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SYNCHRONOUS BALANCED OPERATION OF OPEN-END WINDING MOTOR DRIVE WITH TWO ISOLATED DC-SOURCES
V. Oleschuk, G. Griva, R. Prudeak, A. Sizov
Abstract. Novel method of synchronized pulsewidth modulation (PWM) has been applied for balanced control of dual inverter-fed drive with an open-end-winding induction motor, supplied by two isolated dc-sources. Simulations give the behavior of dual inverter-fed systems with continuous and discontinuous versions of synchronized PWM.
Keywords: cascaded voltage source inverters, open-end winding induction motor, synchronized pulse-width modulation, dc-voltage sources, power balancing.
FUNCTIONARE ECHILIBRATA A ACTIONARILOR ELECTRICE CU MOTOR CU iNFASURARILE iN GOL §I DOUA SURSE IZOLATE DE CURENT
CONTINUU
V. Olesciuk, G. Griva, R. Prudeak, A. Sizov
Rezumat. Noua metoda a modularii in durate a impulsurilor (PWM) sincronizate a fost aplicata pentrn controlul echilibrat de actionar electric pe baza de motorul asincron cu infa§urarile in gol, acesta fiind alimentat de doua surse izolate de curent continuu (CC). Sunt prezentate rezultatele simularilor sistemului cu varianta continua si discontinua a PWM sincronizate.
Cuvinte-cheie: invertoare de tensiune in serie, motor asincron cu infa§urari in gol, modularea in durate a impulsurilor sincronizate, surse de tensiune continua, echilibrarea puterilor.
СИНХРОННОЕ СБАЛАНСИРОВАННОЕ РЕГУЛИРОВАНИЕ ЭЛЕКТРОПРИВОДА НА БАЗЕ ДВИГАТЕЛЯ С РАЗОМКНУТЫМИ ОБМОТКАМИ И С ДВУМЯ ИСТОЧНИКАМИ ЭЛЕКТРОПИТАНИЯ
В. Олещук, Дж. Грива, Р. Прудяк, А. Сизов
Аннотация. Новый метод синхронной широтно-импульсной модуляции (ШИМ) использован для сбалансированного управления электроприводом на базе двигателя с разомкнутыми обмотками, регулируемого при помощи двух инверторов напряжения, связанных с двумя изолированными источниками электропитания. Приведены результаты моделирования системы с непрерывной и прерывистой разновидностями синхронной векторной ШИМ.
Ключевые слова: каскадно-соединенные инверторы напряжения, электродвигатель с разомкнутыми обмотками, синхронная широтно-импульсная модуляция, баланс мощностей источников электропитания.
I. INTRODUCTION
Multilevel and multiphase converters and drives are a subject of increasing interest in the last years due to some advantages compared with conventional three-phase systems. Cascaded (dual) two-level converters which utilize two standard three-phase voltage source inverters are now perspective topology for different high power applications [1]-[3].
The structure of the adjustable speed drive system based on cascaded converter is constructed by splitting the neutral connection of the induction motor and connecting both ends of each phase coil to a two-level inverter. In this case cascaded converters are capable of producing voltages which are identical to those of three-level and four-level converters [3].
Dual two-level inverter-fed open-end winding motor drives have some advantages such as redundancy of the space-vector combinations and the absence of neutral point fluctuations [4]. These new drive topologies provide also the best possible use of different types of semiconductor switches [3]-[5]. So, between perspective fields of application of these systems
there are high power/high current traction drives, like ship propulsion, locomotive, electrical vehicles, etc. In particular, flexible PWM control of dual two-level inverters can provide increased effectiveness of these traction systems.
Different modulation techniques are used for control of dual inverter-fed adjustable speed drives. In particular, phase-shift carrier-based PWM was used for multilevel output voltage generation in [1]. Also some typical schemes of conventional space-vector modulation have been applied to basic topologies of cascaded converters consisting of two standard two-level voltage source inverters [4]-[7].
Almost all versions of conventional space-vector PWM are based on the asynchronous principle, which results in sub-harmonics (of the fundamental frequency) in the spectrum of the output voltage of converters, that are very undesirable in most medium/high power applications [8],[9]. In order to provide synchronous phase voltage control in dual inverter-fed drives, allowing to avoid of undesirable even harmonics and sub-harmonics in the output voltage of the drive systems with increased power rating, a novel method of synchronized modulation has been applied for control of dual inverter-fed open-end winding motor drives with single dc voltage source [10], and with two separate dc sources (without power balancing between sources) [11].
