Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey
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paper_title: Wind energy systems

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Wind energy has matured to a level of development where it is ready to become a generally accepted utility generation technology. A brief discussion of this development is presented, and the operating and design principles are discussed. Alternative designs for wind turbines and the tradeoffs that must be considered are briefly compared. Development of a wind energy system and the impacts on the utility network including frequency stability, voltage stability, and power quality are discussed. The assessment of wind power station economics and the key economic factors that determine the economic viability of a wind power plant are presented. >
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paper_title: Doubly fed induction generator systems for wind turbines

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This article shows that adjustable speed generators for wind turbines are necessary when output power becomes higher than 1 MW. The doubly fed induction generator (DFIG) system presented in this article offers many advantages to reduce cost and has the potential to be built economically at power levels above 1.5 MW, e.g., for off-shore applications. A dynamic model of the DFIG was derived to develop a vector controller to decouple dynamically active and reactive power control. Simulations show excellent response of the DFIG independent of speed. Measurements obtained from 1.5 MW units currently in operation confirm the theoretical results.
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paper_title: High-Power Hard-Driven GTO Module for 4.5 kV/3 kA Snubberless Operation

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The conventionally driven GTO suffers from a limited safe-operating area which necessitates large protective snubber capacitors. A novel low inductance drive technique is presented which endows the GTO with a square safe-operating area, obviating the use of snubbers and reducing storage time to that of IGBTs. Full area, homogeneous switching enhances operational reliability by eliminating current filamentation which in turn allows turn-off current to scale linearly with silicon area. The appropriated circuit techniques for this optimal use of the GTO are presented with reference to a low-cost, compact and reliable phase-leg module with minimal parts-count, thus heralding a new era of converter design. New semiconductor technologies which further enhance the hard-drive concept by reducing losses and gate-drive requirements are discussed and device characteristics presented.
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paper_title: High-performance induction generator-wind turbine connected to utility grid

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This work proposes a wind generation system connected to a grid using self-excited induction generator (SEIG). The stator currents of the induction generator (IG) are optimally controlled using indirect orientation control (IFOC) according to the generator speed in order to maximize the generated power from the wind turbine. The IG is controlled by the maximum power point tracking (MPPT) below the base speed and the maximum energy can be captured from the wind turbine. The paper presents an IFOC for IG with rotor flux to maintain the DC-link voltage at constant value. During voltage buildup, the variation of the magnetizing inductance is taken into consideration. The DC-link power is transferred to the gird via inverter with vector control. By using the vector controller and the synchronous frame current regulators both the input and output currents of the integrated system are characterized with fast current response and low harmonic distortion. The complete system has been simulated with different wind velocities. The simulation results show the effectiveness of the proposed MPPT wind energy system.
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paper_title: Control of DFIG-based wind generation for power network support

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This paper addresses the design and implementation of a novel control scheme for a doubly fed induction generator (DFIG), of the type employed with wind turbines, to provide support to power system operation. It is shown that this controller provides a DFIG-based wind farm with operational and control compatibility with conventional power stations, the ability to contribute to voltage support and recovery following network faults, the ability to provide a power system stabilizer capability that improves overall system damping, and the capability of contributing short-term frequency support following loss of network generation. A simple but realistic test network that combines synchronous and wind farm generation has been modeled and used to assess dynamic performance. Simulation results are presented and discussed that demonstrate the capabilities and contributions of the new DFIG controller to network support.
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paper_title: Multilevel inverters: a survey of topologies, controls, and applications

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Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.
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paper_title: HVDC transmission for large offshore wind farms

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Onshore and offshore wind farms are a rapidly growing worldwide industry. The development of larger, more efficient turbines is opening up new frontiers in wind energy generation in the form of large offshore wind farms. The use of high-voltage direct current (HVDC) technology can fully realise the potential of these developments, overcoming the major technical challenges facing traditional AC solutions and providing an efficient, economic and reliable solution.
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paper_title: Axial-flux permanent-magnet air-cored generator for small-scale wind turbines

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The design and development of an axial-flux permanent-magnet air-cored generator for use as a direct drive generator with small-scale wind and water turbines is described. The generator is designed for simplicity and ease of manufacture and consists of two rotor discs each with permanent magnets located around its periphery. The stator is made of plastic and has a number of bobbin-wound armature coils located around its periphery. A three-phase prototype generator has been designed and built with 16 magnets per rotor disc, 12 armature coils (four per phase) and an overall diameter of 495 mm. The generator produces 1000 W at 300 rpm or 2000 W at 500 rpm with an electrical efficiency substantially greater than 90%. The generator performs as predicted by the design process.
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paper_title: Modeling Strategy for Back-to-Back Three-Level Converters Applied to High-Power Wind Turbines

