sign in sign up
<<
Home , Armature (electrical)

562

SIMULATION OF A SHIP PROPULSION SYSTEM WITH DTC DRIVING SCHEME

G. Diamantis J.M. Prousalidis School of Naval Architecture and Marine Engineering National Technical University of Athens Greece

Keywords: (DTC), Ship Propulsion 2. Direct Torque Control (DTC) of a three- phase AC motor Abstract Since when professors Depenbrock and Takahashi - In this paper an effort is made to represent the behaviour Noguchi [4,10] have initially presented the method, of a ship propulsion system comprising a Permanent several applications exploiting it have been announced. (PMSM) driven by a 6-pulse Among the most representative ones are the marine driving system using the Direct Torque Control (DTC) propulsion applications, which have already been control technique. The DTC technique cited is emulated integrated into recently built ship stxuctures. via MATLAB's Simulink toolbox while all study cases are performed successfully in MATLAB's Power System The basic control scheme of an driven by a Blockset environment. DTC controller is shown in Fig.1 [5,12].

The DC voltage is transformed into AC via the inverter the 1. Introduction switching logic of which is defined by the so-called optimized volfage space-vecfor selecfion table. The In the last decades, there has been significant development voltage space vector reflects the combination of the in electric motor driving schemes. This has lead to their switching states of all consisting the inverter exploitation in several applications including large scale considered. For the 6-pulse configuration considered in ones, among the most important of which are related to this paper, as there are three pairs of switches, there are 8 ship electric propulsion. Thus, complicated propulsion (2"3) space vectors. It is worth noting that the magnitude arrangements have been developed comprising the state- of two of them (u7 and US) equals 0, while each one of the of-the art of electric motor control. other six corresponds to one sector of a hexagon (see Fig.2). More specifically, the AC motor control techniques developed can be grouped as follows [1,2,8,12]: The operating point of the motor is estimated by measuring the motor supply voltage and currents, see Fig. * V/f (scalar) control I. Moreover, considering a balanced supply system, only Field two out of three currents and only a single line voltage 0 . Direct Torque Control (DTC) need to be measured. These quantities are used to calculate the operating torque and flux using the well known The last two are the most sophisticated ones, and hence dqo - transforination and a detailed mathematical motor they have been extensively exploited in the ship electric model, often called adaptive, the numerical parameters of propulsion domain. which i.e. resistances, inductances and time constants must be as more accurately known as possible. This paper aims at modelling and simulating the DTC control technique in a 6-pulse driving system of a Moreover, the current switching state of all inverter Permanent Magnet Synchronous Motor (PMSM) intended switches is identified; hence the active voltage space to be used as one of the alternative options in a small scale vector is defined. It is underlined that no measurement of ship propulsion scheme. In an attempt to validate their speed or position is taken. Furthermore, the next optimum effectiveness before integrated in the entire propulsion space vector is selected from the corresponding table, see dynamic system, the propulsion motor and its DTC Table 1, according to the active space vector (i.e. controller are modelled and tested in representative study switching state of all switches) and the values of torque cases. All simulations have been performed in MATLAB's error (DT) and flux error (DY), i.e. the differences Power System Blockset (PSB) along with Simulink between the desired reference values and their actual Simulation toolbox [I I]. values.

0 2004 The Institution of Electrical Engineers. Printed and published by the IEE, House, Six Hills Way, Stevenage, SGI 2AY Mains P+

I Hysteresis comparators I I I DC lin! I I -I!!

Tome I switching ! New Flux circuit States I I , Inverter I I

4

I Actual Flux I I I I I I I 4 I

I I , I ~ Current Switch states I - I I I Actual Adaptne 4 EX Motor Model I Actual Frequency I 4 wotorCurrent I I I

Fig. 1 Direct Torque Control Scheme of a three-phase motor I I I I 1 I+ I+ I+ s3

-++ +--

--+ +-+ 5 6

Fig. 2 Space vectors of a 6-pulse inGerter 564

sector2 I sector 3 I sector4 I sector5 1 sector6

Table 1. Optimized voltage space-vector selection table according to the requirements for incremental/decremental change in flux (Y)and/or torque (TJ.

However, their numerical values are distinct and are defined by a DTC technique synthesized in MATLAB’s obtained from the corresponding hysteresis comparators. environment.

