ΦAbstract – The development of wave and tidal energy power generation technology is moving steadily towards pre-commercial status. The multitude of widely differing power capture technologies means that it is a difficult industry to analyse in terms of a common approach to optimal generator selection or design. This paper examines some of the overarching requirements that will pertain to all device developers in terms of generator technology. It then examines some of the requirements imposed by the individual device technologies and the sometimes widely differing functionality of the generator operation and control in each case. In conclusion, this study will assist in identifying the design drivers for off the shelf (OTS) machine selection or indeed custom machine design in ocean wave and tidal devices.

Index Terms-- AC generators, AC machines, Marine technology, Energy conversion

I. NOMENCLATURE OEC Ocean Energy Converter WEC Wave Energy Converter TEC Tidal Energy Converter OTS Off-the-shelf SG Synchronous Generator PMG Permanent Magnet Generator DFIG Doubly Fed Induction Generator SCIG Squirrel Cage Induction Generator OWC Oscillating Water Column PTO Power Take-Off OTS Off-the-shelf FS Fixed speed VS Variable speed GBC Gearbox coupled SVC Static Var Compensator Pu Per unit quantity

II. INTRODUCTION HE selection or design of a particular machine technology for the main power generator in an electricity generating device or station is an important

step. In conjunction with any associated control or power conversion equipment, it has a significant influence on the system efficiency, reliability, controllability, and grid code compatibility.

On-line synchronous generators form the backbone of traditional fossil fuel generation power systems. These run at effectively constant speed, are synchronized to the electricity grid, and are optimised for the speed at which they run. However, in the field of renewable power generation, speed variation in the generator control is often vital in order to

This research was funded by the Charles Parsons Energy Research Initiative under the auspices of Science Foundation Ireland.

D.L. O’Sullivan is with Hydraulics & Maritime Research Centre, University College Cork, Youngline Ind. Est. Pouladuff Rd., Cork, Ireland (e-mail: dara.osullivan@ucc.ie).

A.W. Lewis is with Hydraulics & Maritime Research Centre, University College Cork, Youngline Ind. Est. Pouladuff Rd., Cork, Ireland (e-mail: t.lewis@ucc.ie).

maximize the primary power take-off efficiency from the renewable source, which by its nature is usually highly variable in time. For example, in wind power generators the available power take-off increases substantially if the turbine rotational speed is controlled to increase as a defined function of the wind speed [1]. This control strategy is known as Maximum Power Point Tracking and has been responsible for the gradual transition of wind power generator technology from fixed or dual speed, to variable speed, machines. Likewise, under heavy gust or swell conditions, a fixed speed generator will experience severe shock loads on the generator shaft, whereas if the speed is allowed to increase, the inertia of the system will absorb some of the extra power input. This mechanical consideration initially led to the adoption of asynchronous generators in wind turbines where the slip range was utilised to provide a small measure of speed variation [2] and speed compliance. Extensions to the speed range were also provided for by pole changing or rotor resistance variation. However, in recent years, the improvement in cost and performance levels of high power switching transistors has led to the adoption of fully variable speed controlled generators. These have typically taken the form of either gear-coupled DFIGs with power electronics control of the rotor voltage and frequency, or direct-coupled SGs with power electronics control of the stator voltage and frequency, and either brushless field excitation or permanent magnet excitation (PMG).

It would appear then that turbine efficiency optimization, mechanical shock alleviation and, as ever, cost, have been the principal design drivers in the historical development of wind turbine generator technology. These design drivers have led to a clear convergence towards one or two electrical machine technologies. The purpose of this paper is to seek to identify whether similar design drivers can be identified in the case of ocean energy conversion. The dispersed nature of OEC technology makes this a less straightforward task, and there is no ‘one size fits all’ solution in this case. It is possible to identify some common generator design requirements and also some design requirement sub-categories, that will assist in generator machine selection or design.

