the commercial advancement of 16 mw offshore wave power generation technology in southwest of the uk

7/31/2019 The Commercial Advancement of 16 MW Offshore Wave Power Generation Technology in Southwest of the UK

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8th International Conference on Power Electronics - ECCE Asia

May 30-June 3, 2011, The Shilla Jeju, Korea

978-1-61284-957-7/11/$26.00 2011 IEEE

[ThF1-4]

Abstract-- This paper presents the work carried out to

develop the Wave Hub project off the coast of southwest of

the UK, which represents the world's first large-scale wave-

energy farm. Four companies have been chosen to deploy

their own particular wave-energy converters. These include

the Pelamis, the Overtopping Device, the Multiple Point

Absorber System, and the Oscillating Water Column. The

work focuses on integrating offshore wave energy plant into

the UK electrical grid and finds the optimum configuration

for grid integration of multiple WEC devices. It discusses

how to mitigate the grid integration challenges associated

with 16MW commercial implementation of offshore wave

energy technologies in terms of voltage and reactive power

control and investigates their impact on electrical networks.

Index Terms-- Wave energy converters, Grid-integration,

Reactive power control, Voltage control.

I. INTRODUCTION

The worlds oceans constitute one of the largestinventories of untapped renewable power[1]. With adwindling supply of fossil fuels and increasing questionssurrounding the depleting resources, ocean wave energycan play a critical role. The successful deployments of

large-scale grid-connected ocean power plants willcontribute toward reduction in greenhouse gas emissionand ensure local and regional energy security[2-5].

Research priorities should be given to economical systemdesign, long-term and large-scale deployment and high-efficient grid-integration of wave energy plants into the

electric power system. Energy buffering for wave energyconverters (WECs) may represent a serious issue since

the raw power produced by a single unit may causevoltage variations at the connection node. The impactdepends on the grid strength. Due to wave grouping ina given sea state, a large number of devices opportunely

deployed in an array are needed to substantially reducethe short-term variations of the output power[6-9].Most of the ocean wave technologies are at early stages

of development and only a few of the devices have beentested in grid-connected. For commercial development ofWEC arrays a number of issues that have to be addressed

to achieve cost-effective technologies, include:1. Increasing the conversion efficiency of the

conventional wave energy converters.

2. Simplifying the mechanical complexity of thehydraulic- and pneumatic-based WEC.

3. Development of power electronic interfaces for harshconditions.

4. Loss reduction of transformer and transmission linelosses for AC systems.

5. Reactive power control and fault mitigation througheffective use of power electronic systems.

6. The impact of the new Grid Codes for renewableenergy sources upon low harmonics and powerfluctuations.

7. Minimizing environmental impact and significantsafety and operational issues.

In the development of WECs[10-14], grid integration isthe last stage and therefore the least explored. For the

purpose of the Wave Hub project design, a range ofgeneric WECs have been modelled to determine theirtypical power outputs characteristics. In this paper, acost-effecitive and high-effecincy electrical transmissionsystem for grid integration of different offshore WECs

array is desgined to maintain the voltage tolerances andreactive power flows within limits.

II. THE WAVE HUB PROJECT

In Fig.1, the Wave Hub project allows developers ofwave energy converter devices (WECs) from The UK,Norway, Australia and the United States to test theirdevices over several years in a realistic, fully monitoredmarine environment. The project provides a transmission

system to link the WECs with the UK power grid. Thesingle 6 core 300mm

225 km high-voltage subsea cable

16-50MW transmission system is rated at 33kV instead

of 11kV. Thus, when initially operated at 11kV, thecapacity of the system is 16MW however when operatedat 33kV the capacity of the cable increases to 50MW.

Additional cables of 1km 4 km are also required forcoonecting different WECs deployed in an array. Theelectrical parameters of the 6-core 300 mm

2cable are

R=0.0745 :/km, L=0.34155 mH/km and C=0.208F/km.Each WEC, rated at 4 MW, contains a transformer at itsbase in order to convert the generator output from lowvoltage 415/690 V to 11kV. For 16MW of WECs, this

voltage still needs to be elevated to high voltage (33kV)levels for more efficient transmission through subsea

cables. The onshore Hayle substation comprises an11kV/33kV, 20MVA transformer with a nominal

impedance of 10%.