More specifically, the difference between reference and Furthermore, DTC Control logic has been composed from actnal quantity (torque or flux) is examined whether it is scratch via Simulink control blocks, while the power within or not a specified hysteresis hand. Thus, in version system considered has been modelled via Power System presented in this paper, [5,12], the flux comparator is a two Blockset (PSB) modules, see Fig.3. level one, i.e. the flux error is transformed into two possible values +I and -1, with the former corresponding As mentioned above, the dynamic behaviour of the to a demand for a flux increment and the latter to a flux propulsion motor and its DTC controller has been decrement. emulated in representative simulation cases, in an attempt to validate their effectiveness. Thus, the operation of the On the other hand, the torque comparator is often a three DTC driven motor has been simulated in steady-state level one transforming the torque error into the three propeller load operating conditions as well as when an possible values as follows, [5,12]: abrupt polarity inverse takes place in the torque reference +I: torque increment is required signal. -1: torque decrement is required 0: no change in torque is required (the torque error is within the hysteresis hand limits) 3.1 PMSM model

Due to recently performed significant achievements in the 3. Study case permanent magnet technology, PMSM have started being extensively used [5,6] as they combine the low In this paper, the DTC method driving a Permanent maintainability requirements of Induction Motors (IM) Magnet Synchronous Motor (PMSM) has been considered. along with the high performance indices of synchronous This scheme has been regarded as one of the alternative machines (SM). Nevertheless, machines with permanent propulsion options within a construction project of a I m magnet excitation appear lately fairly appealing especially long model of a bulk carrier ship [5]. More specifically, in the domain of ship electric propulsion. That was the the propulsion scheme comprised a 1,l kW/38OV main reason why this type of motor has been included in Permanent Magnet Synchronous Motor (PMSM) supplied the list of alternatives for the ship model considered., by a 6-pulse inverter the switching logic of which is 565

INPUTS

3 level REL4Y dPsi(1.-I) MATWB SPACEMCTOR M Fundion 2 level REL4Y ELEC MOT voltage converter theta I + voltage spaoe vector Selection Table (1.2.3.4.5.6.7or8)

Fig. 3. DTC controller modelled in MATLAB's environment

On the other hand, mathematical modelling progress, has curve, which is an inherent feature of DTC'technique not followed to the same extent, yet. Thus, PMSM models [3,5,121. have not been integrated but in a few electric energy system analysis computer packages, with MATLAB's PSB being one of them [9,11]. Nevertheless, in the Appendix 3.3 Abrupt torque inversion the main equations of a three-phase PMSM with an auxiliary cage winding are cited. The second simulation is of significant importance consisting in the system behaviour in the transient state where an abmpt torque inversion takes place. This is the foundation step for the simulation of the "crash-stop'' case, 3.2 Propeller load where a step change of sign in the reference torque takes place while primarily the propulsion motor and In the case an electric motor drives a ship propeller, the consequently the entire ship reverses its speed so that e.g. a required torque-speed characteristic curve follows the so- crash-accident is avoided. called "propeller's law", with the torque, T being approximately proportional to the square of the speed, n, As the propulsion scheme consisting of the propulsion Eq (1). motor and its controller are supposed to be integrated in T=k.n' (1) the entire hydrodynamic ship model, they have to be where k is the propeller's constant. validated first in this study case. Therefore, at this stage, the dynamic behaviour of the ship model structure or its The characteristic of the propeller for the small scale ship propeller have not been considered as the interest has been model considered in this study case is shown in Figure 4a. focused on the DTC driven PMSM response. The propeller selected at the hydrodynamic study of the project [5] was of B4.70 type, with 0,20 m diameter and Thus, in Figure 5a, the motor torque response is shown, P/D ratio equal to 1,l. The k constant in equation (1) is where again apparently the response follows well the equal to 0,01297. reference signal but with the inclusion of the torque ripples due to DTC. On the other hand, the resultant speed As it can he noticed in Figure 4b, where the simulated T-n response of the DTC driven motor depicted in Figure 5b curve is presented, although in general, the simulated shows a fairly satisfactory sign inversion without any curve follows well the original one shown in Fig. 4a, there oscillations at all. are ripples around the mean value of the characteristic 566

Fig. 4 (a) Propeller’s law (Torque vs Speed) of the ship model considered (b) Simulate! Tyrque vs Speed curve of the PMSM driven by DTC controller

m srnh .t*,ORi I I 4

Fig. 5 Simulated abrupt torque inversion : (a) propulsion motor torque vs time (b) propulsion motor speed vs time