III. COMMON REQUIREMENTS The OEC generator design requirements that are

technology neutral and common to all devices can be summarized as those pertaining to the environmental conditions, and to the grid connectivity. These are addressed in this section.

Suitability to sustained and reliable operation of the generator in the harsh environmental conditions of the offshore marine environment is clearly a significant and important requirement. This is examined in terms of the mechanical and environmental wear sustained by machines in such an environment. One important issue to be tackled is

Generator Requirements and Functionality for Ocean Energy Converters

Dara L. O’Sullivan and Anthony W. Lewis

T

XIX International Conference on Electrical Machines - ICEM 2010, Rome

978-1-4244-4175-4/10/$25.00 ©2010 IEEE

the feasibility of the use of brushed machines such as the DFIG and the brushed SG in such an environment, and the consequent maintenance requirements.

A. Brush Operation Brush wear in brushed machines is the result of

mechanical friction and electrical erosion. Friction produces carbon dust, while the result of electrical erosion is the vaporization of carbon with little physical residue. In order to achieve a good coefficient of friction between the carbon brush and the slip ring, it is necessary to establish a good carbon composite film [3]. A good film layer can reduce the coefficient of friction to 10% of the original bare coefficient. In order to maintain a good working film, brushes should ideally operate close to the rated load current. Operation in over-current causes slip ring blackening and reduced brush life, whereas protracted light load operation results in film removal and increased brush friction and wear. The differing power profiles of typical wave and wind energy converters are depicted in Figure 1 for a time series of 12 s. While the power input to the wind device is oscillatory, its variation around the average power level is significantly less severe than for the wave device, where the power input fluctuates to zero once every half-wave cycle. The high pulsating nature of wave energy power flows is thus clearly not well matched to the desired electrical operating point of a brush-slip ring arrangement.

050

100150200250300350400450500

0 2 4 6 8 10 12

Time(s)

Pow

er(k

W)

(a)

(b)

Figure 1: Power time series for (a) WEC (b) wind turbine over a 12s time slot (average power is indicated by the dotted line).

It is evident from Figure 1 that without significant inherent energy storage, WECs require a high peak to average rating, and so will operate close to their peak or overload current ratings for a significant proportion of the time. This represents a further complication in the employment of brushed machines. In a recent publication sponsored by Vestas Wind Systems [4], it was discovered that under high current operation, brush-slip ring systems can periodically enter film instability modes that are characterized by severe brush wear and high brush temperature, increasing the wear rate by approximately 40% more than the expected wear rate. This is less of an issue for

TECs which will have an input power profile more akin to a wind energy device.

The other factors that inhibit good brush film formation are high humidity and the presence of chemical contaminants in the air. While these environmental factors are issues for offshore wind turbines also, humidity and air vapour control are being built into modern offshore wind turbines for these reasons [5]. The inclusion of such air quality control in the generator enclosure of OECs is usually less feasible since the generator is often situated in an inaccessible location.

The minimum brush life of a general purpose, slip ring machine is typically 3,500-8,750 hrs. The effective operating period of an OEC at a good site is around 5,000 hrs annually. Thus, in order to avoid costly outages, or even generator damage, the brushes ought to be changed at least twice annually, which corresponds with best practice in the wind industry, where brush changes are carried out typically every six months and potentially every three months [6].

