Peter Zacharias

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.

A Single-Stage PV Module Integrated Converter Based on a Low-Power Current-Source Inverter

This paper presents a transformerless three-phase inverter designed for the integration into a special type of a photovoltaic (PV) module, which is capable of providing an output voltage of several hundred volts. The chosen topology, a current-source inverter, features a single-stage power conversion system that directly feeds into the grid. The principle operation and control is described, and a modified modulation strategy is proposed to attenuate common-mode currents. A robust and highly efficient laboratory prototype of a 250-W module integrated converter has been implemented and tested. Its compact and flat design allows the direct attachment to the PV module.

Multi-string-converter with reduced specific costs and enhanced functionality

Abstract The development of a PV-converter based on the advanced Multi-String concept results in significantly reduced specific costs while still profiting from the well-known advantages of the string-converter technology developed by ISET and SMA in the mid-1990s. The paper deals with the basic considerations from a system’s point of view that resulted in the development of the Multi-String-converter. The features of the Multi-String-converter concerning operational behaviour and PV-system design are described. Due to enhanced capacity of the operational control unit the Multi-String-converter can be used additionally for active compensation of harmonics and reactive power in order to improve the power quality. An evaluation of Multi-String-converters in comparison with string-converters or conventional central-converter concepts is presented. The selected topology and the control strategy for the Multi-String-converter are introduced.

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.

Development of a single-stage three-phase PV module integrated converter

A transformerless three-phase power conditioning unit interfacing with a high voltage (>200 V) photovoltaic module is presented. The chosen topology, a Current Source Inverter, features a single-stage power conversion system that feeds directly into the grid. A robust and highly efficient laboratory prototype of a Module Integrated Converter has been implemented and demonstrated by experimental results.

Experimental program of the pulsed corona tar cracker

We are concentrating on the development of pulsed corona discharges to crack heavy tar components (hydrocarbons) into lighter ones. The method has the advantage that it can operate at a high temperature, and be retrofitted to existing installations. The corona discharge is energized by 100-150 ns wide voltage pulses (100 kV) at a continuous repetition rate of 600-1000 pulses per second. The power dissipated by the corona discharges is 1.5 kW average and 50 MW peak in each pulse. To be cracked by discharges, the hydrocarbons of the tar mixture need to be gaseous and therefore, the corona reactor must operate at a high temperature. In the first phase of the experiments the reactor will run at a modest temperature of 150/spl deg/C. The reactor is a 1-3 m long stainless steel cylinder, 0.25 m diameter with a corona wire along the axis. The pressure will be 1 atmosphere.

A Novel Solid State Pulsed Power Module for Excimer aser

The design of solid state pulsed power modules for excimer lasers have been long-established over many years. The primary switch is normally a high power IGBT, a step up transformer, and two or three pulse compression stages. To overcome the problems of resetting and cooling of the pulse compressions stages, a novel design for a pulsed power module with extremely high repetition rates (10 kHz) and low energy (2 mJ) is proposed. The pump energy for the laser tube is switched by a stack of semiconductor switches without step up transformer or pulse compression. The principal design, simulation results, and first measurements are shown