Scholarly article on topic 'A Modified Multiband Hysteresis Controlled DTC of Induction Machine with 27-level asymmetrical CHB-MLI with NVC modulation'

A Modified Multiband Hysteresis Controlled DTC of Induction Machine with 27-level asymmetrical CHB-MLI with NVC modulation Academic research paper on "Electrical engineering, electronic engineering, information engineering"

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{"Direct torque control" / "Multiband hysteresis controller" / "Asymmetrical multilevel inverter" / "Space vector control" / "Induction machine drives"}

Abstract of research paper on Electrical engineering, electronic engineering, information engineering, author of scientific article — Rohith Balaji Jonnala, Ch Sai Babu

Abstract The influence of Direct Torque Controlled Induction Motor Drive in the area of industrial application is very high; it presents foremost area of controllability of load at different states of operation. The major snags to the controller are maintaining Constant Switching Frequency and Infeasibility state. This paper concentrates on rectifying these problems with Modified Multiband Hysteresis Controller and Nearest Vector Control Modulated Asymmetrical Cascaded H-Bridge Multilevel Inverter for the better drive operation. In this case proper modification in MHC gives the optimal utilizations of each control vector to avoid the infeasibility states with a Lookup-Table and Multilevel Inverter gives more number of control voltage vectors with constant switching frequency for flexible operation of drive with low disturbances. Direct Torque Control equipped with these two modules achieves better operating conditions with low Torque ripples, low distorted flux and speed with different loads, and all other satisfactory load operating parameters.

Academic research paper on topic "A Modified Multiband Hysteresis Controlled DTC of Induction Machine with 27-level asymmetrical CHB-MLI with NVC modulation"

Ain Shams Engineering Journal (2015) xxx, xxx-xxx

Ain Shams University Ain Shams Engineering Journal

www.elsevier.com/locate/asej www.sciencedirect.com

ELECTRICAL ENGINEERING

A Modified Multiband Hysteresis Controlled DTC of Induction Machine with 27-level asymmetrical CHB-MLI with NVC modulation

Rohith Balaji Jonnala *, Sai Babu Ch1

EEE Department, UCEK, JNTUK, Kakinada, E.G.Dt, A.P. 533003, India Received 6 April 2015; revised 21 August 2015; accepted 29 August 2015

KEYWORDS

Direct torque control; Multiband hysteresis controller;

Asymmetrical multilevel inverter;

Space vector control; Induction machine drives

Abstract The influence of Direct Torque Controlled Induction Motor Drive in the area of industrial application is very high; it presents foremost area of controllability of load at different states of operation. The major snags to the controller are maintaining Constant Switching Frequency and Infeasibility state. This paper concentrates on rectifying these problems with Modified Multiband Hysteresis Controller and Nearest Vector Control Modulated Asymmetrical Cascaded H-Bridge Multilevel Inverter for the better drive operation. In this case proper modification in MHC gives the optimal utilizations of each control vector to avoid the infeasibility states with a Lookup-Table and Multilevel Inverter gives more number of control voltage vectors with constant switching frequency for flexible operation of drive with low disturbances. Direct Torque Control equipped with these two modules achieves better operating conditions with low Torque ripples, low distorted flux and speed with different loads, and all other satisfactory load operating parameters. © 2015 Faculty of Engineering, Ain Shams University. Production and hosting 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/).

1. Introduction

The escalation of Induction Machine (IM) control requirement in the industrial applications with input side qualitative power,

* Corresponding author. Tel.: +91 9989262655.

E-mail addresses: rohithbalajijonnala@yahoo.com, researchscho-

lar201@jntuk.edu.in (R.B. Jonnala), chsaibabu@jntuk.edu.in (C. Sai

Babu).

1 Tel.: +91 8978788555. Peer review under responsibility of Ain Shams University.

raises the opportunity to the arrival of contemporary converters and controllers capable to machinate all needed operating conditions.

