Abstract: This paper presents the main approaches for a marine
current turbine emulator development. Three design phases are developed: the first one is based on the emulated marine current turbine coupled to a load break, which will be replaced by an electrical generator in the second phase; finally, the third structure is conceived with an additional active load used for inertial tests.
Key words: real time simulators marine energy conversion, electrical powered system, power electronics
I. Introduction
In the last few years, due to the concern for the climate changes, more and more researches are developed in the field of renewable energies, and mostly concentrated on the renewable energies connected to the oceans energies: thermal energy of the seas, offshore wind energy, tidal energy, waves energy, energy of the marine currents.
In this article the interest is focalized on the marine currents. Electrical energy extraction from marine currents offers the promise of regular and predictable energy. The location and viability of such devices to extract energy from marine currents has been a focus on several investigations [1].An example of the existing potential is the estimation made for the area between the Anglo-Normand islands (Alderney) and the French coasts (La Hague). The diameter of turbines has been defined so that the lowest point is at 25% from the bottom of the sea and the highest point at 7 m below the surface [2]. The assumptions define a full installed capacity of about 3243 MW, annual energy productivity is around 7.4 TWh[3]. In many ways the energy conversion from marine currents is quite similar to that of wind energy conversion, the conversion principals remain the same (it is also called the “under water wind turbine”), but there are several differences between them. The underwater placement of a marine current energy converter gives some advantages such as no noise disturbance for the public, low visual exposure and little use of space land,
but adds some challenges concerning the technologies off shore, difficult and costly maintenance etc. Another characteristic is the difference in density between water and air [4]. Marine current benefit from the waters density, which is 832 times bigger than that of the air (almost 1.23 3/kg m at 15° C), which means that the marine current turbine has higher torque at lower speeds. So, despite the marine current speed, generally lower (2-3 m/s), the power obtained per surface unit is bigger for the marine current turbines [5].
In the following of this paper there is presented a Marine Current Turbine Emulator (MCTE), which is conceived using the "Hardware in the Loop Simulation" (HILS) techniques. In the conception of the emulator is used the experience gained in the GREAH laboratory from the studies made in the field of the wind turbine simulator. Because the energy conversion is almost the same, there is the possibility to apply the same principals used in the wind turbine simulator on the MCTE.
II. Marine current turbine modeling
The marine current turbines (MCT) are similar, in concept, with the wind turbines. The MCT is an under-water turbine who uses the kinetic energy of the marine current, like the wind turbine uses the kinetic energy of the wind. In other words, the marine current turbine transforms the water energy into mechanical energy and the generator turns it into electrical energy.
The kinetic energy of the water mass “m” moving with speed “v” is:
2
c1E mv2
= (1)
The turbine purpose is to generate electric energy by
recuperating a part of the kinetic energy of the marine current, which has a speed that can go up to 2-3 m/s. The kinetic energy is recovered and transformed directly into mechanical energy, just like the wind turbine [15].
14th International Power Electronics and Motion Control Conference, EPE-PEMC 2010
If all this energy can be retrieved with a turbine who has the surface “S”, located perpendicularly to the direction of the marine current, then, the power developed (Pm) by the current is:
3
m1P Sv2
= ρ (2)
The marine current energy convert efficiency is given
by the power coefficient Cp, defined by the relationship:
p pm
PC ;C 1P
= < (3)
where, P is the power extracted by the turbine from the available power Pm (P is inferior to the available power Pm) [5].
It is considered a turbine with 3 blades, with a radius of 1.25 m. The modeling of the turbine is strictly dependent with the power coefficient Cp or by its yield. This modeling resembles with the ones made for the wind turbines. Fig. 1 presents the characteristics of the power coefficient (Cp) in dependence with the speed ratio, knowing that the maximum limit is 0.45 and the theoretical limit is 0.59 (Betz formula).
The speed ratio formula is:
T TRvΩ
λ = (4)
where: λ: speed ratio, RT(m): turbine radius, v(m/s): marine current speed and ΩT(rpm): rotation speed of the turbine.
The starting point of the modeling is the representation of the extractible power depending on the speed of the incoming flow of the marine current and the working conditions. [6]
From (3) and (4) the power (P, fig.2) has the form:
3p
1P C Sv2
= ρ (5)
where: ρ(kg/m3): water density, S(m2): area swept by the blades.
The torque (ΓT) obtained at the shaft of the turbine is given by:
TT
PΓ =Ω (6)
The model does not take into consideration the
specific hydrodynamics phenomena of a marine current turbine working in its natural environment.
