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DUAL THREE-PHASE ADJUSTABLE SPEED DRIVE WITH SYNCHRONIZED SPACE-VECTOR MODULATION
V. Oleschuk, R. Prudeak, A. Sizov, E. Yaroshenko
Resume. Split-phase symmetrical motor drive on the base of two voltage source inverters, controlled by algorithms of synchronized pulse width modulation (PWM), has been investigated. Simulation results are presented for dual three-phase power conversion systems with continuous, discontinuous and combined versions of synchronized PWM.
Keywords: split-phase induction motor drive, dual voltage source inverters, pulse width modulation, phase voltage synchronization.
ACTIONARI ELECTRICE DUBLU TRIFAZATE REGLABILE CU MODULATIE SINCRONA SPATIAL -VECTORIALA A IMPULSURILOR
V. Olesciuk, R. Prudeac, A. Sizov, E. Iaro§enko
Resumat. A fost investigat sistemul de actionare electrica pe baza a doua invertoare de tensiune §i motorul electric asincron cu infa^urari disjunctionate tip simetric, reglate in corespundere cu algoritmii modularii sincrone a impulsurilor dupa durata. Sunt prezentate rezultatele modelarii sistemelor de conversie trifazate duble cu diferite tipuri de modulatie: continuu, discontinuu §i combinata.
Cuvinte cheie: actionare electrica, infa^urare disjunctionata; sistem de invertoare dublu; modulatie sincrona a impulsului; sincronizarea tensiunilor de faza.
СДВОЕННЫЙ ТРЕХФАЗНЫЙ РЕГУЛИРУЕМЫЙ ЭЛЕКТРОПРИВОД С СИНХРОННОЙ ВЕКТОРНОЙ МОДУЛЯЦИЕЙ
В.Олещук, Р. Прудяк, А. Сизов, Е.Ярошенко
Аннотация. Исследована система электропривода с двумя инверторами напряжения и асинхронным электродвигателем с расщепленными обмотками симметричного типа, регулируемая на базе алгоритмов синхронной широтно-импульсной модуляции. Приведены результаты моделирования сдвоенных трехфазных преобразовательных систем с непрерывной, прерывистой и комбинированной разновидностями синхронной векторной модуляции.
Ключевые слова: электропривод с расщепленными фазами, сдвоенная инверторная система, широтноимпульсная модуляция, синхронизация фазного напряжения.
1. INTRODUCTION
Multiphase, and, in particular, dual three-phase (six-phase) induction motor drives are a subject of increasing interest in the last years due to some advantages compared with conventional three-phase adjustable speed drives [1]-[5]. Ones of the perspective applications of the six-phase (split-phase) drives are now traction drive systems, in particular, hybrid electric vehicle drives, powered from fuel cell and battery [5].
Fig. 1 presents topology of the electrical vehicle system on the base of the split-phase (six-phase) induction motor supplied by two inverters with two different DC links: 1) Battery DC link with the Vdc1 voltage, and 2) Fuel Cell DC link with the Vdc2 voltage [5]. There are symmetrical and asymmetrical topologies of split-phase (dual three-phase) converters and drives. In particular, in the case of symmetrical dual three-phase systems the induction
machine has two sets of winding spatially shifted by 60 electrical degrees with isolated neutral points [6]-[8].
Fig. 1. The topology of the electrical vehicle system
To provide increased efficiency of symmetrical six-phase drives, novel space-vector-based control and modulation strategies have been proposed and developed for these systems with the single DC-link [6]-[8]. It is known, that for drives with increased power rating it is necessary to synchronize voltage waveforms of power converters for elimination of undesirable sub-harmonics of voltage and current [9],[10]. So, this paper presents results of dissemination of novel method of synchronized PWM to symmetrical traction system with two DC voltage sources with different voltages.
2. BASIC PROPERTIES OF SYNCHRONIZED SCHEMES OF SPACE-
VECTOR MODULATION
In order to avoid asynchronism of conventional versions of voltage space-vector modulation, novel methods of synchronized PWM can be used for control of each inverter in symmetrical six-phase systems [11].
Figs. 2 - 3 present switching state sequences of standard three-phase voltage source inverter inside the interval 00-900. It illustrates schematically basic continuous (CPWM, Fig. 2) and discontinuous (DPWM, Fig. 3) versions of space-vector pulsewidth modulation, which are used typically in adjustable speed drive systems.
The upper traces in Figs. 2 - 3 are switching state sequences (in accordance with conventional designation [11]), then - control signals for the cathode switches of the phases a, b, c (x, y, z) of each inverter. The lower traces in Figs. 2 - 3 show the corresponding quarter-
wave of the line output voltage of inverters. Signals p represent total switch-on durations during switching sub-interval z, signals yk are generated on the borders (Fig. 2) or in the centres (Fig. 3) of the corresponding p. Widths of notches Xk represent duration of zero sequences.