The DTC induction motor drive gives better dynamic performance with a simple control strategy, but has undesired torque and speed ripples due to some internal computational drawbacks in control action such as switching frequency, bands of hysteresis controller and voltage vector selection. There is a significance of simple and better arrangement of controller to fix these flawed parameters at an optimal state to minimize the drawbacks. The most essential part of DTC is inverter and employed to get low disturbances in input side as well as to provide more number of voltage vectors to avoid infeasibility state of DTC operating conditions with fundamental switching states. Multilevel Inverter provides

http://dx.doi.org/10.1016/j.asej.2015.08.007

2090-4479 © 2015 Faculty of Engineering, Ain Shams University. Production and hosting 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/).

more number of vectors and levels in output, but general DTC based MHC does not utilize every vector due to constraints in bands of Hysteresis controller. Classical DTC control strategy has ONE level Flux, TWO level Torque Hysteresis controllers and THREE level inverter. Instead of inverters, Multilevel Inverters are replaced to get low distorted input to the machine; simultaneously, Hysteresis controllers are also changed. But these Hysteresis controllers are modified by the aspects of Torque Ripples, so only torque Hysteresis

controllers are extended to Multiple Bands and Flux Hysteresis controller is in the same level. This modification in Hysteresis Controller gives better results in torque ripple reduction but avoiding some of the vectors generated by multilevel inverter. Modifications in MHC give the maximum utilization of voltage vectors for selection in the control strategy to get better control of induction motor without changing the estimators and conventional DTC logic.

Figure 1 General block diagram for DTC.

Figure 2 Three level space vector hexagon with sector selection approach and hysteresis bands.

1.1. Conventional switching strategy of DTC induction machine drive

different sectors and comparators output (desired flux and torque corrections).

The advancement in Scalar control technique is DTC and this technique has virtually similar performance with vector controlled drive, and this is substitute method for flux-oriented control. The internal activities in the form of mathematical equations (1)-(9) of IM in DTC drive can be represented in terms of space vector with the stator stationary reference frame:

, _ 2 . 1 . 1 . iqs 3 ia 3 ib 3 ic (1)

f - 1 « + 1 « ds ~ P3 b + P3ic (2)

2 11 = 3 Va - 3 Vb - 3 vc (3)

* = - 73Vb+p3Vc (4)

Vds = | (V* - R^dt (5)

w; = / fa - R¿qs)dt (6)

w, = V wd,2+wq,2 (7)

ee = tan-1 (wywds) (8)

Te=2 p {wd/q, - wq/ds) (9)

Block diagram shown in Fig. 1, illustrates the arrangement of DTC-Hysteresis based IM drive, as in [1]. The required stator flux Wj and torque Tj magnitudes are compared with the relevant estimated values, and the error values are analyzed through hysteresis band controller.

The flux and torque hysteresis loop controllers have one and two band levels in that order of digital output according to the following relations:

Hw = 1 for Ew >

Hw = -1 for Ew < -HBw

HTe = 1 for ETe > +HBTe HTe = -1 for ETe < -™Te HTe = 0 for -HBTe < Ere <

Output voltage of the converter is based on the selection of the switching states (Sa, Sb and SC) acquired from the lookup table. All these switching states are picking out based on the requirement as the need of torque and stator flux position. Fig. 2 illustrates hysteresis bands and possible voltage vector generated by the inverter with space vector strategy, here the dashed line indicates possible voltage vector for anticlockwise direction and normal line indicates clockwise working direction of machine. The future sector related voltage signal to be applied to the drive can be determined which will adjust the flux and torque responses, Knowing the Present voltage vector, torque and flux error (ETe, E^) and the stator flux position (sector determined by angle he). Note that the next voltage vector assigned to the converter will always be one of the four nearest vectors to the present, which will regulate the required output and reduce dynamic nature in torque response. Table 1 summarizes vector selection for the

1.2. Multilevel inverter

Multilevel voltage source inverter provides a cost-effective solution in the medium-voltage energy management market. Multilevel inverter has been widely applied to industrial applications such as traction, pumps, marine propulsion, automotive applications, power quality and energy transmission [2,3]. So the researchers concentrate for long period to enhance the multilevel inverter performance such as simplification in pulse generation and the efficiency of various optimized algorithms are utilized to refine the distortions in the output signal [4,11].