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Three-level converters are becoming a realistic alternative to the conventional converters in high-power wind-energy applications. In this paper, a complete analytical strategy to model a back-to-back three-level converter is described. This tool permits us to adapt the control strategy to the specific application. Moreover, the model of different loads can be incorporated to the overall model. Both control strategy and load models are included in the complete system model. The proposed model pays special attention to the unbalance in the capacitors' voltage of three-level converters, including the dynamics of the capacitors' voltage. In order to validate the model and the control strategy proposed in this paper, a 3-MW three-level back-to-back power converter used as a power conditioning system of a variable speed wind turbine has been simulated. Finally, the described strategy has been implemented in a 50-kVA scalable prototype as well, providing a satisfactory performance
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paper_title: Control of a permanent magnet synchronous generator used in a variable speed wind energy system

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To obtain maximum power from a stall controlled wind energy converter (WEC) it has to be operated in the variable speed mode. With the generator connected to the grid via a self-commutated converter with intermediate voltage circuit, the generator speed and hence the speed of the wind turbine can be controlled by the generator side inverter. In a preferred control concept the generator is operated at its optimal working point in order to minimize the losses in the generator and the power electronic devices. The paper considers a synchronous generator with permanent magnet excitation. Such machines have been employed with several 100 kW and are under consideration in the MW range.
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paper_title: Impact of direct-drive WEC Systems on the design of a small PM wind generator

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Direct-drive wind energy conversion systems (WECS) are required to operate at low speeds determined by that of the turbine. This paper addresses the problem of adapting a small, standard, readily available PM synchronous machine for direct coupling to a small wind turbine. In particular, it investigates several minor changes to the machine design, which will ensure that rated power can be delivered to its load, whilst operating at the reduced speeds required by a direct-drive WECS.
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paper_title: Multilevel converters for high power AC drives: a review

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The multilevel approach appears to be very promising in AC motor drives, especially when both reduced harmonic contents and high power are required. Several topologies of multilevel voltage source power converters structures are presented and discussed, with particular reference to the modulation techniques which rule the power converters interfacing with the AC drives processes. >
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paper_title: HVDC Light Experiences applicable for power transmission from offshore wind power parks.

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With big wind power parks becoming a considerable share of the total power generation in a network, wind power farms will have to be as robust as conventional power plants and stay online at various contingencies in the AC network. Compensation will then be needed to preserve the power quality and/or even the stability in the network. HVDC Light is a transmission system that does not require any additional compensation, as this is inherent in the control of the converters. It will therefore be an excellent tool for bringing wind power into a network. To date, six HVDC Light transmission systems have been put into operation, of which two, Gotland Light and Tjaereborg are bringing wind power to networks. The experiences gained from the above installations of HVDC Light have shown that it is capable of handling wind power and of reacting rapidly enough to counteract voltage variations in an excellent way. Another tendency today is to place large wind power farms offshore. An offshore wind farm isolated from the network by an HVDC Light transmission system would constitute a generation block, the operating frequency of which could be freely chosen. With the installation of HVDC Light converters on a North Sea platform, Troll A, experience has been gained of how to adapt converters for the rough environment of an offshore location.
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paper_title: Strong winds on the horizon: wind power comes of age

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Wind power has been the fastest growing energy technology in the world for the last decade, and US wind capacity is expected to increase by 50% by the end of 2001. The factors driving that growth are discussed. Small wind turbines, which have many applications ranging from off-grid to grid-connected to hybrid systems, are cost-competitive with diesel systems while offering substantial environmental advantages. The barriers to increased wind deployment are limited. Siting is not too difficult, particularly in the areas of the Great Plains where so much of the wind resource is to be found. Perhaps the most significant barrier is transmission simply because the wind resource is typically found at a distance from load centers. But states like Texas have helped facilitate wind development through policy decisions that ensure that the transmission system does not discriminate against wind because it is a variable resource. Perhaps the greatest current barrier facing wind energy technology is an information barrier: the fact that so many key decision makers-electric industry, financial community, and public policymakers-have not kept up with wind power's enormous progress over the last decade. But, as wind continues to double in global capacity every three years, that problem appears to be slowly fading. Clearly, wind energy's future is bright.
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paper_title: Control of a three level converter used as a synchronous rectifier