III. Hardware-in the-loop simulation (HILS) concept
In the last couple of years, for physical simulation of systems, the modern controlled equipment and systems development has been based on using the Hardware-In-the-Loop Simulation (HILS) techniques. The HILS concept had been introduced and used for the first time into the field of mechanical developments and testing. Afterwards, this technique made it possible to simulate diverse complex systems, which is utile in the prototyping phase.
The reasons to use this simulation technique are (especially for the test control systems): when the process is not accessible, either the costs are too high, either the test can be dangerous for the system integrity, either the processes nature doesn’t allow to do tests in its natural environment.
HILS systems contain a physical system that emulates a real device which will be investigated. This part is controlled in real time by a software system, build on the basis of the emulated system mathematical model. It offers static and dynamic characteristics, practically identical with the ones of the emulated (investigated) system. In general the HIL system is composed from two parts [7]:
− a software system in which is implemented the mathematical model of the process to be studied, using a numerical resource (computer) on which there are simulation program of the process and of other variable who are used as commands;
− physical system that provides the similar static and dynamic characteristics as the real system (process) that is to be studied. As can be seen in fig.3, the two systems communicate with each other, they are in a closed loop. The real time software unit sends reference signals to the physical system which responds, during the real time simulation. Fig.1: The power coefficient characteristics
Fig.2: Marine Current Turbine Power characteristics
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Simulation into a closed loop gives the possibility to correct the control errors in real time.
IV. The Marine Current Turbine Emulator (MCTE)
This study comes into the continuation of the researches made in the GREAH laboratory (Group de Recherche en Electromécanique et Automatique du Havre) on the field of real time simulation of wind turbine [8] using the HILS concepts. These studies are used and applied to a new renewable resource, the conversion of the marine current energy.
In this paper is presented a real time marine current turbine emulator (RTMCTE), conceived to generate the “marine current power characteristics” (fig.2) on its shaft, with regard to the static and dynamic characteristics for the given turbine.
There are different categories of real time turbines emulators, depending on their conception approach. In fig.4 is shown the general schema of the marine current emulator, conceived with the "hardware-in the-loop" approach and the structure is: − the Informatics system provides, with a small
possible error, numerical models of the marine current system; it supplies references signals for the second
system and it is composed by: • a computation device in which there is
implemented the programs to control and observe the electromechanical sub-system;
• the interface unit that ensures the communication between the computation device and the electro-mechanics components.
The Electro-mechanic system that receives the references from the informatics system and reproduces at its shaft the characteristics of the marine current turbine, then it send's back the information to the control system. It has three subsystems
• the turbine emulation is realized with an asynchronous machine vector controlled (rated power: 2200W);
• the electrical generator is a double-feed asynchronous generator, as the one used in the real system (rated power: 1000 W);
• the active break is based on asynchronous machine vector controlled, used to create an extra charge and a variable inertia momentum (rated power: 2600 W).
Informatics system is a supervising and control system which executes several functions in real time: - it simulates:
a) the marine current turbine characteristics; b) the marine current;
- it computes the references for the electromechanical subsystems;
- it executes digital algorithms for the control structures;
- it ensures real time supervision of the simulation process.
It is considered that the turbine is simulated by knowing its static characteristics Γ =f( Ω ).
In order to reject the errors corresponding to different models and to the running operation of the electromechanical system, are considered three design stages:
Active Break
DFIG
Induction Machine
Motor Control
Break Control
V
Γ
Ω
P.C. Interface
Informatics System Electro-mechanic System
Fig.4: S yn o p s i s of the marine current turbine emulator
Fig. 3: General schema for HIL simulation system
Software simulator
Physical system
Reference
Measurements
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• First stage - the turbine studying, in which the turbine will be emulated by a controlled asynchronous motor (using HILS), and a charge will be created with the help of a controlled break system;
• Second stage - the energy conversion study, in which the turbine will be emulated in the same way and connected with an double feed asynchronous generator, which will be connected to the network;
• Third stage - dedicated to the study of the entire conversion chain; the emulated turbine is mechanically coupled to the asynchronous generator and to an active break acting like a momentum wheel
Furthermore, in this paper there are presented different types of structures concerning the emulator development.
