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Engineering Science and Technology, an International Journal

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Full Length Article

An isolated/non-isolated novel multilevel inverter configuration for a dual three-phase symmetrical/asymmetrical star-winding converter

Sanjeevikumar Padmanaban a,b'*, Michael Pechtc

a Department of Electrical and Electronics Engineering, University of South Africa, Auckland Park, Johannesburg, South Africa b Research and Development, Division of Power Electronics, Ohm Technologies, Chennai, India c Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, MD, USA

article info

Article history: Received 3 May 2016 Revised 20 July 2016 Accepted 8 August 2016 Available online xxxx

Keywords: Multiphase drives Dual three-phase inverters Six-phase inverters Multilevel inverters Symmetrical/asymmetrical inverters Multiple space vector transformation Split-phase space vector transformation Pulse-width modulation

abstract

This article is devoted to the development of a novel isolated/non-isolated multilevel inverter configuration for a dual three-phase star-winding converter. The proposed topology fits the (low-voltage/high-current) applications of medium-power, AC tractions and More-Electric Aircraft (MEA) propulsion systems. The power circuit module consists of voltage source inverters (VSIs) with isolated/non-isolated DC supply. Further, each single phase of the VSI is introduced with one bi-directional switching device (MOSFET/IGBT) and two capacitors with linked neutral points. Also, an original modified single-carrier five-level modulation (MSCFM) algorithm is developed in this article that easy to implement in real digital processors. The proposed modulation algorithm generates 5-level output voltages at each terminal of the VSI as equivalent to standard multilevel inverters. The observed results are presented by numerical modeling of the complete AC converter system with (Matlab/PLECS) simulation software. The investigation confirms that the results are in good agreement with the developed theoretical background and the proposed multilevel inverter is applicable to asymmetrical and symmetrical dual three-phase star-winding converter configurations.

© 2016 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC

BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4XI/).

1. Introduction

Multiphase AC drives are the solutions for limited rating devices (MOSFET/IGBT) configuration than their counterpart three-phase AC drives as proven by many literatures and applications [1-6]. Advantage includes the redundant structure, limited DC link ripple, increased power density, fault tolerance, and reduced per-phase of inverter rating [1,2,7]. Recently, the dual three-phase (six-phase) drives of multiphase configuration are preferred due to the high reliability and fault tolerance capabilities [2-6,8-11]. Renowned for its construction, two adjacent phases are spatially shifted by 30° (asymmetrical type) [2-6] or by 60° (symmetrical) [2,8,9]. Application suits solutions for low-voltage/high current AC tractions and More-Electric Aircraft applications (MEA) [12-15]. With MEA propulsion systems, the hydraulic and pneumatic actuators are replaced by multiphase AC drives, which lead to increased reliability in fault conditions and improved overall aeronautic propul-

* Corresponding author at: Department of Electrical and Electronics Engineering, University of South Africa, Auckland Park, Johannesburg, South Africa. E-mail address: sanjeevi_12@yahoo.co.in (S. Padmanaban). Peer review under responsibility of Karabuk University.

sion [13,14]. Both motoring and generator action during start and flight modes are performed by multiphase AC drives [15].

On the other hand, the viability of AC drives is improved by the introduction of multilevel inverters (MLIs). The benefits of MLIs are reduced total harmonic distortion (THD), lowered dv/dt, and the possibility of obtaining high power ratings with voltage-limited devices [16,17]. The combination of both multi-phase and multilevel inverters is an effective solution for increasing power ratings of voltage- and current-limited devices [3-6,8-12,18-23]. However, MLIs are subject to different potential anomalies, (31-37.9)% of failures occurs due to power parts (IGBT mechanisms) and failures addressed with capacitors and gate control techniques [7,24,25].

Classical voltage source inverters (VSIs) are reliable and reconfigured as standard solutions for dual three-phase AC drives, configured by the proper arrangement of multiple VSIs [2-6,812,18-23]. Topologies generally are addressed as the dual three-phase inverters (six-phase) for both asymmetrical and symmetrical versions. The VSIs (two-level) are connected at the open windings of a six-phase system and termed dual three-phase inverters [26,8-12,18-23]. Each pair of 2-level VSIs is modulating to obtain three-level output voltages. But dual inverters suffer from the restricted levels in the output voltages; each leg is limited to three

http://dx.doi.org/10.1016/j.jestch.2016.08.006

2215-0986/® 2016 Karabuk University. Publishing services by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

levels and the output voltages appear in three levels in lines, which cannot be overcome with the addressed topologies [2-6,10-12,1823]. Such configurations are addressed for limited common-mode components and for increasing the output levels [11,19,26]. Also, these topologies are constructed by open-winding configuration or by increased switches per phases or cascading multiple VSIs [10,11,20-23]. Hence the reliability is limited, low redundancy, and complex pulse-width modulation (PWM) strategies are required. However, proper and optimal switch configuration for more than three-level outputs for multiphase AC drives is still not addressed.

