Owemes 2006, 20-22 April. Citavecchia, Italy.
Control and Operation of Variable Speed Marine Current Turbines Results from a Project funded by the German Ministry for the Environment G. Mattarolo, J. Bard, P. Caselitz, J. Giebhardt ISET, Institut für Solare Energieversorgungstechnik Königstor 59, 34119 Kassel, Germany
[email protected]
Abstract Wind energy and marine current turbines have a common principle. Control engineering methods developed for dynamically loaded wind turbines are valid and applicable also for this renewable energy technology. Similarly to wind energy converters, variable speed operation of marine current turbines presents several advantages compared to fixed speed rotors. A prototype of a variable speed marine current turbine, SEAFLOW, has been realized and brought into operation in the framework of the project “World’s first pilot project for the exploitation of marine current at a commercial scale” funded by EC. Two other parallel projects funded by the UK Department for Trade and Industry (DTI), and by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) integrated the main one. In order to develop and realize the turbine, a dynamic simulation model based on blade element theory has been developed. A speed control algorithm and a plant management concept have been elaborated based on these simulations for a safe operation under normal and extreme current conditions. In June 2003, the system components have been realized and assembled into the SEAFLOW machine developed by the British company Marine Current Turbines (MCT). The plant installation was completed in the Bristol estuary. A special converter concept has been developed and tested in the laboratory in order to cope with the off-grid operation of the plant during the test phase. During the ongoing test phase, the simulation model has been validated and speed and operational control have been adjusted according to the real current conditions. Another aspect to investigate and to develop is the system monitoring, including power control and limitation. This is fundamental in order to guarantee automatic operation. The test results indicate that the plant reaches the planned performance: 300 kW have been achieved at current peaks. A follow up project concerning the development of a MW machine based on a twin rotor concept as a pilot for a small series has been started in August 2005 in close cooperation with MCT. The paper illustrates the electrical system design and the first experimental data, focusing on the control and monitoring system, developed in the German project
is: “Control and management of a variable speed marine current turbine” [1]. All the projects led to a consortium of 7 organizations working on SEAFLOW, bringing a better experience required for the development, the installation and the testing of the turbine (Figure 1). The overall budget was around €5 million. Figure 1 summarizes the structure of the projects. The projects ended in June 2004. The main target of the project was to design, build, install and test a 300 kW prototype of a marine turbine with horizontal axis rotor.
1. THE SEAFLOW PROJECT The realization of the variable speed marine turbine SEAFLOW has been a cooperative effort of multiple partners and multiple funding agencies. The work began with the EC project in September 1998. Two other projects, to support SEAFLOW started at the beginning of 2001 [5]. The first one has been financed by DTI and involved 5 English companies. The second one has been supported by BMU and carried out by ISET. The title of this project
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UK PROJECT Supported by DTI EUROPEAN PROJECT Supported by EC Jahnel-Kestermann Gearbox design and manufacture GERMAN PROJECT Supported by BMU ISET e.V.
IT Power Project coordinator, design, procurement and testing
MCT Design, testing and commercial development of technology Bendall Engineering
Seacore
Design detailing, steel fabrication and assembly
Design operation, offshore operations
Corus Steel supply, technical assistance
Rotor modeling, electrical power and control system
Figure 1: The relationship of the various projects and partners behind SEAFLOW.
representing the marine currents has been implemented, taking into account the effects of waves, tidal cycles and velocity shear through the water column. The purposes of the simulations are two: the evaluation of performances and dynamic loads in different operating conditions, and the concept development for the operation and control of the marine turbine. The two main components of the operation control are the pitch and the speed control. The pitch control limits the power in case of extreme current conditions and allows to start and stop the system. The speed variations enable an operation control based on the current conditions and the compensation of dynamic variations caused by waves or turbulence. This allows to reduce the structural mechanical loads, increasing the output power and offering a potential reduction of maintenance and components fabrication costs. By changing the speed in quasi-stationary current conditions, it is also possible to investigate the rotor characteristics. It can as well be seen as an extra degree of freedom added to the marine turbine concept, which is an advantage, given the uncertainties in a prototype development. These control features, developed previously for wind energy, have been tested and approved by dynamic simulations before the installation of the system.