Multilevel inverter has considerable advantages and is analyzed with conventional or familiar two level converters.

Table 1 Sector selection table.

Present Sector 1 2 3 4 5 6

Flux Torque

1 2 3 4 5 6 1

1 0 0 7 0 7 0 7

-1 6 1 2 3 4 5

1 3 4 5 6 1 2

-1 0 0 7 0 7 0 7

-1 5 6 1 2 3 4

Figure 3 Asymmetrical CHB multilevel inverter.

These advantages are basically focused on improvement in the nominal power and output signal quality in inverters. Currently there exist three commercial topologies of multilevel voltage source inverters such as Flying Capacitor (FC), Neutral Point Clamped (NPC) and Cascaded H-Bridge (CHB) [4,5]. Among these inverter topologies Cascaded H-Bridge Multilevel Inverter reaches the higher output voltage and power levels and the higher reliability for a given number of cells (N) due to its standard topology.

The cascaded H-Bridge inverter consists of power conversion cells, each one energized by an isolated DC source on the DC side which can be obtained from batteries, ultra capacitors (or) fuel cells and series connection with load on the AC side. If all DC voltage sources are equal to VDC the inverter is then known as a Symmetrical multilevel converter otherwise

asymmetrical type of multilevel converter [15]. The use of asymmetrical input voltages can reduce, or when properly chosen, eliminate redundant levels on output side and maximize the number of different levels generated by the inverter. Therefore this asymmetrical multilevel inverter can attain the identical output voltage quality with less number of switches (or) cells. This also reduces volume, cost, and losses.

1.2.1. Asymmetrical multilevel inverter

To emphasize the merits of the asymmetrical multilevel converter, the basic characteristics of different H-Bridge multilevel topologies are as follows:

i. Cascaded H-Bridge Multilevel Inverter,

ii. Hybrid H-Bridge Multilevel Inverter,

Table 2 Comparison between symmetrical and asymmetrical multilevel inverter.

Items Symmetrical Multilevel Inverter Asymmetrical Multilevel Inverter Observations

Type of Sources Equal DC Sources Unequal DC Sources

No. of Cells No. of Switches No. of Cells No. of Switches

Output Level 3 1 4 1 4 Same operating conditions for both inverters

5 2 8 2 8

■ 7 3 I 12 2 8

9 4 16 2 8

11 5 20 3 12

13 6 24 3 12

15 7 28 3 12

17 8 32 3 12

19 9 36 3 12

21 10 40 3 12

23 11 44 3 12

25 12 48 3 12

■ 27' 13 52 I 3 I i 12 i Maximum of 27 level obtained by the same 3 cells'

29 14 56 4 T 16 T 1

Aft 2

— ™ ' Symmetrical Multilevel Inverter 4 4 Z

Asvmmetrical Multilevel Inverter 4 V 2 r

$ 2 - z

1A r

7fi - y

1 jC H r 1 g

1 1 A 11 1 2 1 2 1 2 1 2 1 2 i 2 1 2 1 >

s s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Output Level

Figure 4 Graph between output levels vs. no. of switches.

V i C a

s o h g

w f S w

do di Hi

Table 3 Comparison between various multilevel inverters with switching states and output levels.

Division - I: cascaded H-bridge multilevel inverter output voltages and switching states for N = 3

—3V —2V — 1 V 0V 1 V 2 V 3 V

1V — — — 0 — — + 0 0 — 0 0 — + — + 0 + 0 0 — + + 0 + + +

1V — — 0 — — + — 0 — 0 — + 0 0 + — 0 0 + 0 + - — ++ 0 + +

1V — 0 — — +— — — 0 0 + — + — 0 0 0 0 0 + + + — + + 0 +

Division - II: hybrid multilevel inverter output Voltages and sswitching states for N = 3 Vdc