Motivated by the above facts, this work proposes a novel configuration for a dual three-phase multilevel (symmetrical/asymmetrical) inverter for both isolated (shown in Fig. 1) and non-isolated (shown in Fig. 2) versions [7,13-17,27]. Moreover, this configuration meets the optimal switch requirements and fits for star-winding

loads, medium power(low-voltage/high-current), and MEAapplica-tions. The reconfiguration includes each phase incorporated with a bi-directional switch (IGBT/MOSFET), and two capacitors are introduced in neutral linked. An additional benefit is that each VSI outputs 5-level in their line-to-line irrespective of a non open-winding structure. Overcomes, the drawbacks of standard dual inverter configurations available in the literature. Further, the structure is easily extendable to 9,12, or a higher number of phases. The benefits are compromised the same as for standard multilevel inverters, additionally more reliable under faulty conditions when one or two or more phases fail [2-6,10-12,18-23,26]. Also, an original modified single carrier five-level modulation (MSCFM) algorithm (independent) is developed in this paper and applied to both non-isolated/isolated dual three-phase converters [27-32]. To verify the effectiveness of the proposed converters, the complete system is numerically modelled by Matlab/PLECS simulation software

Fig. 1. Configuration of proposed novel isolated multilevel inverter for asymmetrical/symmetrical dual three-phase AC drives.

Fig. 2. Configuration of proposed novel non-isolated multilevel inverter for asymmetrical/symmetrical dual three-phase AC drives.

and tested. A set of predicted results is presented in this paper, and these results show good conformity with a theoretical background.

2. Multiple and split-phase decomposition space vector transformation

2.1. Multiple space vector transformation

The six-phase system is represented by rotating orthogonal multiple space vectors classically as given below [2-6,8,9]:

' Xi = 3 [xi + X2a4

3L,M i ,v2^ i x3a8 + x4a + x5a5 + x6a'

x3 = 3 [(Xi + X2 + X3) +j{X4 + X5 + Xe)] x5 = 3 [X1 + X2a8 + X3a4

x4a + X5a9 + x6a5]

' xi = 3 [Xi + X2a2 -

X3a4 + X4a + X5a3 + x6a5] x3 = 3 [(Xi + X2 + X3) +j(X4 + X5 + Xe)] x5 = 3 [X1 + X2a4 + X3a2 + x4a + x5a5 + x6a3]

The bi-directional switch per phase and the two capacitors with neutral are removed in Figs. 1 and 2, the resultant circuit is a standard two-level VSI. Now the circuit is referred to as two classical three-phase VSIs (H(1) and H(2)) for the isolated converter (Fig. 1) and classical six-phase VSI for the non-isolated converter (Fig. 2). Hence, modulations are carried out such as standard three-phase and six-phase VSIs. By space vector theory, the output voltage vector v of the dual three-phase windings can be expressed as the sum of the voltage vectors of two three-phase windings v(1) {1} and v(2) {2} as given below [3,12,18]:

v = v(1) + v(2) (8)

By splitting the six-phase windings into standard two three-phase windings then Eq. (8) by taking account Eq. (4) then the modulating vectors can represent the first and second three-phase windings as below:

x(1) =4 Vdc (Sh1 + Sh2ej2p=3 + S^é47'3)

where a = exp(jp/6) for the asymmetrical version in Eq. (1), and a = exp(j2p/6) for the symmetrical version in Eq. (2), the spatial displacement between windings. The multiple space vectors Xi, V3, and x5 are the sub-zones accordingly the first d1-q1, second d3-q3, and third d5-q5 sub-spaces. The dual three-phase system along with isolated/non-isolated DC sources can be introduced by space vectors. For this, the six-phase system is split into two three-phase sub-systems {1} and {2} represented as [3,18,27-30]:

x11) =

xV' = X2 ; {2} <

X22) = X5 •

The stationary rotating space vectors vx(1), xv(2) and the zero-sequence components, x02) for each three-phase sub-system {1} and {2} are defined below:

