Renewable energy sources like wind, sun, and hydro are seen as a reliable alternative to the traditional energy sources such as oil, natural gas, or coal. Distributed power generation systems (DPGSs) based on renewable energy sources experience a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field. Due to the increasing number of DPGSs connected to the utility network, new and stricter standards in respect to power quality, safe running, and islanding protection are issued. As a consequence, the control of distributed generation systems should be improved to meet the requirements for grid interconnection. This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control

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Overview of Control and Grid Synchronization for Distributed Power Generation Systems

Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.

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Recent Advances and Industrial Applications of Multilevel Converters

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

In the last years, the use of distributed uninterruptible power supply (UPS) systems has been growing into the market, becoming an alternative to large conventional UPS systems. In addition, with the increasing interest in renewable energy integration and distributed generation, distributed UPS systems can be a suitable solution for storage energy in micro grids. This paper depicts the most important control schemes for the parallel operation of UPS systems. Active load-sharing techniques and droop control approaches are described. The recent improvements and variants of these control techniques are presented.

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Control of Distributed Uninterruptible Power Supply Systems

Neural networks

A new strategy for direct torque control with imposed switching frequency (DiCoIF) is proposed. This strategy was specially designed to operate with a multicell (flying capacitors) inverter with any number of levels, which means it can also be used for standard two-level inverters. This approach combines the well known advantages of the multicell. inverter with those of a direct torque controlled (DTC) based strategy. It is shown that the multicell topology presents enough degrees of freedom to control both torque and flux with very low ripple and high dynamics on one hand, and to impose the switching frequency and the capacitors voltage balance on the other hand. Experimental and simulation results, obtained with a standard two-level inverter and with a four-level multicell inverter, are presented and discussed. Finally, a comparative analysis either with the classical DTC and field oriented techniques is carried out.

Switching frequency imposition and ripple reduction in DTC drives by using a multilevel converter

The increasing interest in development, operation and integration of large amount of renewable energy resources like offshore wind farms and photovoltaic generation in deserts leads to an emerging demand on development of multi-terminal high voltage direct current (MTDC) systems. Due to preformed studies, voltage source converter based HVDC (VSC-HVDC) system is the best option for realising future multi-terminal HVDC system to integrate bulk amount of energy over long distances to the AC grid. The most important drawback of VSC-HVDC systems is their need for fast HVDC circuit breakers. This paper aims at summarizing HVDC circuit breakers technologies including recent significant attempts in development of modern HVDC circuit breaker. A brief functional analysis of each technology is presented. Additionally, different technologies based on derived information from literatures are compared. Finally, recommendations for improvement of circuit breakers are presented.

A review on HVDC circuit breakers

This paper proposes a new $\boldsymbol {n}$ -port hybrid dc circuit breaker for offshore multiterminal HVDC grid application. The $\boldsymbol {n}$ -port dc circuit breaker can substitute $\boldsymbol {n-1}$ hybrid dc circuit breakers at a dc bus with $\boldsymbol {n-1}$ adjacent dc transmission lines. The system-level behavior of the proposed multiport hybrid dc circuit breaker is similar to other hybrid dc circuit breakers. The operation principles of the proposed multiport dc circuit breaker are introduced, analyzed, and compared to the existing solution in this work. The components ratings are compared to the existing solution, and the functionality of the proposed device is verified by simulation.

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Multiport Hybrid HVDC Circuit Breaker

The parallel operation of static inverters is, in a large amount of cases, the appropriate solution to achieve the high power required by some applications or to improve power system reliability The limited inverter capacity obliges to parallel the individual units to obtain the nominal load power In UPS systems, there are situations where a high reliability/availability is required by critical loads Parallel redundancy appears an immediate solution to satisfy this requirement This paper presents a control system for parallel operation of nonredundant UPSs based on current control The relative phase between the inverters is constant Simulation results as well as experimental studies are presented The overall control system is implemented on a simple and low cost platform

Design and implementation of a current controller for the parallel operation of standard UPSs

This paper proposes a new device by combining features of an interline dual H-bridge current flow controller with the core idea of a hybrid HVdc circuit breaker for meshed HVdc grid application. The proposed device can substitute two dc circuit breakers at a dc bus with at least two adjacent transmission lines. In addition to the current interruption action, the current in one of the adjacent lines can be controlled by the embedded current flow controller. The system-level behavior of the proposed current flow controlling hybrid dc circuit breaker is similar to that of the typical hybrid dc circuit breaker and the interline dual H-bridge current flow controller. The operation principles of the proposed device are introduced and analyzed in this work. The component ratings are compared to the existing solution, and the functionality of the proposed device is verified by simulation.

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Adriano Carvalho

Papers

Switching frequency imposition and ripple reduction in DTC drives by using a multilevel converter

A review on HVDC circuit breakers

Multiport Hybrid HVDC Circuit Breaker

Design and implementation of a current controller for the parallel operation of standard UPSs

Current Flow Controlling Hybrid DC Circuit Breaker