Grid Power Integration Technologies for Offshore Ocean Wave Energy

Tarek Ahmed Katsumi Nishida Mutsuo Nakaoka IEEE Member IEEE Member IEEE Member E & E Department, Assiut University Ube National College of Technology Kyungnam University Assiut, Egypt Ube City, Yamaguchi, Japan Masan, Kyungnam, South Korea tarekahmed@ieee.org nishida5@ube-k.ac.jp nishida5@bronze.ocn.ne.jp

Abstract -- In this paper, the advanced electric technologies for grid power integration of different offshore wave energy conversion devices are presented. The electrical connection configurations for integrating the electric power of the multi wave energy conversion devices such as the Oscillating Water Column, Pelamis, the Wave Point Absorbers and the Wave Dragon are developed by employing the most efficient low cost grid interface electrical technologies based on the advanced power electronics. The bi-directional power converter is employed for wide-range variable speed operation of wave energy conversion device to reduce the power output fluctuations and improve the whole power generation efficiency.

Index Terms-- Ocean Wave Energy Devices, Oscillating Water Column, Pelamis, Wave Point Absorbers, Wave Dragon, Power Conditioner system, Grid Integration.

I. INTRODUCTION In the last five years, ocean energy has received

considerable attention and there has been a resurgence of interest in wave energy, especially in Europe. Wave energy can be extracted and converted into electricity by wave energy devices deployed either on the shoreline or in deeper waters offshore [1-3]. Figure 1 shows an Atlas of the global power density distribution of the oceans where the numbers indicate kW/m. The north and south temperature zones have the best sites for capturing wave power. The prevailing winds in these zones blow strongest in winter. Increased wave activity is found between the latitudes of 30 and 60 on both hemispheres, induced by the prevailing western winds blowing in these regions[4-6]. The oceanic wave climate offers enormous levels of energy. As waves approach the shore, energy is dissipated, leading to lower wave power levels on the shoreline. Therefore, the energy availability is sensitive to location and the distance from the shoreline[7].

The wave energy in the UK has the potential to generate substantial amounts of electricity from its wave resource up to one sixth of the UK's electricity consumption. The potential for integration of wave power into the electric power grid is high, but as in the offshore wind context, the degree of penetration will depend on the adverse impacts it might have on the power network and the technology available to mitigate these impacts[8-10].

Fig.1 Global wave power distribution in kW/m Most of the lessons learned from wind farms for the grid

interface technology can be readily applied to wave farms[10]. However, achievement of a cost-effective technology is the main concern for commercial development of wave energy converters. Research effort needs to be focused in such direction to extract electrical energy from sea waves in a commercially and technologically acceptable manner. A number of issues have to be solved and need to be considered, such as control applied by the generator and power conditioning system for increased production and low voltage harmonics and power fluctuations introduced with the new grid codes for renewable energy sources[11-15].

In this paper, the indirect connection between the power take-off of the wave energy converters and the electrical power transmission system is presented, in addition to the introduction of the ocean wave energy devices in the UK.

II. COMMERCIAL DEVELOPMENT FOR GRID INTEGRATION OF WAVE POWER PLANT

The South west region of the UK is currently experiencing significant activity in the wave energy development to create a major grid-connected project off the north coast of Cornwall. Grid connection will be made via the power distribution substation at Hayle. When operating at 11kV, the wave power plant is capable of delivering 16 MW of power.