IV.1. MCTE structure for the first energy conversion study: kinetic marine energy into mechanical energy
The emulator realizes static characteristic given in the form:
2 2 1( , ,..., ) ( , ,..., )m l p mu u u u+Γ Ω = Γ Ω (7) where mΓ and lΓ are the active and resistant torque
respectively, 1,..., pu u are the input variables that influence the active torque in steady-state operating conditions and 1,...,p mu u+ are the input variables that influence the static characteristics of the resistant torque.
The wind turbine dynamic model is assumed to be: ( , )x F x u= ( , )y G x u= (8)
where the state vector x and the input vector u are: 2( , ,..., )nx x x= Ω (9)
1
2
uu
u⎡ ⎤
= ⎢ ⎥⎣ ⎦
, (10)
with 1 1 2( , ,..., )Tpu u u u= ; 1 1( ..., )T
p mu u u+= (11) In steady state condition, the algebraic equation:
( , ) 0F x u = (12) also includes state condition (7) of the static
characteristics[10].
For a mechanical driving shaft, the emulator should provide given static and dynamic characteristics, such as (7) and (8).
In the first stage of simulation, the marine current turbine emulator has the structure shown in fig. 5 and the two parts are:
• The software system:
- G(v) - imposes the reference for the marine current speed, which sends the command to the function block ( , )F x u ;
- CG: is the load control unit that imposes the loading references;
- G(Ω,v) provides the characteristics of the resistant torque r
lΓ ;
• The physical system: - VC: blocks of vector control; - M: asynchronous machine; - Br: active break. In this development stage, the two systems that
compose the marine current turbine are emulated: the turbine and the generator. Therefore, the turbine is emulated using an asynchronous motor with vector control unit, and the generator using an active break (self-driven synchronous motor), also controlled with a vector control block.
The turbine emulator is formed, like in the general
structure used in HILS systems, from a control system (software unit device) and from a physical system. The parts are operating in a closed loop, in order to correct all the dynamic and static errors. For the physical part is chosen the asynchronous machine with a vector control unit. The latest studies in the field and our initial simulations have shown, the marine current has a low speed, which forces the turbine to work at lower speeds with a great torque charge. The asynchronous machine with short-circuited rotor is able to realize a good dynamics for the speed variation of the marine resource and for the electrical charge evolution. The motor operates on its rotor shaft as the real turbine: it develops the same torque ( Γ ) and the same speed ( Ω ) as a turbine placed under natural conditions. This way of working is
c(t)
G(v F(x,u)
v(t) x1 Is
rlΓ
+ VC M
x -
( )ref tΩ
BrVC+lΓ
-
rlΓ
G(Ω,v
CG
( )ref tΩ
Fig.5: Simulator structure: first simulation stage
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Fig.6: Simulator structure: second simulation stage
-
G
≈=
= ≈
G(v) F(x,u)
v(t) x1 Is
Γ l
+ VC M
x
( )ref tΩ
Tt
Ω(t)
assumed by the vector control device which is able to make the motor work in a large torque range.
The vector control (VC block, in our schema) means controlling the flux and the feeding currents of the electrical machine in order to obtain a certain reference speed. In its composition there are transformation blocks, which are doing the Concordia and the Park transformations, and also a PWM control device, used to control the motor currents.
The generator emulator In this stage of simulation, the generator is emulated
to do a better studying on the turbine. For this it is used a self-driven synchronous motor (active break) with vector control system. This break is used during the developing and the testing, when are established the simulation principles. Practically, the mechanic characteristics of the electrical generator are imposed by a specific reference load procedure.
The vector control on the break device is used, in order to impose a reference torque, needed for the control structure. Like in the case of the asynchronous motor the flux and the feeding currents of the machine are controlled. For the synchronous machine, to control the flux means to control the rotor position and this control is done by the VC block.
IV.2. MCTE structure for the second energy conversion study: mechanical energy to electrical energy
For the second development stage of the marine
current turbine emulator, we propose the structure shown in fig. 6. Like in any HILS structure it is composed from two parts:
• The informatics system - G(v) - imposes the reference for the marine current speed, which sends the command to the function block ( , )F x u ; and afterwards it sends the speed reference ( ( )ref tΩ ) to the motor used to emulate the turbine ;
• The electromechanical system: - VC: block of vector control; - M: asynchronous machine;
conversion from mechanical energy into electrical energy. The turbine emulator provides at its shaft the mechanical energy and the generator will convert it into electrical energy. The emulator realizes the characteristics of a real turbine and can measure the energy obtained at the network level from the generator.