-7 V -6 V

-4V -3V

0 V 1V

4 V 5 V

6 V 7 V

2 V 4 V

0 + 0 0 0 0

Division - III: quasi-linear multilevel inverter output Voltages and switching states for N = 3 Vdc

-9 V —8 V —7 V

-6V —5V

—4V —3V

—2V —1V

0 V 1V

2V 3 V

4 V 5 V

6 V 7 V

8 V 9 V

1V — 2 V — 6 V -

Division - IV: new hybrid multilevel inverter output voltages and switching states for N = 3 Vdc

0 + 0 0 0 0

— 0 + + + +

—0 0 0

13V —12V —11V —10V —9V —8V —7 V —6 V —5V —4V —3 V —2 V —1V 0V 1V 2V 3V 4V 5V 6V 7V 8V 9V 10 V 11V 12V 13V

Figure 5 Classification of multilevel inverter modulations.

Figure 6 Operating logic for NVC.

iii. Quasi-linear Multilevel Inverter and

iv. New Hybrid Multilevel Inverter; their characteristic variations are shown in Table 3 for N =3.

Among the above four types of topologies, New Hybrid multilevel inverter shown in Fig. 3 gives more number of levels for the same number of cells with different voltage ratios and so called as Asymmetrical Cascaded Multilevel Inverter [6,7] by its features. There is a complete view on increased levels with the comparison of symmetrical multilevel inverter as shown in Table 2 and Fig. 4.

The asymmetrical multilevel converter enclosed with fully controlled bridge conversion cells such as cascaded, quasilinear, and Hybrid multilevel inverter. However the DC source voltage along with the cells has the sequence like 1 V, 3 V, 9V...3n— 1 V. Due to this arrangement the level (l) of the output signal for the presented design equals to 3N.

So asymmetrical converter output voltage waveform for N = 3 has 27 levels, i.e. 0, ±1VDC, ±2VDC, ±3VDC, ±4 Vdc, ±5 Vdc, ±6 Vdc, ±7 Vdc, ±8 Vdc, ±9 Vdc, ±10 VDC, ±11VDC, ±12Vdc, ±13 Vdc. The maximum output voltage of this N cascaded multilevel inverter is Vo,Max = ((3N—1)/2) VDC. In Table 3, ' + ' and '—' denote the output signal of the cells withVDC = 1 V, 3 V and 9 V with positive and negative polarities respectively; moreover, '0' indicates that the related cell is in freewheeling state.

1.2.2. Modulation strategies

One of the problems raised by the conventional DTC is ripples in torque. Various techniques are developed to diminish the ripples in torque, one of them is duty ratio control; a zero voltage vector is applied for the rest of the sampling period after the effect of control [8-10]. This logic can significantly reduce the torque ripple, but increase complexity of DTC strategy. Another method to decrease the ripple in torque is based on modulation system of converter.

Multilevel inverter modulation Strategies have great research and advancement attention over the few decades. The modulation strategies were used in multilevel inverter; they can be classified according to switching frequency as

Figure 7 NVC level pattern.

Figure 8 (a) Block diagram for general MHC. (b) Multiband space vector hexagon.

shown in Fig. 5 [2,4-6]. Strategy that works with high switching frequencies has many commutations for the power semiconductor switches in one period of the fundamental output voltage. An extremely accepted strategy in industrial applications is the standard carrier-based sinusoidal PWM (SPWM) that uses the phase-shifting technique to reduce the harmonics in the output voltages. One more attractive substitute strategy is the Space Vector Modulation.

Strategy that works with low operating switching frequencies normally performs maximum of two commutations of the power semi conductor switches during one fundamental cycle of the required voltage signal to generating a staircase waveform. Representatives of this family are the multilevel Selective Harmonic Elimination (SHE), and the Space Vector Control (SVC).

Even though SVM, Multicarrier PWM and SHE are widely recognized and reached a certain state of capability for multilevel industrial applications [11,12], they become very complicated to design and applicable for converter with multiple levels, because of difficulty for computing the switching angles. In this case, other strategies with low switching frequencies are convenient. Generally, Multilevel Space Vector

Control has many advantages of the more number of generat-able voltage vectors by the inverter. This principle of operation works with a fundamental operating switching frequency with minimum switching losses. Vector based modulations are very easy to implement for DTC strategy, because DTC is in vector control approach.

