New Publications are available for Wave power
http://dl-live.theiet.org
New Publications are available now online for this publication.
Please follow the links to view the publication.A linear superconducting generator for wave energy converters
http://dl-live.theiet.org/content/conferences/10.1049/cp.2012.0298
Direct-drive superconducting linear generators can be an alternative for wave energy converter power take-off systems. A novel superconducting generator topology is presented in this paper. The generator has a stationary superconducting field winding fixed to modular magnetic cores. The translator is composed of air-cored copper windings. The generator is suitable for low-speed, high-torque applications. A 640 kW linear generator is designed. The design is validated using FEA tools and the specifications of the generator are presented. (6 pages)Extending the life of a generator through power management
http://dl-live.theiet.org/content/conferences/10.1049/cp.2012.0190
This work will outline the endeavours taken in the attempt of modelling the behaviour of a generator in an OWC with the aim of obtaining in-service load profile. Temperature and voltage stress-levels are calculated using mathematical models. These models, in conjunction with a chosen deterioration model of the electrical insulation systems, are used to predict the life of the generator. The paper then proposes power management strategies to extend the working life of the machine in case no maintenance is possible. (6 pages)Thermal modelling of a tubular linear machine for marine renewable generation
http://dl-live.theiet.org/content/conferences/10.1049/cp.2012.0302
A lumped parameter thermal model has been constructed for a tubular linear machine that has been designed for use in a marine environment. It shows good correlation to both steady state and transient experimental tests on the machine. The model has been developed for a stationary machine in a laboratory environment - the modelling techniques used and enhancements to enable the application of the model directly to marine scenarios are discussed. (6 pages)Optimum power control for arrays of direct drive wave energy converters
http://dl-live.theiet.org/content/conferences/10.1049/cp.2012.0160
This paper proposes control strategies for arrays of direct drive wave energy converters. Two control strategies taken from the theoretical study of the hydrodynamic properties of oscillating bodies are applied to a wave energy converter (WEC) system including a linear generator (LG) and voltage source converter system. The purpose of this study is to investigate the feasibility of the control strategies on WECs working in arrays. Simulations of an array of direct-drive WECs connected together by DC link were developed in Matlab/Simulink. The study concludes with a discussion on their feasibility in terms of electrical power generated and the deployment suitability in real seas. (6 pages)Reducing bearing wear in induction generators for wave and tidal current energy devices
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0220
Marine energy devices such as wave energy converters and tidal current turbines are set to play a significant role in contributing to electricity generating by renewable means. The offshore location where these devices operate demands high reliability and robust fault tolerance. Bearing failure is a key issue for renewable energy devices and is a cause of significant downtime in wind turbines. Bearing wear is accelerated due to unbalanced magnetic pull in the rotor of an electrical generator. This paper presents a review and assessment of methods to reduce bearing wear through both machine design and active control through power electronics. (6 pages)"Snapper" wave energy - the machine and grid interface system development
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0123
This paper introduces the reader to "Snapper", a novel wave energy converter designed to increase relative velocities within the generator, thus creating the potential to reduce the size, weight and cost of the energy conversion system. "Snapper" is being developed under a euro1.3million development grant awarded by the European Union's Framework 7 Programme (FP7). The objective of the project is to develop and validate a prototype "Snapper" system. This paper shares with the readers the "Snapper" concept with a focus on the development and validation of the "Snapper" machine and its corresponding grid interface system. (6 pages)Regulating DC link voltage fluctuations on a grid connected wave power system using energy storage
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0146
The fluctuating nature of power generated from waves combined with the weak nature of the network, makes the connection of arrays of wave energy converters (WECs) to the network a challenge. This paper investigates the research questions raised by the proposed development of a wave farm using coastal WECs, where the spatial layout of the WECs is restricted and cannot be used to damp the fluctuations in generated power. The concept of using energy storage on a common DC link to compensate for the fluctuations in real power, and ensure that the power quality on a weak, rural distribution network is not compromised, will also be discussed. (6 pages)Analysis, design and testing of a novel direct-drive wave energy converter system
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0122
