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

Typical 6-12 kW switched-mode rectifier (SMR) converters operate at switching frequencies of about 40 kHz, employ FET switches, and utilize pulse width modulation (PWM) power control techniques. These operating conditions create a harsh PWM switching environment which makes the switching to conduction loss ratio approximately 3 to 1, and complicates the task of EMI suppression. A zero-voltage switching (ZVS) SMR high power circuit that improves overall converter efficiency by significantly reducing switching losses while moderately increasing conduction losses is presented. A feature of the proposed converter is that ZVS fixed frequency operation is possible, even with the primary of the transformer disconnected. The modes of circuit operation are presented and analyzed. The feasibility of the proposed converter is demonstrated with experimental results on a 7 kW prototype unit. >

A fixed frequency ZVS high power, PWM SMR converter with zero to rated load variation capability

This paper presents a new control technique for voltage fed load-resonant converters. Unlike conventional variable frequency controllers that externally impose the switching frequency, the proposed scheme is based on controlling the phase shift between one of the resonant circuit variables and the voltage at the output of the inverter. This approach ensures the operation above the resonant frequency and zero voltage switching in all operation conditions. The static characteristics, as well as the dynamic models of a series-parallel resonant converter with the proposed controller, are derived. Experimental results on a 1 kW prototype confirm the feasibility of the proposed technique.

Self-sustained oscillating resonant converters operating above the resonant frequency

This paper proposes reduced switching random PWM (RPWM) techniques based on Space Vector PWM (SVPWM) technique for conventional two-level inverters. In the proposed RPWM techniques, the switching frequency is randomized within a band lower than the nominal switching frequency at and around the peaks of the three-phase output currents. For the rest of the fundamental cycle, the switching frequencies are randomly selected within a band higher than the nominal switching frequency. The proposed method is able to spread the narrowband harmonic clusters similar to the conventional RPWM techniques and additionally produce less inverter output current distortion than conventional spread spectrum techniques. The proposed technique has been analyzed and has been verified using experimental results.

Reduced switching random PWM technique for two-level inverters

Series-shunt unified power quality conditioners, including the dynamic voltage restorer, have demonstrated their capability of supporting bus voltages when faults occur on the AC system. This paper assesses the voltage, power and current compensation capabilities and requirements for these devices. For series voltage regulation, three methods are investigated. Sizes of the series and shunt devices, harmonic filtering requirement, and DC bus energy storage requirement are defined and discussed. The paper allows the design of a compensator that minimizes the kVA rating as a function of a given application.

Rating issues of unified power quality conditioners for load bus voltage control in distribution systems

Force commutated current source rectifiers are mostly controlled using PWM patterns generated offline. This procedure simplifies the inclusion of special requirements associated with unidirectional power switches. This paper shows that online techniques developed for voltage source inverters can be readily adapted to current source rectifiers, thus taking advantage of features such as reduced switching frequency and increased gain. Implementation and performance of the proposed carrier-based and space vector techniques are presented. Finally, the proposed carrier-based techniques are verified an a 5 kVA experimental unit. >

Geza Joos

Papers

A fixed frequency ZVS high power, PWM SMR converter with zero to rated load variation capability

Self-sustained oscillating resonant converters operating above the resonant frequency

Reduced switching random PWM technique for two-level inverters

Rating issues of unified power quality conditioners for load bus voltage control in distribution systems

PWM control techniques in current source rectifiers