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We present an adaptive controller for a three level synchronous rectifier. The controller guarantees regulation and balance of the output capacitors voltages, as well as a close to unity power factor, simultaneously. The design is based on a model in /spl alpha//spl beta//spl gamma/-coordinates of the three level converter. Special attention is given to the /spl gamma/-component of the control input, since it has direct influence in the balancing of the capacitors voltages, represented here as the arithmetic difference of voltages. Moreover, it is shown that this difference is polluted by a periodic disturbance, which is also compensated by the proposed controller by adding an adaptive term to the control law. After some transformations, this adaptive term is finally reduced to a bank of resonant filters. Simulations results are presented to asses the performance of the proposed controller.
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paper_title: Application of finite element method in design of a 50 kW direct drive synchronous generator for variable speed wind turbines

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This paper presents the electromagnetic design process and methodology for a direct drive synchronous generator for variable speed wind turbines, with an emphasis on the application of the finite element method. The finite element method has been used for fine-tuning and verifying the design, as well as for evaluating the performance characteristics. The designed 480 V, 50 kW, 80 RPM synchronous generator has been produced, installed and tested. Test results are reported in the paper.
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paper_title: Multilevel converters for large electric drives

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Traditional two-level high-frequency pulse width modulation (PWM) inverters for motor drives have several problems associated with their high frequency switching which produces common-mode voltage and high voltage change (dV/dt) rates to the motor windings. Multilevel inverters solve these problems because their devices can switch at a much lower frequency. Two different multilevel topologies are identified for use as a power converter for electric drives: a cascade inverter with separate DC sources; and a back-to-back diode clamped converter. The cascade inverter is a natural fit for large automotive all-electric drives because of the high VA ratings possible and because it uses several levels of DC voltage sources which would be available from batteries or fuel cells. The back-to-back diode clamped converter is ideal where a source of AC voltage is available such as a hybrid electric vehicle. Simulation and experimental results show the superiority of these two power converters over PWM-based drives.
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paper_title: Power Converters and Control of Renewable Energy Systems

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The global electrical energy consumption is steadily rising and therefore a continous demand to increase the power generation capacity. A significant percentage of the required capacity increase can be based on renewable energy sources. Wind turbine technology, as the most cost effective renewable energy conversion system, will play an important part in our future energy supply. But other sources like microturbines, photovoltaics and fuel cell systems may also be serious contributors to the power supply. Characteristically, power electronics will be an efficient and important interface to the grid for the renewables and this paper will first briefly discuss three different alternative/renewable energy sources. Next, various configurations of small and medium power conversion topologies are presented including their control (mainly for PV-systems). Finally wind turbine configuration and their control are described.
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paper_title: String and module integrated inverters for single-phase grid connected photovoltaic systems - a review

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This work presents an overview on recent developments and a summary of the state-of-the-art in inverter technology for single-phase grid connected photovoltaic (PV) systems. The information provided includes details on commercially available European string and module integrated PV inverters, their efficiency, price trends and market share. This review is given for inverters for a power level up to 6 kW. Furthermore, the paper deals with the recent developments of new inverter topologies and PV system concepts and discusses possible future trends.
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paper_title: A review of single-phase grid-connected inverters for photovoltaic modules

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This review focuses on inverter technologies for connecting photovoltaic (PV) modules to a single-phase grid. The inverters are categorized into four classifications: 1) the number of power processing stages in cascade; 2) the type of power decoupling between the PV module(s) and the single-phase grid; 3) whether they utilizes a transformer (either line or high frequency) or not; and 4) the type of grid-connected power stage. Various inverter topologies are presented, compared, and evaluated against demands, lifetime, component ratings, and cost. Finally, some of the topologies are pointed out as the best candidates for either single PV module or multiple PV module applications.
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paper_title: Power Converters and Control of Renewable Energy Systems

paper_content:

The global electrical energy consumption is steadily rising and therefore a continous demand to increase the power generation capacity. A significant percentage of the required capacity increase can be based on renewable energy sources. Wind turbine technology, as the most cost effective renewable energy conversion system, will play an important part in our future energy supply. But other sources like microturbines, photovoltaics and fuel cell systems may also be serious contributors to the power supply. Characteristically, power electronics will be an efficient and important interface to the grid for the renewables and this paper will first briefly discuss three different alternative/renewable energy sources. Next, various configurations of small and medium power conversion topologies are presented including their control (mainly for PV-systems). Finally wind turbine configuration and their control are described.
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paper_title: Multilevel PWM inverters suitable for the use of stand-alone photovoltaic power systems