A coupled electromechanical and hydrodynamic simulation of a direct-drive generator connected to a heaving buoy for wave energy conversion has been developed. The system is based around a novel linear generator referred to as 'Snapper" which incorporates a magnetic coupling, resulting in a latching power take-off mechanism. The system has been simulated in the time domain using the Matlab differential equation solvers, and a prototype generator designed, built and tested. (6 pages)Multiphase tubular permanent magnet linear generator for wave energy converters
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0111
Direct driven Permanent Magnet Linear Generators (PMLGs) are being used in Wave Energy Converters (WEC). Higher reliability and simplicity are achieved for WEC using PMLG due to the absence of mechanical or hydraulic systems in comparison with WEC using rotational generators. However, due to the relatively slow motion of the waves, the PMLG share large sizes and high magnetic forces. Additionally, a power electronics integration system converting the electrical output to match the grid requirements is needed. In this paper a Multiphase Tubular Permanent Magnet Linear Generator (MTPMLG) designed for WEC, is developed and simulated. The main aim of this paper is to present a performance comparison between the MTPMLG and existing PMLG using the Finite Element Method (FEM). The comparison between the two generators is based on the power output, the magnetic forces and the volumes of the structural materials used in their assembly. The results obtained present the advantages and disadvantages of using a multiphase linear generator over the existing three phase generators. All simulations in this paper are conducted using JMAG. (5 pages)Analytical model of an improved linear generator for seawave energy harvesting
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0230
This paper describes an improved topology of a linear permanent magnet generator for sea wave energy harvesting and presents an electrical model for analysis of this generator. The electrical model is established based on the magnetic equivalent circuit of the machine. This model can be used to design a power electronic interface for capturing maximum power at the resonant frequency of a sea wave. Validity of the model is verified via comparing the calculated inductance of the machine with that of obtained from numerical analysis based on finite element method. (4 pages)Full life cycle assessment of a wave energy converter
http://dl-live.theiet.org/content/conferences/10.1049/cp.2011.0124
The Pelamis wave energy converter is emerging as one of the most promising devices to harness the available power in the waves. This study examines the environmental impacts of the device, presenting the results as a set of impact potentials, and demonstrating that it performs well in comparison to other renewable energy converters and fossil-fuelled generators. (6 pages)Designing and deploying ethernet networks for offshore wind power applications - a case study
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.0247
This paper discusses special considerations that shall be taken into account when designing and deploying Ethernet networks for wind power applications. It also contains description of practical experience from a real project of a big offshore wind farms recently built and about to be put into service in United Kingdom. The paper is organized into two parts. First part describes the characteristics of an environmentally robust Ethernet network for wind farm automation applications and identifies key issues and considerations when designing an Ethernet network. The second part provides the description of a real project of a big offshore wind farm composed of in excess of 140 of wind generators where an Ethernet network of more than 200 switches had been deployed. The authors describe challenges found in designing big network on a geographically extensive area. (5 pages)Comparative investigation of losses in a reduced matrix converter for off-shore wind turbines
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.0096
This paper presents an analysis of losses of a high frequency link located inside the nacelle of wind turbines that will allow series connection offshore. The conversion system is based on a reduced matrix converter operated as a voltage source. Reverse blocking IGBTs are required for the operation and at the same time allowed for reduction of losses. Two carrier based switching strategies and a modified space vector modulation are implemented and discussed. Losses are calculated analytically and by simulation. A behavioral switching model of each IGBT is used. Results are conclusive about the superior performance of the proposed topology from the point of view of the losses. (4 pages)A new compact high power, low voltage variable speed drive for the renewable and offshore markets
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.0006
A new voltage source drive has been developed by Converteam. This drive uses a 3-level Neutral Point Clamped (NPC) inverter topology and has a nominal power rating of 2.5 MW. It is targeted primarily at the Renewable and Offshore markets but ultimately can be used in all of the markets served by Converteam. This paper gives a technical description of our new product and explains how it meets and in some cases exceeds all the requirements of our markets. (5 pages)Grid integration of offshore wind farms using multi-terminal DC transmission systems (MTDC)