978-1-4244-5287-3/10/$26.00 2010 IEEE 2378

11 kV

WECSubstation3 km

HayleSubstation

HayleSubstatio

n25 km

MechanicalCapacitor Bank

NationalGrid11 kV / 33 kV

25 km2 km

4 km1 km

InductionGenerator

WEC # 4OWC

PermanentMagnet

Generator

FixedCapacitor

MainSwitch

SoftStarter

WEC # 1Pelamis

WEC # 3Wave

Dragon

WEC # 2Archimedes withLinear Generator

11 kV

PowerElectronicsInterface

Step-UpTransformer

11 kV

PowerElectronicsInterface

Step-UpTransformer

11 kV

PowerElectronicsInterface

Step-UpTransformer

InductionGenerator

Fig. 2. Single-line diagram of the commercial development for grid integration of multiple ocean wave energy conversion devices

In Figure 2, the wave energy plant comprises for offshore wave energy devices. Each unit rated at 4 MW and is connected through an 11kV bus via 25 km of 6-core 300 mm2 cable initially operated as two independent circuits with each circuit connecting two devices. Additional cables of 1km4km are also required. The electrical parameters of the cable are R= 0.0745 /km, L=0.34155 mH/km and C=0.208 F/km. Power factor correction equipment is installed at the substation to ensure delivery to the grid within the specifications. When initially operating at 11kV, the 11/33kV, 20MVA onshore tap changer transformer is used to raise or lower the onshore 11KV substation voltage which in turn raises or lowers the voltage at the wave device.

One of the objectives, in addition to assisting in the commercial advancement of wave generation technologies, is to evaluate the smoothing effect which appears when the production of the different wave energy devices in the farm is added. It is necessary to create estimates of the active power profiles of the plant with multiple wave energy devices to assess the expected voltage variations and reactive power variations. Figure 3 shows the power output of the wave plant under four typical wave conditions where the level of installed capacity at the wave plant site of 16 MW is estimated based on a range of possible wave electrical technologies for grid integration. Because energy production follows the time variations of the resources, the power output of each system can vary widely and an important energy storage capacity, acting as a buffer, is necessary. Adding the power produced by each system significantly reduces the

time standard deviation of the power output. It leads to lower costs for storage and to a more constant energy production. In general, an irregular sea state gives a lower peak power indicating greater smoothing. Higher power variation is also produced for irregular waves. Figure 4 shows the grid efficiency and power factor of the wave power plant with reactive power compensation. When the wave energy devices are operating at approximately unity power factor, it is possible to achieve a high performance between one and five seconds. The reactive power compensation unit significantly improves the wave plant efficiency up to 96 % by reducing the cable losses and due to greater output diversity across the site, peak power is reduced for array of wave energy devices.

Fig.3 Power output of the plant under four typical wave conditions

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2 4 6 8 10 12 14 16 18 200

10

20

(b)

2 4 6 8 10 12 14 16 18 200.7

0.8

0.9

1

(c)

2 4 6 8 10 12 14 16 18 200.8

0.9

1

time(sec)

(d)

2 4 6 8 10 12 14 16 18 20-5

0

5

10

(a)

a) Reactive power of wave plant b) Active power of plant c) Power factor of wave power plant d) Grid efficiency

Fig.4 Grid efficiency and power factor of wave power plant based on 16 MW installed capacity

III. POWER TAKE-OFF MECHANISMS OF WAVE POWER PLANT Wave energy companies have been highly involved in the

development of new wave energy schemes such as the Oscillating Water Column, Pelamis wave energy converter, the Wave Point Absorbers and the Wave Dragon. The power take-off mechanisms, converting the captured mechanical energy into electrical energy, are a mixture of hydraulic motor, aero-turbine and water turbine as given in Table I. Tools developed and studies carried on in this research will apply to four types of different wave energy devices.

A. Pelamis Wave Energy Converter A wave farm utilizing Pelamis technology was recently

installed in Aguadora Wave Park, about three miles off Portugal's northern coast, near Pvoa do Varzim. The wave farm initially uses three Pelamis P-750 machines developing a total power of 2.25 MW. Other plans for wave farms include a 3MW array of four 750 kW Pelamis devices in the Orkneys, off northern Scotland, and the 20MW Wave hub development off the north coast of Cornwall, England. Only one Pelamis-750 placed on the sea of 55 kW/m average intensity will produce per year a total energy of 2,2x106kWh.