' X(1' = 3 [x11) + X21'«4 + Xv a

31 ' a8 ]

X1) - 1 rX(1)

= 3 [xr + X

Í X(2' = 2 [x12)+ x2

I X0 ' = 3 [x1

(2)a^ X(2)a8

x32)a8]

(2)+x22)+ x32']

Multiple space vectors and split-phase space vectors are now related by substituting Eqs. (3) and (4) in Eqs. (1) and (2) represented as below:

= 2 [X(1) + aX(2)]

'5 = 2 [x(1' — aX(2']

,X3 = X01'+ jx02)

X(1' = x1 X01)= X3

+ x5 í x(2

• 1 1 x0

x(2' = a-1(x1 — x5)

X02) = X3 • j

where the symbols "" (dot) product.

' and "■" are the complex conjugate and scalar

3. Isolated/non-isolated inverter single-carrier-based five-level modulation

The total power P of the dual three-phase (isolated/non-isolated) inverter can be expressed as the sum of the power of the two three-phase windings {1} and {2} [3-6,12,18]:

P = P(1) + P(2) = 3 x(1) • i(1 + 3 x(2' • ¡(2> p = p(1 + p(2) = s [ x(1) • p) + x(2) • i2)]

x(2) =1 Vdc (Sh4a + Sh5ae>2p/3 + Smaê4pl3)

By substituting Eqs. (9) and (10) in Eq. (8), the arbitrary rotating modulating vector for the dual three-phase (isolated/non-isolated) inverter can be predicted as [3-6,12,18]:

x = 3 Vdc (Sh1 + SH2ej2p¡3 + Srae'4p/3)+

1 Vdc (Sh4a + S^aé271'3 + S^a e4p=3 )

Instant switching, upper-states {SH, SH1, Sh2, Sh3 Sh4, Sh5, Sh6}, lower-states {SH1/, SH2/, SH3,, SH4,, SH5,, SH6/} = {1}, {0} of the inverter legs (applicable to both isolated/non-isolated). Assumed there are no zero-sequence currents in the system (balanced condi-

Fig. 3. Multilevel modulation scheme with one carrier for phase 'a' of inverter VSlHl

Fig. 4. PWM pattern of inverters legs 'a', 'b', 'c', modulation index = 0.8).

tions), Eq. (11) can be rewritten as two separate three-phase VSIs (non-isolated). Preliminary investigations are executed with single carrier based 5-level modulation algorithm in this article [27-32]. The modulating reference signals are compared against

Table 1

Main parameters of the isolated/non-isolated VSIs.

DC Bus Vdc 400 V

Load Resistances R 8 X

Load Inductances L 10 mH

Fundamental Frequency F 50 Hz

Switching Frequency Fs 10 KHz (non-isolated)

5 KHz (Isolated)

Capacitors Vc 2200 iF

the standard triangular carrier to provide maximum utilization of DC buses and the ability to generate 5-level operation.

Fig. 3 shows the single carrier (MSCFM) modulation algorithm to generate 5-level operation across the leg-phases 'a'. The strategy is the same and is applied to all other leg-phases (b, c, d, e, f, g), keeping the proper phase-shift (60° or 30°), between reference modulating signals for symmetrical or asymmetrical for both converter operations (Figs. 1 and 2). Bi-directional switches (SHa, SHb, SHc, SHd, SHe, SHf) are modulated throughout the fundamental period with reference to their arbitrary phase shifts (60° or 30°), i.e., swaps between {1, 0} with switching periods [31,32]. Correspondingly, the switch patterns of the proposed 5-level modulation for inverter-specific legs are shown in Fig. 4, with fixed modulation index equal to 0.8.

Time [S]

(A). Line-to-line voltage of the inverter (VSIh"') measured between windings 'a' and 'b'.

(B). Line-to-line voltage of the inverter (V SIh'2') measured between windings'd' and 'e'.

-1 -300

(G). Line-to-line voltage of the six-phase VSI measured between winding 'a' and 'b'.

.3 -300 J

(H). Line-to-line voltage of the six-phase VSI measured between winding'd' and 'e'.

