of the dc current component injected/absorbed by electric loads or grid-connected converters connected at the same PCC. This paper proposes a low-cost nonlinear sensor for an accurate detection, free from offset problems, of the dc voltage component present in the grid voltage. The detection of the dc voltage component was used to realize an active filter of the dc current component. The proposed solution is outlined, and then the stability issue is addressed by means of a simplified model. Experimental results confirmed that the simplified model closely approximates the real system.

Review of active thermal and lifetime control techniques for power electronic modules

Lifetime of power electronics modules can be extended with passive methods (condition monitoring) and active ones. This paper intends to give an overview in the second category, namely active thermal control or lifetime control, offering a critical comparison based on a comprehensive reference list. Mission profiles are compared to evaluate the potential of the controllers.

A new proposal for ground leakage current reduction in transformerless grid-connected converters for photovoltaic plants

In domestic grid connected PV applications a single phase converter is usually used. In such a low power plants it is possible to adopt converter topologies without galvanic isolation between the photovoltaic panels and the grid. The absence of a high or low frequency transformer permits to reduce power losses, cost and size of the converter. On the other side in presence of a galvanic connection a large ground leakage current could arise due to parasitic PV panel capacitance. Leakage currents cause electric safety problems, EMI increase and, consequently, a reduction of the converter power quality. This paper presents a converter topology able to minimize the ground leakage current also in case of unipolar PWM modulation without increasing inductive common mode filter size and preserving efficiency. Simulations and experimental results show the feasibility of the proposed solution.