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

This paper presents a new multilevel converter topology suitable for very high voltage applications, especially network interties in power generation and transmission. The fundamental concept and the applied control scheme is introduced. Simulation results of a 36 MW-network intertie illustrate the efficient operating characteristics. A suitable structure of the converter-control is proposed.

An innovative modular multilevel converter topology suitable for a wide power range

Overview of current development in electrical energy storage technologies and the application potential in power system operation

Active filtering of electric power has now become a mature technology for harmonic and reactive power compensation in two-wire (single phase), three-wire (three phase without neutral), and four-wire (three phase with neutral) AC power networks with nonlinear loads. This paper presents a comprehensive review of active filter (AF) configurations, control strategies, selection of components, other related economic and technical considerations, and their selection for specific applications. It is aimed at providing a broad perspective on the status of AF technology to researchers and application engineers dealing with power quality issues. A list of more than 200 research publications on the subject is also appended for a quick reference.

A review of active filters for power quality improvement

The authors propose a topology based on a practical and reliable pulse-width-modulation (PWM) AC controller connected to the line side of the power transformer, the low voltage winding supplying an uncontrolled diode rectifier. The AC controller power circuit structure ensures reliable switch operation, and the equal PWM pattern is efficient and simple to implement. Advantages of the proposed topology include increased power factor, low AC input current and DC output voltage harmonic distortion, high efficiency, and high reliability. The authors describe the operation of the proposed topology and derive the characteristic equations. Simulation results are verified on an experimental 10 kVA prototype unit. >

A PWM AC controller-based high current power supply

Four distinct uninterruptible power supply topologies are presented here for powering hybrid fiber/coaxial (HFC) networks. Topologies based on a low frequency isolation transformer are found to have better efficiency than the topologies with high frequency transformer. However, the latter topologies have much better performance in terms of transient response, quality of the output voltage, input power factor, total harmonic distortion of the input current, size and weight. The series-parallel resonant based UPS topology is found to have best overall performance for the emerging HFC applications.

A comparison of UPS for powering hybrid fiber/coaxial networks

One original purpose of FACTS controllers is to increase the transmission capacity of existing lines thus deferring the necessity of building new lines. FACTS, such as the unified power flow controller (UPFC), have also been regarded as controllers for routing power in the market-driven, deregulated power systems. Hitherto, the UPFC has been conceived to control the complex powers through a radial line. This paper presents the concept of a multi-terminal UPFC (M-UPFC), which serves to control the complex powers of several transmission lines converging towards (or radiating from) a transmission node. In order to show that a M-UPFC can be built, that it has stable operation and that it has all the control degrees of freedom of UPFCs, a laboratory model based on 3 units of 3-phase, voltage-source converters has been assembled and tested.

Multi-teriminal UPFC for power system deregulation

This paper proposes the neural network controller as a viable alternative to the PI controller used in DC/DC converters of the buck type for voltage regulation. The PI controller, although robust and simple, requires a priori knowledge of the system characteristics and once designed for a specific load, its parameters remain fixed. The neural controller, in the on line mode has the ability to learn from experience, thus eliminating the need for a priori knowledge of the system dynamics. The neural network can adapt to variations in the load, and still allow the system to track a specific reference without redesign. Performance comparisons made with the standard PI regulator clearly bring out the superior performance of the neural network regulator. >

A neural network based approach to the regulation of DC/DC buck converters

The interconnection of Type-4 wind farms into weak distribution feeders may result in voltage rise and flicker concerns. This paper investigates a wind farm control scheme employing a slow-acting reactive power controller to regulate the steady-state voltage at the point of common coupling (PCC) and a fast-acting active power controller, streaming from an embedded energy storage system (ESS) to mitigate flicker at the wind turbine terminals. Real-time hardware-in-loop (HIL) results have been obtained for the proposed power control strategy, emulating the distribution-feeder and wind turbines on a real-time digital simulator (RTDS) and the power control loops on a digital controller, communicating using the IEC 61850 GOOSE Messaging protocol. Flicker studies are conducted on a 25 kV distribution feeder using a flickermeter compliant to the IEC 61000-4-15 standard. Results demonstrate a short-term flicker level reduction up to 83%. Applicable utility grid codes and standards are considered throughout.

Geza Joos

Papers

A PWM AC controller-based high current power supply

A comparison of UPS for powering hybrid fiber/coaxial networks

Multi-teriminal UPFC for power system deregulation

A neural network based approach to the regulation of DC/DC buck converters

Real-time testing of power control implemented with IEC 61850 GOOSE messaging in wind farms featuring energy storage