The turbine is emulated like in the first structure of the research, and the break is replaced with a double feed asynchronous generator, with an electronic cascade for the rotor connection to the grid [11].
The double-fed machine is chosen for its significant advantage that comes from the design of low power converter for the power electronics part. This advantage is already familiar with his hypo-synchronous operation in which the power from the rotor is recuperated via an electronic converter into the power source. They are used when the system has variable speed, from the synchronous speed sΩ , down to a much smaller speed
minΩ . Therefore, to establish a variable speed with a
constant frequency system, an induction generator is considered attractive due to its flexible rotor speed characteristic with respect to the constant stator frequency. One solution to expand the speed range and reduce the slip power losses simultaneously is to doubly excite the stator and rotor windings. The power converters in the rotor circuit recuperate the majority of the slip power. Both voltage and frequency need to be controlled to feed the power to the load. In order to feed the active power to the grid, the machine should run at a speed greater than the synchronous speed and it feeds electrical power to the grid via both stator as well as rotor side[12].
Most important, is that in the case of hydro-generators, like in the case of aero-generators, there is no need for a special starting system, it will start once the turbine blades start to spin, because the marine current imposes it's speed.
IV.3. General Structure of the Marine Current Turbine Emulator In the last design stage the structure shown in fig. 8 is
proposed. All the previous parts, presented before, are connected in order to study the marine current conversion chain. The turbine remains emulated with the asynchronous motor that is connected with the asynchronous generator and with the active break. The purpose is to study and optimize the energy produced. The active break is used to create an extra charge (variation of ( ),mcT vΩ ) and to simulate an adjustable moment of inertia (J), different interactions between the turbine and the environment can modify mechanical characteristics of the conversion system.
The simplest dynamic model of the turbine is taken into consideration first, and it is given by of the motion equation:
( ) ( ),mc g ldJ T v TdtΩ = Ω − Ω − Γ (13)
where: Ω - the shaft rotation speed, v- marine current speed, J- the total moment of inertia, ( ),mcT vΩ - the marine current turbine (MCT) torque characteristic,
( )gT Ω - the static characteristics of the electrical generator. The MCT torque is calculated using the relation:
( ) 2 312mc TT C v Rρπ λ= (14)
where: R- the blade radius, λ is the speed ratio[13].
Rv
λ Ω= (15)
and ( )TC λ is the torque coefficient characteristic, which is obtained by simulating the turbine on the informatics environment using the blade characteristics (fig.7) [14].
This structure allows to study the problems related to the connection to the grid, and also it can be used to
simulate the energy production of a marine current power plant to be used for isolated sites.
IV. Conclusion
In this paper is presented the structure of a real time
emulator for a marine current turbine. In the first part of this paper is presented the general
context of our research work and the implications of this research on the field.
Furthermore it is shown the general model of the marine current turbine that determined the choice made for the structuration of the emulator. The characteristics are obtained from the simulation of the 3kW marine turbine and will be implemented in the first structure that is proposed as the characteristics of a real turbine.Afterwards, is shown the general principle of the "hardware-in the-loop simulation" technique which is used for the development of the emulator.
Moreover it is shown the application of HILS for the marine current turbine emulator, the structure of the main units that composed the emulator: the software system and the electro-mechanic system. In the continuation of the paper are presented different structures of the
Fig.7: Torque characteristics of the marine current turbine
G(v) F(x,u)
v(t) x1 Is
+ VC M
x -
( )ref tΩ
Γ rl +
Br VC Γ l
-
Γ rl
G(v)
CG c(t)
G
≈=
= ≈
( )Ωref t
Fig.8: General structure of the marine current emulator
emulator, conceived to study the energy conversion chain.
The first structure is developed to study the conversion from kinetic marine energy into mechanical energy, which takes place at the level of the turbines blades. The turbine is emulated with an asynchronous machine with vector control and the electrical generator is emulated with an active break (synchronous machine).
In the second variant proposed, the marine turbine current emulator is mechanically coupled to a real generator in order to study the conversion from mechanical energy into electrical energy.
Finally, is presented the complete structure of the emulator, which is used to study the marine current turbine and the entire energy conversion chain. This structure included an active load utilized for inertial tests.
Actually the emulator is in a developing state, the main devices are interfaced, the models for the turbine are established and we are working on the control implementation devices.
Acknowledgements: The work of CARAIMAN George was supported by Project SOPHRD - SIMBAD 6853, 1.5/S/15 - 01.10.2008. Project sponsored by the European Union and the Government of Romania.
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