Grid Connection of Multi-Megawatt Clean Wave Energy Power Plant under Weak Grid Condition Kai Rothenhagen*, Marek Jasinski†, and Marian P. Kazmierkowski+ * Institute for Power Electronics and Electrical Drives / University of Kiel, Kiel, Germany,
[email protected] †
Institute of Control and Industrial Electronics / Warsaw University of Technology, Warsaw, Poland,
[email protected]
+
Institute of Control and Industrial Electronics / Warsaw University of Technology, Warsaw, Poland,
[email protected]
Abstract— The WaveDragon is a 7 Megawatt Wave Energy Converter, that is currently developed for clean offshore energy production. The system will be floating several kilometres off the Pembrokeshire coast, Wales, UK, and transfers energy using a submarine power cable. Key interest is the control of the power take-off system at disturbed voltages and under weak-grid condition due to long submarine cable and remote, rural location. A multireference frame controller is implemented for harmonics compensation. A parameter estimator is implemented to estimate the grid impedance. Simulation and measurements results from a laboratory test setup that illustrate properties of developed method are shown. Keywords— Renewable energy systems, Voltage Source Converter (VSC)
I. INTRODUCTION It has been widely discussed that the use of clean renewable energy is one of the important future task our society has to take on. The remarkable progress in the field of Wind Energy may serve as an example for other sources of clean, carbon-dioxide free power sources. Next to bio-fuels, solar and hydropower, wave energy is a very interesting and not yet intensively covered field. Oceans cover roughly 75 percent of the earth surface, and offer a tremendous amount of Renewable Energy in form of waves, which may serve as an extension to the current energy mix. Waves contain an average of 75 kW per meter [1]. Table 1 shows the potential of Renewable Energy according to [1]. TABLE I. RENEWABLE ENERGY RESOURCES [1]
Biomass Hydro Solar Wind Goethermal Ocean Total
Available [1018 J] 283 50 1570 580 1401 730 4614
Current Use [1018 J] 50 10 0.2 0.2 2 0 62.4
In order to harvest this energy and convert into AC electric power, wave energy plants are needed. Several different types have been researched in the last decades, such as the Pelamis [12] and AquaBuoy [13] which are floating devices, or so called Oscillating Water Column
(OWC) devices located on-shore or off-shore such as described in [14] and [15]. One very promising approach is the Wavedragon, which uses water turbines. Converting the rotational energy of these turbines into the public grid is most efficiently done by using a variable speed generator. This generator is connected to the grid using a PWM inverter, a DC-link and a PWM converter. A submarine cable of about 5 km length links the Wave Dragon to the point of common coupling at the coast. A transformer will be used to step up the voltage from 690 V rms converter output to 11 kV submarine cable voltage. The long submarine cable and the weak grid impose two difficulties to the converter control. Firstly, harmonics in the grid voltages lead to non-sinusoidal currents. Secondly, the parameters of the cable and the grid should be known for high control performance and unity power factor at the point of common coupling. The goal of this paper is to analyze the problem and present a solution to achieve sinusoidal currents and unity power factor. A multi-reference frame controller and a parameter estimator is used to achieve this. It is shown that it is easily possible to compensate the current harmonics by implementing a harmonic compensator. It is further shown that parameter estimation shows limited performance. It is analysed what factors are responsible for this and what can be done to improve parameter estimation. The paper is organised as follows. An introduction was given in I. The Wave Dragon and its control scheme is described in II. Harmonics compensation is presented in theory in III, while simulation and measurements are shown in IV. Parameter estimation is presented in theory in V, while measurements and Simulation results are shown in VI. The paper is summed up in a conclusion in VII. References are given.
Fig. 1 Wave Dragon Energy Converter [1]
DC Link Voltage
Grid Current
Grid Voltage Estimation PWM Power and Virtual Flux Estimation Fig. 2: The WaveDragon with the water reservoir and the wave focussing arms.
-
+ II.