1.2.2.1. Nearest vector control. Nearest Vector Control was introduced as a substitute to staircase or SHE modulation to facilitate a low switching frequency modulation strategy, with reduction in poor dynamic performance and other requirements of SHE [11]. The fundamental scheme is to take advantages of the more number of generated voltage vectors of the multilevel inverter by comparing the required reference to the nearest voltage vector that can be selected in the d-q plane, without conventional modulation computations. Therefore, this strategy is specified as nearest vector control instead of modulation.

The operating principle of NVC is shown in Fig. 6, where the state-space vectors for 11-level inverter are visualized, with a magnified portion to the NVC operating strategy. Each dot of hexagon is one of the feasible vectors compatible to the

Table 4 Sector selection table for general MHC for DTC.

Flux Torque Sector -1 Sector -1

(for +ve flux error) (for — ve flux error)

13 14 27

12 13 26

11 12 25

10 11 24

9 10 23

8 9 22

7 8 21

6 7 20

5 6 19

4 5 18

3 4 17

2 3 16

1 2 15

-1 78 65

-2 77 64

-3 76 63

-4 75 62

-5 74 61

-6 73 60

-7 72 59

-8 71 58

-9 70 57

-10 69 56

-11 68 55

-12 67 54

-13 66 53

inverter operation; they are surrounded by a hexagon that indicates the boundary of the area in which they are the closest available voltage vectors.

The dashed line shown in Fig. 6 is the voltage space vector reference (Vs ) trajectory through the complex plane. Hence, when the Vs* comes into any hexagon, the respective nearest vector will be selected for the inverter operation. The generated vector according to the illustrative example in Fig. 6 and the level pattern are shown in Fig. 7 [4].

The fundamental selection of the nearest vector to the reference gives a significant reduction in the commutations, since no commutations are forced by a modulator, reducing in this way the switching losses [13]. This strategy, however, does not eliminate low-order harmonic distortions due to it's inherent low and variable switching frequency nature. This can be compensated with higher number of levels by using multilevel converter (usually above seven), which have a more dense state-space vector availability, resulting in a better vector approximation and smaller error.

This method has very simple operating principle; however, an algorithm capable of finding numerically the closest vector needed to be programmed. Hence it is aimed to be used in converters with a high number of levels to avoid important low-order harmonics at the ac side. The main advantage is its conceptual and implementation simplicity and the efficiency achieved with this strategy. This issue can be found in detail in [13].

1.3. Multiband Hysteresis Controller

The improved DTC has modification in torque hysteresis controller for reducing torque ripples and avoiding infeasibility states [16,17] in control operation with more number of voltage vectors, by dividing the comparator levels into several sublevels (0, ±1, ±2 ...) that will provide the different error levels and their respective voltage vectors instead of three level process with six vectors. The block diagram shows the modification in hysteresis controller and major researchers are mostly interested to modify the torque hysteresis controller [18] as shown in Fig. 8(a) because of impact of torque ripples in DTC.

Some of the researchers are involved to change alignment of band position either on upper side or on lower side [19], but selection of alignment angle is very difficult. So symmetrical sectionalization of hysteresis band is simple and meets results of all the requirements of multilevel operation. With reference to torque hysteresis controller's band values, the space vector selection hexagon also having subsectors to provide intermediate levels in voltages with subsectional phase

Figure 9 Modified multiband hysteresis controller.