In particular the objectives of the German project were to create a model of the rotor and to design, develop and install the electrical system and the control, in order to realise a fully automatic variable speed operation. 2. DESIGN AND CONTROL SYSTEM DEVELOPMENT APPROACH The development and the implementation of a new energy conversion technology concept have to face, at the beginning, the lack of experience and knowledge concerning the resource to be used, the operation conditions and the occurring load characteristics. Therefore, beside model tests and detailed measurements, dynamic simulations of operating systems can be very powerful tools in order to develop such technology [2]. These simulations are based on realistic models that describe mathematically the relevant processes and components. Since several years ISET has been developing a model library for the simulation of energy systems and generators. Today this library is composed by more than 100 models and includes all kinds of generators like PV, hydro- and windturbine, gensets with ICEs, and mechanical and electrical components like pumps, gearbox, electrical generators and inverters. In the SEAFLOW project an existing rotor model of a wind turbine based on the blade element theory has been modified for the turbine. In addition a new model necessary for 2
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frequency converter have been manufactured by VATech ELIN EBG .
3. TURBINE DESIGN 3.1 General description The turbine is mounted on a steel tube fixed into the seabed. The horizontal-axis rotor is mounted directly onto the shaft of a gearbox that drives a generator. The powertrain (gearbox and generator) is fixed to a collar that can slide up and down the pile. Underwater operations should not be required during the life of the machine. Inspection and maintenance are allowed by lifting the collar out of the water using a hydraulic mechanism integrated into the turbine (Figure 2). Apart from the power train, all the other systems are housed in a pod on the top of the pile. This means that they can be kept in a controlled, dry environment, which is especially important for the electrical and control components.
Gear box
ASG
Figure 3. Generator-frequency converter system.
4. OFF GRID OPERATION For a commercial machine, a submerged cable would provide the connection of the turbine to the local electrical network on shore. Due to the high cost for a grid connection with respect to the expected electricity production of the prototype, it was decided to run the turbine off grid. This approach implied the development of an electric system concept that would allow to dissipate the power generated, to excite the generator and to provide the back up power for all the complementary systems. Consequently, the same operating conditions could be achieved, as if the turbine was connected to the electrical network. Figure 4 shows a scheme of the electrical system realised. An air-cooled power resistor is used to dissipate the generated power and a standalone power system composed by a diesel generator and batteries has been installed. 5. CONTROL SYSTEM IMPLEMENTATION In order to develop and implement the operation control system, it was necessary to define all the different turbine operating conditions, which can be realised with the controlled operation. Once the operating modes and the conditions to switch from one mode to the other are defined, the control system based on Matlab Simulink can be implemented and tested first in a dynamic simulation model and then in an experimental bench tests. The direct transfer of the control software from the simulation environment to the final control hardware using the “Real Time Workshop” reduces the development time and avoids mistakes. To operate the turbine manually and change the control parameters during operation, the running program can be connected with the Matlab Simulink environment. In this way it is possible to set speed and pitch control from the Matlab interface, while some functions are still automatic, like the start-up process and the supervision of the limit values. Figure 5 shows the turbine management realised by the control system.
Figure 2. Concept of SEAFLOW (picture from MCT).
3.2 Rotor The first step in order to realize the turbine design has been the evaluation of the rotor performances and loads, by using the rotor model developed by ISET. The final rotor has a diameter of 11 meter, designed to achieve 300 kW of electrical power output and to be installed in a sea depth of 15 m at lowest tide. It is a two-bladed rotor, able to reverse the blades by pitching them through 180º in order to operate both on flood and ebb tide. 3.3 Power train The gearbox has been designed and produced by the German partner Jahnel Kestermann. It increases the speed in order to drive a nominal 1000 rpm asynchronous generator. Gearbox and generator are submerged in the seawater. The generator is connected to a frequency converter placed in the pod, allowing to control the generator speed (Figure 3). Generator and 3
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Pitch
ASG
Last
Peripheral loads
Batteries
3-phase inverter
IPC Navigation aids
3-phase inverters
Power Profibus
Sensors
Frequencyconverter
digital, Controller analog I/O Host
Diesel genset WLAN
Figure 4. Simplified schemes of the electrical system.
S1 plant examination
off S2 release/ trundle
on S6 stopping
S3 start up S7 emergency shut-down S4 load operation
S5 controlled shut-down
Figure 5. Operating modes used for the control. 4
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variable speed regime and to validate the function of the control system. A torque-controlled Dc motor is coupled to the turbine generator. In such a way the dynamic loads and the rotor characteristic can be reproduced and the behaviour of the control system can be evaluated.