The Commercial Advancement of 16 MW

Offshore Wave Power Generation Technologies in

the Southwest of the UKTarek Ahmed1, Katsumi Nishida2, and Mutsuo Nakaoka3

1Electrical Engineering Department, Faculty of Engineering, Assiut University, Assiut, Egypt

2Ube National College of Technology, 2-12-1, Tokiwa-Dai, Ube, Japan

3Kyungnam University, 449 wolyong-dong, masan, Kyungnam, Korea


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Fig.1 Single-line diagram of the Wave Hub project under study.

It should be noted that although the nominal cablecapacity is 50MW at 33kV (including a derating of 20%)the 33kV onshore connection to the Hayle substation isonly capable of a maximum of 30MW. In order toaccommodate 50MW, this connection would have to be

upgraded to a 132kV system. Also, at various criticalconnection points, circuit breakers are placed in order toisolate faults. In order to ensure delivery to the grid

within the specifications, power factor correctionequipment is installed at the substation.

III. SEA WAVE POWER

In Fig.2, the distance between two consecutive crests, or

two consecutive troughs, defines the wavelength .Wave height H (crest to trough) is proportional to windintensity and its duration. The wave period T (crest tocrest) is the time in seconds needed for the wave travelers

the wavelength and is proportional to sea depth. The

frequency f = 1/T indicates the number of waves thatappears in a given position. Consequently the wave speed

is v = /T = f. The ratio /2H is called the wave

declivity (downward inclination or a descending slope)and when this value is greater than 1/7 can be proved that

the wave becomes unstable and vanishes. Longer periodwaves have relatively longer wavelengths and move

faster. Generally, large waves are more powerful. The power associated with a wave of wavelength and

height H and a front b is given by[12]:

(1)

where (kg/m3) is the water specific weight and g(m/s2)

is the gravity acceleration. The power across each meter

of wave front associated to a uniform wave with height

H(m) and wavelength (m) is then:

(2)

Pu is expressed in W/m. For irregular waves of heightH(m) and period T(s), an equation for power per unit ofwave front can be derived as[13]:

(3)Pi is expressed in kilowatts per meter (kW/m) of wavefront.

Fig. 2. Propagation of sea waves

It is significant to note that wave power varies with thesquare of wave height. Then, when wave height isdoubled generates four times as much power.In the UK, the best site for capturing wave power is

chosen at 16 km off the north coast of Cornwall nearHayle where the maximum power density distribution ofthe ocean is reaching 50 kW/m and the average wave

height at the site is approximately 2.3 meters, with amaximum height reaching 8.8 meters and a period ofabout 11 seconds. Fig.3 shows Wave Hub power outputat irregular sea state and based on 7.5MW installed

capacity while Fig.4 indicates the probabity of Wave Hubpower output level over an annual period..

Fig.3 Wave Hub power output at irregular sea state and based on

7.5MW installed capacity

Fig.4 Probabity of Wave Hub power output level over an annual period

at irregular sea state and based on 7.5MW installed capacity


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IV. SIMPLIFIED REPRESENTATION OF WECS

In Fig.5, wave energy converters operate on diverse

principles and may require cascaded conversionmechanisms. Different systems operate on differentmethods of wave-device interaction such as heave, pitch

or surge and may need pneumatic, hydraulic or

mechanical power take-off stages. In addition, placementof these devices (distance from shore, depth from surface

and orientation with respect to the wave-front) and subtlestructural aspects (resonance, directionality, etc,) mayblur the definition operating principles. Although the

front-end stages may have significant diversity in design,the final stages of conversion (i.e., electric machines andequipment) are very similar for wind power plants.

Fig.6 indicates the system schematic diagrams of the fourwave-energy converters deployed in Hayle. Wave energyconversion devices such as Hinged Contour Device(Pelamis), Overtopping Device (Wavedragon), and

Oscillating Water- Column (OWC) systems exhibit

noticeable storage with an intermediate conversionprocess, may inherently allow energy storage for shorttime durations. This acts as a low-pass filter removingsome of the high frequency power oscillations generatedfrom wave variations or device operation shown in

Fig.7[2]. Devices such as Archimedes Wave Swing(AWS), where direct drive permanent magnet generator(PMG) and power electronics (PE) interface are used,

do not exhibit this feature and may deteriorate thefrequency spectrum of the captured power unless externalbuffer mechanisms are in place.Being a technology still in its early phase, the controlalgorithms used by the leading wave device developers

are closely guarded commercial secrets and have neverbeen published and part-scale prototype testing,numerical modelling and algorithm synthesis are inprogress. Wave energy converters are subject to varying

wave heights and frequencies. While a large number ofconcepts exist, each based on a different operationalprinciple, it is challenging to generalize the controlrequirements for wave energy devices. However, several

fundamental aspects are as follows[6]:

Elecromechanical & Electrical Process

Final Conversion

Mechanical

Process

Front-endConversion

Power

Take-off

IntermediateConversion

G

Ocean

Wave

Air Flow Water Flow

Hydraulic

Pumps

Mechanical

Transmission

Relative Motion

between Bodies

Mechanical

Gear

Hydraulic

MotorsAir Turbine

Water

Turbine

Fig. 5. Simplified representation of different WECs technologies.