Uq
B. Direct Power Control Space Vector Modulation Because of the good experience in past work, a Direct Power Control using Space Vector Modulation (DPTCSVM) is used for the control of the converters, as is presented in fig. 3. The control scheme has been proved reliable and offers unity power factor towards the grid. Since the power generated by the turbines is known, it is fed forward using an Active Power Feedforward (AF) in order to decrease the actual DC-link voltage fluctuation during transients [2]. For the grid side converter control, the grid currents, DClink voltage and grid voltages are measured. Grid voltage sensorless operation is generally possible with this scheme [3], but for the benefit of grid impedance estimation, which is based on calculating ΔU/ΔI in a synchronous reference frame, voltage sensors are used on the capacitor of the LCL-filter. These voltage sensors may also be used to derive the grid angle by means of a Phased Locked Loop, which is needed for coordinate transformation α-β/d-q and d-q/α-β. The angle derived from the voltage measurement is not only used for power control, but also for transformation into the reference frames synchronous to the 5th and 7th harmonic.
+ PC
+
γ ψ dq
WAVE DRAGON AND POWER T AKE OFF SYSTEM
A. Description of the Wave Dragon The Wave Dragon is extracting energy from the waves using the overtopping principle to store water in a reservoir above the sea level. The water is released downwards through low head turbines, much like in small hydro power plants, as is sketched in fig. 1. In total, up to 20 individually controllable turbines will be used, which are switched on or off depending on the amount of incoming water. Two arms extend the structure to both sides, focussing the waves towards the ramp and thereby increase their height. The Wave Dragon, as shown in figure fig. 2, is 300 m wide at the tips of the arms, each arm has a length of 145 m. It is 170 m long and raises up to 6 m over the sea level.
SVM
Ucomp
Q=0 C
Q
P
Harmonic Compensator
Ud
-
α-β
+ DC Link Voltage Reference
Fig. 3: The grid side control scheme used for Wave Dragon power takeoff system.
III.
PWM RECTIFIER UNDER DISTORTED VOLTAGES
The system is likely to work under distorted grid voltage condition, meaning that 5th, 7th and other harmonics are present in the grid voltages UG due to nonlinear loads as presented in Eq. (1). These harmonics lead to distortion in the grid currents with the same frequencies, if the control of the active rectifier is not altered to compensate these voltage harmonics. This is explained in fig. 5 and fig. 6. In order to feed sinusoidal currents into the power grid, a harmonic compensator, also called multi reference frame controller, is used for each of the harmonics that shall be compensated. Note that the commonly present 5th harmonic is always a negative sequence, while the 7th is a positive sequence. The grid rotational frequency is denoted as ωg=2πfg. The harmonic compensator consists of a vector rotation into a reference frame that is synchronously rotating with the harmonic to be compensated, as is shown in fig. 4. This way, the harmonic is transformed to a DC component. An integral controller is used to control this component to zero, thus eliminating the respective current harmonic. The controller output generates the necessary compensating voltage UComp in synchronous reference frame. A second vector rotation transforms it back to the stationary reference frame. It is then added to the converter reference voltage, which is used by the PWM modulator to derive the duty cycles of the IGBT-converter bridges. Note that the current signal in the rotating reference frame will contain the fundamental now at six times the
grid frequency, as is shown in Eq. (2). Therefore, the controller should be sufficiently slow, thus not trying to pick up on these frequencies. The same is true for the reference frame synchronous to the 7th harmonic Eq. (3). A similar scheme may be used for 11th and 13th harmonic. Other authors propose a resonant controller for harmonics compensation, which does not need vector transformation [4], [5]. Iα
5
th
Har mon ic Sync h. R ef eren ce Frame
α-β Iβ
Uα,C
d5q5
d5q5
+ α-β
5 ωg
+
Uβ,C
-1 7
th
Har mon ic Sync h. R ef eren ce Frame
d7q7
α-β d7q7
α-β
- 7 ωg
-1
Fig. 4 Harmonic compensator for the 5th and 7th harmonics
U G = U1e UGe
j5 ωg t
UGe
− j7 ωg t
jωg t
+ U 5e
= U1e
j6 ωg t
= U1e
− j5 ωg t
+ U7e
+ U5 + U7e
− j6 ωg t
+ U5e
j7 ωg t j12 ωg t
− j12 ωg t
+ U7
(1) (2) (3)
TABLE II. HARMONIC CONTENT IN EXPERIMENT AND SIMULATION
Frequency 1st Fundamental 5th Harmonic 7th Harmonic 11th Harmonic 13th Harmonic
Amplitude Simulation 400 V rms 2.9 % 2.7 % 0 0
Amplitude Lab 150 V rms 2.9 % 2.7 % 1.1 % 0.3 %
Fig. 5: Distorted Grid Voltage and Sinusoidal Converter Reference lead to Distorted Inductor Voltage, and thereby to distorted current.