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.0094
This paper discusses the control flexibility and fault ride-through capability of the bi-polar multi-terminal DC transmission system based on voltage source converters used for integration of large-scale offshore wind farms. Issues such as voltage support and short-term frequency stabilization of the AC network utilizing the stored energy in the wind turbine inertia and DC link capacitors are discussed. Regarding control flexibility, two aspects are presented: power sharing between the grid-side converters by any ratio, and provision of an alternative path for the power flow in case of a permanent DC fault or loss of one grid-side converter. In this investigation, the wind farms are based on fixed-speed wind generators, while the converters are modelled as a neutral-point clamped converter using the universal bridge. The investigation was conducted in Matlab/Simulink. (6 pages)Investigating the potential contribution of future offshore wind turbines to frequency stability during major system disturbances
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.1004
This paper investigates the limits to frequency response capabilities of wind turbines equipped with full converters and permanent magnet generators. It takes as a base case real frequency excursions on the GB network and demonstrates control methods to emulate a conventional power plant. However, it shows that droop may be limited to 4% or less and the effective inertial constant may not exceed 3 seconds without increased drive ratings. (5 pages)Dynamic modelling of VSC-HVDC for connection of offshore wind farms
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.1014
A VSC-HVDC (Voltage Source Converter - High Voltage Direct Current) dynamic model with a set of control strategies is developed in DIgSILENT Power-Factory with the objective of analyzing the converter's operating capability for grid support during grid faults. The investigation is carried out based on a 165 MW offshore wind farm with induction generators and a Low Voltage Ride-Through solution of the offshore wind turbines and Static Voltage Compensator units in the point of connection with a grid represented by a reduced four-generator power grid model. VSC-HVDC promises to be a reliable alternative solution for interconnection with off-shore wind farms as they become larger, with a higher installed power capacity, increased number of wind turbines, and geographically situated at larger distances from suitable connection points in the transmission grids. (4 pages)Thermal model of an induction generator in oscillating water column wave energy converter
http://dl-live.theiet.org/content/conferences/10.1049/cp.2010.0020
A thermal model for an induction generator in a renewable energy application is presented. The thermal resistances used in the model are found from steady state testing in an appropriate environment. The cooling in this application is inherently better than that normally experienced by this type of generator and thus a rating above nameplate can be achieved continuously. (6 pages)Wave power generation and its feasibility in Hong Kong
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.1788
Wave energy harnessing becomes a hot issue in exploiting renewable resources. In order to convert wave energy into mechanical energy and further using generators to produce electricity, various wave energy converters have been proposed. In this paper, these wave energy converters, including both the rotational generator type and the linear generator type, are reviewed. Consequently, a direct-drive linear generator is proposed and analyzed for wave power generation. Finally, the wave energy potentiality in Hong Kong and the corresponding wave power generation feasibility are discussed. (6 pages)Onshore power system, connection to reduce emissions
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0985
The presentation gives information about the actual status of emissions from ships in harbors. It outlines possibilities to reduce these emissions by using onshore power supply to provide ships with electrical power in harbors. Economical and environmental aspects are considered. (4 pages)Requirements for different components in cables for offshore application
http://dl-live.theiet.org/content/conferences/10.1049/cp.2009.0531
The installation of offshore wind-farms and the increase in electricity trading between countries has resulted in an increased number of offshore submarine cable installations as an efficient way to solve many electricity supply issues. Although offshore cables with XLPE insulation are similar to land based cables, the offshore environment adds to the challenges of very long lengths, much more onerous installation conditions and an aggressive environment. This paper discusses how these issues are confronted in distribution cable designs. In addition, the necessities for long-lasting medium voltage cables up to the lower high voltage level in wet conditions plus the increase in performance needs are discussed. It also covers the different requirements of 3 Core installation and water tree retardency tests in sea water applications. This paper will focus on medium and high voltage cables as they are used extensively for offshore applications connecting wind farms, for lake crossings and for inter- island connections. (4 pages)Ocean energy: the wave of the future
http://dl-live.theiet.org/content/conferences/10.1049/ic_20080651