The Pelamis wave energy converter, a Scottish invention, is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints. The wave-induced motion of these joints is resisted by hydraulic rams, which pump high-pressure oil through hydraulic motors via smoothing accumulators. The hydraulic motors drive the 6*125 kW electrical generators to produce a total power of 750 kW for each Pelamis unit as shown in Fig. 5. Power from all the joints is fed down a single umbilical cable to a junction on the sea bed. Several devices can be connected together via an 11 kV three phase power transformer and linked to shore through a single seabed cable.

TABLE I POWER TAKE-OFFS AND GENERATORS [4-9]

Device Power take-off Generator Speed Pelamis Hydraulics Induction Fixed Archimedes wave swing

Hydraulics Permanent magnet/ Induction

Variable

Wavedragon Water turbine Rotary permanent magnet

Variable

Oceanlinx OWC and variable-pitch turbine

Induction Variable

Fig. 5 Inside view of the power module

Fig.6 Power conversion system schematic

A general schematic of the power take-off and conversion

system is shown in Fig.6. Certain details have been omitted for commercial reasons[15].

B. Offshore Wave Dragon System Figure 7 shows a photograph of the Wave Dragon system

installed nearshore. The Wave Dragon, which works much like a hydroelectric dam, is a floating tapered channel device that uses a pair of curved reflectors to gather and focus waves. The waves overtop a ramp which elevates water to a reservoir above sea level. This creates a head of water which is subsequently released through a number of low-head turbines and converted into electricity. Water is returned to the vents in the base of the unit. The only moving part is the low-head turbine. Figure 8 illustrates this principle[1].

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Fig. 7 Wave Dragon system installation nearshore

Fig. 8 Wave Dragon system principle

C. Oscillating water column (OWC) The Oscillating Water Column operates much like a wind

turbine via the principle of wave-induced air pressurization as illustrated in figure 9. Some sort of closed containment housing (air chamber) is placed above the water, and the passage of waves changes the water level within the housing. The turbines are coupled to generators which produce the electricity as illustrated in figure 10[12-14].

Fig. 9 Oscillating column of water system

Fig. 10 Generator / rectifier air turbine group

Fig.11 FO3 wave point absorbers

D. Wave Point Absorbers In Fig.11, the FO3 has a series of wave point absorbers

mounted in vertical hydraulic cylinders which work in both directions. The vertical movements of the floating point absorbers will be transformed to hydraulic pressure. The hydraulic pressure is utilized to generate power by generators. With working surfaces moving at velocities in the region of 1 m/s, high-pressure oil hydraulics offer the best solution because of their high energy density and possibility of on-board energy storage for smoother delivery. A mechanical interface is employed to convert the slow-speed rotation or reciprocating motion into high-speed rotational motion for connection to a conventional electrical generator.

IV. ELECTRICAL POWER GENERATION SYSTEM FOR WAVE ENERGY CONVERSION DEVICES

The standard electrical power generation system of the power take-off of the wave energy devices can fundamentally be divided into two categories: (a) a direct connection power generation system for a fixed speed device, and (b) indirect connection systems for variable speed devices[10].

A. Fixed Speed Power Generation System Pelamis device uses a 125kW squirrel-cage induction

generator directly connected to the grid. Therefore, this wave device must operate at a constant speed with an allowable variation of 1-2% and adjusts the rotor speed by controlling the hydraulic motor which drives the induction generator to produce electricity. The 50Hz/60Hz, 415V/690V output voltage of the generator used is low and so there will be an 11KV, 950 kVA step-up transformer on the device. The induction machine requires local power-factor correction (PFC) usually in the form of mechanically switched capacitors. The PFC may be thyristor switched capacitor or even be dynamic although these typically have a higher failure rate. The reactive control is likely to be very coarse unless a continuous dynamic PFC has been installed. In addition, the induction machine generally requires a soft-starter to reduce the inrush current during start-up.

Fig.12 shows the basic configuration of this design while Fig.13 shows its d-q axis equivalent-circuit. The stator of the induction generator is connected to the grid and to a capacitor bank. The capacitor bank provides the reactive current needed to excite the induction generator and acts as an uncontrolled reactive current source that provides the required excitation.