Sector I (for —ve torque errors) ^t- m ^t- "xl" «o i> ^t oo ^t- ON o «O «ri «ri m «ri «0 «ri «ri «ri i> «ri oo «Tl ON »/i o m «ri oo ^ ON o l> m i> -xf «ri i> oo 0/79

o o o C/3 oj > + ê <D U* o cn 5-1 O f-i <D ON m oo m i> m m «ri m m m m <N m m o m ON <N oo i> «ri <N m <N o ON OO «T) m (SI 1 o ON o OO o i> o o «ri o o m o <N O ON

7 m o ON o oo o i> o 'O o «O o o m o <N O O o o

7 m o ON o oo o i> o 'o o «ri o o m o <N O O o o

7 m o ON o oo o i> o 'o o «O o o m o <N O o o o

7 m <N o ON o oo o l> o o «O o "xl" o m o O O o o

m <N - o ON o oo o l> o ^D O «O o "xl" o m o <N O O o o

CO m <N - o ON o oo o l> o ^D O «O o "xl" o m o <N O O o o

m <N - o ON o oo o l> o ^D O «T) o "xl" o m o <N O O o o

m <N - o ON o oo o l> o ^D O «T) o "xl" o m o <N O O o o

V) m <N - o ON o oo o l> O ^D O «T) o "xl" o m o <N O o o o

m <N - o ON o oo o l> O ^D O «T) o "xl" o m o <N O o o o

C-, m <N - o ON o oo o l> O ^D O «T) o "xl" o m o <N O o o o

rl m <N - o ON o oo o l> O ^o o «T) o "xl" o m o (N O O o o

7 m <N - o ON o oo o l> o O «T) o "xl" o m o O o o o

+ m <N o ON o oo o l> O ^D O «T) o "xl" o m o (N O o o o

+ m <N o ON o oo o l> O ^o o «T) o -xf o m o (N O o o o

+ m - o ON o oo o l> o 'o o «Tl o o m o <N O o o o

+ m o ON o oo o l> o o «ri o o m o <N o o o o

+ m <N - o ON o oo o l> o 'o o «ri o o m o <N O o o o

CO + m - o ON o oo o l> o 'o o «Tl o o m o <N O o o o

Ü ffi S •ö o m + m - o ON o oo o l> o 'o o «Tl o o m o <N o o o o

+ V) + m <N - o ON o oo o l> o ^o o «T) o -xf o m o (N O o o o

o a £ m (N ^ o ON o oo o l> o 'o o «Tl o o m o <N O o o o

+ m (N - o ON o oo o l> o ^D O «T) o "xl" o m o (N O o o o

a m + m - o ON o oo o l> O 'O o «ri o o m o <N O o o o

Ö _o o JH rl + m - o ON o oo o l> o 'o o «ri o o m o <N O o o o

5-1 o + m o ON o oo o l> o o «ri o o m o <N O O o o

Table 5 Set t X p 1-1 Ph w p c/ o H T

Table 6 Simulation System Parameters.

Simulation parameters

Execution time 5s

Solver ODE 23tb

Max step size 1e-5 Sec

Min step size Auto

Sample time 1e-5 Sec

Fundamental frequency 50 Hz

Inverter parameters

Type of configuration Asymmetrical multilevel inverter

Number of cells 3

Source (multiplication) 1-3-9 (28.461 V: 85.384 V:

factor 256.153 V)

Total voltage value 370 V

Type of switches IGBT with antiparallel Diode

Snubber parameters Rsn = 10 X, Csn = 1 iF

Control technique Nearest Vector Control

Modulation type Space vector control (Adjacent

Machine parameters (induction ! motor)

Nominal power 7.5 KW (10 HP)

Line to line voltage 400 V

Frequency 50 Hz

Speed 1440 RPM

Stator resistance 0.7384 X

Stator inductance 0.003045 H

Rotor resistance 0.7402 X

Rotor inductance 0.003045 H

Moment of inertia 0.0343 Kg m2

Friction 0.000503 Nm S

Number of poles 4

angles is shown in Fig. 8(b). At present 27-level 3-Cell AS-CHB multilevel inverter takes the better chance to coordinate with any other control strategy because of its simplicity in generation of gating pluses, so this 27 level inverter has totally 78 sectors in that hexagon [20] with ±13 sublevels in torque

hysteresis controller with zero level, and one level flux hysteresis controller.

