2013 21st Mediterranean Conference on Control & Automation (MED) Platanias-Chania, Crete, Greece, June 25-28, 2013
Experimental Investigation of an Emulator "Hardware In the Loop" for Electric Naval Propulsion System K. MAROUANI, H. GUENDOUZ, B. TABBACHE, F. KHOUCHA, and A. KHELOUI
selected propulsion motor, in order to ensure that it is appropriate for the application concerned. However, if the choice leads to conduct these tests on a real ship in the sea, this choice is costly in terms of money, time and human resources involved. For these reasons, engineers are opted, since the beginning, for experimental test benches that use other types of loads to emulate the real propeller and to test the performances of the propulsion motor. Moreover, among the emulators having an operational flexibility with better reproduction of the dynamic characteristic of the ship, are those based on the Hardware In the Loop (HIL) principle [4], [5]. Accordingly, the main objective of this paper is the realization of an emulator to reproduce the real operation of a ship propulsion system. The work consists of two parts: the first one concerns the modeling of the propeller and the different resistance forces opposed to the movement of the ship, allowing the estimation of the necessary propulsion power, and the second part presents the emulator of the ship propulsion chain, based on the Hardware In the Loop (HIL) principle, and using an electric machine operating as a generator to emulate the dynamic characteristic of the ship. The resulting model is validated by numerical simulations and then tested on an experimental test bench constituting the emulator. Different experiments are conducted taking into account the real operation of the ship.
Abstract – The purpose of this paper is the realization of an emulator for electric naval propulsion system. The main objective of an emulator is to reproduce the real system operation. The work consists of two principal parts: the first part concerns the modeling of the propeller and the different resistance forces opposed to the movement of the ship, allowing the estimation of the necessary propulsion power, and the second part presents the emulator of the ship propulsion system, based on the Hardware In the Loop (HIL) principle, using an electric machine operating as a generator to emulate the dynamic characteristic of the ship. The resulting model is validated by numerical simulations and then tested on an experimental test bench constituting the emulator. Different experiments are conducted taking into account the real operation of the ship. Index Terms : DSP, Emulator, HIL, Multi-phase Machines, Propeller, Propulsion, Ship.
I. INTRODUCTION
S
hip propulsion with an electric motor instead of a diesel engine or a gas turbine had emerged from a hundred years go. Furthermore, the strong demand for propulsive power was provided by synchronous electric motors, powered by turbo-generators. In addition, the variation of the speed of the ship is done by varying the frequency of the generator which is related to its rotational speed. However, because of this rigid solution which has drawbacks, such as a complicated structure, low efficiency, high cost etc..., electric propulsion has not been widely adopted. Today, electric propulsion has become the most appropriate solution, thanks to advances in the fields of power electronics, static converters, electric motors, high power, microcontrollers and digital technology [1], [2]. The difficulty of the motorization resides in the selection and sizing of the propulsion motor [3]. By definition, propulsive power is equal to the product of the resistance forces opposed to the movement of the ship and its speed. Thus, the determination of the different resistance forces allows the prediction of ship power propulsion to be installed. Then, it is necessary to carry out tests on the
II. SHIP DYNAMIC MODEL The ship conventional propulsion chain is composed of a combustion engine, a gearbox, a driveshaft and a propeller. Contrary, the propulsion chain for an electric ship is limited to an electric motor driving a propeller, as shown in Figure1. Thus, the model of the electric propulsion chain is based on the model of the drive motor and the dynamic model of the ship. The dynamic model of the ship expresses the interaction between the propeller and hull, which results in a resistant torque image of resistance forces opposed to the movement of the ship. Q Propulsion motor
The Authors are with the Laboratoire Commande des Machines, Unité d’Enseignement et de Recherche en Electrotechnique, Ecole Militaire Polytechnique (EMP), 16046 Bordj El-Bahri, Algiers, Algeria (E-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected].).
978-1-4799-0997-1/13/$31.00 ©2013 IEEE
n
T Propeller
V
Hull
Fig.1 : Diagram of the propulsion chain of an electric ship. 125
1. Ship resistance forces
propelling efficiency. Certain propellers are made up only of one piece, others have directional blades reported on the hub with the orientation can be fixed or controlled by an appropriate mechanism. The propellers with directional blades allowed the improvement of ships efficiency, whose operation requires frequent changes of operation regimes, like the tugboats. Based on the operation principle of the propeller, the main variables describing it can be defined as follows.