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This paper presents a new multilevel pulse width-modulation (PWM) inverter scheme for the use of stand-alone photovoltaic systems. It consists of a PWM inverter, an assembly of LEVEL inverters, generating staircase output voltages, and cascaded transformers. To produce high-quality output voltage waves, it synthesizes a large number of output voltage levels using cascaded transformers, which have a series-connected secondary. By a suitable selection of the secondary turn-ratio of the transformer, the amplitude of an output voltage appears at the rate of an integer to an input dc source. Operational principles and analysis are illustrated in depth. The validity of the proposed system is verified through computer-aided simulations and experimental results using prototypes generating output voltages of an 11 level and a 29 level, respectively, and their results are compared with conventional counterparts.
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paper_title: A New Neutral-Point-Clamped PWM Inverter

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A new neutral-point-clamped pulsewidth modulation (PWM) inverter composed of main switching devices which operate as switches for PWM and auxiliary switching devices to clamp the output terminal potential to the neutral point potential has been developed. This inverter output contains less harmonic content as compared with that of a conventional type. Two inverters are compared analytically and experimentally. In addition, a new PWM technique suitable for an ac drive system is applied to this inverter. The neutral-point-clamped PWM inverter adopting the new PWM technique shows an excellent drive system efficiency, including motor efficiency, and is appropriate for a wide-range variable-speed drive system.
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paper_title: Power inverter topologies for photovoltaic modules-a review

paper_content:

This review-paper focuses on the latest development of inverters for photovoltaic AC-modules. The power range for these inverters is usually within 90 Watt to 500 Watt, which covers the most commercial photovoltaic-modules. Self-commutated inverters have replaced the grid-commutated ones. The same is true for the bulky low-frequency transformers versus the high-frequency transformers, which are used to adapt the voltage level. The AC-module provides a modular design and a flexible behaviour in various grid conditions. It hereby opens the market for photovoltaic-power for everyone at a low cost due to the plug and play concept, which also makes a further enlargement of the system possible.
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paper_title: Fundamentals of Power Electronics

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Preface. 1. Introduction. I: Converters in Equilibrium. 2. Principles of Steady State Converter Analysis. 3. Steady-State Equivalent Circuit Modeling, Losses, and Efficiency. 4. Switch Realization. 5. The Discontinuous Conduction Mode. 6. Converter Circuits. II: Converter Dynamics and Control. 7. AC Equivalent Circuit Modeling. 8. Converter Transfer Functions. 9. Controller Design. 10. Input Filter Design. 11. AC and DC Equivalent Circuit Modeling of the Discontinuous Conduction Mode. 12. Current Programmed Control. III: Magnetics. 13. Basic Magnetics Theory. 14. Inductor Design. 15. Transformer Design. IV: Modern Rectifiers and Power System Harmonics. 16. Power and Harmonics in Nonsinusoidal Systems. 17. Line-Commutated Rectifiers. 18. Pulse-Width Modulated Rectifiers. V: Resonant Converters. 19. Resonant Conversion. 20. Soft Switching. Appendices: A. RMS Values of Commonly-Observed Converter Waveforms. B. Simulation of Converters. C. Middlebrook's Extra Element Theorem. D. Magnetics Design Tables. Index.
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paper_title: A bidirectional inverter with high frequency isolated transformer

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There is a growing interest to develop renewable energy, particularly the solar energy to meet the growing energy demand. In this paper, a bidirectional photovoltaic (PV) inverter using high frequency (HF) transformer is proposed. The HF transformer provides electrical isolation between the DC supply and AC output. Besides, by using minimum power switches at the active rectifier on the secondary side of HF transformer, the system has low switching losses. Therefore, the proposed inverter has advantages of reliable, lightweight, low cost and increased efficiency. A prototype inverter is implemented using a low cost microcontroller and laboratory experiments are carried out to verify the system viability.
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paper_title: Stability of grid-connected PV inverters with large grid impedance variation

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Photovoltaic (PV) inverters used in dispersed power generation of houses in the range of 1-5 kW are currently available from several manufacturers. However large grid impedance variation is challenging the control and the grid filter design in terms of stability. In fact the PV systems are well suited for loads connected in a great distance to the transformer (long wires) and the situation becomes even more difficult in low-developed remote areas characterized by low power transformers and long distribution wires with high grid impedance. Hence a theoretical analysis is needed because the grid impedance variation leads to dynamic and stability problems both in the low frequency range (around the current controller bandwidth frequency) as well as in the high frequency range (around the LCL-filter resonance frequency). In the low frequency range the possible variation of the impedance challenges the design of resonant controllers adopted to mitigate the effect of the grid harmonic distortion on the grid current. In the high frequency range the grid impedance influence the frequency characteristic of the filter and the design of passive or active damping (to ensure stability) becomes more difficult. In this paper both topics are addressed and discussed with simulation and experimental results.
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paper_title: Developing a "next generation" PV inverter