B. Operation and Maintenance The option of using the DFIG machine or the brushed SG

machine requires the presence of brushes in the system which, as previously mentioned, must be maintained and replaced on a regular basis, typically twice a year. Depending on the site specifics this is potentially a more serious issue for OECs than for wind energy, even offshore wind. In the case of WECs, if on-board maintenance is considered safe up to a 1m swell, then statistically this only allows a probable 7 days in the year for maintenance, in a typical North Atlantic offshore location. If on-board maintenance is allowed up to a 1.5m swell, there will be on average about 55 days in the year when such maintenance is possible. This problem became apparent for the first time in the Bockstigeen wind farm approximately 12km of the Swedish west coast [2]. Docking of the maintenance boat proved to be extremely difficult even at a wave height of a little over 1m. Subsequent to this experience, alternative approaches are being explored in accessing offshore wind turbines, including movable docking rails, submarine vehicle and diver access points, as well as helicopter pads located on the nacelle [2, 7]. Moreover, once access is possible, the actual maintenance procedure can proceed in a relatively stable and protected environment. Clearly the situation is even less straightforward for floating offshore WECs. Access is likely to only be by boat, and the working environment itself may not be stable. These factors and the consensus of the industry and research community [8] appear to strongly support long lifetime, low or even zero on-board maintenance designs being a requirement. These considerations would appear to rule out the use of the DFIG (as well as the brushed SG) in offshore WECs, despite its clear advantage in terms of size, cost and efficiency.

In the case of TECs, the issue of access for maintenance is technology dependent. For seabed mounted devices, again low maintenance designs are essential, however, some TECs such as the SeaGen device [9] provide access techniques, in this case a moving platform that enables the blades and generator nacelles to be raised above the water surface for maintenance.

Some WEC technologies [10, 11] locate the generator onshore, alleviating the issue of access and maintenance, thus opening up the option of brushed machines.

C. Corrosive environment All machine types will be protected to a high degree from

the worst effects of the environment. However, if the generator is located offshore in the marine environment, it is vulnerable to the effects of saline air or moisture. This will have the most detrimental effects on a PMG machine since NdFeB, which is the material of choice for high performance permanent magnet machines, is very sensitive to corrosion [12]. These materials can be destroyed within days if certain forms of corrosion take hold. This fact represents a disadvantage for the PMG in this regard, although epoxy coatings such as VACCOAT™ have been developed for the protection of such rare earth materials in saline environments [13].

D. Mechanical Issues Typical heave motions of a WEC device during a severe

sea state are illustrated in Figure 2. This data is taken from wave basin model tests, scaled to a full scale prototype. Motions up to 6m in amplitude can occur in time periods of 3s, along with pitching motions leading to angular accelerations greater than 7 deg/s2. It is evident that such conditions apply severe mechanical stress on the system components, and bearings and couplings will have to be rated to absorb these shock loads. Moreover the pitching motions of the device will induce gyroscopic loadings on the bearings.

Initial analysis of the impact of these motions on bearing design indicates that the static loading of the bearings for a horizontal axis rotary generator, within a typical full scale device layout, increases at least twofold.

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2

4

6

8

0 5 10 15 20 25 30

Time(mins)

Heave(m)

Figure 2: OWC heave motions

Generators with a higher power to weight ratio will have an advantage in this regard, as the shock loading and bearing ratings will not be as severe. In the power range of interest, the 4 pole SCIG has a ratio of 4-4.5 kg/kW, as compared to 3-3.8kg/kW for the 4 pole SG and PMG [14]. Hence the SCIG has a weight penalty of 20-30%. The surface magnet PMG, however, has an additional problem in that the permanent magnets are brittle and can be cracked under mechanical shock unless precautions are taken. This is less of an issue in TECs as they are generally quite stationary.

E. Grid Connection The connection requirements for generators connecting to

the distribution and transmission networks are specified in an appropriate distribution or grid code [15]. These connection codes have been typically updated to incorporate the connection of renewable power generation, with the emphasis being on wind energy. It is anticipated that similar connection conditions will be imposed on wave energy generators. The most pertinent requirements of the grid connection code that are influenced by the generator

selection are as follows: • Flicker • Voltage distortion • Voltage limits • Reactive power requirements • Fault ride-through • Power ramp rates

In terms of grid connection, the presence or absence of power electronics converters in the generator system has an important effect, and grid connection is examined for both cases, illustrated in Figure 3.