(C). First-phase voltage of first three-phase windings {1}. (I). First-phase voltage of second three-phase windings

Fig. 5. Observed simulation behaviour of the proposed multilevel six-phase star-winding converters. Modulation index = 0.8, kept for balanced operation. Voltages are depicted with their corresponding time-averaged fundamental components. Left: Asymmetrical isolated/non-isolated converters. Right: Symmetrical isolated/non-isolated converters.

05127215

0.03 Time [Sec]

(D). First-phase voltage of second three-phase windings (J). First-phase voltage of second three-phase windings

{!}• {2}.

(E). Three-phase currents of the first three-phase

windings {!}.

(F). Three-phase currents of the second three-phase windings {2}.

(K). Three-phase currents of the first three-phase

windings {!}.

(L). Three-phase currents of the second three-phase windings {2}.

Fig. 5 (continued)

4. Numerical modeling-based simulation results and discussion

To verify the theoretical developments and the performances of the proposed converters are numerically developed by means of Matlab/PLECS simulation software. Correspondingly, the parameters used for testing are elaborated in Table 1. The test is conducted under the balanced conditions by a modulation index fixed to 0.8.

Fig. 5 shows the complete simulation behaviour of the asymmetrical isolated/non-isolated multilevel converter (left) and symmetrical isolated/non-isolated multilevel converter (right).

Fig. 5(A) and (G) are the observed line-to-line voltage of isolated and non-isolated converters of the first three-phase windings {1}. Fig. 5(B) and (H) are the observed line-to-line voltage of isolated and non-isolated converters of the second three-phase windings {2}. Note that the voltages are depicted with their corresponding time scale averaged fundamental components. In the case of

isolated/non-isolated symmetrical converters, it is observed that line-to-line voltages shifted exactly by 30° (Fig. 5(A) and (B)). In the case of isolated/non-isolated symmetrical converters, line-to-line voltages shifted exactly by 60° (Fig. 5(A) and (B)). As expected, the converters produced the 5-level output voltages with exact spatial displacements between the windings {1} and {2} for symmetrical and asymmetrical converters. All of the fundamental components are of the same amplitude and confirm the balanced operation of the converters. It is proved that each VSI is capable of generating 5-level outputs with the developed MSCFM algorithm. Hence, the drawbacks of addressing dual inverter configurations are overcome [2-6,10-12,18-23,25].

Fig. 5(C) and (I) are the generated phase voltages of the first phase of the first three-phase star-winding (phase 'a') {1} by the isolated and non-isolated converters. Fig. 5(D) and (J) are the generated phase voltages of the first phase of the second three-phase

star-winding (phase'd') {2} by the isolated and non-isolated converters. Also, the voltages are depicted with their corresponding time scale averaged fundamental components. It is observed that phase voltages are 7-level stepped waves, which are actually predicted.

Note that the isolated/non-isolated symmetrical converters phase voltages shifted exactly by 30° (Fig. 5(C) and (D)). Whereas, the isolated/non-isolated symmetrical converters phase voltages shifted exactly by 60° (Fig. 5(I) and (J)). Further, the generated fundamental components are in agreement with Eq. (9) first three-phase star-winding and Eq. (10) second three-phase star-winding. Further, the fundamental components confirm that they are of the same amplitude and demonstrate abalanced, smooth operation (modulation index = 0.8).

The first three-phase star-winding currents {1} are shown in Fig. 5(E) for isolated and non-isolated asymmetrical converters and in Fig. 5(K) for isolated and non-isolated symmetrical converters. Similarly, the second three-phase star-winding currents {2} are shown in Fig. 5(F) for isolated and non-isolated asymmetrical converters and in Fig. 5(L) for isolated and non-isolated symmetrical converters.

It could be observed that currents are sinusoidal and equally balanced in amplitude with proper phase shifts of 30° between the first three-phase windings {1} and second three-phase windings {2} for isolated and non-isolated asymmetrical converters (Fig. 5(E) and (F)). Similarly, it could be observed that currents are sinusoidal and equally balanced in amplitude with proper phase shifts of 60° between the first three-phase windings {1} and second three-phase windings {2} for isolated and non-

isolated symmetrical converters (Fig. 5(K) and (L)). The six-phase currents generated by both isolated/non-isolated and symmetrical/asymmetrical converters confirms that they modulated sinu-soidally with the developed MSCFM PWM algorithm and balanced operation.

