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A 225?kW Direct Driven PM Generator Adapted to a Vertical Axis Wind Turbine
S. Eriksson,H. Bernhoff,M. Leijon
Advances in Power Electronics , 2011, DOI: 10.1155/2011/239061
Abstract: A unique direct driven permanent magnet synchronous generator has been designed and constructed. Results from simulations as well as from the first experimental tests are presented. The generator has been specifically designed to be directly driven by a vertical axis wind turbine and has an unusually low reactance. Generators for wind turbines with full variable speed should maintain a high efficiency for the whole operational regime. Furthermore, for this application, requirements are placed on high generator torque capability for the whole operational regime. These issues are elaborated in the paper and studied through simulations. It is shown that the generator fulfils the expectations. An electrical control can effectively substitute a mechanical pitch control. Furthermore, results from measurements of magnetic flux density in the airgap and no load voltage coincide with simulations. The electromagnetic simulations of the generator are performed by using an electromagnetic model solved in a finite element environment. 1. Introduction The use of wind power is increasing all over the world, and there are several different types of electrical systems available for converting the wind power to electricity, but no single technology is dominating the market [1]. In this paper a direct driven permanent magnet (PM) synchronous generator is presented [2–4]. The generator presented here has been specifically designed to be directly driven by a vertical axis wind turbine (VAWT) [5] and to be placed on ground level. One of the advantages of using a VAWT is that it is omnidirectional; that is, it can accept wind from any direction and does not need a yawing mechanism. The present vertical axis turbine is a straight-bladed Darrieus turbine [6]. A presentation of the design of the same type of wind turbine can be found in [7]. A more extensive presentation of the generator type, generator experiments, and verification of simulations can be found in [8, 9]. A direct driven generator is spared from losses, maintenance, and costs associated with a gearbox. However, direct drive yields a larger generator than with a generator connected through a gearbox. For a vertical axis wind turbine where the generator can be placed on ground level, the size and weight are of less concern. The generator can therefore be optimized considering efficiency and cost instead of focusing on lowering the weight. Thus, the vertical orientation of the axis allows for a general study of generator design freed from weight and size constrains. The presented generator is unique in several ways.
Parking Strategies for Vertical Axis Wind Turbines
F. Ottermo,S. Eriksson,H. Bernhoff
ISRN Renewable Energy , 2012, DOI: 10.5402/2012/904269
Abstract:
Electric Control Substituting Pitch Control for Large Wind Turbines
Jon Kjellin,Sandra Eriksson,Hans Bernhoff
Journal of Wind Energy , 2013, DOI: 10.1155/2013/342061
Abstract: A completely electrical control of a variable speed wind turbine is experimentally verified. A vertical axis wind turbine with a direct driven generator and an electrical system with diode rectification and full inverter connected to the electric grid is presented. This is the first paper that presents this novel 200?kW wind power plant erected at the west coast of Sweden. The turbine has fixed pitch and is only controlled electrically accommodated by passive stall of the blades. By electrically controlling the generator rotational speed with the inverter, passive stall regulation is enabled. The first results on experimental verification of stall regulation in gusty wind speeds are presented. The experiments show that the control system can keep the turbine rotational speed constant even at very gusty winds. It is concluded that electrical control accommodated by passive stall is sufficient as control of the wind turbine even at high wind speeds and can substitute mechanical control such as blade pitch. 1. Introduction Increased amount of electricity produced from wind power is one of the ways to reach the goal of lowering emissions of greenhouse gases from energy production. Installed wind turbines and wind power plants have increased both in size and number in the last 25 years. However, research is still needed, in order to increase the reliability of turbines [1]. The research in wind power at the division of electricity at Uppsala University is focused at vertical axis wind turbines of straight-bladed Darrieus type. Darrieus turbines are described in [2]. The novel concept with an all-electric control and a variable speed turbine connected to a direct driven PM generator placed on ground reduces the number of moving parts compared to conventional wind turbines. Hence, the maintenance cost is expected to be lower than for conventional wind turbines. The turbine is controlled electrically by controlling the power output and rotational speed of the generator; that is, no mechanical control as described in [3] is needed. In wind speeds above nominal, the electrical system is accommodated by passive stall. Passive stall has also been used historically in fixed speed horizontal axis wind turbines [4]. However, in this concept it is used together with a variable speed turbine instead of fixed speed. In conventional horizontal axis wind turbines, the gearbox, pitch, and yaw system stand for a substantial part of the downtime [5]. Further, by placing the generator on ground, it is easier to mount and maintain it. The generator can be designed for
Parking Strategies for Vertical Axis Wind Turbines
F. Ottermo,S. Eriksson,H. Bernhoff
ISRN Renewable Energy , 2012, DOI: 10.5402/2012/904269