The power point presentation discussed the developing technology of ocean energy with design convergence on tidal but not on wave. Today's technologies will help solve the immediate needs, but we need to work hard nurturing tomorrow's low carbon technologies today. Ocean energy represents one of the more difficult forms of renewable energy to harness. The UK is leading internationally in the development of marine energy but further development investment is needed to move the technology forward. Marine energy could supply up to 2 GW of UK electricity demand by 2020 and significantly more than this by 2050. The development of ocean energy and promising ocean driven machines are briefly reviewed, their operating conditions and the suitability of different types of hydro turbines for use as power take off options, the recent international experience, and how the technology is developing. (16 pages)Phase and amplitude control of a linear generator for wave energy conversion
http://dl-live.theiet.org/content/conferences/10.1049/cp_20080484
Direct drive power take-off for wave energy conversion has been proposed as a viable alternative to hydraulic and pneumatic based systems found in conventional wave energy converters. Allowing for further benefits to be realised, this paper presents a reaction force control scheme to maximise energy extraction, and investigates the modelling and simulation of a direct drive wave energy converter. The control scheme is applied to an experimental test rig with a prototype linear machine with results presented and analysed.Measurement and modelling of induction generator performance in an oscillating water column wave energy converter
http://dl-live.theiet.org/content/conferences/10.1049/cp_20080486
A study has been made on power takeoff from an oscillating water column wave energy device. An induction generator can be overloaded to meet peak powers. The maximum overload of an induction generator is limited by internal temperature which is determined by the cooling ability of its environment and the induction generator losses. Measurements of the cooling present in an OWC and its effects on the induction generator temperatures are detailed. A thermal modelling is carried out in MotorCAD and used to extend the measurements.Wave energy emulator and AC/DC rectifiers for direct drive wave energy converters
http://dl-live.theiet.org/content/conferences/10.1049/cp_20080485
This paper describes a solid state electrical emulator devised for laboratory testing of power conditioning electronics for direct drive linear wave energy converters (DDLWEC). Two rectification strategies are considered; a uni-directional boost topology, and an H-bridge which may be controlled in either uni- or bidirectional modes.Simulation of joint wind and wave loading time histories
http://dl-live.theiet.org/content/conferences/10.1049/cp_20060423
To harness vast amounts of energy with offshore wind turbines, it is necessary to analyze the dynamics of these structures. This requires simulation of the forces that these tall, flexible and slender structures are subjected to. Since sea waves are caused by wind blowing for a sufficiently long time, the state of the sea is related to wind parameters and the possibility of correlating wind and wave loading conditions on structures exists. Traditionally the literature available for wind and wave loadings has isolated the respective loadings on the offshore structures, possibly neglecting combined modes that may induce large structural vibrations in the process. Time simulation histories of joint wind and wave loading are not commonly available in literature. The Harris spectrum for wind loading is combined with the JONSWAP wave spectrum to formulate correlated wind and wave loadings. The simulation results for the time histories of wind and wave loading are presented and concluded upon.The dynamics of wave energy
http://dl-live.theiet.org/content/conferences/10.1049/cp_20060408
This paper examines the challenges of efficiently harnessing wave energy. A variety of energy conversion device types is reviewed and a generic heaving buoy device selected for detailed examination. A number of modelling and control challenges are detailed and a hierarchical control structure is indicated. Both potable water production and electricity generation are included as possible uses of such devices and each presents separate control challenges.Design and control considerations for a wave energy converter
http://dl-live.theiet.org/content/conferences/10.1049/cp_20040587
This paper discusses the main power take-off design issues and the resulting control strategy of a wave energy converter with the dual function of producing potable water and generating electricity.A low speed reciprocating permanent magnet generator for direct drive wave energy converters
http://dl-live.theiet.org/content/conferences/10.1049/cp_20020162
The design of a prototype linear variable reluctance permanent magnet machine is outlined in this paper. It has been built to investigate operation at speeds of 0.5 m/s, which is typical of a wave energy converter. Results are presented to verify the design, and to investigate the performance of the machine. In order to optimise power conversion, methods of compensating for the effects of the high inductance of these machines are discussed.Power quality assessment from a wave-power station
http://dl-live.theiet.org/content/conferences/10.1049/cp_20010828