DC

Full rating power converter

LV

MV

LV

LV

LV

MV

MV

MV

Partial rating power converter

SG

SCIG

DFIG

PMG/SG/SCIG

Direct Grid Connection

Converter Connection

Figure 3: Generator grid connection options 1) Direct grid connection

Some of the grid code requirements put direct grid-coupled SCIGs in WECs at a significant disadvantage. With their very limited speed variability (typically <5% above synchronous speed although the exact slip range is dependent on machine power rating and speed), these generators can only provide minimal power smoothing, and then only in conjunction with a very large inertia. Thus, the full power variation inherent in the wave energy source is transferred to the electrical network unless the WEC contains significant inherent energy storage. This represents the worst-case situation for voltage variation, ramp rates, and flicker requirements, particularly in a weak network region. Furthermore, the SCIG cannot provide reactive power to the grid, nor can it maintain its magnetization through a sustained voltage dip, without an external reactive power and voltage support mechanism, such as a Static Var Compensator. These in turn introduce a significant cost element [16]

It is possible to use a direct grid-connected SG where the OEC contains significant energy storage (e.g. an overtopping device) or in TECs where the resource input is not as variable. Such generators can provide reactive power to the grid and have the capability to ride through faults. 2) Converter connection

In the subgroup of converter connected generators, the DFIG is at a disadvantage to the PMG, SCIG and SG. The fault ride-through performance of the DFIG has come under considerable scrutiny, due to its prevalence in wind turbine

generators. It has been shown to be significantly more sensitive to grid faults than those generator options with a full power electronics conversion interface [17]. The PMG, SCIG and SG all have full frequency converters in place between the machine stators and the grid, and so are more significantly decoupled from its influence. The presence of a full frequency converter with a capacitive dc bus also allows the potential to provide reactive power to the grid and thus participates in grid voltage support. The availability of wide-ranging speed control also gives more options in terms of inertial power smoothing which can help in the reduction of grid voltage variations and flicker levels. Moreover a frequency converter will not feed a grid fault beyond the overcurrent rating of the converter. This can be a significant advantage in areas where the existing network equipment is operating close to the limit of its fault level capacity. It can also, however, be seen as a disadvantage in that frequency converters may not be able to provide enough fault current in the event of a fault occurrence to trip the system breakers, resulting in a fault remaining undetected. In summary, the generator options with full frequency converters have greater flexibility in meeting the requirements of existing renewable energy grid connection codes.

IV. REQUIREMENTS BY DEVICE CATEGORY As outlined previously, there exists a profusion of ocean

wave and tidal energy devices under development. It is not easy to come to any clear conclusions regarding which category or categories of device will end up being commercially successful. In this section, a range of some of the major device categories are examined, and the specific generator machine characteristics assessed for each category. The device categories to be considered are:

1. Oscillating water columns (OWC) with air turbines

2. Point absorbers with hydraulic PTO 3. Overtopping or pump devices with hydro PTO 4. Point absorbers with direct PTO 5. Tidal turbines 6. Tidal current oscillating hydrofoils

A. OWC [18, 19] In an OWC device, wave action is converted to an

oscillating air flow by means of hydrodynamic pressure variations across a water column inside the device which in turn pressurizes and depressurizes the air in an air chamber. The oscillating airflow enters and exits the air chamber through a ducting arrangement in which is usually inserted a bidirectional air turbine, such as a Wells or impulse turbine [20]. The presence of the duct acts effectively as a gearing mechanism converting low velocity airflow in the chamber to high velocity flow across the turbine blades. The consequences of this for the generator, is that the turbine shaft can usually be gearless and still operate in a relatively high speed range. In Wells turbine designs OTS 2 or 4 pole machines are possible. Impulse turbines tend to operate at a somewhat lower speed, but can still operate within the range of a 6 or 8 pole machine. A wide operating speed range is generally optimal in these situations, since the applied airflow profile is typically highly variable, and the efficiency of the air turbine is optimized by varying the speed to match an instantaneous or averaged operating point of maximum efficiency yield [21]. These devices do not tend to have any significant inherent energy storage, apart from the inertia in

the mechanical system. Thus, generator machines with high peak to average torque ratios are suitable in order to keep the machine rating reasonable. However, if there full rated power electronics in the system, this will have to be rated close to the peak generator power.

