SIMULATION OF A SHIP PROPULSION SYSTEM WITH DTC DRIVING SCHEME

Transcription

1 562 SMULATON OF A SHP PROPULSON SYSTEM WTH DTC DRVNG SCHEME G. Diamantis J.M. Prousalidis School of Naval Architecture and Marine Engineering National Technical University of Athens Greece Keywords: Direct Torque Control (DTC), Ship Propulsion Abstract n this paper an effort is made to represent the behaviour of a ship propulsion system comprising a Permanent Magnet Synchronous Motor (PMSM) driven by a 6-pulse driving system using the Direct Torque Control (DTC) control technique. The DTC technique cited is emulated via MATLAB's Simulink toolbox while all study cases are performed successfully in MATLAB's Power System Blockset environment. 1. ntroduction n the last decades, there has been significant development in electric motor driving schemes. This has lead to their exploitation in several applications including large scale ones, among the most important of which are related to ship electric propulsion. Thus, complicated propulsion arrangements have been developed comprising the stateof-the art of electric motor control. More specifically, the AC motor control techniques developed can be grouped as follows [1,2,8,12]: * V/f (scalar) control Field Vector control 0. Direct Torque Control (DTC) The last two are the most sophisticated ones, and hence they have been extensively exploited in the ship electric propulsion domain. This paper aims at modelling and simulating the DTC control technique in a 6-pulse driving system of a Permanent Magnet Synchronous Motor (PMSM) intended to be used as one of the alternative options in a small scale ship propulsion scheme. n an attempt to validate their effectiveness before integrated in the entire propulsion dynamic system, the propulsion motor and its DTC controller are modelled and tested in representative study cases. All simulations have been performed in MATLAB's Power System Blockset (PSB) along with Simulink Simulation toolbox [ ]. 2. Direct Torque Control (DTC) of a threephase AC motor Since when professors Depenbrock and Takahashi - Noguchi [4,10] have initially presented the method, several applications exploiting it have been announced. Among the most representative ones are the marine propulsion applications, which have already been integrated into recently built ship stxuctures. The basic control scheme of an electric motor driven by a DTC controller is shown in Fig.1 [5,12]. The DC voltage is transformed into AC via the inverter the switching logic of which is defined by the so-called optimized volfage space-vecfor selecfion table. The voltage space vector reflects the combination of the switching states of all switches consisting the inverter considered. For the 6-pulse configuration considered in this paper, as there are three pairs of switches, there are 8 (2"3) space vectors. t is worth noting that the magnitude of two of them (u7 and US) equals 0, while each one of the other six corresponds to one sector of a hexagon (see Fig.2). The operating point of the motor is estimated by measuring the motor supply voltage and currents, see Fig.. Moreover, considering a balanced supply system, only two out of three currents and only a single line voltage need to be measured. These quantities are used to calculate the operating torque and stator flux using the well known dqo - transforination and a detailed mathematical motor model, often called adaptive, the numerical parameters of which i.e. resistances, inductances and time constants must be as more accurately known as possible. Moreover, the current switching state of all inverter switches is identified; hence the active voltage space vector is defined. t is underlined that no measurement of speed or position is taken. Furthermore, the next optimum space vector is selected from the corresponding table, see Table 1, according to the active space vector (i.e. switching state of all switches) and the values of torque error (DT) and flux error (DY), i.e. the differences between the desired reference values and their actual values The nstitution of Electrical Engineers. Printed and published by the EE, Michael Faraday House, Six Hills Way, Stevenage, SG 2AY

2 Current Mains P+ Hysteresis comparators -! DC lin! Tome switching! New Switch Flux circuit States, nverter 4 4 Actual Flux 4, ~ - Actual Adaptne 4 Motor Model Actual Frequency Switch states EX w o t o r Current Fig. 1 Direct Torque Control Scheme of a three-phase motor s Fig. 2 Space vectors of a 6-pulse ingerter