IV.
SIMULATION AND MEASUREMENT OF GRID HARMONICS COMPENSATION
Simulations have been carried out to test the harmonic compensator. The results are shown in fig. 7 and fig. 8 for generating mode. The total harmonic distortion is decreased from 11.16% to 6.97%. Good performance is also achieved for motoring mode, as shown in fig. 11 and fig. 12. The Harmonic Compensator has also been implemented in a laboratory test setup. It is used on the grid side converter of a back to back Squirrel Cage Induction Machine variable speed drive, which is controlled by the DPTC-SVM control scheme [2]. The Harmonic Compensator is implemented as is shown in fig. 4. The transformation angle is derived as a multiple of the fundamental grid angle, which is derived by a Phased Locked Loop. The PLL is implemented on the LCL filter voltage using the dq-PLL-method described in [10]. Measurements are taken via the dSPACE DS1103 Rapid Prototyping System, which is equipped with enough analogue digital converters, encoder inputs and PWM outputs to control a back to back frequency converter, as is needed for a variable speed generation system. A 5 kHz sampling frequency is used. The drive is a 3 kW squirrel cage induction machine. Since the focus of this work is not on the drive itself but on the grid connection, the drive can be regarded as an active load. The back to back converter consists of two Danfoss VLT 5005 frequency converters with adapters, which allow external control using the dSPACE system. For the measurements, a California Instruments 5000 iX AC power supply has been used. Using this device, harmonics were implemented into the grid voltage according to table II. The converter DC link voltage was set to 500 V, the load torque in generating mode to –20 Nm at a rotational speed of 1420 rpm. Results are given in fig. 9 and fig. 10. Harmonic distortion is decreased from 10.6 % to 6.0 %, which also fit very well to the simulation.
Fig. 6: Compensating the distortion leads to sinusoidal Inductor Voltage
Fig. 7: Disturbed currents due to 2.9% of 5th and 2.7% of 7th harmonic. Simulated THD of line currents is 11.16%.
Fig. 8: Sinusoidal Currents with Harmonic Compensator: Simulated THD of line currents is 6.97%.
Fig. 9: Disturbed currents due to 2.9% of 5th and 2.7% of 7th harmonics Measured THD of line current is 10.6%.
Fig. 10: Currents with Harmonic Compensator. Measured THD of line current is 6.0%.
Fig. 11: Motoring mode: Sinusoidal Currents after activating the Harmonic Compensator. THD of simulated current is 7.01%
Fig. 12: Motoring mode: Sinusoidal Currents after activating the Harmonic Compensator. THD is 4,6%
V.
GRID PARAMETER ESTIMATION
Trigger
In order to estimate the grid impedance, the method proposed by [6] is first tested in simulation and later implemented in a laboratory test setup. Unlike as in this method, the LCL Filter voltage is used rather than the grid voltage. Other methods, such as [7] and [8], employ complicated extra devices or extensive algorithms and are therefore not suitable for implementation on a floating device, where ruggedness is required. Most work on Grid Impedance Detection focuses on anti-islanding detection, for example for distributed energy production. One approach [9] excites oscillations in an LCL filter, and estimates the grid impedance using the resonance frequency. This approach is also most likely too complicated for industrial use. Basically, the voltage drop over the grid impedance is used to estimate its value. Therefore, a step in the grid current (power) is required to eliminate the unknown actual grid voltage from the equations. Since the grid side converters real power 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 converter is already used by active current, e.g. in full power operation. UGrid
UL 1
LG
L1
RG
C
Ug
L=
ΔU d ΔId + ΔUq ΔIq ΔI2d + ΔIq2 2 d
)
+ ΔIq2 ω
Uβ
Iα
dq
γ
PLL Id
α- β Iβ
Uq
dq
Iq
a z-b
Sample & hold
a z-b
Sample & hold
1 z
a z-b
Sample & hold
1 z
a z-b
Sample & hold
1 z
1 z
+
+
_ ΔU d _
ΔUq
Uq=0
+
+
_
_
ΔId
ΔIq
Fig. 14: Calculating the terms needed for equations (4) and (5).