B. Point absorbers with hydraulic PTO [22, 23] Point absorber devices are generally axi-symmetric about

a vertical axis and are characterized by being small in comparison to the incident wave length. A buoyant body, termed a displacer which can be either surface piercing or submerged, moves with the wave motion against a stationary or slow moving reactor. The relative motion of the two drives a hydraulic ram. This slow moving pumping action is converted to a higher speed rotating motion in a hydraulic circuit which may involve one or more accumulators and one or more hydraulic motors. The hydraulic motor is generally directly shaft connected to the main generator. The speed and torque characteristics of the generator in this case are greatly influenced by the hydraulic circuit design. If significant accumulator energy buffering is present in the hydraulic circuit, the rotational speed of the generator can be constant [22] or have a narrow range of variation, although variable speed designs offer some advantages in terms of efficiency [23]. Rotational speeds can be designed to match OTS generator designs. If there is sufficient flexibility in the hydraulic circuit design, the torque and power rating of generators in these devices can be relatively benign, and be selected close to the device rated power.

C. Overtopping/pump devices with hydro PTO [24, 25] Overtopping devices extract energy from the sea by

allowing waves to impinge on a structure such that they force water up over that structure into a reservoir, thus raising its potential energy. Pump type devices use the direct forces of the waves on a hinged device structure to pump water into a raised reservoir in a similar manner. The potential energy of the water is converted to kinetic energy using a conventional hydro turbine. After exiting the turbine, the water is then returned to the sea. Due to the presence of the large reservoir, the turbine-generator combination can run at fixed speed, however, allowing some variation in the speed range enhances the hydro turbine efficiency. Variable discharge operation is then possible with fixed guide vanes and fixed runner blades [24]. As with conventional hydro turbines, speed levels tend to be quite low, of the order of 100-300rpm, generally necessitating a gearbox. The presence of the large reservoir buffers the generator from the inherently large peak to average power ratio of the resource, and allows for total generator power rating close to the device rated power level.

D. Point absorber with direct PTO [26] Instead of driving a hydraulic ram or pump, the motion of

a point absorber can be directly connected to the electrical generator via a mechanical linkage of some sort. One example of this would be a direct drive linear generator [27], where the motion of the point absorber is directly coupled to a linear translator, which acts as the moving element in a linear generator. Another example would be a direct drive rotary generator [28], where the bidirectional linear motion of the point absorber is converted to bidirectional or unidirectional rotary motion of a generator via mechanical couplings such as pulleys, clutches, belts or gears.

Linear generators are most definitely a non-OTS design, and represent a very difficult generator design problem due to their low velocity. This results in very large torque and current requirements, which in turn are limited by magnetic saturation and thermal limits in the generator windings. A further issue is the attractive forces between stator and translator. This can be alleviated by utilizing a non-ferromagnetic core in either the translator or stator [27]. However, these generators are still very much at the development stage. They require converter connection to the grid as they are inherently variable speed. PMGs are realistically the only feasible machine design in this case.

Direct drive rotary generators can be OTS designs, given appropriate gearing arrangements. These require converter connection to the grid also as they are inherently variable speed. If a clutching mechanism is present, the generator can be unidirectional, and the presence of inertial storage will improve the peak to average rating requirements of the machine. Clutching mechanisms can present maintenance issues, so bidirectional generator operation can also be considered. High part load efficiency is very important in this case [29], as the operating point of the generator us constantly changing making PMGs the most attractive option again.