3 564 sector2 sector 3 sector4 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 obtained from the corresponding hysteresis comparators. More specifically, the difference between reference and actnal quantity (torque or flux) is examined whether it is within or not a specified hysteresis hand. Thus, in version presented in this paper, [5,12], the flux comparator is a two level one, i.e. the flux error is transformed into two possible values + and -1, with the former corresponding to a demand for a flux increment and the latter to a flux decrement. On the other hand, the torque comparator is often a three level one transforming the torque error into the three possible values as follows, [5,12]: +: torque increment is required -1: torque decrement is required 0: no change in torque is required (the torque error is within the hysteresis hand limits) 3. Study case n this paper, the DTC method driving a Permanent Magnet Synchronous Motor (PMSM) has been considered. This scheme has been regarded as one of the alternative propulsion options within a construction project of a m long model of a bulk carrier ship [5]. More specifically, the propulsion scheme comprised a 1,l kw/38ov Permanent Magnet Synchronous Motor (PMSM) supplied by a 6-pulse inverter the switching logic of which is defined by a DTC technique synthesized in MATLAB s environment. Furthermore, DTC Control logic has been composed from scratch via Simulink control blocks, while the power system considered has been modelled via Power System Blockset (PSB) modules, see Fig.3. As mentioned above, the dynamic behaviour of the propulsion motor and its DTC controller has been emulated in representative simulation cases, in an attempt to validate their effectiveness. Thus, the operation of the DTC driven motor has been simulated in steady-state propeller load operating conditions as well as when an abrupt polarity inverse takes place in the torque reference signal. 3.1 PMSM model Due to recently performed significant achievements in the permanent magnet technology, PMSM have started being extensively used [5,6] as they combine the low maintainability requirements of nduction Motors (M) along with the high performance indices of synchronous machines (SM). Nevertheless, machines with permanent magnet excitation appear lately fairly appealing especially in the domain of ship electric propulsion. That was the main reason why this type of motor has been included in the list of alternatives for the ship model considered.,

4 565 NPUTS 3 level REL4Y dpsi(1.-) 2 level REL4Y theta MATWB Fundion voltage spaoe vector Selection Table ( or8) SPACEMCTOR M voltage converter + ELEC MOT Fig. 3. DTC controller modelled in MATLAB's environment On the other hand, mathematical modelling progress, has not followed to the same extent, yet. Thus, PMSM models 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 the main equations of a three-phase PMSM with an auxiliary cage winding are cited. 3.2 Propeller load n the case an electric motor drives a ship propeller, the required torque-speed characteristic curve follows the socalled "propeller's law", with the torque, T being approximately proportional to the square of the speed, n, Eq (1). T=k.n' (1) where k is the propeller's constant. The characteristic of the propeller for the small scale ship model considered in this study case is shown in Figure 4a. The propeller selected at the hydrodynamic study of the project [5] was of B4.70 type, with 0,20 m diameter and P/D ratio equal to 1,l. The k constant in equation (1) is equal to 0, As it can he noticed in Figure 4b, where the simulated T-n curve is presented, although in general, the simulated curve follows well the original one shown in Fig. 4a, there are ripples around the mean value of the characteristic curve, which is an inherent feature of DTC'technique [3,5, Abrupt torque inversion 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, where a step change of sign in the reference torque takes place while primarily the propulsion motor and consequently the entire ship reverses its speed so that e.g. a crash-accident is avoided. As the propulsion scheme consisting of the propulsion motor and its controller are supposed to be integrated in the entire hydrodynamic ship model, they have to be validated first in this study case. Therefore, at this stage, the dynamic behaviour of the ship model structure or its propeller have not been considered as the interest has been focused on the DTC driven PMSM response. Thus, in Figure 5a, the motor torque response is shown, where again apparently the response follows well the reference signal but with the inclusion of the torque ripples due to DTC. On the other hand, the resultant speed response of the DTC driven motor depicted in Figure 5b shows a fairly satisfactory sign inversion without any oscillations at all.

