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Vertical Axis Turbines
Vertical axis turbines that operate in marine currents are based on the same principles as the land based Darrieus turbine. The Darrieus turbine is a cross flow machine, whose axis of rotation meets the flow of the working fluid at right angles. In marine current applications, cross flow turbines allow the use of a vertically orientated rotor which can transmit the torque directly to the water surface without the need of complex transmission systems or an underwater nacelle.
The vertical axis design permits the harnessing of tidal flow from any direction, facilitating the extraction of energy not only in two directions, the incoming and outgoing tide, but making use of the full tidal ellipse of the flow . In this kind of turbines as in the horizontal axis ones the rotation speed is very low (around 15 rpm).
The Enermar Project (Italy)
The core of the Enermar project is the patented Kobold turbine (Fig. I.17a). Among its main characteristics, the Kobold turbine has a very high starting torque that makes it able to start spontaneously even in loaded conditions. A pilot plant is moored in the Strait of Messina, close to the Sicilian shore in Italy, in an average sea tidal current of about 2 m/sec (Fig. I.17b). With a current speed of about 1.8 m/sec, the system can produce power of 20 kW .
The Blue Energy Project (Canada)
Four fixed hydrofoil blades of the Blue Energy tidal turbine are connected to a rotor that drives an integrated gearbox and electrical generator assembly (Fig. I.18a). The turbine is mounted in a durable concrete marine caisson which anchors the unit to the ocean floor, directs flow through the turbine further concentrating the resource supporting the coupler, gearbox, and generator above it. These are located above the water surface and are readily accessible for maintenance and repair (Fig. I.18b). The hydrofoil blades employ a hydrodynamic lift principal that causes the turbine foils to move proportionately faster than the speed of the surrounding water. The rotation of the turbine is unidirectional on both the ebb and the flow of the tide. A unit turbine is expected to be about 200 kW output power. For large scale power production, multiple turbines are linked in series to create a tidal fence across an ocean passage or inlet .
The Gorlov Helical Turbine (USA)
The Gorlov Helical Turbine (GHT) is shown in Fig. I.19a. The turbine consists of one or more long helical blades that run along a cylindrical surface like a screw thread, having a so- called airfoil or airplane wing profile. The blades provide a reaction thrust that can rotate the turbine faster than the water flow itself .
Floating Tidal Energy System
The concept of floating tidal energy system is based on the use of a moored floating structure (e.g. a catamaran) to support the weight of a wheel/generator system (Fig. I.25). The use of a moored device allows easy access to remote coastal communities where it is often expensive to build a fixed infrastructure, and also facilitates removal to a place of refuge when extreme storm conditions occur. Such systems can also exploit shallow water sites or those with large variations in water level .
ELECTRIC GENERATOR TOPOLOGIES IN MARINE CURRENT TURBINES
Marine current turbines can operate with either fixed speed (small speed changes due to the generator slip) or variable speed. Variable speed is here understood as 5% to 100% variation of nominal speed. For fixed-speed turbines, the generator (induction generator) is directly connected to the grid. Since the speed is almost fixed to the grid frequency, and most certainly not controllable, it is not possible to store the turbulence of the tidal resource in form of rotational energy. Therefore, for a fixed-speed system the turbulence of the tidal resource will result in power variations, thus affect the power quality of the grid . This is why the fixedspeed system is not proposed for marine tidal current turbine application. For a variable speed, the generator is controlled by power electronic equipment, which makes it possible to control the rotor speed. In this way the power fluctuations caused by marine current variations can be more or less absorbed by changing the rotor speed . Hence, the power quality impact caused by the marine tidal current turbine can be improved compared to a fixed-speed turbine .
Furthermore, one should distinguish two topology categories: the direct and the indirect driven technology. In this section the following marine current turbine concepts will be briefly presented:
1. Full range variable speed system (with gear).
2. Full range variable speed system (without gear).
3. Limited range variable speed system.
There are also some other concepts, which could be considered but they tend to increase the complexity and therefore the tidal turbine cost .
Full Range Variable Speed System (with Gear)
The system presented in Fig I.26. consists of a marine current turbine equipped with an inverter connected to the stator of the generator. The generator could either be a singly-fed induction generator or a synchronous generator. The used inverter has to have active switches in both the grid and the generator side converters, which allow the converter, at the generator side, to supply reactive power, while active power is flowing from the generator.
The gearbox is designed so that the maximum rotor speed corresponds to the generator rated speed. Since this full-power converter system is commonly used for other applications, one advantage with this system is its well-developed and robust control [54-56]. However, the disadvantage for the full speed range system is that all the power has to be converted in the inverter.