Abstract: Strategies for parking a vertical axis wind turbine at storm load are considered. It is proposed that if a directly driven permanent magnet synchronous generator is used, an elegant choice is to short-circuit the generator at storm, since this makes the turbine efficiently damped. Nondamped braking is found to be especially problematic for the case of two blades where torsional oscillations may imply thrust force oscillations within a range of frequencies. 1. Introduction It has become increasingly important to broaden the search for potential technologies for renewable energy conversion, as part of the joint effort to cut greenhouse gas emissions. Within wind power, the established and through incentives now commercially viable technology of horizontal axis wind turbines (HAWTs) has attracted most of the attention during the last decades. Modern HAWTs rely on a quite impressive list of moveable parts for its function, for example, individually pitchable blades and usually a gearbox-expensive and maintenance demanding matters [1], which in part explains the incentives dependence. Another concept for harnessing wind power, the vertical axis wind turbine (VAWT), has the inherent potential to reduce the number of moving parts as pitch and yaw mechanisms are not needed [2]. The generator of VAWTs may conveniently be placed on ground level, facilitating use of bulky direct drive generators which may be optimized for low cost rather than low weight. Patented already in the 30’s by Darrieus [3], the VAWT concept was studied quite intently during the 70’s and 80’s [4, 5], but since then the major resources have been directed towards HAWTs. VAWT activities in the mean time have mostly been concentrated to small-scale turbines where the technology has indeed been successfully commercialized [6, 7]. Renewed interest in larger-scale turbines has arisen lately, especially in the context of offshore applications where the low center of gravity may be advantageous [4]. For the large-scale VAWT prototypes that have been demonstrated so far (i.e., those constructed in the 70’s and 80’s), the aerodynamic efficiency is of the order of 35–40%, which is somewhat lower than for HAWT. The challenge is, therefore, to reach a point where manufacturing and maintenance costs of VAWTs, as compared to HAWTs, are at least 20% lower per swept area, in order to render the concept viable. In this study, a straight-bladed VAWT, also called H-rotor, is considered. A 200?kW prototype H-rotor with direct drive has recently been manufactured based on research from Uppsala University [8–10];
Power Balance Control in an AC/DC/AC Converter for Regenerative Braking in a Two-Voltage-Level Flywheel-Based Driveline
Janaína G. Oliveira,Johan Lundin,Hans Bernhoff
International Journal of Vehicular Technology , 2011, DOI: 10.1155/2011/934023
Abstract: The integration of a flywheel as a power handling can increase the energy storage capacity and reduce the number of battery charge/discharge cycles. Furthermore, the ability of recovering energy of the vehicle during breaking can increase the system efficiency. The flywheel-based all-electric driveline investigated here has its novelty in the use of a double-wound flywheel motor/generator, which divides the system in two different voltage levels, enhancing the efficiency of the electric driveline. The connection of two AC electrical machines (i.e., the flywheel and the wheel motor) with different and variable operation frequency is challenging. A power matching control applied to an AC/DC/AC converter has been implemented. The AC/DC/AC converter regenerates the electric power converted during braking to the flywheel machine, used here as power handling device. By controlling the power balance, the same hardware can be used for acceleration and braking, providing the reduction of harmonics and robust response. A simulation of the complete system during braking mode has been performed both in Matlab and Simulink, and their results have been compared. The functionality of the proposed control has been shown and discussed, with full regeneration achieved. A round-trip efficiency (wheel to wheel) higher than 80% has been obtained. 1. Introduction There is a clear trend in the automotive industry to use more electrical systems in order to satisfy the ever-growing vehicular load demands. Automotive electrical power systems are expected to undergo a drastic change in the next 10–20 years [1]. In electric vehicles reported in the literature, the power is transferred from the wheels directly to the main energy storage device (e.g., batteries) during regenerative braking [2]. Traditionally, the battery is directly connected to the wheel motor [3]. However, the combination of a primary energy source, for example, batteries, and a power buffer can be used to meet the peak energy/power requirements of an electric vehicle. Electric vehicle traction systems that combine a supercapacitor or flywheel peak power buffer with the battery energy source are also called dual power sources. The battery-supercapacitor combination for vehicular applications has been reported in the literature [4–7]. Results claim that supercapacitors offer high internal efficiency and can be charged and discharged a large number of times without performance deterioration. However, the supercapacitor kW/h cost is estimated to be between 10000–20000?$/kWh. Flywheels, on the other hand, have a kW/h
Battery Recharging Issue for a Two-Power-Level Flywheel System
Janaína Gon alves de Oliveira,Hans Bernhoff
Journal of Electrical and Computer Engineering , 2010, DOI: 10.1155/2010/470525
Abstract: A novel battery recharging system for an all-electric driveline comprising a flywheel with a permanent magnet double wound synchronous machine (motor/generator) is presented. The double winding enables two voltage levels and two different power levels. This topology supersedes other all-electric drivelines. The battery operates in a low-power regime supplying the average power whereas the flywheel delivers and absorbs power peaks, which are up to a higher order of magnitude. The topology presents new challenges for the power conversion system, which is the focus of this investigation. The main challenge is the control of the power flow to the battery when the vehicle is parked despite the decay of the flywheel machine voltage; which is dependent on its charge state, that is, rotational speed. The design and simulation of an unidirectional DC/DC buck/boost converter for a variable rotational speed flywheel is presented. Conventional power electronic converters are used in a new application, which can maintain a constant current or voltage on the battery side. Successful PI current control has been implemented and simulated, together with the complete closed loop system.