5 566 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 4 m s rnh.t*,ori Fig. 5 Simulated abrupt torque inversion : (a) propulsion motor torque vs time (b) propulsion motor speed vs time t is worth noting that as underlined in the literature [3,5], the torque ripple phenomenon introduced by DTC can he alleviated significantly either by increasing the armature winding resistance which results in increased losses or by introducing more sophisticated DTC schemes comprising multi-pulse (multiples of 6.) inverter bridges and multilevel hysteresis comparators. n this latter case, there more voltage space vectors and more switching states leading up to smoother torque step changes [3]. 5. Conclusions n this paper an effort is made to represent the behaviour of a ship propulsion system comprising a Permanent Magnet Synchronous Motor (PMSM) driven by a 6-pulse driving system using the Direct Torque Control (DTC) control technique. The DTC technique cited is emulated via MATLAB s Simulink and toolbox while all study cases are performed in MATLAB s Power System Blockset environment. The behaviour of the DTC driven motor is represented satisfactorily well including the undesired hut inherent to DTC torque ripples.

6 References [ABB: Direct Torque Control, Technical Guide No 1, Finland, [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: mplementation of a Direct Torque Control Algorithm for nduction Motors based on Discrete Space Vector Modulation, leee Transactions on Power Electronics, Vol. 15, No. 4, pp (2000). [4] M. Depenbrock: Direct Self Controlled (DSC) of nverter Fed nduction Machine, EEE Transactions on Power Electronics, Vol. 3, No. 4, pp (1998). [S G. Diamantis: Digital Emulation of Direct Torque Control (DTC) Electric Motor Control in MATLAB graduation thesis (in greek), Athens (Greece) [6]J. Gieras, M. Wing: Permanent Magnet Motor Technology Design & Application, Marcel Dekker, New York, (1997). [7]J. N. Nash, Direct Torque Control, nduction Motor Vector Control Without an Encoder, EEE Transactions on ndustry Applications, Vol. 33, No 2, pp , (1997). [8]J M Prousalidis, N D Hatziargyriou, B C Papadias, On studying ship electric propulsion motor driving schemes, Proceedings of Sh nternational Conference on Power System Transients, pp , Rio de Janeiro (2001). [9]J M Prousalidis, Simulation tools for ship electric power and control system studies, Proceedings of 6 nternational Naval Engineering Conference and Exhibition NEC2002, pp , Glasgow(2002). [O]. Takahashi, T. Noguchi, Quick Torque Response Control of an nduction Motor using a New concept, EEE J. Tech. Meeting on Rotating Machines, paper RM 84-76,pp , (1984). [ ] The Mathworks 1nc; MATLAb User s Manua1,2000. [21 P. Vas, Sensorless Vector & Direct Torque Control, Oxford Science Publications, New York, (1998). Appendix - PMSM equations The, permanent magnet flux is considered working as an excitation winding of constant flux in the d-axis. Thus: Armature equations in dqo-frame: V =ri +>+Ad- da der dt dt dad vd =<id +dt- do, 4 dl Auxiliary cage equations (short-circuited windings) in dqo-frame: Armature winding Linkage Flux equations: A, = L,i, + L,,,,,i, Ad =Ldid+Lrndi M+;l m 4 = L,& Auxiliary cage winding Linkage Flux equations: A, = Lmqiq + Lkh i, aim = Lmdid + LMM iru+ Aim Electromagnetic Torque equation 7- =E(A i -1 i ),3E(L -L )j i +ZP(L i j -Ldi kq id) dll l d 22 * 0 d 22 * M O 2 the permanent magnet linkage flux k h can be regarded as being caused by an equivalent current source Nomenclature 1 =L i rn md rn Ld, L,: self inductance of armature winding on d-, q- axis, respectively Lmd: mutual inductance between two armature windings on the d-axis Lmq: mutual inductance between two armature windings on the q-axis Lkdkd: mutual inductance between an armature winding and an auxiliary cage winding, both on the d-axis L kqkq: mutual inductance between an armature winding and an auxiliary cage winding, both on the q-axis LS: leakage inductance of armature windings rs: stator resistance r kd: auxiliary cage d-axis winding resistance fkq: auxiliary cage q-axis winding resistance id, iq, io : armature winding currents on d-, q- and 0-axis, respectively i kd, i kq: auxiliary cage winding current on d- and q-axis, respectively i m: permanent magnet equivalent current source &: linkage flux of d-axis armature.winding hq: linkage flux of q-axis armature winding L: linkage flux of 0-axis armature winding h. kd: linkage flux of d-axis auxiliary cage winding Yk& linkage flux of q-axis auxiliary cage winding Ym: excitation linkage flux of permanent magnets