PE Interface

DFIG

Wells TurbineAir Chamber

Grid

PE Interface

PMG

Impulse TurbineHose Pump

GridStorage

(Accumulator)

PE Interface

PMG

Propeller

Turbine

Augmentation

(Reflector with Ramp)

GridStorage

(Reservoir)

Transformer

with Soft-Starter

SIG

HydraulicMotorReciprocatingPump

GridStorage

(Accumulator)

Fluid Power Mechanical Power

(c)

(d)

(b)

(a)

Electric Power

Fig.6 Converter schematics (a) Pelamis (b) WaveDragon (c) OWC (d)

Multiple Point Absorber

Production Estimate Hydraulic

Time (s)

Time (s)

(a)

Power(kW)

Power(kW)

(b)

Instantaneous absorbed GeneratedMean absorbed

Fig.7 Averaged power output from wave energy device, examples of

(a) hinged contour device [3] and (b) overtopping device [4]

A. Resonance and phase control

The theoretical optimum operation of a device isachieved when the natural frequency of device oscillationis matched to the incident wave frequency. Thisresonance or phase control approach is complicated bydevice construction with cascaded multi-stage

architecture and wave irregularity.

B. Optimum amplitude control

In addition to the phase match, the level of power capture

is directly related to the amplitude of the deviceoscillation compared to the incident wave. This can berealised by dynamically adjusting the operation of one or

more elements within the power-take-off system such asfluid flow or turbine-generator rotational speed.


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V. MULTIFARM AVERAGING EFFECT

The present trend in the ocean power sector is to develop

modular devices that can be placed in a spatiallyseparated array. In such cases, it is expected that intra-and interfarm superposition of extracted power will

induce an averaging effect on the net power output of

wave farm. At farm level, other options may beconsidered and, for some devices, near-optimum control

strategies may involve power exchange between thedevices themselves. This of course changes the offshorestructure of the electrical connections, and long-distance

power transmission schemes may be affected.Fig.8 indicates that the single WEC unit has a highly

variable power output based on the WECs device type

characteristics. As the power variability is measured overseveral to 60 seconds, the peaks of the instantaneouspower will need to be considered during the voltage anddissipation calculations. The instantaneous peak power is

used for equipment sizing to ensure that the cable thermal

rating is adequate but mainly to ensure that the offshorevoltage remains within acceptable limits.Since wave variations have significant periodiccomponents, multi-unit wave energy devices couldpotentially produce more energy if they are placed in

optimum locations and achieve near-steady cumulativeoutputas shown in Fig.9. Peak power is reduced for largerarrays of WEC devices due to greater output diversity

across the site.

VI. REACTIVE POWER AND VOLTAGE CONTROL SYSTEMANALYSIS

The objectives of this study is to investigate all possibleoperating conditions for the Wave Hub electrical systemin order to provide an optimized solution for transformer

voltage ratio, impedance and tap range, compensationrating, and WEC operating constraints. This study isundertake for both the system operating as an initial 11kVsystem and for the future possible change to 33kV in

order to offer some form of future proofing in particularfor subsea equipment.

A. Distribution code Requirements andReactive Compensation

According to the connection code, embedded generators

with a User System Entry Point at 33kV or below orGenerators in England and Wales directly connected tothe GB Transmission System at 33kV or below are

excluded from the grid code reactive requirements. These

Fig. 8. Power output of a typical WEC device

The instantaneous power of a linear device working in a 10 s Pierson-Moskowitz

spectrun over 8 min

The combined output of 16 widely separated devices working in the same sea state

Fig.9 Analytical study on multi-unit wave power plants averaging

effect on overall power output [5]

Fig.10 Distribution code reactive requirements for generators connected

at 33kV

generators are only required to maintain reactivecapability within the relaxed requirements shown in

Fig.10 below for voltage changes at the relevant GridEntry Point or User system Entry Point. For this study it

is assumed that the voltage at the grid entry point (33kV)

varies 6 %. As a result of the inductive operation ofgenerating units and high reactive losses on the longconductor, the operating point at rated power would lie tothe left of the curve i.e. inductive. Any compensation willtry to pull the operating point onto the highlighted sectionin Fig.10.