Special signals X (X5 in Fig. 2, X4 in Fig. 3) with the neighbouring p (p5 in Fig. 2, PA in Fig. 3) are formed in the clock-points (00, 600, 1200..) of the output curve of inverters with synchronous PWM. They are reduced simultaneously till close to zero value at the boundary frequencies Fi between control sub-zones. This control principle provides continuous adjustment of the voltage waveforms of inverters, with smooth pulses-ratio changing.
yl y2 y3 yA
Vab -
Fig.2. Switching state sequences of standard three-phase voltage source inverter inside the
interval 00-900
phase a
phase b
phase c Vab —
@2 |3l
;.i
Fig.3. Switching state sequences of standard three-phase voltage source inverter inside the
interval 00-900
Equations (1)-(8) present a set of control functions for determination of parameters of signals of inverters with synchronized PWM in absolute values (seconds) for scalar V/F
4
phase a
phase b I—
4
yl y2
y3
control mode of the system during the whole control range including the zone of overmodulation [11]:
For j=2,...i-1:
pj = p cos[(j -1 - K3 )tKovX ]
7j = pt-J+i (0-5 - 0.87 tan[(/- j - K3 )t]}K
p, = p = pcos[(,-K3 - 1)tKoVi]Ks yy =p"(0.5-0.87tan[(i-K3 -2)z +
(p-1 + £ +X-i)/2]}KsKov2 Xj = T- (pj +pj+1)/2 x,=x=T-p KviK,
ov2
F =
1
6(2, - 1)z
F =--------1----
'-1 6(2, - 3)T
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
where: p - total switch-on duration inside switching interval; y - minor parts of the total switch-on durations; X - duration of notches; z -switching interval (sub-cycle); m = F/Fm -modulation index; Fi and Fi-1 - boundary frequencies between control sub-zones (index i is equal to the number of notches inside a half of the 600-clock-intervals, including the notch on the border of the clock-intervals); p = 1.1vn until Fov1 = 0907Fm, and px =T after Fov1 ; Ks = [1 - (F - F )/(F-i - F )] - coefficient of synchronization; the first coefficient of overmodulation ^ =1 until F^, and ^1 =[1 - (F - Fov1)/(Fov2 - FovJ] between Fov1 and = 0.952F; the second coefficient of overmodulation Kov2 = 1 until Fov2, and
Kov2 = [1 - (F - Fov2 ) /(Fm - Fov2 )] in the zone between Fov2 and Fm '; K3 = 0 for CpWM, and
Ks=0.25 for DPWM.
III. SYNCHRONIZED PWM IN SYMMETRICAL SIX-PHASE TRACTION SYSTEM WITH TWO DC VOLTAGE SOURCES
Control of symmetrical six-phase induction machine drives is based on the 600-phase-shift of control and output signals of two inverters [6]-[8]. In accordance with the theory of
vector space decomposition, the basic six-dimensional space (as, bs, cs, xs, ys, zs) of a dual-three phase induction machine with isolated neutral points can be transformed into two orthogonal two-dimensional subspaces (sa, sb) and (ml, m2) [1]. Voltage components Vsa and Vm1 in these subspaces, and also the phase voltage Vas = Vsa + Vm1 , are calculated for symmetrical six-phase system with two isolated neutrals as [7]:
Vsa = 0.333(Va - 0.5Vb - 0.5Vc + 0.5Vx - Vy + 0.5Vz) (9)
Vmi = 0.333(Va - 0.5Vb -0.5Vc - 0.5Vx + Vy - 0.5Vz) (10)
Vas = Vsa + Vml = Va - 0.333(Va + Vb + Vc) (11)
where Va, Vb, Vc, Vx, Vy, Vz are the corresponding pole voltages of each three-phase inverter.
In this case, the Vsa component, which produces useful rotating MMF k-th order
voltage harmonics ( k = 12 m + 1, m=1,2,3,.), is the useful component. But the Vm1 component,
which generates loss-producing harmonics (k = 6m+1, m=1,3,5,..), is the undesirable voltage component.
In order to provide equivalence of the output fundamental voltages of two inverters (with the same fundamental frequency) during scalar V/F control of the system, it is necessary to provide linear correlations between its modulation indices and magnitudes of the DC voltages:
mi Vdci = m2 Vdc2 (12)
Figs. 4 - 6 present the pole (Va and Vx), and phase Vas and Vxs voltages, and also the useful Vsa component of the phase voltage, of two inverters (with spectral characteristic of the Vsa voltage) of the six-phase system (Fig. 1) with two DC sources, where Vdc1 = 0.75Vdc2. Fig.