In both cases, the generator system must absorb high torque pulsations, since there is no inherent energy storage. This can be absorbed somewhat by inertial speed changes, but it is likely that the peak power rating of both generator and power electronics will be significantly greater than the average device power rating.

E. Tidal turbines [30, 31] Horizontal axis tidal turbines are the marine equivalent of

wind turbines, with energy being extracted through the lift forces of the moving water on the turbine blades. The significantly higher density of sea water relative to air results in significantly smaller diameter turbine blades than equivalent power rated wind turbines. Vertical axis turbines are typically cross flow turbines, whose rotation direction is perpendicular to the prevailing flow direction. In terms of energy conversion and dynamic performance, they are similar to horizontal axis turbines.

These turbines operate at very low rotational speeds, typically <10rpm, and hence a gearbox is an absolute necessity. An OTS generator can then be utilized, either fixed or variable speed, although, as in wind turbines, variable speed offers advantages in terms of power production and mechanical drive-train shock loading, particularly with a high ratio multi-stage gearbox in the system. Peak torque and power ratings are quite benign, in similar fashion to wind turbines.

F. Tidal current oscillating hydrofoils [32] Oscillating hydrofoils utilize the aerodynamic principle of

lift due to a moving fluid passing over a hydrofoil. The hydrofoil is typically fixed to a moving arm by which the lift force is transferred to a pumping mechanism for driving reciprocating hydraulic ram pumps which in turn power a prime mover such as a hydraulic motor. The angle of attack of the hydrofoil is actively controlled in order to maintain optimum power take-off throughout the stroke, and in order to reverse direction at the ends of the stroke. The generator characteristics will be similar to point absorbers with hydraulic PTO, although the power input is not pulsating,

and so peak power and torque rating will be benign.

V. GENERATOR FUNCTIONALITY: DESIGN IMPLICATIONS It can be understood from the previous section that the

specifics of an OEC’s inherent operation greatly influence the generator choice and design. Of equal or greater impact as a design driver is the generator functionality within the context of the device operation. This functionality can vary significantly even within the same device. In this section the different operational functionalities of the generator are categorized, and their influence on the generator design parameters is discussed.

A. Power conversion The most basic functionality requirement of a generator

in an OEC is that of mechanical to electrical power conversion – in similar manner to the functionality of a generator in a fossil fuel power plant. In this case, the generator does not participate in controlling the power take-off action, but simply converts the incoming mechanical power to an electrical output. Some examples of this would be:

• Constant speed tidal turbines. • Point absorbers with hydraulic PTO, accumulator

smoothing, and constant speed hydraulic motor and generator.

• Overtopping devices with multiple constant speed hydro turbines.

The torque range of the generator in this case will be completely dependent on the primary power capture and prime mover mechanisms, and any inherent energy storage. It is likely that the generator will have to operate over a wide load range continuously, and so part-load efficiency becomes an important variable. Typically, the generator will operate at constant speed and will be directly grid connected.

B. Prime mover efficiency optimization This is probably the most common requirement of a

generator in ocean energy devices, and corresponds to the generator functionality of the majority of variable speed wind turbines. The generator speed and/or torque are specifically controlled to optimize the performance and efficiency of the prime mover[21]. Some examples of this are:

• OWCs with variable speed air turbines. • Variable speed tidal turbines. • Overtopping or pump devices with variable

discharge or variable jet hydro turbines. In these cases, the generator control is the means of prime

mover efficiency optimization. It is important for the generator to have a wide speed and torque control range in order to optimize this efficiency. Generator torque ratings can be eased due to the wider speed range and consequent absorption of some of the input power in the system inertia.

C. Power smoothing The generator control can allow for smoothing of the

electrical output power, in conjunction with the inertia of the rotating system, which can be enhanced by additional flywheel inertia. This is beneficial to output power quality, and may overlap in terms of control strategy with prime mover efficiency optimization, but may also compromise it somewhat due to the limitation in response time if there is significant added inertia.