B. Reactive Power control

Reactive power transfer to the power distribution systemmust be maintained within limits under the connection

agreement. As no connection agreement currently exists,the standard distribution code limits have been appliedi.e. operating within the requirements of Fig.10.


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Wave power generating systems with inductiongenerators such as Pelamis and OWC may consume a

significant amount of reactive power (35% to 40% atidling, 60% at rated capacity)[14]. The subsea cable

draws reactive power even with no operating WECs.Transformer impedance generally has an impact of

reactive power flow but in this case is not significant dueto the dominant effect of the cable impedance. Therefore,

to minimise the system losses and to increase the voltagestability, compensation techniques are essential.In order to maintain reactive power flow within the limitsdefined by the distribution code some means of reactivepower control must be incorporated. This can be either:reactive power compensation or WEC power factor

control. The use of additional switched reactors andcapacitors can be installed to absorb or generateadditional reactive power and thus control the amounttransferred to power distribution system at the grid entry

point (33kV). The generators of the WECs are able to

absorb( lagging power factor) or generate (leading powerfactor) reactive power. For voltage control, a combinationof both could be used although it is unlikely to be adynamic closed loop control.

C. Voltage control

In a time-varying generation wave power generatingsystems, the voltage profile throughout the network takes

a more complex shape. In the vicinity of the generationsource, the steady-state voltage rises and may exceed theacceptable limits. The extent of this rise also depends on

load conditions and network parameters.In order to maintain voltages within acceptable limits of

11kV 10%, some means of voltage control must beincorporated. This can be a combination of onshoretransformer tap-changer voltage control and/or powerfactor control. A tap-changer on the transformer can be

used to raise or lower the onshore 11kV substationvoltage which in turn raises or lowers the voltage at theWECs. Generators of the WECs operating at an inductivepower factor cause a reduction in terminal voltage whilst

those operating at a capacitive power factor cause a risein voltage. For both practical reasons, to ensure ranges

fall within manufactures standards, and in order tominimize cost a combination of the two methods isgenerally used.

VII. RESULTS AND DISCUSSUION OF 11KVOPERATINGVOLTAGE AND REACTIVE COMPENSATION

The Wave Hub system from the 33kV Hayle substationto the wave energy convertors has been modellled as

shown in Fig1. The model comprises 4 offshore WECs,each connected through a collector hub (Hub A or HubB) to an 11kV onshore substation via 25km 6core double-circuit cable. Each unit, rated at 4 MW, is assumed to be

operating at fixed power factor. Also, it is assumed forthis study that the onshore transformer tap-changer is

fixed at different positions depending on the location ofreactive compensation. Manually employing a fixed

position tap allows the onshore 11kV substation voltageto vary around lower voltages than nominal. In Fig.11, 6

conditions in addition to the standard distribution codehave been considered for the provision of reactive

compensation equipment.

a) No compensation with WECs operating at unitypower factor(PF) i.e. zero MVAr.

b) Compensation via individual WEC units.c) Compensation at onshore 11kV-board, with WECs

operating at 0.98 inductive PF to prevent excessive

voltage rise.

d) Compensation at onshore 11kV-board through LVstep-up transformer, with WECs operating at 0.98

inductive PF to prevent excessive voltage rise.

e) Compensation at onshore 11kV-board through LVstep-up transformer , with WECs operating at unity

PF to prevent excessive voltage rise.

f) Compensation at 33kV with WECs operating at 0.98inductive PF to prevent excessive voltage rise.

Voltage setpoint of the substation transformer can beused to bring down the 11kV voltage levels to preventexcessive voltage rise at the generator terminals. Asimilar affect can also be achieved if the transformer taps

are fixed which could reduce costs. As the powerinjection increases, generator terminal voltages increaseand 33kV onshore voltage decreases as a result of

inductive operation. Provision of a 6 MVAr switchedcapacitor bank compensator minimises the subsea cablelosses within the acceptable voltage levels. However,even with 100% power fluctuation, 33kV voltage

variation is fairly small (< 1%). At minimum systemvoltage (-6%) at the Hayle susbstation, the generator

voltages are too low then an on-load tap-changer isrequired.The effects of minimum compensation on voltagevariations at differnt network locations of the Wave Hub

systems are shown in Fig.11. Fig.11(b) indicates the bestsolution for the wave device voltage control. Therequirement for reactive power compensation of the

WECs can be split between the four WECs in foursmaller ratings. However a larger operating voltage rangebetween no-load and full power requires a more reactive

power which inceases the ratings of the WEC electricalgenerators and requires control units installed in harsh seaconditions.