Based on the logic of conventional DTC, two level flux and three level torque hysteresis comparators are giving six possible ways of error combination. Those are like {Ew, Exe}, if the combination is {1, 1} then the Conventional DTC Strategy chooses the One Step in progressive or anticlockwise direction to reduce the error and to achieve the required state of machine, similarly, One Step in regressive or clockwise direction for {1, —1}, Two Steps in progressive or anticlockwise direction for {—1, 1}, Two Steps in regressive or clockwise direction for {—1, —1} and choosing of inactive vectors for 0 torque error. With those combinations and the status of present sector the next control voltage vector will be selected through the lookup table and the approach was clearly explained in Section 1.1. With the same logical approach, 13 level Space Vector Hexagon has 78 Voltage Vectors i.e., 6 sets of 13 Vectors. Here the total 54 ways of combinations are available with the modified torque comparator. Any logical pattern of selection of voltage vector will be constant at 54 over 78. Positive Torque errors are 13 in combination with the positive Flux error choosing One Step in progressive or anticlockwise direction of voltage vector of hexagon i.e., first set of 13 vectors. Positive Torque errors are 13 in combination with the negative Flux error choosing Two Steps in progressive or anticlockwise direction of voltage vector of hexagon i.e., next set of 13 voltage vectors, similarly in negative Flux error also. The total approach of selection of Voltage vectors for this multilevel hysteresis controllers is given in Table 4. Here the operation is fixed with the sets of voltage vectors, because of only Flux hysteresis controller having one level band. And the only drawback is neglecting 24 vectors due to combinations of comparators, because this 24 infeasibility states are raised when the controllers need to select intermediate sectors.

Section 2 presents the working principle and features of proposed modulation strategy and their sub-sectionalization of bands, and also the modification of lookup table with proper alignment to avoid the conflicted positions.

Figure 10 Required speed and load torque curves.

2. Proposed Multiband Hysteresis Controller

Several combinations in comparators and different patterns of MHC for DTC are introduced for individual specified requirements such as reduction in torque ripple, effective utilization of control vector, and avoiding dead zone of sampling period [14,15,18]. This section concentrates on the modification in multiband hysteresis controller to get effective utilization of every voltage vector for avoiding infeasibility states and reduction in torque ripple.

As discussed in Section 1.3 for 27 level inverter's vector hexagon, 54 sectors are utilized over 78. In basic level conventional DTC logic, present and opposite to that sectors are avoided as ineffective zones and remaining four sectors are considered as an effective regions. But general MHC is having more than two ineffective regions. To avoid these ineffective regions and getting more number of control vectors, some

Figure 13 Flux trajectories for proposed DTC.

Figure 11 Output voltage waveform for symmetrical multilevel inverter with NVC.

Time (Sec) ■

Figure 12 Output voltage waveform for asymmetrical multilevel inverter with NVC.

Table 7 THD and other Parameters for the performance of the NVC Techniques.

Type of multilevel Type of No. of Output Current Voltage Total demand Power factor Displacement P.F

inverter modulation cells/phase level THD (%) THD (%) distortion (%) (True) (lag) (displacement factor) (%)

Symmetrical NVC 3 7 7.40 11.98 7.68 0.71812 8.9340

Asymmetrical NVC 3 27 6.65 6.66 6.284 0.8526 1.0240

needed modifications in hysteresis controller are made and it is shown in Fig. 9.

In this controller both flux and torque comparator bands are divided into subsections, and for this type of 27 level based comparators having 702 combinational ways. All these combinational ways will raises the complexity of computations of Controller. But 25 common zero torque error combinations have same type of control action, and here the combination is reduced to 677 by neglecting these 25 common zero torque error combinations and actually it is not a great reduction. By making some need of modifications on MHC, optimized

combinations are selected with a number of 77 including zero torque error combinations. These combinations are selected with individual voltage vectors of space vector hexagon, here the flux and torque bands are divided into symmetrical in level and both controllers are having equal bands. So each and every combination will take the one step variation in vector selection based on the same conventional DTC strategy with the optimal placement i.e., {1, 13}, {2, 12}, {3, 11}, {4, 10}, {5, 9}, {6, 8}, {7, 7}, {8, 6}, {9, 5}, {10, 4}, {11, 3}, {12, 2} and {13, 1} error combinations are having same operational requirement in drive, so all these combinations are having only

Figure 14 Resultant flux wave for proposed DTC.