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Step-UpTransformer Grid

System ControlMechanicalSpeed Sensor

InductionGenerator

IG

r

SoftStarter

FixedCapacitor

vABC

Storage &Smoothing

(Accumulator)

Mechanical PowerTake-Off See Model

Pin

h

MechanicalPower Control

Digital SpeedControl

HydrodynamicsResponse Model

(Mooring+Structure)

HydraulicMotor

11 kVMain

Switch

vabc

Fig. 12. Schematic configuration of the direct connection of the induction

generator to the Grid

-vqs

+ iqs rs iqr Llr

Lgi d- +

Csvds Cs

Induction Machine Capacitor Grid

iq ds- +

Lls

Lm qs

sdr rr

qr vq

Lg rg

(a) q-axis

- + ids rs idr Llr

Induction Machine Capacitor Grid

qs-+ Lls

Lm ds

sqr rr

dr vds Lgi q-+

Cs

Grid

id

vd

Lg rg

Csv qs

(b) d-axis

Fig. 13. Simplified d-q axis equivalent-circuit during the direct connection of induction generator to the Grid

where, the subscripts s, r and g denote stator, rotor and grid quantities, respectively. iqs, ids , vqs and vds are the d-q axis induction machine stator currents and voltages. iq, id , vq and vd are the d-q axis grid currents and voltages. rs and rr are the stator and rotor resistances, respectively. Lm is called the magnetizing inductance. Lls and Llr are the stator and the rotor leakage inductances, respectively. s (s = r ) is called the slip frequency which is the frequency of the actual rotor current. and r are the angular speeds of the arbitrary reference frame and the rotor, respectively.

The capacitor-bank is installed to supply the induction machine with the required reactive current where the grid reactive current (id ) can be minimized in order to reduce the grid rms current and transmission losses. In order to select the value of the capacitor bank (Cs), a simple and useful rotating reference frame is chosen to align the stator voltage vector vs of the induction machine on the quadrature axis. Using this d-q reference frame, the direct and quadrature voltage components are: vds=0 and vqs=vs respectively, then the instantaneous active and reactive power are defined by:

qsqsivp 23= (1)

dsqsivq 23= (2)

The d-axis and q-axis current components are the active and reactive power producing, respectively. The power factor is defined as follows:

22dsqs

qs

ii

ipf += (3) As the capacitor bank has been selected to provide the

whole stator reactive current at 1 pu voltage, the grid reactive current is near to null. Under these conditions, the stator reactive current becomes:

qssds vCi = (4) From the above equation, the stator reactive current (ids)

depends on vqs. The main drawback with that, the induction machine efficiency may be very low because it works in the saturation region. The stator voltage is defined as follows: ( )dsmqs iv = (5) ( )dsm i is simply approximated by a straight-line tangent to the magnetizing curve at the operating point( msatL at the rated iqs ) which is represented by : ( ) dsmsatdsm iLki += (6)

Then, the generated voltage is simply expressed by:

smsatqs CL

kv 21

= (7) The quadrature stator voltage (vqs ) must stay within the

acceptable limits by selecting the suitable capacitor bank in order to avoid the saturation of the induction machine that may occur due to an excessive reactive current. Solving the above equation, the capacitance Cs can be expressed by:

qsmsatmsats vL

kL

C = 21

(8)

B. Variable- Speed Power Generation System The main components of the wave energy system are

illustrated in Figure 14, including the mechanical power take-off, accumulator, hydraulic motor, generator, transformer, and potential power electronics. Moreover, Figure 14 shows the control block diagram of an induction generator with a bi-directional PWM converter. The grid integration system consists of a three-phase square-cage induction generator and two PWM converters connected back-to back. The simplified vector control is applied to the induction generator by using the so-called volts-per-hertz control for the generator-side PWM rectifier, so that the generator can produce a variable electric power to the dc bus. Furthermore, in the grid-side PWM converter, the active power transferred or supplied to the electric power grid is adjusted in order to keep the dc bus voltage at the desired constant value.