(a) No Compensation-WECs at Unity PF


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(b) Compensation-WECs

(c) 11kVWECs at -0.98 PF

(d) 11kV via LV transformers-WECs at -0.98PF

(e) 11kV via LV transformersWECs at unity PF

(f) 33kV-WECs at -0.98 PF

Fig.11 Minimum reactive compensation effects on the per unit voltage

variation at critical network locations with fixed tap-changer

Second best solution for the voltage control shwon inFig.11(e) is operating the WECs at unity PF

compensation provided onshore at 11kV via LV-transformers.

As obvious from Fig.11, the reactive power control isrequired to keep the voltage levels of the WECs withinthe acceptable limits. It also reduces the rms currents in

order to improve the wave power plant efficiency throughloss reduction of transformer and transmission line lossesfor the AC system. In general, allowing the WECs togenerate at a leading power factor minimizes the voltagerange at the WEC terminals and reduces the transformertap-changer range required, but increases the required

reactive power compensation. Allowing the WECs togenerate at a lagging power factor maximizes the voltage

range at the WEC terminals and increases the transformertap-changer range required, but reduces the requiredreactive power compensation. A variety of options forwave power farm operation to maintain the voltage

tolerances and reactive power flows within limits are;I. provision of a 33/11 kV transformer with a tap

changer having a range of 10%;

II. operating the WECs generation at unity powerfactor;

III. provision of a 6MVAr switched capacitor bankcompensator; and,

IV. using larger WECs connecting cables.Fig.12, Fig.13 and Fig.14 shows the changes in the WaveHub system efficieny and reactive power flow (MVAr)into the Wave Hub at the Hayle substation from 33kV

bus under different locations of reactive powercompensation. For this study it is assumed that the

voltage at the grid entry point (33kV) varies 6 %.When WECs operate at 0.98 inductive, the reactive andreal losses increase significantly, and as a result, high

levels of capacitive compensation are required to meetthe distribution code requirement. Similarly, if it can beconfirmed that 33kV voltage at the Hayle susbstation

does not fall


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(a) Reactive power flow into Wave Hub from 33kV bus (MVAr )

(b) Reactive power compensation (MVAr)

(c) Wave Hub system efficiency

Fig.12 Changes in Wave Hub system efficieny due to different locations

of reactive compensation at (1+ 6%) pu of operating voltage.

(a) Reactive power flow into Wave Hub from 33KV bus (MVAr )




of reactive compensation at nominal operation voltage.

(a) Reactive power flow into Wave Hub from 33KV bus (MVAr )



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of reactive compensation at (1-6%) pu of operating voltage.

VIII. CONCLUSIONS

In this paper, a simple study for the Wave Hub project iscaried out to find the optimum configuration for grid integration of multiple WEC devices and investigate theirimpact on electrical networks. A variety of options forWave Hub operation to maintain voltage tolerances and

reactive power flows within limits are:

a) Provision of a 33/11kV transformer of nominal 10%impedance with an on-load transformer tap-changer

having a range of 10%. The transformer tap

position is selected dependent on 33kV voltage

variation not WEC power flow.b) Operating the WEC generation at unity power factor.

Operating at this power factor, which is measured on

the LV side of the WEC transformers, offers the

most efficient solution.

c) Provision of a 6 MVAr Switched capacitor bankcompensators.

From the load flow study, the optimum compensation atfull capacity (16MW) is provided onshore at 11kV via

LV-transformers with the WECs operating at unity PF.For the future possible change from 11kV to 33kV, thetransformer is no longer required and compensation

changes, due to high cable capacitive charging current (9times that at 11kV), from 5MVAr reactive at no load to1MVAr capacitive at full load. Operating at 33kVeffectively reduces the cable voltage drop and increasesreactive support. Voltage regulation through 0% to 100%generation is not as demanding as before (2% compared

to 9%). As opposed to 11kV operation, at 33kV inductivecompensation is required due to higher charging currents.Total capacity of the Wave Hub is increased from 16MWto 30MW by increasing the WEC unit ratings to 7.5MW.

ACKNOWLEDGMENT

This work was supported by the Peninsula Research

Institute for Marine Renewable Energy (PRIMaRE)

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the commercial advancement of 16 mw offshore wave power generation technology in southwest of the uk

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