6. LABORATORY TESTING After the development and testing in the simulation environment, the electric power and control systems have been assembled and tested in ISET’s “Demotec” laboratory, in a HIL (Hardware In the Loop) arrangement (Figure 6 and 7). The test bench allows to operate the generator in a
System control and monitoring
SEAFLOW components
test drive unitDEMOTEC
grid
GM
grid
ASG
I
U
Mlim,nlim real time
I
U
flim P1
M,n
SEAFLOW model
Figure 6. Scheme of the experimental test stand in Demotec.
Figure 7. Electrical system testing at ISET. 5
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The communication to the machine is realised via a WLAN connection. That allows the remote control and monitoring of the turbine by using a PC onshore.
The tests covered also the frequency converter, the power resistor, the 3-phase grid consisting of batteries and battery inverter, the inverter and the controller PC. In order to recharge the batteries a Diesel genset could also be used, as in the real machine. With the combination of dynamic simulations and hardware tests it has been possible to check the system in operating conditions similar to the real ones, allowing to adjust and optimise the control system and the power train components prior to the installation of the turbine.
8. INSTALLATION After a successful submergence test, in which the power train was kept under water for 24 hours, the turbine was installed in the selected site off the North Devon coast. The installation was
7. MONITORING AND COMMUNICATION SYSTEM Because of its environment, any offshore technology requires high reliability and more detailed monitoring than systems onshore with relatively easy access. Furthermore a prototype, developed to analyse and understand better the power extraction from tidal currents, requires a number of sensors to monitor the operation of the turbine in detail. Table 1 summarises the main instrumentation installed in the turbine. Table 1. Main instrumentation.
Area Environment
Forces
Operation
Measurement Current Water depth and waves Wind speed and direction Blade bending moment and forces Pile bending moment and forces Pile movement Power, voltage, current, etc. Pitch angle Rotor position
Condition monitoring
Gearbox oil and bearing temperature Generator winding and bearing temperature Water in hub
Sensor Magnetic meter Pressure transducer Anemometer Blade strain gauges Pile strain gauges Pile accelerometer Frequency converter Rotary shaft encoders Shaft encoders Temperature sensors Temperature sensors Leakage sensors Figure 8. Schematic of installed turbine (picture from MCT) 6
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9.2 Experimental results Because of the uncertainty on the current behaviour and therefore on the dynamic loads acting on the turbine, precautions have been taken to bring safely the turbine at the full operating regime. The output power was increased gradually, during several tests, to finally get to the nominal operating conditions. Figure 10 gives an overview of the tests run from June 2003 until March 2004. SEAFLOW reached electrical power peak just under 300 kW, very close to the initial target of the project. Figure 10 shows typical test results during an ebb tide cycle. The graph shows both unfiltered and filtered data, which give a better indication of the average power output. It has been found that the rotor is more efficient than predicted, but the velocity current at hubdepth is lower than expected (Figure 11). Even filtered data show a highly unsteady power output profile of the turbine. Variations of up to 30-50 kW with no particular period can be observed. They can be mainly related to the fluctuations of the current velocity, given by the turbulence of the water. Figure 12 shows the profile of the current velocity and the corresponding turbine power plotted against time over a 20 minute period. Two other short period fluctuations that can be seen in the power output are given by the pile and current effects related to the rotor speed and by the oscillation of the control system.
completed in June 2003. A jack up barge realised the foundation by inserting a casing in the seabed and placed the mono pile in it. The pile was reinforced with injected grout. Once the structure achieved a sufficient strength the rest of the turbine was assembled (Figure 8). 9. RESULTS 9.1 Dynamic simulation results One purpose of the simulation work was to evaluate the effect of the variable speed operation on the reduction of the dynamic loads. Figure 9 shows the results of a simulation where loads have been calculated both in variable speed operation and in fix speed operation. The conditions assumed in this case were a stationary current with a velocity of 2 m/s, and imposed waves with amplitude of 3.5 m and period of 5.6 s. In the case of the variable speed operation, in front of an average power output increase of 10%, the average and the variation range of the thrust have increased respectively only 8% and 4% and there is even a reduction of the torque range of 18%. Such results are not relevant for the design and the sizing, but show a significant reduction of fatigue loads from waves. This implies longer lifetime and reduced costs, which are fundamental aspects for the commercial success of a technology. 350
333 297
300
307
variable speed
294
fix speed 242
[kW],[kNm], [kN]
250
217
200
186
178 154
150
168
154 139
198
137
146
135
100
79 79 50
0
Average Maximum Range
Power [kW]
Average
Maximum Range
Torque [kNm ]
Average
Maximum Range
Thrust [kN]
Figure 9. Dynamic loads resulting from simulations: comparison between variable speed and fix speed operation.