4 shows basic signals of the drive system with continuous synchronized PWM (CPWM). Fig.
5 presents the corresponding signals for the six-phase drive with discontinuous PWM with the 300-non-switching intervals (DPWM). Fig. 6 shows basic voltage waveforms for the system with combined CPWM+DPWM control. The switching and fundamental frequencies of each inverter are, respectively, equal to 900 Hz and 35 Hz (modulation indices of two inverters in accordance with (12) are equal correspondingly to m1 = 0.75 and m2 = 0.56).
Ol der of voltage harmonics
Fig.4. Basic signals of the drive system with continuous synchronized PWM (CPWM)
Order of voltage harmonics
Fig.5. Signals for the six-phase drive with discontinuous PWM with the 300-non-switching
intervals (DPWM)
Ol der of voltage harmonics
Fig.6. Basic voltage waveforms for the system with combined CPWM+DPWM control
The motor phase voltages Vas and Vxs of the six-phase vehicle drives with both continuous and discontinuous synchronized PWM have symmetry during the whole control range (see Figs. 4 - 6), and its spectra do not include even harmonics and sub-harmonics, which is especially important for high power/high current systems.
Fig. 7 presents calculation results of Weighted Total Harmonic Distortion factor (WTHD) for the useful component Vsa of the phase voltage (averaged values of Ii000 "
WTHD = (1/ Vsai) E (Vsak /k)2 ) of symmetrical dual three-phase traction system with
V k=2
continuous (CPWM), discontinuous (DPWM), and combined (CPWM+DPWM) schemes of synchronized modulation. DC-link voltages are Vdc1 = 0.75Vdc2 in this case. In accordance with (12), modulation indices for the first and the second inverters have to be in linear dependence m2=0.75m1. The average switching frequency of each three-phase inverter is 900 Hz, control mode corresponds here to standard V/F control during the whole undermodulation zone.
0.02
I 0.018
o «
O 0.016 c o E
® 0.014 75
£ 0.012
— o>
I* 0.01
o.oos
0.2 0.4 0.6 0.8 1
modulation index ml
Fig.7. Calculation results of Weighted Total Harmonic Distortion factor
The spectral characteristics, presented in Fig. 7, show, that algorithms of combined CPWM+DPWM synchronized PWM provide better spectral composition of the useful component of the phase voltage of symmetrical dual three-phase systems (in comparison with two identical (CPWM or DPWM) schemes of modulation for control of two inverters) during the whole linear control range.
IV. CONCLUSIONS
Novel method of synchronized PWM has been disseminated for control of symmetrical split-phase (dual three-phase) traction system, powered from fuel cell and buttery. Control algorithms of synchronized PWM, based on space-vector approach for determination of the pulse patterns, allow minimum number of switchings in the system and minimal switching losses. The phase voltages of traction drive with synchronized PWM have quarter-wave symmetry during the whole control range, and its spectra do not contain even harmonics and sub-harmonics, which is especially important for the systems with increased power/current ratings. Combined scheme of synchronized PWM provide better spectral composition of the useful component of the phase voltage of symmetrical split-phase systems (in comparison with two identical PWM schemes for control of two inverters) during the whole linear control range.
REFERENCES
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Valentin Oleschuk, D.Sc., is Director of the Research Laboratory of the Power Engineering Institute of the Academy of Sciences of Moldova. He is author and co-author of two books and more than 190 publications in the area of Power Electronics and Electric Drives, including more than 50 publications in the IEEE transactions and proceedings. He holds also 89 patents and authors certificates in this field. His research interests include control and modulation strategies for perspective topologies of power converters and drives.
Roman Prudeak is PhD Student of the Power Engineering Institute of the Academy of Sciences of Moldova. He is author of several technical papers in the field of Power Electronics and Electric Drives. His research interests include both feedforward and feedback control methods and techniques for power converters and drives.
Alexandr Sizov is Scientific Collaborator of the Laboratory of Automated Electric Drives of the Power Engineering Institute of the Academy of Sciences of Moldova. He is author and co-author of more than 50 publications and 10 patents and authors certificates. His research interests include elaboration, modelling and simulation of control algorithms and control systems for power electronic converters and drive systems.
Evgenii Yaroshenko is Scientific Collaborator of the Laboratory of the Automated Electric Drives of the Power Engineering Institute of the Academy of Sciences of Moldova. He is author and co-author of about 60 publications and 9 patents and authors certificates. His research interests are connected with elaboration, modelling, simulation and implementation of modern topologies of adjustable speed drive systems
