Power Electronic Grid-Interface for Renewable Ocean Wave Energy Marian P. Kazmierkowski and Marek Jasiński Institute of Control and Industrial Electronics Warsaw University of Technology Warsaw, POLAND
[email protected],
[email protected],
Abstract - This paper presents information on ocean wave energy converters and power electronics grid-interface. In the introduction a basic terms and methods of ocean wave energy capture are discussed. Further several most important ocean wave energy conversion prototypes are briefly described. The generators and power electronics solutions for Power Take Off (PTO) system are presented on the example of Wave Dragon MW ocean wave energy converter. The Wave Dragon MW captures power from the ocean waves by means of low-head Kaplan turbines and converts it into rotating mechanical power. Problems which can appear in mechanical power to electrical power conversion in Wave Dragon MW can be expected to be similar as in wind turbine. However, subject of the mechanical energy conversion from ocean waves to electrical energy is not well identified and further research should be carried out. Additionally, specific problems of AC-DC-AC grid-interfacing converters under grid voltage distortions - including grid impedance estimation, higher harmonics and voltage dips compensation - are discussed. Some simulated and experimental oscillograms that illustrate properties of the presented systems are shown.
I.
INTRODUCTION
Recently, development of renewable energy sources (RES), including offshore wave energy, arises from the requirement to increase the security of supply, reduce greenhouse effect, emission of carbon dioxide and acid rain gases. The oceans cover 75% of the world surface and as such ocean energy is a global resource. There are different forms of renewable energy available in the oceans: • waves, • currents, • thermal gradients, • salinity gradients, • the tides, • and others.
Fig. 1. Energy of sea waves [9]
Ways to exploit these high energy densities resources are being investigated worldwide. The power in a wave is proportional to the square of the amplitude and to the period of the motion (Fig. 1). Long period (~7-10 s), large amplitude (~2 m) waves exceeds energy fluxes about 40-50 kW per meter width of oncoming wave. High ocean wave power resources are located along the Western Europe coast. Higher levels of wave power can be found only in the southern part of South America and in the Antipodes. The available wave power resource in the area of north-eastern Atlantic including the North Ocean and in the Mediterranean achieves about 290 GW and 30 GW, respectively (Fig. 2). The potential of world wide ocean wave energy contribution is estimated to be about 10% (2 000 TWh/year) of the total energy consumption. Among the main difficulties in wave power generations are: • difficulty to obtain maximum efficiency because of irregularity in wave direction, amplitude and phase, • problem of coupling irregular slow motion of wave to electrical generators, • very high loading in the case of extreme weather conditions (hurricanes) over 100 times higher as average conditions. However, among important advantages are: • environmental compatibility, • free of pollution energy conversion, • low visual and acoustic impact.
Fig. 2. Available sea wave power resources in Europe [9]
978-1-4244-8807-0/11/$26.00 ©2011 IEEE
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II. OVERVIEW OF OCEAN WAVE ENERGY CONVERTERS Recently, the ocean wave energy conversion has been developed strongly, particularly in EU countries, Canada, Japan, USA, India, China and others. In contrast to other RES the number of concepts for wave energy conversion is very wide. Although over 1000 wave energy conversion techniques are patented worldwide, the apparent large number of concepts for wave energy converters can be classified within a few basic types: Oscillating Water Columns, overtopping devices, heaving devices (floating or submerged), pitching devices and surging devices. Below some selected European pilot wave plant are listed [9].
C.
Archimedes Wave Swing (AWS) The AWS device has been developed by Teamwork Technology BV (Nederland), http://www.waveswing.com, the rights now owned by AWS Ocean Energy LTD (UK), http://www.awsocean.com, consists of a hollow, pressurized steel structure, the upper part of which is initiated to heave motions by the periodic changing of hydrostatic pressure beneath a wave. Being submerged, the device is characterized by low visual and acoustic impact. New model AWS II is under development.
A. Oscillating Water Columns (OWC) The power plant is located at the island of Pico, on the Azores (Portugal). It is a shoreline OWC equipped with a 400 kW Wells turbine. This plant was built between 1995 and 1999, under the coordination of Instituto Superior Técnico, Lisboa (Portugal) and has been co-funded by the EC (http://www.ist.utl.pt). Currently an commercial project is developed.
Fig. 5. Prototype of 2 MW AWS energy converter
Fig. 3. Prototype of 400 kW OWC plant at the Pico island (Portugal)
D. Wavebob The Wavebob energy converter (Fig. 6) has been developed by Wavebob Ltd (UK), http://www.clearpower.ie. The device comprises a wave energy absorber and a hydraulic power take-off system driving synchronous alternators.
