The authors discuss high-voltage power conversion. Conventional series connection and three-level voltage source inverter techniques are reviewed and compared. A novel versatile multilevel commutation cell is introduced: it is shown that this topology is safer and more simple to control, and delivers purer output waveforms. The authors show how this technique can be applied to either choppers or voltage-source inverters and generalized to any number of switches.<<ETX>>

Multi-level conversion : high voltage choppers and voltage-source inverters

The annual world photovoltaic (PV) cell/module production is growing at almost an exponential rate and has reached 1727 MW in 2005. Building integrated PV (BIPV) projects are emerging as the strongest part of the PV market and grid interactive inverters are a key component in determining the total system cost. Module integrated converter (MIC) technology has become a global trend in grid interactive PV applications and may assist in driving down the balance of system costs to secure an improved total system cost. This paper concentrates on the topology study of the PV MICs in the power range below 500 W and covers most topologies recently proposed for MIC applications. The MIC topologies are classified into three different arrangements based on the dc link configurations. A systematic discussion is also provided at the end of the paper that focuses on the major advantages and disadvantages of each MIC arrangement. These are considered in detail and will provide a useful framework and point of reference for the next generation MIC designs and applications.

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A Review of the Single Phase Photovoltaic Module Integrated Converter Topologies With Three Different DC Link Configurations

This paper presents an overview of single-phase inverters developed for small distributed power generators. The functions of inverters in distributed power generation (DG) systems include dc-ac conversion, output power quality assurance, various protection mechanisms, and system controls. Unique requirements for small distributed power generation systems include low cost, high efficiency and tolerance for an extremely wide range of input voltage variations. These requirements have driven the inverter development toward simpler topologies and structures, lower component counts, and tighter modular design. Both single-stage and multiple-stage inverters have been developed for power conversion in DG systems. Single-stage inverters offer simple structure and low cost, but suffer from a limited range of input voltage variations and are often characterized by compromised system performance. On the other hand, multiple-stage inverters accept a wide range of input voltage variations, but suffer from high cost, complicated structure and low efficiency. Various circuit topologies are presented, compared, and evaluated against the requirements of power decoupling and dual-grounding, the capabilities for grid-connected or/and stand-alone operations, and specific DG applications in this paper, along with the identification of recent development trends of single-phase inverters for distributed power generators.

Topologies of single-phase inverters for small distributed power generators: an overview

This paper presents a novel approach for the connection of renewable energy sources to the utility grid. Due to the increasing power capability of the available generation systems, a three-level three-phase neutral-point-clamped voltage-source inverter is selected as the heart of the interfacing system. A multivariable control law is used for the regulator because of the intrinsic multivariable structure of the system. A current source (playing the role of a generic renewable energy source) is connected to the grid using a three-level inverter in order to verify the good performance of the proposed approach. Large- and small-signal d-q state-space averaged models of the system are obtained and used to calculate the multivariable controller based on the linear quadratic regulator technique. This controller simultaneously regulates the dc-link voltage (to operate at the maximum power point of the renewable energy source), the mains power factor (the power is delivered to the grid at unity power factor), and the dc-link neutral-point voltage balance. With the model and regulator presented, a specific switching strategy to control the dc-link neutral-point voltage is not required. The proposed controller can be used for any application, since its nature makes possible the control of any system variable. The good performance of the presented interfacing solution in both steady-state and transient operation is verified through simulation and experimentation using a 1-kW neutral-point-clamped voltage-source-inverter prototype, where a PC-embedded digital signal processor board is used for the controller implementation

Interfacing Renewable Energy Sources to the Utility Grid Using a Three-Level Inverter

The introduction of microgrids in distribution networks based on power electronics facilitates the use of renewable energy resources, distributed generation (DG) and storage systems while improving the quality of electric power and reducing losses thus increasing the performance and reliability of the electrical system. The hierarchical control structure, which consists of primary, secondary, and tertiary levels for microgrids that mimic the behavior of the mains grid, is reviewed. The main objective of this article is to give a description of state of the art for distributed power generation systems (DPGS) based on renewable energy and explore the power converters connected in parallel to the grid which are distinguished by their contribution to the formation of the grid voltage and frequency and are accordingly classified in three classes. This analysis is extended focusing mainly on the three classes of configurations: grid-forming, grid-feeding, and grid-supporting. The article ends up with an overview and a discussion of the control structures and strategies to control distribution power generation system (DPGS) units connected to the network.