L2
Uinv
UMeas
(4)
ΔU q ΔId − ΔU d ΔIq
( ΔI
Ud
α- β
UL 2
Fig. 13: Voltage drop on grid impedance used for grid impedance estimation.
R=
Uα
(5)
As it can be seen in fig. 13, 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 LCLfilter 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 reference frame are low pass filtered 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. (4) and Eq. (5). This scheme is shown in fig. 14.
Fig. 15: Pictures of the laboratory setup, 1–isolating interface and control desk, 2- AC/DC/AC converter constructed based on two VSIs (Danfoss VLT 5005 with AAU replaced control boards), 3–resistive load, 4– power supply simulator, 5– reversible rectifier, 6– induction machine set, 7– permanent magnet synchronous machine set
Simulations are carried out with and without antialiasing filter. For filtering, a 3rd order chebychev type 1 as shown in fig. 16 filter is chosen, that results in a phase shift of 6.13° for the 50 Hz grid frequency, but suspends the switching frequency by the factor 0.0059. Filter coefficients are given in the appendix.
Fig. 16: Chebychev Filter Bode Diagramm
VI.
SIMULATION AND EXPERIMENTAL RESULTS OF GRID PARAMETER ESTIMATION
Simulations were carried out in order to evaluate the performance of the described algorithm. In simulation, the LCL filter parameters were L2=361 µH, L1=288 µH, R1=R2=2 mOhms, and C=84.9 µF. The grid impedance was set to LG=192 µH and RG=2mOhms. The used grid voltage was Ug=690 V phase to phase, feeding a current of 100 A to the grid. The reactive power reference was increased to 60 kVAr at time t=0.5 and set back to 0 kVAr at t=0.9. This is shown in fig. 17. U1 I1
Qref Qmeas
Iq
ΔId
Id
ΔUd
Ud
ESTIMATED I NDUCTANCE WITH AND WITHOUT ANTI-ALIASING FILTER
Implemented Inductance
Calculated Inductance with Anti-Aliasing Filter
480 µH 580 µH 680 µH
478 µH 578.6 µH 679.7 µH
Calculated Inductance without AntiAliasing Filter 771 µH 902 µH 1020 µH
Calculation of the ohmic resistance did not deliver usable values on a step of the reactive power. The same scheme was implemented in a laboratory test setup. The total impedance of grid and grid side inductor is approximately 1.435 mH and 0.745 Ohms. No antialiasing filter was used. Using a step in reactive power, an impedance of 1.8mH and 0.022 Ohms was calculated. While the inductance is within a tolerance of 30%, the ohmic resistance is also not well estimated in the laboratory. VII.
Fig. 17: Step in reactive power used to line parameter estimation
ΔIq
TABLE III.
SUMMARY AND CONCLUSION
The Wave Dragon is a 7 MW renewable energy source, which encounters weak grid problems due to the remote nature of an offshore floating platform. Due to the long cable and its inherent phase shift, a parameter estimation of the grid is desirable. It is shown in this paper that by using a harmonic compensator, it is possible to feed sinusoidal currents into a distorted grid. Estimation of the grid impedance is generally possible using a step response in reactive power, but suffers from inaccuracy in the real world implementation. Further research is necessary to improve this scheme. ACKNOWLEDGEMENTS
478μH
Lest
Fig. 18: Step in reactive power used to line parameter estimation
The measured and filtered currents and voltages are shown in fig. 18. During simulation, 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. The remaining error results from the ripple found in the measured signals, which could be further reduced using heavier filtering, which in return would also make the system slower. In order to verify that anti-aliasing filtering is necessary for correct estimation, the estimation procedure has also been run without proper filtering, as is shown in table III. Sampling is synchronized to the PWM of the rectifier.