This paper describes the development and testing of a software based flickermeter used in order to assess the supply quality from the LIMPET wave-power station on Islay. It describes the phenomenon of voltage flicker and the effect that a wave-power station has on this quantity. The paper also explains techniques developed in order to improve flickermeter performance when used with pre-recorded data. It also shows that the standard flickermeter sample frequency may be reduced for wave-station applications. Finally the paper presents flicker results from preliminary data collected from the LIMPET station and shows that the device is operating well within acceptable limits. (5 pages)The role of converters and their control in the recovery of wave energy
http://dl-live.theiet.org/content/conferences/10.1049/ic_19970916
Although mass generation of power from wave energy is still in its infancy, the OSPREY project has made a significant contribution to the development of the necessary technologies necessary to make the process viable. The power electronic interface plays a vital role in the system. It not only gives the opportunity to control the generator torque profile to ensure that optimum energy recovery is achieved for but also affords the luxury of being able to act and react to changes in environmental conditions. Cegelec proposed a common DC bus fed inverter solution. This configuration employs IGBT bridges for control of the generators. Energy recovered is then fed into a common DC link and returned to the supply system via a single fully rated anti-parallel thyristor supply bridge. An anti-parallel supply converter is used so that at times of exceptionally low wave energy, low speed rotation of the turbine can be achieved by reversing power flow (i.e. motoring). This ensures the OSPREY will start and be ready to absorb wave energy. (7 pages)Mathematical modelling of wave power devices
http://dl-live.theiet.org/content/conferences/10.1049/ic_19970556
It is not possible in a short paper to summarise the contribution of mathematical modelling to the development of wave power devices (WPDs) and for this reason it is not attempted. Instead a slightly different perspective is adopted; one in which a paper on mathematical modelling does not contain any mathematics, nor does it mention specific devices, but instead considers the background, contribution and challenges associated with WPD modelling. (7 pages)Wave energy plants - the way forward
http://dl-live.theiet.org/content/conferences/10.1049/ic_19970559
Successful large-scale implementation of offshore wave energy converters can only be achieved in stages. The technology development requires funding and a multi-disciplinary open co-operation. Practical engineering experiments and real sea experience combined with generic R&D investigating feasibility, reliability and survivability of larger units must gradually select the device types and principles best fit for offshore operation and commercial production. Existing expertise from cognate industries, such as offshore engineering, must be employed whenever possible. (5 pages)From wave to wire - control techniques for acceptable power
http://dl-live.theiet.org/content/conferences/10.1049/ic_19970558
The author describes how the use of control equipment on a wave power station can result in higher productivity, but it is important to balance the gains against the losses due to decreased reliability. At the end of the day, if the quality of the power supply is inadequate, the wave power system will be unacceptable. Also if the cost of the power supply is not competitive with other sources, then it will not prove applicable. At present, pressure must be placed on reducing the cost per kWh. Increased sophistication of control will only be of value if it results in a reduction in the cost per unit of electricity of energy of acceptable quality. (6 pages)Introduction to wave power
http://dl-live.theiet.org/content/conferences/10.1049/ic_19970555
Wave power, resulting from the transfer of energy from wind into water, represents a large potential source of energy for the UK, Europe and some other parts of the World. The wave power around the UK and Eire has an estimated annual average of 120 GW. When factors such as the need to avoid heavy shipping areas, the need to leave space between devices and the inability of devices to capture all of the incident energy are taken into consideration, the technical resource may be of the order of 25 GW. The technically achievable total world potential might be around 2000 GW. (8 pages)Learning form the Islay wave power plant
http://dl-live.theiet.org/content/conferences/10.1049/ic_19970557
In 1985, the wave power team at the Queen's University of Belfast commenced work on the prototype shoreline wave power plant on the Isle of Islay. A natural rock gully 4 m wide was selected in the south facing side of a headland to reduce the amount of construction and provide a partially sheltered environment in which to gain `sea experience' and conduct fundamental research, During 1988, a concrete plenum chamber 4 m wide, 9 m long and 9.7 m high was constructed and the pneumatic power output monitored for two years using a simple orifice. Finally, in May 1991, the plant was fully commissioned with the installation of the mechanical electrical plant and connection to the electrical distribution grid. The plant comprised a 1.2 m diameter biplane Wells turbine direct coupled to a 75 kW wound rotor induction motor operating as a generator above its synchronous speed of 1500 RPM. The design, construction and subsequent operation of the wave power plant has provided a significant insight into the feasibility of wave power conversion. The development of wave power plant poses a significant design challenge for civil, mechanical and electrical engineers. The plant must withstand the immense forces imposed during storms yet efficiently convert the slow cyclic motion of waves into a useful energy source such as electricity and do so at a mice competitive with other forms of generation. The step from the laboratory to prototype installations is considerable and was undoubtedly underestimated at the outset of the project. (5 pages)The role of converters and their control in the recovery of wave and wind energy