There are several components composing the total resistance force opposed to the movement of the ship [2], [6], [7]. When, the ship is supposed moves on a calm sea, the main components are : - Rf : Friction resistance due to friction between the water and the hull. - Rw : Waves resistance reflecting the energy spent in the formation of the field of waves accompanying the ship in its movement. - Ra : Aerodynamic resistance expressing the hull roughness effect and the air resistance. - Rapp : Additional resistance describing the effect of appendages (rudders, fin stabilizer...). - Rb : Resistance due to the presence of bow bulbs near the water surface. - Rr : Resistance eddy that can be neglected with a shapely hull. Then, the total resistance force opposed to the movement of the ship can be written as follows:
Propeller torque : Propeller thrust : Propeller efficiency : Propulsion efficiency : Advance coefficient : with : Thrust coefficient. Torque coefficient. n : Propeller rotational speed . D : Propeller diameter. V : Ship speed. The coefficients and are generally derived from experimental measurements on the propeller in basins, and given in the form of abacus as a function of the advance coefficient J. Now, the interaction between the propeller and hull can be expressed according to the motion equation as follows:
with :
ρ = 1025 kg/m3: Density of seawater. υ = 2,1x10-6 m2/s : Kinematic viscosity of seawater. S : Wetted surface of the hull. Sapp : Wetted surface of the appendages. Cf : Friction coefficient of the ship. Ca : Correlation coefficient of the ship. Rw, Rb et Rr are evaluated and given by experiments done in hulls basins.
With M is the total mass of the ship. Once the dynamic model of the ship is obtained, the simulation and emulation of the ship propulsion chain can be performed easily. III. STRUCTURE OF THE EMULATOR The emulator of the ship electric propulsion chain is shown in Figure-2 [8]. It consists of three main parts: the propulsion motor, the hardware part and the software part of the emulator of the ship dynamic model. An induction motor (IM) is used as a propulsion motor and a DC machine (DCM) operating as a generator to emulate the dynamic characteristic of the ship. The IM is fed by a voltage source inverter, and its speed is controlled using the vector control technique [9], [10]. The generator is coupled to the IM and feed an electric load through a chopper. The current reference of the chopper is the image of the resistant torque representing the interaction between the propeller and the hull. The resistant torque is generated by the simulation program of the ship dynamic model.
It should be noted that this simplified model does not take into account all hydrodynamic phenomena specific to the ship. Nevertheless, it presents a statistical method of estimation of the propulsion power at the initial stage of design. There are other more complicated models introducing other factors in the total resistance. 2. Propeller model The propeller, with its special profile and complicated geometry, allows the conversion of the power developed by the propulsion engine into a thrust to propel the ship. The principal characteristics of a propeller are the diameter, the advance coefficient, the number and the shape of the blades. These characteristics have a direct influence on the 126
Fig. 2 : Block diagram of the emulator of the ship electric propulsion chain.
IV. SIMULATION RESULTS
propulsion chain emulator is shown in Figure-5. The block diagram of this experimental test bench is exactly similar to those shown in Figure-2 and Figure-3 [8], [9], [10]. It is composed of three main parts: - Electric motors: Two electric motors mounted on the same shaft. A dual star induction machine (DSIM: 5.5kW, 6 poles) configurable as symmetrical or asymmetrical sixphase induction machine, and DC machine (DCM: 3kW). The asynchronous machine is used to emulate the propulsion motor and the DCM to emulate the dynamic characteristic of the ship. - Static converters: Two three-phase voltage source inverters (VSI: IGBTs-1200V/50A) are used to feed the DSIM, and a four-quadrant chopper (IGBTs-1200V/50A) associated to the DCM. - Control board: The control algorithms for both propulsion motor and the emulator of the dynamic characteristic of the ship are implemented on an eZdspF2812 board programmable with Simulink/MATLAB©. This DSP card performs the acquisition of the different measured variables, the calculation of the control laws and sends the control signals to the inverters and chopper. The dynamic model of the ship is also implemented on the same control board.
The simulation of the whole model of the ship electric propulsion chain emulator, shown in Figure-2, is performed under Simulink/MATLAB©. The block diagram of the simulation scheme is shown in Figure-3. Figures-4-(a) and (b) present the responses of the ship dynamic model for different ascending increments of the reference speed. This scenario corresponds to a real operation of the ship, which passes by different regimes known as maneuvers speeds before reaching the cruising speed. The regulation of various variables (speed and currents) is ensured by PI controllers. These simulation results show that the thrust and resistant torque of the ship are strongly linked to the rotational speed of the propeller. The transient regime causes an acceleration of the ship which increases the resistant torque, and therefore the thrust increases making an overshoot. After, the thrust decreases until its value reaches the resistant torque, corresponding to the establishment of the steady state. So, the choice and sizing of the propulsion engine should take into consideration the operation regime of the ship, especially the transient and cruising regimes. V. EXPERIMENTAL RESULTS The experimental test bench of the ship electric 127
n*
+-
Indirect Field Oriented Control (IFOC)
DSIM
n
I
I
Speed and Current mesurement Propulsion Motor Emulator Ship Dynamic Model
n Fig. 5 : Photography of the experimental test bench.