This paper is focused on investigation of dual inverter-fed drives with algorithms of synchronized PWM, providing continuous control of power sharing between two separate dc sources, in dependence with the magnitudes of its voltages and the required power ratio between the dc-link sources.
II. TOPOLOGY OF A DUAL INVERTER-FED OPEN-END WINDING DRIVE
Fig. 1 presents the basic structure of a dual inverter-fed open-end winding induction motor drive with two standard voltage source inverters with pulsewidth modulation, which are supplied by two separate dc-link sources with voltages Vdci and Vdc2 [3]. Separate dc supply is used for each inverter to block the flow of third harmonic currents.
Fig. 1. Basic topology of dual inverter-fed open-end winding motor drive with two dc-
links
III. PROPERTIES OF THE METHOD OF SYNCHRONIZED PWM In order to avoid asynchronism of conventional space-vector modulation, a novel method of synchronized PWM [12],[13] can be used for control of each inverter in dual inverter-fed drives with open-end winding of induction motor.. In particular, this method of synchronized PWM is based on continuous synchronization of the positions of all central switch-on signals in the centres of the 600-clock-intervals (to fix positions of these signals in the centres of the 600-cycles), and then - to generate symmetrically around the centres of the 600-clock-intervals all other active control signals.
Table 1 presents generalized properties and basic control correlations for the proposed
method of synchronized PWM. It is also compared here with conventional asynchronous space-vector modulation. Basic control functions are available for both undermodulation and overmodulation control zones in this case. A more detailed description of laws and algorithms of synchronized PWM based on either algebraic or trigonometric control functions is in [12],
[13].
Table 1. Basic parameters of PWM methods
Control (modulation) parameter Conventional schemes of vector PWM Proposed method of modulation
Operating and max parameter Operating & max voltage V and Vm Operating & maximum fundamental frequency F and Fm
Modulation index m vivm F ! Fm
Duration of : sub-cycles T T
Center of the Ar-signal at (angtes/degr.) r(A -1) (sec)
Switch-on durations T# = 1. lm!T[sin(600 -ak) + sin Of*] tak = 1.1/7*7’sin ce4 —1.1 mTx sin(60° — at) Algebraic PWM Trigonometric PWM
А-АП-Аж (k-\yrFK0¥i] Г, =A-1„[0.5-6<i - Рк-Гк A - P\ * cosiot - і Yt = 0i-*-n№-5 - 0ЛЦІ - Pk-Yt
Switch-off states (zero voltage) *0* — T ~ tak ~ tbk II 1 2*
Special parameters providing synchronization of the process of PWM Д"=АР -Ax (k-XytFK^K K„,K, fi"= Д X cos A’= (•г - P") x.
IV. SYNCHRONOUS BALANCED CONTROL OF DUAL INVERTER-FED DRIVE
Synchronous symmetrical control of the output voltage of each inverter of the dual inverter-fed drive system in accordance with algorithms of synchronized PWM provides synchronous symmetrical regulation of voltage in the induction machine phase windings. Rational phase shift between output voltage waveforms of the two inverters is equal in this case to one half of the switching interval (sub-cycle) t [1].
In the case, when the two dc-link voltage sources have the same voltage (Vdc2 = Vdci), the resulting voltage space-vectors are equal to the space-vector patterns of conventional three-level inverter [1],[3],[7]. The phase voltage Vas of the dual inverter-fed drive with two insulated dc-link sources (Fig. 1) is calculated in accordance with (1)-(2) [4]:
Vo = 0.333(Va1 + Vb1 + Vc1 + Va2 + Vb2 + Vc2) (1)
Vas = Va1 + Va2 - Vo (2)
where Va1, Vb1, Vc1, Va2, Vb2, Vc2 are the corresponding pole voltages of each three-phase inverter, Vo is the zero sequence (triplen harmonic components) voltage.
In order to provide the rated ratio P1/P2 between the powers of two dc sources with equal dc-link voltages for standard scalar V/F control of dual inverter-fed drives, it is necessary to provide a simple correlation between modulation indices m1 and m2 of the two inverters and
the rated power ratio in accordance with (3):
m^= P_ mn P
As an illustration, Fig. 2 and Fig. 3 present the pole voltages Va1, Va2, line-to-line voltages
Vaibi, Va2b2 of the two inverters, and the phase voltage Vas (with its spectrum) of the dual inverter-fed drive with equal power distribution (P1 = P2) between the two dc sources with equal voltages. Continuous (Fig. 2) and discontinuous (Fig. 3) schemes of synchronized PWM have been used for control of the two inverters with low average switching frequency equal to 900 Hz. The fundamental frequency is F=35Hz (modulation indices m1=m2=0.7).