The use of a generator in this mode of operation can be beneficial to its rating specification, as significant torque pulsations can be absorbed by the inertia. The generator and power converter peak power rating can then be relatively close to the maximum mean power rating of the OEC.

D. Device damping control In this case, the generator is controlled to directly

influence device motions. This is the most effective approach to enhancing overall OEC efficiency, but also places the most stringent demands on the generator ratings. The generator control is utilized to adjust the reaction force to the device motion in, for example:

• Point absorbers with hydraulic PTO. • Tidal current oscillating hydrofoils. • Point absorbers with direct PTO.

Directly controlling the motions of the device can optimize the primary power take-off efficiency, which is the most significant optimization from a system efficiency point of view. However such control schemes can lead to significant over-rating of the generator and associated power electronics to meet the peak torque requirements of such a design [33].

VI. CONCLUSION In this paper the design requirements imposed on the

generator technology by a wide range of ocean energy device categories have been considered. These have been analysed firstly in terms of common requirements to all OECs as pertaining to the marine environment and grid connectivity. The generator design characteristics impacted by the specifics of each device category have then been assessed. Finally, the different functionalities that can be assigned to the generator and its control system have been outlined, and their influence on generator design and rating considered.

TABLE 1: SUMMARY OF GENERATOR CHARACTERISTICS WITH OEC

CATEGORY (REFER TO NOMENCLATURE SECTION FOR ABBREVIATIONS)

OEC Prime Mover Speed Range

Generator Peak Torque Range

Generator Type

A 600-1500 rpm

2-4 pu; reduces with added inertia

VS SCIG/SG/PMG

B 1000-3000 rpm

Close to 1 pu with high accumulator storage, up to 4 pu as storage reduces

• FS SG/SCIG-SVC for high storage designs • VS SG/PMG/SCIG for low storage designs

C 100-250 rpm

Close to 1 pu • GBC: FS SG/SCIG-SVC or VS PMG/SCIG • Low speed VS PMG

D 0-3m/s 0-400 rpm

2-5 pu; reduces with added inertia

• Linear: PMG (custom) • Rotary: GBC VS SG/PMG

E 0-10 rpm Close to 1 pu GBC, FS SG/SCIG-SVCor VS SG/PMG/SCIG

F 1000-3000 rpm

Close to 1 pu FS SG/SCIG-SVC for high storage designs or VS SG/PMG/SCIG for low storage designs

In summary, a broad indication of possible generator types and speed and torque ranges for the different OEC categories, as listed in Section IV. is presented in Table 1. These parameters are dependent on the functionality of the generator in each case, but they should provide a starting

point for a system designer. The speed and torque ranges in the table are derived from the literature and from offshore WEC test data from the wave energy test site in Co. Galway, Ireland. The per unit data is referenced to maximum mean output power.

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energy conversion," Renewable Power Generation, IET, vol. 1, pp. 17--24, 2007. Biographies Dara O’Sullivan graduated with a B.E. degree in electrical engineering from the University College Cork (UCC) in 1995. He went on to obtain the M.Eng. Sc. and Ph.D. qualifications from the same university in 1997 and and 2001 respectively. He was employed as a post-doctoral researcher with the power electronics research centre PEI Technologies, in UCC until 2007. He is currently a senior research fellow in the Hydraulics & Maritime Research Centre. His research interests are in the grid integration of renewable energy and the general area of power conversion and motion control. He has published 23 papers and is the holder of 3 patents. Anthony W. Lewis graduated with a B.Tech. in Civil Engineering from University of Bradford in 1971. He went on to obtain his M.Sc. from Bangor University in 1973 and Ph.D. qualifications from the same university in 1983. He was appointed as a lecturer in UCC in 1983. He is currently a Senior Lecturer and also Director of the Hydraulics & Maritime Research Centre. He has published widely in the field of ocean energy and has been centrally involved in policy development in this area at an EU level.