Full Range Variable Speed System (without Gear)
The generator used here is a multipole synchronous one, either with an excitation system or with permanent magnets. The lack of gear box losses compensates partly the large losses in the full power converters (Fig.I.27).
Limited Variable Speed System
This system consists of a marine current turbine with a variable speed constant frequency induction generator (Fig I.28). It is also known as the doubly fed induction generator system. This means that the stator is directly connected to the grid while the rotor winding is connected via slip rings to an inverter.
Table of contents :
CHAPTER I: State of the Art Survey of Marine Current Turbine: Concepts & Technologies
I.3 TIDAL RESSOURCE
I.4 KINETIC ENERGY EXTRACTION
I.5 TURBINE TECHNOLOGIES AND CONCEPTS
5.1 HORIZONTAL AXIS TURBINES
5.2 VERTICAL AXIS TURBINES
5.3 OSCILLATING HYDROFOIL
5.4 FLOATING TIDAL ENERGY SYSTEM
I.6 DEVELOPMENT OF TIDAL TECHNOLOGIES AND DESIGN OPTIONS
I.7 ELECTRIC GENERATOR TOPOLOGIES IN MARINE CURRENT TURBINES.27
7.1 FULL RANGE VARIABLE SPEED SYSTEM (WITH GEAR)
7.2 FULL RANGE VARIABLE SPEED SYSTEM (WITHOUT GEAR)
CHAPTER II: Marine Current Turbine Modeling
II.2 MODELING REQUIREMENTS
II.3 HYDRODYNAMIC MODEL
3.1 RESOURCE MODELING
3.2 THE ROTOR MODELING
II.4 MECHANICAL MODEL
II.5 POWER CONVERTER
II.7 GENERATOR MODEL
7.1 DOUBLY FED INDUCTION GENERATOR (DFIG)
7.2 PERMANENT MAGNET SYNCHRONOUS GENERATOR (PMSG)
CHAPTER III: Control Strategies and Evaluation of the Tested Technologies
III.2 PRINCIPALS OF MCT OPTIMAL CONTROL
2.1 CASE OF FIXED-SPEED VARIABLE-PITCH MCT
2.2 CASE OF VARIABLE-SPEED FIXED-PITCH MCT
III.3 PI CONTROL
3.1 CONTROLLER DESIGN
3.2 OPTIMAL CONTROL OF MCT DRIVEN DFIG BY PI SPEED CONTROL
3.3 OPTIMAL CONTROL OF MCT DRIVEN PMSG BY PI SPEED CONTROL
III.4 HIGH-ORDER SLIDING MODE CONTROL
4.1 CONTROLLER DESIGN
4.2 OPTIMAL CONTROL OF MCT DRIVEN DFIG BY HOSM SPEED CONTROL
4.3 OPTIMAL CONTROL OF MCT DRIVEN PMSG BY HOSM SPEED CONTROL
4.4 HOSM CONTROL ROBUSTNESS AGAINST PARAMETER VARIATIONS
CHAPTER IV: Validation and Experimental Analysis
IV.2 HARDWARE PECIFICATION
IV.3 CONTROL OF A DOUBLY-FED INDUCTION GENERATOR
3.1 EXPERIMENTAL RESULTS FOR A FILTERED RESOURCE
3.2 EXPERIMENTAL RESULTS FOR A TURBULENT RESOURCE
IV.4 CONTROL OF A PERMANENT MAGNET SYNCHRONOUS GENERATOR116
4.1 EXPERIMENTAL RESULTS FOR A FILTERED RESOURCE
4.2 EXPERIMENTAL RESULTS FOR A TURBULENT RESOURCE
CONCLUSIONS & PERSPECTIVES
EXTENDED FRENCH ABSTRACT
II. ETAT DE L’ART SUR LES HYDROLIENNES
II.2 POTENTIEL ENERGETIQUE ET CHOIX DU SITE
II.3 HYDROLIENNE: CONCEPTS ET TOPOLOGIES
III. MODELISATION D’UNE HYDROLIENNE
III.1 MODELISATION DE LA RESSOURCE
III.2 MODELISATION DU CAPTEUR
III.3 MODELISATION DE LA CHAINE DE TRANSMISSION MECANIQUE
III.4 MODELISATION DE LA GENERATRICE
IV. LES STRATEGIES DE COMMANDE
IV.1 COMMANDE PI
IV.2 COMMANDE NON LINEAIRE PAR MODE GLISSANT D’ORDRE SUPERIEUR
V. VALIDATION EXPERIMENTALE
VI. CONCLUSION ET PERSPECTIVES