This ship electric propulsion chain emulator consists of a hardware part represented by the DSIM coupled to a DCM, and a software part allowing the reproduction of the dynamic model of the ship (interaction propeller-hull). So, this hardware-software combination describes the emulation technique based on the hardware in the loop (HIL) simulation principle. Thus, complex physical systems can be tested easily by this HIL technique, allowing saving in terms of money, time and human resources involved. Experimental tests were performed for different cases of operation and in the same conditions as the simulations. The DSIM is controlled using the vector control technique in order to impose the desired maneuver speed. The DC generator feeds a RL load through the chopper. The reference current of the chopper is the image of the resistant torque developed by the ship. The dynamic model of the ship uses the speed of the propulsion motor as input and the resistant torque as output. To test the dynamic model of the ship, three modes of ship operation were tested: the first at a constant speed, the second at a variable speed profile and the third at a variable sea state. Due to limitations of the DSP control board, particularly the absence of a digital to analog converter to view the variables calculated from the dynamic model of the ship, only the measurable quantities will be presented, such as the rotational speed of the propulsion motor instead of the speed of displacement of the ship, or the RL load current instead of the calculated resistant torque. Figure-6 shows the curves of the RL load current image of the resistant torque, the phase currents of the propulsion motor and its rotational speed. In order to test the speed regulation loop of the propulsion motor, at first, the reference load torque is imposed equal zero, corresponding to a no-load operation represented by a zero current induced by the DC generator. Then, the torque reference, obtained
Ship Dynamic Model Emulator
Fig. 3 : Block diagram of the simulation scheme of the ship electric propulsion emulator.
(a)
(b) Fig. 4 : Responses of the dynamic model of the ship. (a) : Curve of the ship speed. (b) : Curves of the thrust and resistant torque. 128
from the dynamic model of the ship for a constant speed operation, is introduced at t = 50s instant. This torque reference is converted to a current reference input for the chopper. The responses, shown in Figure-6, prove that the speed regulation of the propulsion motor is ensured, either during startup or following the application of a resistant torque. Also, the phase current undergoes an increase during the transient startup of the motor and then decreases to its corresponding value at steady sate after the establishment of the speed. After the application of a resistant torque, the DSIM phase current is increased to impose a torque equal to the load torque. The variation of the load torque is reflected by the variation of the RL load current, which has been increased due to the introduction of the emulation program of the ship dynamic model. In order to show the evolution of the resistance force regarding the ship speed variations, a second test was carried out, and shown in Figure-7. The selected speed profile is composed of five increasing levels, which corresponds to a real ship operation scenario, which passes by different regimes known as maneuvers speeds before reaching the cruising speed. Figure-7 shows the curves of the rotational speed of the propulsion motor and the RL load current. It is shown that the ship resistant torque, which is proportional to the square of the speed, increases strongly when the ship speed increases. Another test is carried out to emulate the case of navigation in a variable sea state. Figure-8 shows the evolution of the resistance force due to variations of the ship wetted surface. In this test, the propulsion motor speed is kept constant. It is shown that the increase or decrease of the wetted surface causes an increase or decrease in resistant torque and influence the dynamics of the ship.
Fig. 7 : Ship speed (yellow curve) and resistance force (green curve).
Fig. 8 : Ship speed (yellow curve) and resistance force (green curve) in case of navigation in a variable sea state.
VI. CONCLUSION In this paper, the principle of electromechanical energy conversion for a ship propulsion chain and the different interactions between the engine, the propeller and the hull has been analyzed. Also, the ship dynamic model is developed and introduced into the realized emulator allowing the reproduction of the ship propulsion system operation, by generating the different resistance forces using the technique of Hardware In the Loop (HIL). Then, experimental results are presented taking into account the ship real operation at different cases, such as : a constant speed, a variable speed profile and a variable sea state. These results shown that the total resistance force of the ship increases strongly when the ship speed or the wetted surface increases and also influences the dynamics of the ship. Therefore, the choice and sizing of the propulsion engine should take into consideration the operation regime of the ship, especially the cruising regime. Also, this HIL platform can serves as a test bench for
Fig.6 : Curves of the RL load current (yellow curve), the phase currents of the propulsion motor (blue and green curves) and its rotational speed (violet curve). 129
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