In the case of unequal power distribution between the dc-link sources, the modulation indices of the two inverters have to be controlled in accordance with (3). In particular, if P1 = 0.5P2, m1 = 0.5m2. Figs. 4 - 5 show the corresponding voltage waveforms for this control mode, together with the spectrum of the phase voltage Vas, for the dual inverter-fed drive system (with unequal power balancing between dc sources) with synchronized continuous (Fig. 4) and discontinuous (Fig. 5) PWM.
Fig. 2. Pole voltages Va1 and Va2, line voltages Va1b1 and Va2b2, and phase voltage Vas (with its spectrum) for dual inverter-fed drive with continuous synchronized PWM (P1=P2).
Fig. 3. Pole voltages Va1 and Va2, line voltages Va1b1 and Va2b2, and phase voltage Vas (with its spectrum) for dual inverter-fed drive with discontinuous synchronized PWM (P1=P2) Algorithms of synchronized PWM provide continuous synchronization of the motor phase voltage Vas of dual inverter-fed drives with both balanced and unbalanced power of two dc sources, and its spectra do not include even harmonics and sub-harmonics, which is especially important for high power drives.
Fig. 4. Pole voltages Va1 and Va2, line voltages Va1b1 and Va2b2, and phase voltage Vas (with its spectrum) for dual inverter-fed drive with continuous synchronized PWM (P1=0.5P2)
Fig. 5. Pole voltages Va1 and Va2, line voltages Va1b1 and Va2b2, and phase voltage Vas (with its spectrum) for dual inverter-fed drive with discontinuous synchronized PWM (P1=0.5P2). Fig. 6 presents the calculation results of the Weighted Total Harmonic Distortion factor
(WTHD) for the phase voltage Vas (averaged values of WTHD = (1/ V)J Z (V /i) ) in dual
i=2
inverter-fed drive with continuous (CPWM) and discontinuous (DPWM) synchronized PWM for the systems with both equal (P1=P2) and unequal (P1=0.5P2) power distribution between the two dc sources with the same voltage magnitude (Vdc1=Vdc2). The average switching frequency for each inverter is 900 Hz, the control mode corresponds to standard V/F control. The results presented in Fig. 6 show that discontinuous scheme of synchronized PWM provides better spectral composition of the phase voltage at higher modulation indices for the systems with both balanced and unbalanced power sharing between two dc sources.
0.016
Q
o 0.012
«2 0.01 £ o.oos
T3
1=
~ o.oos
WTHD of the phase voltage Vas
0.004
-b—CPWM. P1/P2=1 —*— DPWM, P l/P2= l -E>--CPWM, P1/P2=0.5 -«---DPWM, P1/P2=0.5
0
0.2
0.4 0.6 0.8
modulation index m2
Fig. 6. Averaged WTHD factor of the phase voltage versus modulation index m2
For the dual inverter-fed drive with different voltages of the dc-links, in order to provide the rated power ratio P1/P2 between two power sources (for scalar V/F control mode), it is necessary to provide the corresponding correlations between magnitudes of dc voltages, modulation indices of the two inverters and the rated power ratio in accordance with (4):
miK
del
P
m V P
m2V del 1 2
(4)
In particular, in the case of equal power distribution between two dc sources (P1=P2), it is necessary to provide a simple linear correlation between magnitudes of dc voltages and modulation indices of the two inverters:
mVdc1 = m2Vdc2
(5)
If, as an example, Vdc1 = 0.6Vdc2, in this case m2 = 0.6m1.
In the case of unbalanced power distribution between the two dc voltage sources with different voltages, (4) should be used for determination of the ratio of modulation indices of the two inverters of the dual inverter-fed drive system. If, as an example, Vdc1=0.6Vdc2 and P1/P2=0.5, in this case m1=0.833m2. For illustration of this control mode, Fig. 7 and Fig. 8 present the pole voltages Va1, Va2, line-to-line voltages Va1b1, Va2b2 of the two inverters, and the phase voltage Vas (with its spectrum) of the dual inverter-fed drive with unequal power distribution (P=0.5P2) between the two dc sources with different voltages (Vdc1 = 0.6Vdc2). Curves in Fig. 7 correspond to continuous version of synchronized PWM, and curves in Fig. 8 correspond to discontinuous synchronized PWM. The average switching frequency is 900 Hz, and the fundamental frequency F=35Hz. Modulation indices of the two inverters in accordance with (4) are (for scalar V/F control): m2=0.7 and m=0.58.