B. Pelamis The Pelamis, developed by Ocean Power Delivery Ltd (OPD, UK), http://www.oceanpd.com, is a semi-submerged, articulated structure composed of cylindrical sections linked by hinged joints.
Fig. 6. Prototype of 500 kW Wavebob energy converter
Fig. 4. Pelamis commercial scale prototype
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 electrical generators to produce electricity.
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E. Wave Dragon The Wave Dragon (Fig. 7 and 8) is an offshore overtopping device developed by a group of companies led by Wave Dragon ApS (DK), http://www.wavedragon.net. It utilizes a patented wave reflector design to focus the wave towards a ramp and fill a higherlevel reservoir. Electricity is produced by a set of low-head Kaplan turbines. The development work is to a large extent built on the concept: use proven technologies when going offshore. The plant consists of two wave reflectors focusing the incoming waves towards a ramp, a reservoir for collecting the overtopping water and a number of hydro turbines for converting the pressure head into mechanical power. Generators connected with turbines convert mechanical
grid side AC-DC converter as well as for generator side DCAC converter have a major impact on final parameters of the RES.
Fig. 7. Prototype of Wave Dragon in fjord Nissum Bredning [13]
energy into electrical power which is controlled by power electronics AC/DC/AC converters.
Fig. 9. The system is based on 16 PM generators coupled with low-head Kaplan turbines (upper view) and each generator is controlled by one dedicated converter [7]
III. DIRECT POWER AND TORQUE CONTROL OF AC/DC/AC CONVERTER WITH PERMANENT MAGNET SYNCHRONOUS GENERATOR
Fig. 8. Wave Dragon MW [1]
The utilization of the overtopping principle as opposed to power absorption via moving bodies means that the efficiency grows with the size of the converter. This means that only practical matters set limits for the size of this WEC. Additionally Wave Dragon due to its large size can act as a floating foundation for MW wind turbines, thus adding a very significant contribution to annual power production at a marginal cost [2]. Wave Dragon (WD) has been developed during the last nine years. Grid connected prototype presented in Fig. 7 (a scale 1:4.5 of a North Ocean production plant) of the WD MW is presently being tested in a Danish fjord Nissum Bredning. Every activity is focused on one goal: to produce electricity with the highest efficiency in the lowest possible costs – and in an environmental friendly and reliable way [2]. The only way to achieved this goal (from power electronics point of view) is choosing proper generating system and power electronics converter with robust and accuracy control methodology. Therefore, 16 parallel units of full controlled AC/DC/AC converters with PMSG are considered to be implemented (Fig. 9). However, well designed control schemes (estimators, controllers’ parameters in control loops etc.) for
Developed by Warsaw University of Technology, Direct Power Control with Space Vector Modulation (DPC-SVM) and Direct Torque Control with Space Vector Modulation (DTC-SVM) [6] are very promising for control of an AC/DC/AC converter. When both algorithms are joined together for control of the AC/DC/AC converter connecting electrical machine and supply grid the Direct Power and Torque Control with Space Vector Modulation is obtained – DPTC-SVM [4,5]. A. Direct Torque Control with Space Vector Modulation – DTCSVM The DTC-SVM control scheme joins the switching table based DTC and FOC features in one control structure as in Fig. 10. The command electromagnetic torque M ec of generator is delivered from outer PI speed controller. Then, M ec and command stator flux ΨSc amplitudes of generator are compared with estimated actual values of torque M e and stator flux ΨS in generator. The torque eM and flux eψ errors are fed to two PI controllers. The output signals are the command stator voltage components U Syc , and U Sxc in stator flux coordinate system respectively. Further, these signals are transformed into αβ stationary coordinates using γ ΨS flux angle position. Obtained stator voltage vector U Sc is delivered to SVM which generates duty cycles vector D2(D2A, D2B, D2C) for the DC-AC converter generator side. Proper design of the PI torque and flux controller parameters is given in [6].
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P
Q
VM
γΨ
UL
IL
L
D1 + Qc = 0 − + eU dc −
+
U dc
− eQ
U pqc
ep
U ppc
+ PVSIc 2
U pc
αβ
S1
+ + +
C1
PFUI Ψ Sc
Ω mc eΩ +
Ωm
m
−
U dc
+
M ec +
eψ − eM
U Sxc
ΔU d ΔI d + ΔU qCΔI q 2
xy
RestU=
ΔI d2 +S ΔI q2
Sc
U Syc
−
αβ
Lest = γΨ
S
2
ΔU q ΔI d − ΔU d ΔI q D2
(ΔI d2 + ΔI q2I)Sω
(2)
(3)
ΨS Me
Ωm
U dc
3~
Fig. 10. Direct Power and Torque Control-Space Vector Modulated (DPTC-SVM) with active filter power feedforward (PFUI), higher harmonics and voltage dips compensation. Where VM - virtual machine, SVM - space vector modulator.