Fig. 6 depicts the trajectories of the six-phase currents represented in alpha-beta (a-b) rotating planes. The total electrical power is under balanced conditions, as predicted, the rotating component moves along a circular trajectory (at constant frequency) as shown by Fig. 6(left). Again, conformity is shown that each VSI is modulated sinusoidally with the developed PWM strategy. The fifth sub-space (leakage components) and the third subspace (common-mode/zero-sequence components) trajectories confirm that null as is given by Fig. 6(middle) and (right).

Similarly, Fig. 7 illustrates the trajectories of the six-phase voltages represented in the alpha-beta (a-b) rotating planes. As the adapted modulations are 2-level, 7-level open-winding phase voltages and 5-level line-to-line voltages always shift from zero to maximum levels. Therefore, dv/dt is obviously higher by the developed multilevel PWM algorithm, and it is clearly verified from Fig. 7(left). The fifth sub-space (leakage components) of voltages is shown by Fig. 7(middle). It confirms that all the odd harmonic components of order 6n ±1, (n = 1,3,4...) are exposed to subspace five and the first subspace are free of odd harmonic components. The third sub-space (common-mode/zero-sequence components) of voltages depicted by Fig. 7(right) verifies that it is practically null as expected under balanced operating conditions. Finally, it is concluded after the comprehensive test that the balanced operation of the VSIs is guaranteed.

? if im

£ 5 £

I § 10

-10 0 10 20 Alpha Components or Three-Phase Currents

£ If fa.

I -f I

0 1« 0 10 20 Alpha Components of Three-Phase Currents

]0 CI id 20

Alpha Components of Three-Phase Currents

di-qi first sub-space. ds-c/5 fifth sub-space. ds-qs third sub-space.

Fig. 6. Trajectories of dual three-phase currents in sub-spaces of a-b rotating planes first sub-space (right), fifth sub-space (middle), and third sub-space (left).

Alpha Components or Three-Phase Open-winding Voilages

d-q! first sub-space.

Alpha Components of Three-Phase Open-Winding Voltages

d5-q5 fifth sub-space.

'i. 100

o" 50 S

■25Sol

Alpha Components of Three-Phase Open-Winding Voltages

d3-q3 third sub-space.

Fig. 7. Trajectories of dual three-phase voltages in sub-spaces of a-b rotating planes first sub-space (right), fifth sub-space (middle), and third sub-space (left).

Total harmonic distortion (THD) of the generated line-to-line, phase (open-winding) voltage and phase current is shown in the spectrum by Figs. 8-10. It is observed that by the developed multilevel PWM technique, the output generated is free of odd harmonics. Moreover, the phase current THD was found to be 7.2% and which is appreciable without output filter according to IEEE standards. As expected, THD response of the phase voltage is higher (37.8%) than line-to-line voltage (28.7%) due to the fact high dv/dt effect of the developed modulation technique. But the limitation is within the margin of standard THD ranges as by IEEE again without a filter. By adopting proper 5-level modulation, either by carrier based or space vector modulation the effect of dv/dt will be limited and the generated output quality can be much improved. These tasks are kept under development and investigations, will be decimated in future articles.

Finally, this preliminary investigation verifies the satisfactory performances by the proposed isolated/non-isolated, symmetrical/asymmetrical star windings multiphase-multilevel inverters good in agreement with the developed theoretical background.

0 50 100 150 200

Harmonic order

Fig. 8. Total harmonic distortion spectrum of line-to-line voltage.

Harmonic order

Fig. 9. Total harmonic distortion spectrum of phase voltage (open-winding).

Fig. 10. Total harmonic distortion spectrum of phase current.

5. Conclusions

This article devoted novel isolated/non-isolated multilevel asymmetrical/symmetrical multiphase star-winding converters based on reconfiguration of the standard VSI configuration. A modified single carrier five-level modulation (MSCFM) original algorithm also proposed and allows the VSIs to modulate 5-level multi-stepped waveforms at the outputs. Therefore, overcomes the drawback of standard dual open-winding and other standard multilevel inverter configuration addressed to multiphase AC drives. Depicted numerical simulation results confirm that the lower order harmonics are suppressed in the outputs with balanced power operations with both isolated and non-isolated converters. Proposal effectively utilized for batteries or fuel-cells fed system for medium power, AC tractions and More-Electric Aircraft (MEA) applications. Still the investigations are under examination to frame optimized 5-level carrier based or space vector modulation PWM generation methods for near future works.

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