Samuel Vasconcelos Araujo

Highly Efficient Single-Phase Transformerless Inverters for Grid-Connected Photovoltaic Systems

Driven by worldwide demand for renewable sources, the photovoltaic market saw in the last years a considerable amount of innovations regarding the construction and operation of inverters connected to the grid. One significant advance, among some that will be here discussed is, for example, the abolition of the galvanic isolation in inverters installed in Germany. There, transformerless topologies, like the H5 and Heric, can reach very high levels of efficiency and allow the best cost-benefit ratio for low-power grid-tied systems. This paper will follow this direction and propose a single-phase transformerless inverter circuit being composed of the association of two step-down converters. Each one modulates a half-wave of the output current, as the correct polarity of the connection to the grid is provided by low-frequency switches. Due to its straightforward design, reduced amount of semiconductors, and simple operation, it is possible to achieve a high level of efficiency and reliability. These and some other characteristics will be benchmarked against other existing circuits, being followed by a theoretical analysis on the properties of the proposed solution. The project of a laboratory prototype will be presented, along with a discussion about the obtained experimental results.

Highly Efficient High Step-Up Converter for Fuel-Cell Power Processing Based on Three-State Commutation Cell

The interest toward the application of fuel cells is increasing in the last years mainly due to the possibility of highly efficient decentralized clean energy generation. The output voltage of fuel-cell stacks is generally below 50 V. Consequently, low-power applications with high output voltage require a high gain for proper operation. Several solutions were so far proposed in the literature, ranging from the use of high-frequency transformers to capacitive multipliers. This paper proposes the modification of a boost converter operating with a three-state commutation cell that is already well suited for high current stress in the input due to the current sharing between the active switches. Here, an additional winding is added to the autotransformer to provide not only the required high gain but also to significantly reduce the voltage stress across the active switches. Moreover, by employing the three-state switching cell, the size of the inductor is reduced because the operating frequency is double of the switching frequency. A prototype for the verification of the circuit was built for a 30-45-V input-voltage range, 400-V output voltage, and 250-W output power. The operation is evaluated, and the experimental waveforms and efficiency curves are presented.

LCL filter design for grid-connected NPC inverters in offshore wind turbines

This paper deals with the analysis, design and optimization of a LCL filter topology to connect a 7MW NPC inverter to the grid. Following the requirements based on the IEEE 519-1992 recommendation and the German Guideline VDEW, simulation results were evaluated in order to access the performance of the proposed filter and the quality of the current injected into the grid.

Renewable Energy Operation and Conversion Schemes: A Summary of Discussions During the Seminar on Renewable Energy Systems

A short summary of some speeches given during Seminar on Renewable Energy system (SERENE) is presented. The contributions have been mainly focused on power electronics for photovoltaic (PV) and sea wave energies, pointing out some aspects related to efficiency, reliability, and grid integration. Finally, main issues concerning fuel cell (FC) systems as generators based on hydrogen as a low environmental impact energy vector are presented.

Comparative Evaluation of Three-Phase Current Source Inverters for Grid Interfacing of Distributed and Renewable Energy Systems

Despite the rapid growth of self-commutated inverter technology, the well-known current source inverter (CSI) has not achieved significant practical application so far. This raised the question regarding the fundamental criteria that could either prevent or promote the use of such inverters, especially considering emerging fields of application such as grid integration of renewable sources. This paper will, thus, focus on a systematic comparison between circuits based on the CSI and voltage source inverter (VSI). For such purpose, dimensionless benchmark factors were derived. The pulse width-modulated CSI with its inherent step-up capability alongside the indirect CSI (ICSI) with only two high-frequency switches features very interesting characteristics in terms of low switching losses and high inductor power density. However, both topologies show constraints with respect to the provision of reactive power. Two experimental setups have been implemented demonstrating the technical feasibility of different CSI solutions as a grid interface.

Novel grid-connected non-isolated converters for photovoltaic systems with grounded generator

In the photovoltaic branch, transformerless inverters are nowadays the majority of the systems installed in Europe, mainly because of the higher level of efficiency and reduced cost and weight when compared with their counterparts with transformer, whether this concerns low- frequency or high-frequency models. Nevertheless, some limitations arise for the operation of such systems, mainly regarding the fact that normally the panel output cannot be grounded, what is, for example not allowed in USA. Non- grounded operation of panels composed of some new cells technologies can bring undesired effects like cell aging and efficiency reduction. In addition, security matters regarding leakage current due to the panel parasitic capacitance are also a common problem in such systems. In order to address such matters, this paper proposes a novel DC-DC converter for the application in transformerless photovoltaic systems. As main characteristic, it allows the grounding of the negative output of the photovoltaic array and provides a bipolar output voltage. Following the proposition of the circuit, the analysis and evaluation will be performed. A prototype was built and the experimental results are presented in the end.

Analysis and proposition of a PV module integrated converter with high voltage gain capability in a non-isolated topology

Usually considered as one of the future solutions for grid connection of photovoltaic systems, module integrated converters were already the focus of several researches and projects. Most of the proposed approaches relied so far on the use of high frequency step-up transformers either in isolated operation or integrated in isolated dc-dc topologies. This paper analyses the possibility of using non-isolated topologies to achieve the necessary high-voltage gain for grid connection. Several circuits were analyzed and the best suited one for the current application was evaluated and optimized. Experimental results are presented in the final section.

Cosmic radiation ruggedness of Si and SiC power semiconductors

A measurement campaign to investigate the robustness of ten power semiconductor devices (Si as well as SiC) with voltage ratings between 650 V and 1.7 kV was conducted. Three of the tested devices were previously tested by other researchers. This allowed the results to be compared in order to show that the measurements can be reproduced. The results of the measurements show that the tested SiC MOSFETs are significantly more robust than Si MOSFETs and IGBTs. Further analysis of the results has shown that part of the robustness of the SiC devices is related to their high breakdown voltage. Yet, even if devices are compared and the breakdown voltage difference is taken into account the high robustness of most of the SiC devices remains visible.

Loss measurement of magnetic components under real application conditions

Due to the increasing demand for highly efficient power electronic converters, a detailed design targeting each individual circuit component like magnetic components is getting more and more important. Several electric and calorimetric methods for the loss measurement of magnetic components are known from literature. Nevertheless, only the last approach can be used to properly perform measurements under real application conditions, i.e. when the magnetic component is driven with high frequency rectangular voltages. As a common drawback, calorimetric methods usually require long measurement periods. This paper will present a calorimetric measurement method to reduce the measurement time, thus enabling the realisation of large test series in order to obtain extended core loss data of magnetic materials. Furthermore, a method to extract core losses of magnetic materials with the help of calorimetric setups will be presented. A mathematical approach for the multidimensional fitting of measurement data will be shown in order to allow the modelling of the specific core losses regarding the multitude of influencing parameters (e.g. induction, frequency, DC bias, temperature, duty cycle). It will be shown that a core loss model based on data obtained by measurement under real application conditions can lead to superior accuracy.

A novel 5-level hybrid rectifier using bypass concept for electric vehicle onboard battery charging

This paper introduces a novel 5-level three-phase PFC rectifier, appropriate for electric vehicle (EV) onboard battery charging systems, using an approach known as bypass concept. In contrast to the standard approach, where two power stages always work in series (PFC + step-down), such serial operation takes place here only during a brief interval of time. The characteristics of the proposed system, including the principle of operation, modulation strategy, dimensioning equations and calculated losses, are described in detail. Finally, the feasibility of the 5-level hybrid rectifier with bypass function is demonstrated by means of a 22 kW hardware prototype.