5. Conclusions It is worth noting that as underlined in the literature [3,5], the torque ripple phenomenon introduced by DTC can he In this paper an effort is made to represent the behaviour alleviated significantly either by increasing the armature of a ship propulsion system comprising a Permanent winding resistance which results in increased losses or by Magnet Synchronous Motor (PMSM) driven by a 6-pulse introducing more sophisticated DTC schemes comprising driving system using the Direct Torque Control (DTC) multi-pulse (multiples of 6.) inverter bridges and multi- control technique. The DTC technique cited is emulated level hysteresis comparators. In this latter case, there more via MATLAB’s Simulink and toolbox while all study voltage space vectors and more switching states leading up cases are performed in MATLAB’s Power System to smoother torque step changes [3]. Blockset environment. The behaviour of the DTC driven motor is represented satisfactorily well including the undesired hut inherent to DTC torque ripples. 567

6. References Auxiliary cage equations (short-circuited windings) in dqo-frame: [IIABB: “Direct Torque Control”, Technical Guide No 1, Finland, 1999. [2]B.K. Bose,”Power Electronics and Variable Frequency Drives: Technology and Applications”, ZEEE Press, New York], (1997). [3] D. Casadei, G. Serra, A. Tani:” Implementation of a Direct Torque Control Algorithm for Induction Motors Armature winding Linkage Flux equations: based on Discrete Space Vector Modulation”, lEEE Transactions on Power Electronics, Vol. 15, No. 4, A, = L,i, + L,,,,,i’, pp.769-777 (2000).

[4] M. Depenbrock: “ Direct Self Controlled (DSC) of Ad =Ldid+Lrndi‘M+;l‘m Inverter Fed Induction Machine”, IEEE Transactions on Power Electronics, Vol. 3, No. 4, pp.420-429 (1998). 4 = L,& [SI G. Diamantis: “ Digital Emulation of Direct Torque Auxiliary cage winding Linkage Flux equations: Control (DTC) Electric Motor Control in MATLAB” A‘, = Lmqiq+ LIkh i‘, graduation thesis (in greek), Athens (Greece) 2001. [6]J. Gieras, M. Wing: “ aiM= Lmdid+ LIMM iru+Aim Technology Design & Application”, Marcel Dekker, New Electromagnetic Torque equation York, (1997). [7]J. N. Nash, “Direct Torque Control, 7- =IE(Ai -1 i ),3E(L -L )j i +ZP(L i’ j -Ldi’kq id)+-3 Vector Control Without an Encoder”, IEEE Transactions - 22 dll ld 22 * 0 d“ 22 *MO 2 on Industry Applications, Vol. 33, No 2, pp. 337-341, (1997). the permanent magnet linkage flux k’h can be regarded as [8]J M Prousalidis, N D Hatziargyriou, B C Papadias, ‘On being caused by an equivalent current source studying ship electric propulsion motor driving schemes’, 1‘ =L i’ Proceedings of Sh International Conference on Power rn md rn System Transients, pp. 87-92, Rio de Janeiro (2001). [9]J M Prousalidis, ‘Simulation tools for ship electric Nomenclature power and control system studies’, Proceedings of 6” International Naval Engineering Conference and Ld, L,: self inductance of armature winding on d-, q- axis, Exhibition INEC2002, pp. 263-276, Glasgow(2002). respectively [IO] I. Takahashi, T. Noguchi, “Quick Torque Response Lmd:mutual inductance between two armature windings on Control of an Induction Motor using a New concept”, the d-axis IEEE J. Tech. Meeting on Rotating Machines, paper RM Lmq:mutual inductance between two armature windings on 84-76,pp. 61-70, (1984). the q-axis [I I] The Mathworks 1nc;’MATLAb User’s Manua1’,2000. LIkdkd:mutual inductance between an armature winding [I21 P. Vas, “Sensorless Vector & Direct Torque and an auxiliary cage winding, both on the d-axis Control”, Oxford Science Publications, New York, (1998). L’kqkq:mutual inductance between an armature winding and an auxiliary cage winding, both on the q-axis Appendix - PMSM equations LIS:leakage inductance of armature windings rs: stator resistance The, permanent magnet flux is considered working as an r’kd: auxiliary cage d-axis winding resistance excitation winding of constant flux in the d-axis. Thus: fkq:auxiliary cage q-axis winding resistance id, iq, io : armature winding currents on d- , q- and 0-axis, Armature equations in dqo-frame: respectively i’kd,i’kq: auxiliary cage winding current on d- and q-axis, respectively V =ri +>+Ad-dA der i’m:permanent magnet equivalent current source ‘ ’‘ dt dt &: linkage flux of d-axis armature.winding dAd do, hq: linkage flux of q-axis armature winding vd =