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The most serious reliability issue for photovoltaics (PV) is the inverter. Mean time to first failure (MTFF) is estimated to be about five years. Recent efforts to rapidly expand the production of grid-tied inverters have not resulted in improved reliability. At this time the inverter industry is uniquely positioned to develop a 'next generation' inverter that has ten-year MTFF, better performance and lower cost. The recent advents of new technologies such as DSP (digital signal processing), the growth in sales to a few hundred thousand inverters per year and the emergence of larger companies with interest in PV inverters make this possible today. At this time the Department of Energy is considering the development of a new inverter for use in distributed energy applications. This report summarizes the status of power electronics today, identifies technology issues, and identifies inverter manufacturer attributes thought to be essential for the production of a ten-year lifetime inverter.
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paper_title: Can your roof provide your electrical needs? - The growth prospect of building-integrated photovoltaic

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There has been an unprecedented growth of photovoltaic market in the last five years. Industry analysts estimate the world market to exceed 15 GW in 2020. Which are the technologies that would dominate the market then? In this paper, I shall outline the growth prospect for the thin film amorphous silicon technology developed at United Solar. The roll-to-roll approach for production lowers manufacturing cost; the flexible products can also be easily integrated with the roof. A marketing strategy has been developed where alliances have been made with roofing contractors or roofing material manufacturers to sustain the growth. The superior performance of the product and the new sales strategy have resulted in a 100% growth in sales volume from 2003 to 2004. Future plans for expansion will be discussed.
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paper_title: Control of power circuit interface of a flywheel-based energy storage system

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This paper presents a flywheel energy storage system, which uses a switched reluctance motor/generator. It is shown this device operating as a static shunt compensator, and how the power circuits can be controlled in distribution systems, to deal with loads that, for short periods of time, demand a considerable supplement of alternating real power. This device can also be used to attenuate the effects of short interruptions in energy supply. The implementation of the control system was simulated with a commercial program. The energy loss, due to friction, will be reduced using a superconducting magnetic bearing.
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paper_title: Grid frequency regulation by recycling electrical energy in flywheels

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Grid frequency regulation function addresses the balance between the network's load and power generated. The system operator generates a signal, area control error (ACE) signal at PJM, based on the difference between these two parameters. The expected goal is to keep the system near nominal 60 or 50 Hz. Traditionally, frequency regulation is managed by varying the output of fossil fuel or hydro generators connected to the electric grid. The authors present a new method in which electric energy is recycled. First, absorbing energy when it is in abundance, then discharging the same for the desired frequency regulation effect. The proposed system is based on kinetic energy storage technology in high-speed flywheels. Performance modeling results, based on actual ACE signals, indicate that a modular flywheel system matrix is effective in providing frequency regulation. The cost of this system is attractive compared with today's generator-based approach. The controlled injection or absorption of reactive power, inherent to the system, extents its capability to provide voltage support. Further, fast acting control electronics allow for the damping of grid oscillations. All the above functions, once deployed in moderate numbers, could help prevent frequency and voltage shifts that have been linked to significant blackouts in recent history
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paper_title: Active filters and energy storage systems for power conditioning in Japan

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This paper describes advanced power electronics technology relevant to active filtering and energy storage for the purpose of power conditioning. The combination of active filtering and energy storage leads to a versatile system in terms of power conditioning. However, energy storage is much more difficult and costly in realization than active filtering because modern science offers only chemical action, electromagnetic or electrostatic field, and kinetic or potential energy as viable ways of energy storage. The author cannot survey the whole spectrum of active filtering and energy storage. Therefore, this paper is focused on the present status of active filters for power conditioning, along with a 200-MJ/20-MW flywheel energy storage system which has been commercially installed on a 66-kV power system for the purpose of line-frequency regulation in Japan.
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paper_title: Investigation of joint operation of a superconducting kinetic energy storage (flywheel) and solar cells

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Superconductors provide unique possibilities for storage of electricity, which is especially important for the reliability of networks supplied by renewable energy sources such as solar energy and wind energy. A system consisting of an HTS-based levitated flywheel as the energy storage unit and solar cells as the power supply was installed and investigated as a model of a viable variant of the mini-power plant concept. Measurements were performed to obtain information about relevant storage characteristics such as efficiency and stability. Possibilities of fitting the storage unit to the solar supply were also investigated in order to get maximum charge and discharge efficiency.
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paper_title: Control strategies for enhanced power smoothing in wind energy systems using a flywheel driven by a vector-controlled induction machine

paper_content:

This paper presents a novel control strategy for power smoothing in wind energy applications, especially those feeding a stand-alone load. The system is based on a vector-controlled induction machine driving a flywheel and addresses the problem of regulating the DC-link system voltage against both input power surges/sags from a wind turbine or sudden changes in load demand. The control is based on a feedforward compensation scheme augmented by a nonlinear controller. Two feedforward compensation schemes are discussed and the limitations and performance of each scheme are analyzed. Experimental results are presented which verify the excellent performance of the feedforward compensation technique.
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paper_title: Voltage sags compensation using a superconducting flywheel energy storage system

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This paper presents a voltage sag compensator, which uses a flywheel energy storage system with superconducting magnetic axial thrust bearing (SMB) and a permanent magnet radial bearing (PMB). The SMB was built with Nd-Fe-B magnet and YBCO superconducting blocks, refrigerated with liquid Nitrogen. The magnets are assembled with magnetic flux shapers in order to increase the levitation force and the stiffness. The radial PMB is used to positioning the vertically arranged switched reluctance machine (SRM) used as motor/generator. Simulations of the power electronics and SRM show that the system can work up to 30,000 rpm supplying the required energy during disturbances.
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paper_title: Power quality improvement for wave energy converters using a D-STATCOM with real energy storage

paper_content:

Wave energy converters promise to be an important source of low-carbon electrical energy, but produce output power which fluctuates at a low frequency. The usual solution is to use flywheel energy storage on the shaft output. This paper proposes a novel alternative which uses aggregation to smooth most of the power oscillation and a fast-response power electronics device, the D-STATCOM, to smooth out the residual change. Novel control schemes are examined to prolong the life of the D-STATCOM energy storage element.
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paper_title: An integrated flywheel energy storage system with homopolar inductor motor/generator and high-frequency drive

paper_content:

The design, construction, and test of an integrated flywheel energy storage system with a homopolar inductor motor/generator and high-frequency drive is presented in this paper. The work is presented as an integrated design of flywheel system, motor, drive, and controller. The motor design features low rotor losses, a slotless stator, construction from robust and low cost materials, and a rotor that also serves as the energy storage rotor for the flywheel system. A high-frequency six-step drive scheme is used in place of pulsewidth modulation because of the high electrical frequencies. A speed-sensorless controller that works without state estimation is also described. A prototype of the flywheel system has been demonstrated at a power level of 9.4 kW, with an average system efficiency of 83% over a 30000-60000 r/min speed range.
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paper_title: TEX: the next generation of energy storage technology

paper_content:

Engineers at active power have taken three mature energy storage technologies and combined them in a new and ingenious way. The resulting products will be technically and commercially viable replacements for batteries in the near future.
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paper_title: Power smoothing control using sensorless flywheel drive in wind-diesel generation systems

paper_content:

This paper presents a novel control strategy for power smoothing. The system is based on a sensorless vector controlled induction machine driving a flywheel. The problem of regulating the voltage against input power surges or sudden changes in load demand is addressed. The induction machine is controlled to operate in a wide speed range by using flux weakening above rated speed. A speed observer is used to obtain the rotational speed in the whole speed range. The observer parameters are adapted during flux weakening in order to obtain close tracking of the flywheel speed. Experimental results for the operation of the flywheel drive between zero to more than twice base speed are presented and discussed. The system described in this paper is suitable for wind-diesel generation or stand alone wind energy systems.
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paper_title: Performance of a stand-alone renewable energy system based on energy storage as hydrogen

paper_content:

Electrolytic hydrogen offers a promising alternative for long-term energy storage of renewable energy (RE). A stand-alone RE system based on energy storage as hydrogen has been developed and installed at the Hydrogen Research Institute, and successfully tested for autonomous operation with developed control system and power conditioning devices. The excess energy produced, with respect to the load requirement, has been sent to the electrolyzer for hydrogen production. When energy produced from the RE sources became insufficient, with respect to the load requirement, the stored hydrogen was fed to a fuel cell to produce electricity. The RE system components have substantially different voltage-current characteristics and they are integrated through power conditioning devices on a dc bus for autonomous operation by using a developed control system. The developed control system has been successfully tested for autonomous operation and energy management of the system. The experimental results clearly indicate that a stand-alone RE system based on hydrogen production is safe and reliable.
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paper_title: Liquid hydrogen tank with cylindrical superconducting bearing for automotive application

paper_content:

