Davide Barater

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.

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.

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.

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.

Performance Evaluation of a Three-Level ANPC Photovoltaic Grid-Connected Inverter With 650-V SiC Devices and Optimized PWM

Photovoltaic (PV) energy conversion has been on the spotlight of scientific research on renewable energy for several years. In recent years, the bulk of the research on PV has focused on transformerless grid-connected inverters, more efficient than traditional line transformer-based ones, but more critical from a power quality point of view, especially in terms of ground leakage current. Neutral-point-clamped (NPC) inverters have recently gained interest due to their intrinsically low ground leakage current and high efficiency, especially for MOSFET-based topologies. This paper presents an active NPC (ANPC) topology equipped with 650-V silicon carbide (SiC) MOSFETs, with a new modulation strategy that allows to reap the benefits of the wide-bandgap devices. An efficiency improvement is obtained due to the parallel operation of two devices during the freewheeling intervals. Simulations and experimental results confirm the effectiveness of the proposed converter.

Performance analysis of SiC MOSFET based 3-level ANPC grid-connected inverter with novel modulation scheme

Performance analysis of three-level active neutral point clamped (ANPC) inverter with 650V SiC MOSFETs by ROHM is presented with a new switching pattern that utilises the active rectification capability of SiC devices. Performance analysis of the converter with 700V DC link and 230Vrms grid voltage are presented for different switching frequency, device case temperature and load conditions. The switching frequency is varied from 10kHz to 80kHz at four different output power and four different heat sink temperature conditions. The experimental results show that the converter can maintain high efficiency under wide load, frequency and heat sink temperature conditions. Robust performance of SiC devices can lead to reduction in passive component size, by utilizing high switching frequency and heat sink weight and volume by operating SiC at higher case temperature conditions.

Unipolar PWM for transformerless grid-connected converters in 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 topology with no 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 leakage ground current could arise due to parasitic PV panel capacitance. This leakage current causes electric safety problems, an increase of EMI and a reduction of the power quality. This paper presents a converter topology based on unipolar PWM modulation able to minimize the leakage ground current locking the common output voltage to VDC/2 as the converters topology based on bipolar modulation do. Simulation results show the effectiveness of the proposed topology. Eventually a comparison of the power losses of the proposed architecture with respect to some established topologies is presented.

Review of Oscillating Water Column Converters

Ocean waves are a huge largely unexploited energy resource, and the potential for extracting energy from waves is great. Research in this area is driven by the need to meet renewable-energy targets, but it is relatively immature compared to other renewable-energy technologies. This review introduces some device types that represent the state of the art of oscillating water column technology, a kind of wave energy converter (WEC). Unlike other works in literature, typically limited to specific aspects of WECs, in this paper, a system-wide perspective will be pursued, from the sea waves to the grid connection.

Diagnosis of incipient faults in PMSMs with coaxially insulated windings

Early detection of incipient faults in AC drives is one of the most difficult challenges for condition monitoring, especially for faults related to insulation degradation of the windings. This is because insulation degradation is very difficult to be detected on-line, so these faults may pass unnoticed before turning into a major faults. This paper looks at the feasibility of using coaxially insulated windings not only to increase reliability against winding insulation problems, but as an alternative to advance condition monitoring that may be suitable for the detection of insulation degradation, and other faults not easily detected by conventional schemes such as demagnetization issues, and the presence of inter-turn short circuits. The concept of insulation degradation for the case of coaxially insulated windings is analytically considered in this work and a novel condition monitoring alternative proposed. The proposed scheme and winding topology are extensively evaluated numerical to determine their feasibility to drive condition monitoring.