http://dl-live.theiet.org/content/conferences/10.1049/cp_19960909
Although mass generation of power from wave energy is still in its infancy, the OSPREY project has made a significant contribution to the development of the necessary technologies necessary to make the process viable. The power electronic interface plays a vital role in the system. It not only gives the opportunity to control the generator torque profile to ensure that optimum energy recovery is achieved, but also affords the luxury of being able to act and react to changes in environmental conditions. The author describes the synchronous and induction generator drive converters. The latter is based on the use of voltage source force commutated technology due to the need for four quadrant operation. IGBT (insulated gate bipolar transistor) based inverters offer the most advanced control and highest performance. At present this technology is available up to ratings of approximately 1.5 MW. GTO (gate turn off thyristor). The author also discusses constant speed, power and torque control. The addition of a wind turbine to OSPREY is also considered.Experimental results from the operation of aggregated wave energy converters
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2010.0234
Wave energy comes in pulses and is unsuitable for direct conversion and transmission to the grid. One method to smooth the power is to deploy arrays of wave energy converters (WECs), the geometrical layout and damping optimisation of which many have studied analytically and numerically, but very few by experiments at sea. In this study, the standard deviation of electrical power as function of various parameters is investigated. Two offshore experiments have been conducted. During the longer run, three WECs were operated in linear damping during 19.7 days. It is shown that the standard deviation reduces with the number of WECs in the array up to three WECs. The reduction compared to single WEC operation was found here to be 30 and 80% with two and three WECs, respectively, as a mean for an arbitrary array member. It is found that in sea states above ∼2 kW/m, the standard deviation is independent of sea state parameters. This is contradictory to a previous study on the same device. The results are, however, in accordance with numerical results of the SEAREV device but show larger reduction in standard deviation with number of WECs. This could be because of suboptimal damping conditions.Multi-phase air-cored tubular permanent magnet linear generator for wave energy converters
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2011.0190
Direct driven permanent magnet linear generators (PMLGs) are an alternative solution for wave energy converters (WECs). Generally, problems such as high magnetic attraction forces between the permanent magnets and the magnetic core are associated with direct driven PMLG. To eliminate the attraction, air-cored generators can be used. They do not contain any stainless steel in either the stator or the rotor and therefore there is no magnetic attraction between the moving and the stationary parts. In this study, a novel design of multi-phase air-cored PMLG is proposed. The main advantage of the generator is the reduction in the Lorentz forces acting on the bearings by addressing the force in the direction parallel to the motion axis and elimination of cogging forces. Additionally, in the study a new system bypassing inactive coils is proposed and simulated as part of the grid integration system. The system achieves implementation of a small number of elements connected in series with the coils and hence the thermal losses in the grid integration system are reduced. All simulations are made by means of finite-element (FE) software working simultaneously with Matlab/Simulink.Integrated structural and electromagnetic design of direct-drive linear machines for wave energy
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2011.0002
Combined electrical and structural models of four types of permanent magnet linear electrical machines suitable for direct-drive power take-off in wave energy applications are presented. Electromagnetic models were developed using polynomial approximation to finite-element analysis results. The structural models are based on simple beam theory and other classical techniques. Unoptimised example designs are provided for each machine using the integrated tools, demonstrating their potential usefulness in an integrated optimisation process.Power conditioning of the output from a linear vernier hybrid permanent magnet generator for use in direct drive wave energy converters
http://dl-live.theiet.org/content/journals/10.1049/ip-gtd_20045134