B.
Direct Power Control with Space Vector Modulation – DPCSVM
Direct power control with space vector modulation – DPC-SVM guarantees high dynamics and static performance via an internal power control loops. This method joins the concept of hysteresis based DPC and virtual flux (VF) oriented control (V-FOC) [6]. The DPC-SVM with constant switching frequency uses closed active and reactive power control loops (Fig. 10). The command active power Pc are generated by outer DC-link voltage controller, whereas command reactive power Qc is set to zero for unity power factor operation.
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These values are compared with the estimated P and Q values, respectively and calculated errors e p and eQ are delivered to PI power controllers. Voltages generated by power controllers are DC quantities, which after transformation to stationary αβ coordinates, using γ ΨL VF position angle, the voltages are delivered to SVM block. The proper design of the power controller parameters is very important especially in respect to line side power quality [7].
C. Active power feedforward – PF In spite of very good dynamics behaviors of DPTC-SVM scheme, the stabilization of the DC-link voltage can be improved. Therefore, active power feedforward – PFUI from generator side DC-AC converter to grid side AC-DC converter was introduced. The PFUI deliver information about machine states directly to active
power control loop of the AC-DC. Thanks to faster control of power flow between generator and grid, the fluctuation of the DC-link voltages will be significantly reduced. So, the life time of the DClink capacitors can be enlarged. Estimation of this power is quite difficult, because the parameters of the machine and states of power switches are needed. Therefore, U simplified power estimator based on command stator voltage Sc and actual stator current can be used [3]:
PLSC =
3 (I SxU Sxc + I SyU Syc ) 2
(1)
D. Grid parameter estimation In order to estimate the grid impedance, the method proposed in [1, 10] has been applied. Basically, the voltage drop UGrid over the grid impedance is used to estimate its value. Therefore, a step in the grid power is required to eliminate the unknown actual grid voltage from the equations. Since the real power of the grid side converter controls the DC-Link voltage, the reactive power is increased to provide a current step without influencing the active power control. Note that this is not possible whenever the current rating of the inverter is already reach by active current, e.g. in full power operation. As can be seen in Fig. 11, there is a voltage drop on the grid impedance denoted UGrid. There is also a voltage drop on the known grid side inductor of the used LCL-filter titled UL1. The measured Voltage UMeas will therefore show a drop whenever the current is increased. A three phase PLL is used to transfer the three phase voltages in a two phase synchronous reference frame, where Uq is always zero. The voltage and currents in synchronous coordinates dq are low pass filtered by 3rd order Chebychev filter to remove ripple which will distort and influence measurement accuracy. The values are measured twice at different grid current. The difference that results from this is used to calculate the grid impedance according to equations Eq. (2) and Eq. (3). This block scheme is shown in Fig. 12. For the LCL filter parametersL2=361 µH, L1=288 µH, R1=R2=2 mOhms, C=84.9 µF. and the grid impedance LG=192 µH and RG=2mOhms. The step in reactive power is shown in Fig. 13.
Qref 2
Qref
t1 U gα
Ud αβ
Uq U gβ I gα
dq
γ
MEASUR. 1
t1
MEASUR. 2
t2 −
PLL
αβ
Id
Iq
ΔUd
+ −
U qref = 0
− −
dq
Igβ
FILTER DP
Qre f1 = 0
t2
FILTER DP
MEASUR. 1
MEASUR. 2
t1
t2
+ ΔUq
ΔId
LEST
+ ΔI q +
Fig. 12. Algorithm of grid impedance estimation according to Eqs. (2) and (3).
U1 I1
Qref Qmeas
Fig. 13: Step in reactive power used for grid parameter estimation Q = 0 → 60 kVA →0
ΔIq
478μH
Iq
ΔId
Id
ΔUd
Ud
Lest
Fig. 14: Measured and filtred grid currents and voltages during step in reactive power used for grid parameter estimation
Fig. 11: Voltage drop on grid impedance UGRID used for grid impedance estimation.
The measured and filtered currents and voltages are shown in Fig. 14. During test, the quadrature component of the measured voltage stays zero, due to the used PLL. The inductance of the grid is very well estimated to be Lest=478 µH, compared to LG+L1=480µH.