Fig. 7. Pole voltages Va1 and Va2, line voltages Va1b1 and Va2b2, and phase voltage Vas (with its spectrum) for dual inverter-fed drive with continuous synchronized PWM (Vdc1 =0.6Vdc2,
P=0.5P2)
Analysis of spectral characteristics of the phase voltage of the dual inverter-fed drive show, that due to the algorithms of synchronized PWM the spectra of the phase voltage do not contain even harmonics and sub-harmonics for any control regime of the drives, with both equal and different voltages of the two dc sources, for any power ratio between the power sources. It is especially important for power conversion systems with increased power ratings. Voltage synchronization in the system is provided continuously for any ratio between the switching and fundamental frequencies. In particular, phase voltage spectral compositions in Fig. 9 and Fig. 10 present the calculation results of the Weighted Total Harmonic Distortion
factor (WTHD) for the phase voltage Vas of the dual inverter-fed drive with continuous (CPWM) and discontinuous (DPWM) schemes of synchronized PWM for the system with different dc-link voltages (Vdc1 =0.6Vdc2) with both equal power distribution between dc sources (Fig. 9: P1=P2,) and unequal power of dc-links (Fig. 10: P1/P2=0.5). Figs. 2 - 5 and Figs. 7 - 8 correspond to a fractional ratio of these frequencies (900Hz/35Hz=25.7).
Fig. 8. Pole voltages Va1 and Va2, line voltages Va1b1 and Va2b2, and phase voltage Vas (with its spectrum) for dual inverter-fed drive with discontinuous synchronized PWM
( Vdc1 = 0. 6Vdc2, P1 = 0.5P2)
In accordance with (4)-(5), m2=0.6m1 for the first case, and m1=0.833m2 for the second case. The average switching frequency of each modulated inverter is 900 Hz, control mode corresponds to standard scalar V/F control.
Fig. 9. Averaged WTHD factor of the phase voltage versus modulation index m1
(Vdc1= 0.6Vdc2, P1=P2)
Fig. 10. Averaged WTHD factor of the phase voltage versus modulation index m2
(Vdc1=0.6Vdc2, P1=0.5P2)
Analysis of the simulation results for dual inverter-fed drives shows that the choice of the rational PWM scheme is in strong dependence from magnitudes of dc voltages and required power balancing between dc-links. In particular, for the system where Vdc1=0.6Vdc2 and P1=P2 (see Fig. 9), both continuous and discontinuous versions of synchronized pulsewidth modulation provide almost the same quality of the phase voltage. And, as an example, for other operating conditions, when Vdc1=0.6Vdc2 and P1=0.5P2 (see Fig. 10), discontinuous scheme of synchronized PWM provide better spectral composition of the phase voltage at medium and higher modulation indices.
Method of synchronized modulation, applied to dual inverter-fed drives, is well suited for synchronous control of the motor phase voltage of the drive system during overmodulation. In particular, basic control correlations of this method (see Table I) include two special linear
functions (coefficients) of overmodulation Kov1 and Kov2 [14]. Both in the undermodulation and overmodulation control zones, the spectra of the phase voltage of an open-end winding motor drive with synchronized PWM contain only odd harmonics (without triplen harmonics), for any ratios (integral or fractional) between the switching and fundamental frequencies of dual inverters. The proposed PWM techniques provide also smooth shock-less pulses-ratio changing in cascaded inverters of balanced ac drive system during the whole control range.
V. CONCLUSION
Algorithms of synchronized PWM, applied for control of dual inverter-fed drives with two isolated dc-links and based on simple phase-shift technique of signals of dual inverters, provide both continuous phase voltage synchronization and required sharing of the output power between two dc sources during the whole control range, including the zone of overmodulation.
Regulated power sharing between two dc sources is provided in dual inverter-fed drives with synchronized PWM in accordance with simple linear correlations between the rated power ratio of the dc-links, the magnitudes of the dc voltages and modulation indices of the two inverters. The spectra of the motor phase voltage of the drive systems with synchronized pulsewidth modulation do not contain even harmonics and sub-harmonics for any ratio between the switching frequency and fundamental frequency, and for any variations of voltage amplitudes and power distribution between dc sources, which is especially important for the medium power and high power systems.
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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 210 publications in the area of power electronics and electric drives, including more than 60 publications in the IEEE transactions and proceedings. He is also the author of 89 patents and authors certificates in this field. His research interests include control and modulation strategies for perspective topologies of power converters and drives.
Giovanni Griva, PhD, is Associate Professor of the Politecnico di Torino, Turin, Italy. He is author and coauthor of more than 90 technical papers published in international journals and proceedings of international conferences. His scientific interests regard power conversion systems, adjustable speed electric drives and non conventional actuators.
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.