Using the concept of superconducting suspension of the inner tank in an outer vessel, the authors have designed and constructed a prototype of a liquid-hydrogen storage tank for automotive application. In contrast to the earlier model with planar design of the bearing, they used a rotationally symmetric arrangement with the inner tank suspended over a central frame bar. The bearing consists of superconducting rings fixed to the central tube of the inner tank and a system of permanent magnets mounted on the frame bar. No additional cooling of the superconductors (YBaCuO rings prepared by a modified multi-seeding process and Bi-2212 rings prepared by the melt cast process) is required. For suspension of the tank in the warm state above Tc, they used newly developed actuators with main springs made of shape-memory alloy. When the tank is filled with liquid hydrogen, the actuators release the tank, thereby providing absolutely passive activation of the bearing.
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paper_title: Experimental study for wind power - hydrogen energy system with energy capacitor system

paper_content:

The most serious defects of a wind turbine system are instability of the output and power transmission problem. This report aims at a stand-alone wind turbine system with a hydrogen generator (wind power - hydrogen energy system). In comparison with electric, hydrogen is suitable for energy storage and transport. So this system improves fluctuating output power of the wind turbine generator and the energy transmission problem occurred by converting electric to hydrogen. Firstly, the I-V characteristic of a small-scale wind turbine generator was analyzed to investigate how the wind turbine generator operates under the influence of connecting with the hydrogen generator directly. As a result, it has been turned out that the system was not matching at all. Therefore, it is necessity to install controlling devices between the wind turbine generator and the hydrogen generator. Based on the result, we considered that the energy capacitor system (ECaSS) would have good performance in wind power - hydrogen energy system. Then, we assembled the wind power-hydrogen energy system with ECaSS actually, and evaluated the utility.
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paper_title: Use of carbon nanostructures for hydrogen storage for environmentally safe automotive applications

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Carbon nanostructures are considered materials with a high potential for hydrogen storage. CCVD technique was used to grown nanotubes with diameters ranging from 50 to 100 nm on a Ni:Cu catalyst and nanofibers with diameters ranging from 10 to 100 nm on a Pd/La/sub 2/O/sub 3/ catalyst. The hydrogen uptake experiments performed volumetrically in a Sievert-type installation, showed the quantity of desorbed hydrogen (for pressure intervals ranging from 1 to 100 bars) by the nanotubes was less than 1 % by weight. The hydrogen sorption capacities of the carbon nanofibers show saturation value of about 1.5 wt. % and to be in a good correlation with the Pd/C ratio, revealing a catalytic effect of Pd that supplies atomic H.
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paper_title: Gone with the wind: innovative hydrogen/fuel cell electric vehicle infrastructure based on wind energy sources

paper_content:

The development of sustainable, clean, and efficient transportation systems is a critical factor for the improvement of overall air quality and the reduction of the nation's dependence on diminishing domestic resources and imported fuels. This article presents the development issues for an innovative fuel cell electric vehicle (EV) infrastructure and bidirectional grid connection, derived from renewable and clean energy sources, with a focus on wind energy. The effective use of wind power to drive water electrolysis plants to produce hydrogen for advanced fuel-cell EV is also illustrated in this paper.
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paper_title: Hybrid energy storage systems based on compressed air and supercapacitors with maximum efficiency point tracking

paper_content:

Beside the high-capacity storage facilities based on hydro-power technologies, electrochemical solutions are the today's candidate for storage for renewable energy sources. However, limited life-cycles and sustainability of batteries are often inhibiting factors. This paper presents a hybrid energy storage system with high life cycle, based on compressed air energy storage (CAES). The storage and discharge are done within maximum efficiency conditions. As the maximum efficiency conditions impose the level of converted power, an intermittent time-modulated operation mode is applied to the thermodynamic converter to obtain a variable converted power. A smoothly variable output power is achieved with the help of a supercapacitive auxiliary storage device used as a filter. The paper describes the concept of the system, the power-electronic interface circuits and especially the maximum efficiency point tracking (MEPT) algorithm and the strategy used to vary the output power. In addition, the paper describes the principles of higher efficiency storage systems where the air machine is replaced by an oil hydro-pneumatic converter, used under isothermal conditions. Practical results are reported, that have been recorded from a low-power pneumatic motor coupled to a small DC generator for the purely pneumatic system, and from a first prototype of the oil-hydraulics and pneumatic system. Some economical considerations are also made, through a comparative cost evaluation of the presented hydro-pneumatic systems and a lead acid batteries system, in the context of a stand alone photovoltaic home application. This evaluation confirms the cost effectiveness of the studied hybrid storage systems
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paper_title: TEX: the next generation of energy storage technology

paper_content:

Engineers at active power have taken three mature energy storage technologies and combined them in a new and ingenious way. The resulting products will be technically and commercially viable replacements for batteries in the near future.
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paper_title: Elimination of high power delivery rates by energy storage in supercapacitors

paper_content:

Temporary coverage of high peak power consumption requires a high power delivery rate also at low average power consumption. Consequently peak load coverage becomes economically expensive for the user. Using supercapacitors as energy storage can help to reduce the power delivery rate. The capability of storing energy can even substitute UPS for safety relevant systems. Two examples, an electrodynamic rail brake and a charging station for AutoTram, demonstrate applications of temporary high power requirement. By using supercapacitors the required power delivery rate could be reduced significantly. Applications in the field of low power networks have become feasible through the capability of parametrisation of the power consumption. The examples focus on the characteristic of the capacitors and the interaction with the power electronic and the load
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paper_title: Dynamic Performance of a Static Synchronous Compensator with Superconducting Magnetic Energy Storage

paper_content:

This paper discusses the integration of a static synchronous compensator (STATCOM) with a superconducting magnetic energy storage (SMES) system for controlling the frequency of large power systems (PS). At present, the new energy storage systems (ESS) are a feasible alternative to store excess energy for substituting the primary control reserve thus improving the primary frequency control (PFC). In this way, the performance of the STATCOM controller can be enhanced with the addition of the SMES coil through a dc-dc chopper. This combination allows to greatly improve the PFC and consequently the PS security. In this study, the dynamics of active and reactive power responses of the integrated STATCOM/SMES system to power system oscillations is analyzed. This analysis is performed for the case of a tie-line tripping on a test power system by using SimPowerSystems of MATLABtrade. This paper also discusses the detailed modeling of the STATCOM based on a multi-pulse voltage source inverter composed of three-level elementary inverters, the two-quadrant multi-phase dc-dc chopper and the SMES coil. Finally a multi-level controller is presented for the integrated compensator by using concepts of instantaneous power on the synchronous-rotating d-q reference frame
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paper_title: Wind farms linked by SMES systems

paper_content:

The objective of this paper is to introduce the concept of wind farms linked by SMES systems. In this work, the SMES system is applied to a wind farm that is interconnected with a grid through a back-to-back DC link for the variable speed operation of the wind turbines. This system enables the output power leveling of the wind farm depending on the power demand and can reduce the capacity of the converter system by selecting an optimal discharge/charge rate of the SMES. By using the stored energy of the SMES, this system can also compensate the inertia of the blades so that the wind turbine speed can be rapidly controlled depending on the wind condition. This paper describes the design condition of the SMES for the output power leveling of the wind farm and discusses the SMES configuration for a 100-MW class wind farm.
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Title: Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey

Section 1: INTRODUCTION
Description 1: Introduce the significance of renewable energy sources and the role of power-electronic systems in integrating these sources into the grid.

Section 2: Variable-Speed Wind Turbines
Description 2: Discuss the advancements and benefits of variable-speed wind turbines, including technological developments and economic viability.

Section 3: Current Wind-Power Technology
Description 3: Detail the existing technologies in wind power, including different concepts like doubly fed induction generators (DFIGs) and full-power converters.

Section 4: Voltage Fault Ride-Through Capability of Wind Turbines
Description 4: Explore the requirements and solutions for wind turbines to remain stable and connected during voltage dips and other disturbances.

Section 5: Power-Quality Requirements for Grid-Connected Wind Turbines
Description 5: Examine the standards and methods for ensuring power quality in grid-connected wind turbines.

Section 6: Trends in Wind-Power Technology
Description 6: Analyze the future trends in wind power technology, focusing on offshore installation, HVDC systems, high-power medium-voltage converters, and direct-drive technology.

Section 7: PV TECHNOLOGY
Description 7: Review recent developments in power-electronic converters for PV systems and the state-of-the-art implementations.

Section 8: Market Considerations
Description 8: Consider market trends, growth, and economic factors impacting the adoption of photovoltaic (PV) technology.

Section 9: Design of PV-Converter Families
Description 9: Offer an overview of existing inverter topologies for interfacing PV modules with the grid.

Section 10: PV Topologies
Description 10: Provide a classification of PV topologies, detailing configurations with and without dc/dc converters and the implications of using transformers.

Section 11: Future Trends
Description 11: Predict upcoming trends in PV technology, including improvements in efficiency, standards compliance, and cost minimization of grid-connected inverters.

Section 12: Energy-Storage Systems
Description 12: Describe various energy-storage technologies used for grid integration of renewable energy, including flywheels, hydrogen, compressed-air energy storage, supercapacitors, superconducting magnetic energy storage, batteries, and pumped-hydroelectric storage.

Section 13: CONCLUSION
Description 13: Summarize the critical role of power-electronic technology in renewable energy integration, highlighting current advancements and future research directions.