A linear electrical generator may be used to convert the energy in sea waves directly into electrical energy. The reciprocating movement of the generator caused by the waves results in an electrical output that is variable in voltage and frequency, with a low-frequency amplitude oscillation of the same frequency as the reciprocating vertical movement of the waves. Power conditioning is required to extract the energy from the linear generator, and invert it into a mains supply of constant voltage and frequency. A type of power conditioner based on an AC/AC converter is illustrated. Inversion into the mains must take place at constant power by the power conditioner, which must therefore buffer the energy fluctuations produced by the wave energy device. The energy storage requirement for the power conditioner is derived as a function of the generated power, the reciprocating frequency of the wave energy device, the average AC/AC converter DC voltage and the capacitor voltage ripple. Results are presented for the operation of a prototype linear generator and power conditioner inverting into the three-phase supply.Electrical generators for direct drive wave energy converters
http://dl-live.theiet.org/content/journals/10.1049/ip-gtd_20020394
Wave power devices traditionally use conventional rotary electrical machines for power conversion. However hydraulic systems or air turbines are required to convert the low reciprocating motion of the wave device to rotation at 1500 rpm. The concept of a direct drive system is introduced, in which a reciprocating electrical machine is driven at the same speed as the device. A linear permanent magnet synchronous machine is compared to the transverse flux machine in a basic design study for this application. The latter machine is identified as offering the best potential. Electromagnetic and electric circuit models are developed to investigate the performance of the transverse flux machine in a wave energy converter. Measures are suggested to optimise the performance of the machine..Ocean wave energy absorption in response to wave period and amplitude – offshore experiments on a wave energy converter
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2010.0124
The ability of a wave energy converter to capture the energy of ocean waves has been studied in offshore experiments. This study covers 50 days during which the converter was subjected to ocean waves over a wide range of frequencies and amplitudes as well as three different electrical loads. The results present the wave energy converter's energy absorption as a function of significant wave height, energy period and electrical load. It is shown that the power generated overall continues to increase with wave amplitude, whereas the relative absorption decreases towards the highest periods and amplitudes. The absorption reached a maximum of approximately 24% with the used combination of buoy, generator and electrical load. Absorption to cover for iron and mechanical losses has not been included. A brief study of the nature of the electromagnetic damping force has also been included in the study. The wave energy converter is of the technology that is being researched at Uppsala University and experimented on off the Swedish west coast at the Lysekil wave energy research site.Electrical issues associated with sea-water immersed windings in electrical generators for wave- and tidal current-driven power generation
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg_20080083
For the majority of proposed wave- and tidal current-driven power generation applications, the electrical generators are submerged in sea water, frequently at many metres of depth. The environment places significant stress on the rotating or translating seals between the driven shaft and the electrical generator leading to reduced reliability and lifetime. A potential solution is to eliminate the seal, thereby flooding the generator and allowing sea water to circulate around the shaft, windings and rotor of the machine. The impact of immersing the windings of the machine in sea water is assessed here. Specifically, the impact that the insulation has on the leakage capacitance as well as the consequent impact the leakage capacitance has on current and voltage oscillations in the switching converter used to excite the winding is assessed. Thermal tests are conducted to assess the impact of the insulation on the thermal conductance of the coil–insulation system. Experimental evidence is provided.Reaction force control of a linear electrical generator for direct drive wave energy conversion
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg_20060028
Direct drive wave energy converters have been proposed in view of the disadvantage of mechanical complexity and low conversion efficiencies in conventional wave energy converters. By directly coupling a linear generator to a reciprocating wave energy device, it is suggested that direct drive power take-off could be a viable alternative to hydraulic- and pneumatic-based systems. To further realise the benefits of a direct drive system, a control scheme based on reaction force control to maximise energy extraction is presented. It focuses predominantly on the theoretical analysis of the linear generator reaction force. The modelling, simulation and control of direct drive wave energy conversion are systematically investigated by computer-aided analysis via Matlab/Simulink.Power conversion and control for a linear direct drive permanent magnet generator for wave energy