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su

gate signal

U+

U

W+V+

WV

VdcCdc

i

VoltageDetection

ZeroCross

RC, LCvs vvs w

vs vw

isv

vs uv cu vcv vcw isw

Crystal Oscillator

Counter Ciruuit

Sampling TriggerGenerator

PWM Generator

eCounter

vc*3 d-q

sqi

Space VectorModulator

Vdc*

+Vdcsdi

Inverter OutputCurrent

Controller

Y

UtilityGrid

i g ui g v

ig w

U+

U

W+V+

WV

PWMGenerator

vg uv g vv g wIG

+

png

s fg

AD

DA

TachoGenerator

Storage &Smoothing

(Accumulator)

MechanicalPower Take-

Off

WaveModel

Pin

h

MechanicalPowerControl

DigitalSpeed

Control

HydrodynamicsResponse Model

(Mooring+Structure)

HydraulicMotor

Idc

DSP TI C5416PI

fg

k (png)3

Cube + PI

Moving Averageof isq

Fig.14. Generalized electrical and power take-off modeling for variable-speed wave energy device.

1. Control Method of Generator-side Rectifier The ac output voltage vg of a three-phase square-cage type

induction generator and the generator output frequency fg are controlled in accordance with volts-per-hertz control and expressed by:

gig fKv = (9) Based on the above equation, the air-gap flux linkage s of

the induction generator can be almost kept at constant value. So the rotor current ir and the stator current ig, which are proportional to the slip frequency s fg , are defined as follows:

r

ssrg r

fsNNi

NNi = 2

1

2

1

2 (10)

where the rotor slip s is ( ) ( ) |///| pfnpfs ss = , fs is the generator frequency, rr is the rotor resistance, n is the rotor speed and p is the number of pole pair. The power output Pg of the induction generator is given by:

2gggg fsivP (11)

From (11), the generator power output Pg is proportional to s fg2 i.e. Pg is proportional to the cube of the generator output frequency which is similar to the wind-turbine characteristics.

2. Control Method of Grid-side Inverter The PWM rectifier converts the variable-frequency

variable-voltage power generated by the induction generator to dc power. The dc power is inverted to a 60 Hz and a fixed ac voltage through a PWM inverter. The dc link voltage controller determines the active current reference isq* by detecting the dc link voltage so that a balance between the dc input power (Pdc= dcdc IV ) of the generator-side rectifier and the ac output power(Pac = SqS iv 3 ) of the grid-side inverter is carried out.

Furthermore, the moving average of isq for one period of the source voltage is required in order to adjust the generator frequency. From (11), the generator frequency should be set based on the fact that the generator power output is proportional to the cube of the generator speed. In order to find the generator frequency, a PI controller is required and the parameters of the PI controller are determined from the experiments. The proportional gain is 0.033 [Hz/A], and the integral gain is 0.99 [Hz/(A.s)].

V. RESULTS AND DISCUSSIONS The proposed system of the fixed and variable speed wave

energy devices given in figures 12 and 14 respectively have been tested in the lab on small-scale induction machine with 2.1 kW rating. The observed waveforms of fixed-speed type and variable-speed type of wave energy converters at the generator and grid-side are measured.

A. Operating Performance of Fixed Speed Wave Device Figure 15 shows the measured waveforms of the direct

connection of the 60Hz, 4 poles induction generator in terms of the current igu, the current igv and the terminal voltage vgvw, respectively under the condition of a fixed-speed operation. The measured rotor speed is 31.0 rps, and the rms terminal voltage of the induction generator is 100 V. The measured active current is 9.0 A and the reactive current is 5.0 A and the measured leading power factor is 0.87.