The authors gratefully acknowledge the financial support of the European Union FP6 (contract no. 019983 Wave Dragon MW).
APPENDIX TABLE IV.
Control Sampling Time Inverter (Grid) DC-Link Voltage
DATA OF EXPERIMENTAL SETUP
dSPACE DS1103 200µs IGBT 2-level Voltage Source Inverter Danfoss VLT 5005 500 V
TABLE V. DATA OF PROPOSED SUBMARINE CABLE
Type Ohmic Resitance Inductance Rated Current Rated Voltage Length TABLE VI.
Numerator Denominator
2XS(FL)2YRAA6/10(12) kV 0.16 Ohm / km 0.34 mH / km 363 A 11 kV Approx 5 km CHEBYCHEV FILTER COEFFICIENTS
[0 0 0 17755e11] [1 7872 60595243 17755e11]
REFERENCES [1] L. Christensen, E. Friis-Madsen, J. Kofoed: The Wave Energy Challenge The Wave Dragon Case, PowerGen 2005 Europe conference [2] M. Jasinski, Direct Power and Torque Control of AC/DC/AC Converter-Fed Induction Motor Drives, PhD-thesis Warsaw University of Technology, Warsaw 2005. [3] M. Malinowski, Sensorless Control Strategies for Three-Phase PWM Rectifiers, PhD-thesis Warsaw University of Technology , Warsaw 2001. [4] H Jae-Wong, M. Winkelnkemper, P. Lehn: Design of an Optimal Stationary Frame Controller for Grid Connected AC-DC Converters, IECON 06, pp 167-172, Paris. [5] R. Teodorescu, F. Blaabjerg, U. Borup, M. Liserre: A New Control Structure for Grid-Connected LCL PV Inverters with Zero SteadyState Error and Selective Harmonic Compensation, 19th annual IEEE Applied Power Electronics Conference and Exposition, pp 580-586, 2004. [6] M. Ciobotaru, R. Teodorescu, P. Rodriguez, A. Timbus, F. Blaabjerg Online grid impedance estimation for single-phase gridconnected systems using PQ variations, PESC’07, Orlando, Florida. [7] M. Harris, A. Kelley: Instrumentation for Measurement of Line Impedance, Proc. On APEC’94, Vol. 2, pp 887-893, 1994 [8] B. Palethorpe, M. Sumner, D. Thomas: System Impedance Measurement for use with active filter control, Proc. Of Power Electronics and Variable Speed Drives, pp 24-28, 2000. [9] M. Liserre, F. Blaabjerg, R. Teodorescu: Grid Impedance Detection via Excitation of LCL Filter Resonance, IEEE Transactions on Industry Applications, vol 45, no 5, pp 1401-1407, 2005 [10] A. Timbus, R. Teodorescu and F. Blaabjerg, M. Liserre: Synchronization Methods for Three Phase Distributed Power Generation Systems An Overview and Evaluation, 36th PESC 07, pp 2474-2481, 2005. [11] Cichowlas, PWM Rectifier with Active Filtering, Warsaw University of Technology, Ph.D. Thesis, Warsaw, Poland, 2004. [12] A. Weinstein, G. Frederikson, M. Parks, K. Nielsen: AquaBuoy – The Offshore Wave Energy Converter Numerical Modeling and Optimization, OCEANS’04, MTS/IEEE Techno-Ocean’04, Vol. 4, pp 1854-1859, 2004. [13] www.oceanpd.com, website of the Pelamis manufacturer. [14] L. Christensen, E. Friis-Madsen, J. Kofoed: The Wave Energy Challenge The Wave Dragon Case, PowerGen 2005 Europe conference [15] H. Polinder, M. Scuotto: Wave Energy Converters and Their Impact on Power Systems, International Conference on Future Power Systems, 2005.