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A Survey on Control of Electric Power Distributed Generation Systems for Microgrid Applications

The paper discusses the analysis and control of single-phase AC systems by means of rotating frames. Two methods for the D-Q transformation are compared in terms of computational requirements, transient response and sensibility. One of the transformation methods is based on sine and cosine composition, while the other uses a bi-phase transformation. The comparison has been done using simulation and an experimental prototype of a three-level NPC boost rectifier.

Comparison between two methods of DQ transformation for single phase converters control. Application to a 3-level boost rectifier

This paper deals with the analysis, design and operation of a control system for a single-phase three-level rectifier with a neutral-point-clamped (NPC) topology. Usually the desired operating conditions for this type of converter are: unity displacement factor, output DC voltage regulation and neutral point voltage balancing. A d-q reference frame has been used in this work to model the rectifier behaviour in order to exploit the results obtained in the field of three-phase converters. In this way, a space vector modulation PWM method has been used, with the possibility of using redundant switching states to achieve charge balancing of the capacitors. The time assignment of each redundant switching state is accomplished by utilizing a closed-loop control system. Validity of the modeling and control strategies are confirmed by the transient and steady state simulation and experimental results.

D-Q modeling and control of a single-phase three-level boost rectifier with power factor correction and neutral-point voltage balancing

This paper introduces a DC/AC converter with high-frequency isolation and bidirectional power flow capability for photovoltaic energy applications. The topology is based on a high-frequency inverter, a high-frequency transformer and an AC/AC converter. No additional DC-link stages are required, providing a reduction of components. A modulation strategy for the commutation of the AC/AC converter switches is presented with the benefits of soft-switching commutations with no extra components. Simulation and experimental verification are provided.

Synthesis and modulation of a single phase DC/AC converter with high-frequency isolation in photovoltaic energy applications

A new method for guaranteeing the capacitor voltage balance in a three-level AC/DC single-phase converter is presented. Using a nonlinear control loop and combinations of redundant switching functions, the capacitor charge balance for any power factor is achieved. Experimental results validate the performance of this proposal.

Nonlinear control of neutral point in three-level single-phase converter by means of switching redundant states

One of the main features to consider in the development of new pulsewidth modulations (PWM) for multilevel converters is the high-frequency output-voltage distortion. In this letter, a novel per-switching-cycle figure, the harmonic distortion of order n for switching cycle k(HD/sub n,k/), is introduced to quantitatively characterize the output three-phase voltage harmonic distortion of multilevel converters around all the integer multiples of the switching frequency. This figure allows for the decomposing of the modulation design problem within an output voltage fundamental cycle into an independent set of smaller problems for every switching cycle. The expression of HD/sub n,k/ as a function of the switching states' duty-ratio is presented for the three-level three-phase neutral-point-clamped voltage source inverter and it can be easily obtained for any other multilevel converter. From the evaluation of HD/sub n,k/ over 1/6th of the output-voltage fundamental-period the value of HD/sub n/ is obtained, providing a measure of the output voltage distortion in a fundamental period. This information is obtained at a lower computational cost than conventional fast Fourier transform (FFT) analysis. The accuracy of the HD/sub n/ distortion predictions is verified by comparing it to FFT-based results obtained from simulation and experiments. The expression to compute the total harmonic distortion (THD) as a function of HD/sub n/ is also derived.

A. Gilabert

Papers

Comparison between two methods of DQ transformation for single phase converters control. Application to a 3-level boost rectifier

D-Q modeling and control of a single-phase three-level boost rectifier with power factor correction and neutral-point voltage balancing

Synthesis and modulation of a single phase DC/AC converter with high-frequency isolation in photovoltaic energy applications

Nonlinear control of neutral point in three-level single-phase converter by means of switching redundant states

Output voltage distortion characterization in multilevel PWM converters