Figure 16 shows the dynamic responses of the direct connection of the induction generator in terms of the measured active and reactive current and the generator power factor, when the rotor speed changes within a small speed variation range of 1%-3.3%. From Fig.16, the power factor

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From the top, Ch-1: u-phase current iguof induction generator 5[A/div]

Ch-2: v-phase current igv of induction generator 5[A/div] Ch-3: terminal voltage vgvwof induction generator 250[V/div] Fig.15 Measured steady-state waveforms of the direct connection of the

induction generator to the Grid (slip= 0.033, rated current= 8A)

0 1 2 3 4 5 6

2

4

6

8

10[A]

Time [s]

Active Current

Reactive Current

Power Factor

Fig.16 Dynamic responses of the direct connection of the induction generator

to the Grid (slip: from 0.01 to 0.033).

of the generator is almost constant 0.87 and the PFC unit should be installed on the wave device to control the power factor between 0.95 to unity for maintaining the device voltage within the acceptable limit and improving the efficiency of the electrical power generation system when integrating the wave energy to the Grid.

B. Operating Performance of Variable- Speed Wave Device Figure 17 shows the measured steady-state waveforms

of the proposed grid integration system for variable speed wave energy device such as Oscillating Water Column while Figure 18 shows the dynamic responses of the variable-speed induction generator system. From Fig.18(a), the dc bus input current, Idc changes linearly in proportion to the speed variations or the frequency reference. The dc bus input current Idc varies between 1A to 3A. Furthermore, Grid-side inverter current isv in Fig.18(b) varies with the changes in the Idc And for maintaining the dc bus voltage constant, a balance between the dc input power, Pdc of the generator-side rectifier and the ac output power, Pac of the

grid-side inverter is carried out by using a PI controller. From the experimental results, the proposed control

system for grid integration of wave energy supplies sinusoidal currents into the grid and maintains the dc link voltage constant.

From the top, Ch-1: Generator speed 1000 rpm/div, Ch-2: u-phase IG current iGU 12.5 A/div (45Hz, 5.7 A rated), Ch-3: IG terminal voltage vGUV or the output voltage of the PWM rectifier 500 V/div and Ch-4: DC Bus Input Current Idc5 A/div

(a) Observed waveforms of the induction generator-side PWM rectifier (fg=45Hz, rotor speed=23.33rps, slip= 0.037, the average of Idc =2.5A).

From the top,

Ch-2: Grid-side inverter current isv 5 A/div Ch-3: Grid voltage vsvw 250V/div. Ch-4: DC Bus voltage Vdc100V/div

(b) Measured waveforms of the grid integration inverter (f=60Hz, Isv=3.2 A rms, Vsvw =100 Vrms, Vdc=220 V, P=550 W).

Fig.17 Measured steady state waveforms of the variable-speed induction generator system.

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From the top,

Ch-1:Generator speed50 rps/div, (from 31 to 43Hz) Ch-2: u-phase IG current iGU 12.5 A/div ( 6.2 A rated), Ch-3: IG terminal voltage vGUV or the output voltage of the PWM rectifier 500 V/div and Ch-4: DC Bus Input Current Idc5 A/div (a) Observed waveforms of the generator-side PWM rectifier (1 s/div)

From the top,

Ch-3: Generator speed 100 rps/div Ch-2: Grid-side inverter current isv 5 A/div Ch-4: DC Bus voltage Vdc100V/div

(b) Measured waveforms of the grid integration inverter (f=60Hz, Vsvw =100 Vrms, Vdc=220 V),. (1 s/div) Fig.18. Dynamic responses of the measured waveforms of the variable-speed

induction generator system.

VI. CONCLUSIONS A combination of electrical generation systems of the

power take-off of the wave energy conversion devices including directly connected generation (asynchronous) and indirectly connected generation with bi-directional converter is used to integrate all wave power to the grid. Suitable control of the bi-directional converter is suggested to ascertain satisfactory performance of the proposed scheme under different operating environments. The performance of the scheme is demonstrated in experimental results. The measurements show the effect of the bi-directional converter for the grid integration of the wave energy to meet the good operating performance of the proposed system under different speeds.

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