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2009.0125
This study describes the power conversion and control solution used in the electrical power take-off of a 35 kW test rig developed to investigate a linear, direct drive, air-cored, tubular, permanent magnet generator for an offshore wave energy device. The solution proposed is to collect the power extracted directly from individual coils of the generator, which have different induced voltages and cannot easily be connected into a small number of phases. Local energy storage is integrated into the system to smooth the electrical output power and reduce the rating of the downstream inverter for grid interfacing. The solution is demonstrated by analysis, backed up by simulation and test results. This shows the potential and limitations of the proposed conversion technology solution.Comparison of wave power extraction by a compact array of small buoys and by a large buoy
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2009.0166
Wave energy converters are usually designed to achieve maximum efficiency by impedance matching with the incoming waves. One of the simplest design is a buoy attached to an energy absorber (power-takeoff devise). For maximum efficiency, the buoy must be large enough in order to resonate at the peak frequency of the incident sea, and the extraction rate is the same as the rate of radiation damping. With one or few degrees of freedom the resonance peak(s) is(are) usually band-limited. The latter feature is common to all wave-power absorbers based on impedance matching. Inspired by the FO3 system being developed in Norway by Fred Olsen and ABB Associates, and the Manchester Bobber from UK, the authors examine theoretically power extraction by a compact array of small buoys, that do not resonate but possess many degrees of freedom. It is shown that such systems have certain advantages over a single large buoy.Fault ride through of fully rated converter wind turbines with AC and DC transmission systems
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2008.0018
Fault ride through of fully rated converter wind turbines in an offshore wind farm connected to onshore network via either high voltage AC (HVAC) or high voltage DC (HVDC) transmission is described. Control of the generators and the grid side converters is shown using vector control techniques. A de-loading scheme was used to protect the wind turbine DC link capacitors from over voltage. How de-loading of each generator aids the fault ride through of the wind farm connected through HVAC transmission is demonstrated. The voltage recovery of the AC network during the fault was enhanced by increasing the reactive power current of the wind turbine grid side converter. A practical fault ride through protection scheme for a wind farm connected through an HVDC link is to employ a chopper circuit on the HVDC link. Two alternatives to this approach are also discussed. The first involves de-loading the wind farm on detection of the fault, which requires communication of the fault condition to each wind turbine of the wind farm. The second scheme avoids this complex communication requirement by transferring the fault condition via control of the HVDC link to the offshore converter. The fault performances of the three schemes are simulated and the results were used to assess their respective capabilities.Operation analysis of a wave energy converter under different load conditions
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2010.0153
This study analyses the electrical behaviour of a direct-driven linear generator under different load conditions. The studied generator is used in a wave energy converter (WEC) that converts the energy in ocean waves into electric energy. To enable a grid connection of a WEC, the voltage must be converted, and thereby, the generator will be subjected to a non-linear damping. Depending on how the conversion system is designed, the damping will be different. In the case studied, the voltage is first rectified, and on the dc-side of the rectifier the voltage is kept constant by controlling the power through a converter. In order to study the electrical behaviour of the generator in this operation mode, a simulation model was made in MATLAB Simulink. The model of the generator was verified with experimental data from an offshore operating WEC. The result of the study shows that the model of the generator agrees with the real generator and can be used for analysing the electrical behaviour of the WEC. Moreover, the results show that the operation with a non-linear load will be different compared to a linear load case.Multi-megawatt offshore wave energy converters – electrical system configuration and generator control strategy
http://dl-live.theiet.org/content/journals/10.1049/iet-rpg.2009.0090
Considerations of the electrical system configuration and the control strategy for the wave dragon (WD) multi-megawatt (MW) offshore wave energy converter (WEC) are presented in this study. The electrical system consists of 16–20 turbine–generator–AC/DC/AC converter units. The characteristics of the low-head water-turbine and generator control strategy enabling maximum energy capture and the best performance for the MW offshore wave energy power take-off system are described in detail. Simulation results from a single turbine–generator–frequency converter unit at different turbine heads are presented at the end of the study.