Filter Influence on Rotor Losses in Coreless Axial Flux Permanent Magnet Machines
SANTIAGO, J.,GONCALVES de OLIVEIRA, J.,BERNHOFF, H.
Advances in Electrical and Computer Engineering , 2013, DOI: 10.4316/aece.2013.01014
Abstract: This paper investigates the eddy current losses induced in the rotor of coreless Axial-Flux machines. The calculation of eddy currents in the magnets requires the simulation of the inverter and the filter to obtain the harmonic content of the stator currents and FEM analysis of the magnets in the rotor. Due to the low inductance in coreless machines, the induced eddy current losses in the rotor remain lower than in traditional slotted machines. If only machine losses are considered, filters in DC/AC converters are not required in machines with wide airgaps as time harmonic losses in the rotor are very low.The harmonic content both from simulations and experimental results of a DC/AC converter are used to calculate the eddy currents in the rotor magnets. The properties of coreless machine topologies are investigated and some simplifications are proposed for time efficient 3D-FEM analysis. The time varying magnetic field can be considered constant over the magnets when the pole is divided in several magnets.The simplified FEM method to calculate eddy current losses is applicable to coreless machines with poles split into several magnets, although the conclusions are applicable to all coreless and slotless motors and generators.
Implementation and Control of an AC/DC/AC Converter for Double Wound Flywheel Application
J. G. Oliveira,H. Schettino,V. Gama,R. Carvalho,H. Bernhoff
Advances in Power Electronics , 2012, DOI: 10.1155/2012/604703
Abstract: An all-electric driveline based on a double wound flywheel, connected in series between main energy storage and a wheel motor, is presented. The flywheel works as a power buffer, allowing the battery to deliver optimized power. It also separates electrically the system in two sides, with the battery connected to the low voltage side and the wheel motor connected to the high voltage side. This paper presents the implementation and control of the AC/DC/AC converter, used to connect the flywheel high voltage windings to the wheel motor. The converter general operation and the adopted control strategy are discussed. The implementation of the AC/DC/AC converter has been described from a practical perspective. Results from experimental tests performed in the full-system prototype are presented. The prototype system is running with satisfactory stability during acceleration mode. Good efficiency and unity power factor could be achieved, based on vector control and space vector modulation. 1. Introduction Extensive research has been recently done on Electric Vehicles (EVs) [1, 2]. The development of an efficient and robust propulsion system is essential to the feasibility of EVs [3]. But, even though sophisticated engines and advanced electrical power trains exist, the main issue, the long-term energy storage, has not been resolved. All-electric drivelines based on battery, supercapacitor, flywheel, and combinations of these are being widely discussed and tested, attempting to lower the requirement on power density from the batteries [4, 5]. The propulsion system in development at Uppsala University is based upon a double wound flywheel energy storage device [6]. The flywheel under study is physically divided into two voltage levels through the stator winding. The high voltage side of the flywheel is connected to the wheel motor, whereas the low voltage side is connected to the battery, as shown in Figure 1. The system is bidirectional and the power can either flow from the battery to the wheel motor (acceleration mode) or from the wheel-motor to the battery (regenerative braking) [7]. Figure 1: All-electric driveline schematics: The power can flow in both directions. In acceleration mode, the flywheel function is basically to provide the variant power requested by the wheel motor, so that the battery delivers a smoother output power. In braking mode, the wheel motor acts as a generator, and the flywheel is responsible for storing the regenerated energy. The system needs a considerable number of power electronics converters and electronic controllers in order
On the Efficiency of a Two-Power-Level Flywheel-Based All-Electric Driveline
Johan Abrahamsson,Janaína Gon?alves de Oliveira,Juan de Santiago,Johan Lundin,Hans Bernhoff
Energies , 2012, DOI: 10.3390/en5082794
Abstract: This paper presents experimental results on an innovative electric driveline employing a kinetic energy storage device as energy buffer. A conceptual division of losses in the system was created, separating the complete system into three parts according to their function. This conceptualization of the system yielded a meaningful definition of the concept of efficiency. Additionally, a thorough theoretical framework for the prediction of losses associated with energy storage and transfer in the system was developed. A large number of spin-down tests at varying pressure levels were performed. A separation of the measured data into the different physical processes responsible for power loss was achieved from the corresponding dependence on rotational velocity. This comparison yielded an estimate of the perpendicular resistivity of the stranded copper conductor of 2.5 × 10 ?8 ± 3.5 × 10 ?9. Further, power and energy were measured system-wide during operation, and an analysis of the losses was performed. The analytical solution was able to reproduce the measured distribution of losses in the system to an accuracy of 4.7% (95% CI). It was found that the losses attributed to the function of kinetic energy storage in the system amounted to between 45% and 65%, depending on usage.
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