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350
300
Power ADCP - filtered Shaft Power - Filtered
Shaft Power [kW]
250
200
150
100
50
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Time from HW [hrs]
Figure 10. Typical test run results over an ebb tide cycle.
2,5 2 Vx - Prediction based on Tide Tables
1,5
Vx - Current Meter Raw Data Current Speed [m/s]
1 0,5 0 -0,5 -1 -1,5 -2 -2,5 21:36
22:36
23:36
00:36
01:36
02:36
03:36
04:36
05:36
06:36
07:36
Time
Figure 11. Comparison of calculated and measured current speed profile over a period of 12 hours.
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08:36
09:36
Owemes 2006, 20-22 April. Citavecchia, Italy. 220 Leistung Power [kW]
200
3,0
Current velocity Strömungsgeschwindigkeit [m/s] 180
Power (Kw) Leistung kw
120
2,0
100
80
Current velocity (m/s)
2,5 140
Strömungsgeschwindigkeit [m/s]
160
60 1,5 40
20
0
1,0 0
300
600
900
1200
Zeit (s) [s] Time
Figure 12. Profiles of power and current velocity measured in front of the rotor over a period of 23 minutes.
Another important concept which could be implemented for marine current turbines, is the fault prediction realised by condition monitoring. This method is based on the real time analysis of frequencies generated by the system (e.g. mechanical vibrations from different components). All the relevant frequencies are measured since the very beginning of the system operation. Any new frequency that appears later can be identified by the condition monitoring system and, in case, evaluated as fault signal. [4]
10. FURTHER STEPS The next project phase, “SEAGEN”, funded by the British DTI and the German BMU has started in 2005 as a continuation and improvement of the work done to realise SEAFLOW. The target of the new project is to design and build a larger pre-commercial turbine with two rotors, each generating at least 500 kW, to give 1 MW output. The twin-rotor design would allow to get more power from a single pile installed, reducing cost, and to keep the rotors far from the pile, enabling a more regular bi-directional operation (Figure 13). On the other hand, with such a concept, new loads given by the interaction between the two rotors have to be taken in account. Therefore the control and monitoring system become even more important to guarantee a safe, reliable and performing operation of the turbine. Concepts and techniques already in use or under development in wind energy can be modified and implemented in marine current turbines for such purposes. One of these concepts is the use of dynamic pitching. Here the blade angle is changed during the rotation of the rotor, depending on the actual load situation and on the blade position. That requires individual blade pitching systems with a fast and precise sensor integrated in the main control. Such a control technique will help e.g. to reduce peak loads and tower oscillations [3].
11. CONCLUSIONS A variable speed marine current turbine has been successfully designed, tested and installed in the framework of an EC project. The turbine is able to provide an electric power output of up to 300 KW. The electric, control and monitoring systems have been developed by ISET and implemented in the turbine after dynamic simulations and HIL testing. This work has been carried out in the framework of a parallel German project funded by BMU. During the test phase, the machine has been in continuous operation over weeks with out any malfunction. The concept and the development process have proven to be adequate for this new technology. This project can therefore be considered as a mayor step forward towards a commercial use of tidal currents for electricity production.
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Figure 13. Future farm of twin-rotor marine current turbines (picture from MCT)
[3] Caselitz P., Geyler M., Giebhardt J., Panahandeh B., “Hardware-In-the-Loop Development and Testing of New Pitch Control Algorithms”, Proceedings EWEC 2006, European Wind Energy. [4] Caselitz P., Giebhradt J., “Rotor condition monitoring for improved operational safety of offshore wind energy converter”, Journal of Solar Energy Engineering, May 2005, Vol. 127, pp 253-261. [5] Thake, J.R., “SEAFLOW World’s First Pilot Project for the Exploitation of Marine Currents at a Commercial Scale”, 2004, final publishable report.
A follow-up project has started in 2005 with the objective to develop a twin-rotor marine current turbine able to provide an output power of 1 MW. REFERENCES [1] Bard J., “Regelung und Betriebsführung drehzahlvariabler Meerströmungsturbinen”, 2004, final publishable report. [2] Bard J., Schmid J., Caselitz P., Giebhardt J., “Electrical Engineering Aspects of Ocean Energy Converters”, Proceedings 6th European Wave and Tidal Energy Conference, 2005.
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