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V. VOLTAGE DIPS AND HARMONIC COMPENSATION For voltage dips and harmonic compensation the control scheme has to be expanded as shown by dashed blocks. Typical test considers a 60% non-symmetrical voltage dip in phase “a”. In Fig.15 operation in generating mode with 100ms dip is presented. It can be seen that after 70 ms from the sag beginning actual powers return to command values. Small 50W (3%) oscillation of active power are caused by little current unbalance in phase “b”.
Fig. 16 Harmonics compensation process: a) before compensation, b) with compensation and power controllers tuned based on symmetry optimum, c) with compensation and power controllers tuned based on modulus optimum; I – 2A/div, U-100V/div
. Fig. 15. Simulations results in generating mode: under 60% voltage sag in phase “a”: a) for classical control structure, b) with grid voltage compensation
Experimental results for higher harmonic compensation are presented in oscilograms of Fig. 16. Finally, in Fig. 17 grid phase currents, powers and DC voltage waveforms during voltage dip are presented: a) classical control – compensation is off (during 30% voltage dip), b) grid voltage dip (60%) compensation is on. As it can be observed compensation assures stable operation during voltage dip and power level is the same as before the dip. IV. CONCLUSIONS The paper has reviewed development of ocean wave energy converters with special focus on Wave Dragon MW device. Power train consists of 16 PM generators with AC-DC-AC grid interfacing converters. The AC-DC-AC grid interfacing converter system based on the Direct Power and Torque Control with Space Vector Modulation (DPTC-SVM) algorithm has been described. Also, problem of grid impedance estimation higher harmonic and voltage dips compensation has been illustrated with experimental oscilograms. V. ACKNOWLEDGEMENT Fig. 17 Experimental results in generating mode under 60% voltage dip in phase
Dr M. Jasinski is grateful for the support of the Centre of Advanced “a”: a) 30% for classical control, b) 60% for control with sag compensation I – Study, Warsaw University of Technology. 5A/div, U-100V/div, P,Q – 1kW/div, Udc-200V/div.
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REFERENCES [1] M. Citabaru, R. Teodorescu, P. Rodriguez, A. Timbus, F. Blaabjerg, "On-line Grid Impedance Estimation for single-phase grid connected systems using PQ variations" In Proc. IEEE-PESC '07, Orlando, Florida 2007 [2] L. Christiansen, E. Friis-Madsen, J. P. Kofoed, ”Worlds Larges Wave Energy Project 2007 in Wales”, PowerGen 2006. [3] H. Hur, J. Jung and K. Nam, ”A Fast Dynamics DC-link PowerBalancing Scheme for a PWM Converter-Inverter System,” IEEE Trans. on Industrial Electronics, vol. 48, No. 4, August 2001, pp. 794–803. [4] M. Jasinski, Direct Power and Torque Control of AC/DC/AC ConverterFed Induction Motor Drives, Warsaw University of Technology, Ph.D. Thesis, Warsaw, Poland, 2005. [5] M. Jasinski et. al: “Control of AC-DC-AC Converter for Multi MW Wave Dragon Offshore Energy Conversion System”, in Proc. Of the IEEE – Intern. Symp. On Industrial Electronics ISIE’07, 2007, pp. 26852690. [6] M. Kazmierkowski, R. Krishnan, and F. Blaabjerg, Control in Power Electronics, Academic Press, 2002, pg. 579.
[7] M. P. Kazmierkowski, M. Jasinski and H. Ch. Soerensen: „Ocean Waves Energy Converter – Wale Dragon MW”, Electrotechnical Review, vol. 84, 2007, No. 2, 2008, pp.1-13. [8] M. P. Kazmierkowski, M. Jasinski, „Power Electronics for Sea Wave Renewable Energy” International Conf. OPTIM 2010, [9] Ocean Energy Conversion in Europe, Centre for Renewable Energy Sources (CRES), 2006 [10] K. Rothenhagen, M. Jasiński, M.P. Kazmierkowski, "Grid Connection of Multi-Megawatt Clean Wave Energy Power Plant under Weak Grid Condition", in Proc. of International Conf. EPE-PEMC, pp. 1927-1933, Poznan, Poland, 2008. [11] D. Swierczynski, Direct Torque Control with Space Vector Modulation (DTC-SVM) of Inverter-Fed Permanent Magnet Synchronous Motor Drive, Warsaw University of Technology, Ph.D. Thesis, Warsaw, Poland, 2005. [12] P. Vas, Sensorless Vector and Direct Torque Control, Oxford University Press. [13] www.wavedragon.net
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