^ 2000 ■g 1500 v a. 1000 l

0 -500 -1000 -1500 -2000

4 4.5 s Time (Sec) —^

Figure 15 Actual controlled machine speed curve for proposed DTC with the required command.

Figure 16 Three phase current and voltage curves for proposed DTC.

one voltage vector (V14). Similarly all the combinations are optimized with the same logic, and this proposed pattern is shown in Table 5.

This Modified Multiband Hysteresis Controller (MMHC) gives the foremost accessibility to the voltage vector without changing the logic of conventional DTC i.e., only missing

Figure 17 Current wave form during speed reversal condition.

Figure 18 Electromagnetic torque response for required load torque. (a) Symmetrical multilevel inverter with NVC with hysteresis controller. (b) Asymmetrical multilevel inverter with NVC with MHC. (c) Asymmetrical multilevel inverter with NVC with MMHC.

vectors for sector I are 40, without neglecting any effective regions of space vectors (sectors of hexagon) and provide more number of voltage vector than general MHC. Finally the selection of next sector by the DTC is transferred to logic and gating circuit. It will generate gate pulses based on the modulation. These pulses are used to change the operating sequence of switches and results required output for the control action.

Furnishing all these Modified MHC and NVC based Asymmetrical Multilevel Inverter with DTC gives better results in torque ripple reduction, reversal operating conditions and THD reduction by total utilization of vectors with MMHC. The resolution of DTC control action and their result analysis with and without MMHC is discussed in Section 3.

3. Simulation results and analysis

The conventional DTC with NLV-SVC based Asymmetrical CHB multilevel inverter and modified Multiband Hysteresis Controller is simulated with MATLAB/Simulink for the parameters and required values are shown in Table 6 and Fig. 10.

The comparison of symmetrical and asymmetrical multilevel inverter with NVC modulated output signals is illustrated in Figs. 11 and 12 and also the Table 7 presents various parameters related to quality in conversion processes.

The main and simple representation of a proper DTC logic is perfect d-q flux trajectory. Fig. 13 shows that for proposed DTC with faultless smooth circle with 1.01 radius, this value will be computed by the control estimator for the specified machine parameters, and the resultant flux signal is illustrated in Fig. 14.

When the DTC controller controls the machine for required values shown in Fig. 10, the simulated machine speed waveforms are shown in Fig. 15. It is clear by the observation of these speed waveforms have low distortion, when the system is equipped with NVC modulated inverter and MMHC.

Fig. 16 illustrates voltage and current signals, when AS-CHB MLI with NVC and MMHC based DTC controller action occurs, Speed reversal action will be done by changing phase sequence using modulation and it is shown in Fig. 17.

The illustrative comparison between existing and the proposed DTC of Induction Machine's torque waveforms is shown in Fig. 18(a-c), waveform (a) is related to conventional DTC so it has more disturbances in it, but waveforms (b) and (c) are having slight difference on torque ripple value due to modification in MHC. Clean examination of waveform (c), there is a great reduction in ripple magnitude for the proposed system due to availability of more voltage vectors from the existing systems.

One of the major problems in DTC is torque ripples. In the control action generally switching actions are very high, and depend on modulation. The proposed DTC scheme has low torque ripples compared with conventional and MHC based DTC scheme. Approximately torque ripple reduction is 45.8% and 23.2% (No load and Load) from conventional and 4.2% and 6.6% (No load and Load) from MHC based DTC to the proposed MMHC based DTC system and all other parameter comparisons are shown in Table 8. In MHC system some vectors i.e., from 28 to 52 are ineffective when the machine is in sector I position, but in case of MMHC system all these vectors are utilized and they are identified by using detectors. These detector's outputs are high when these vectors from 28 to 52 are identified and they are shown in Fig. 19. In

Table 8 Comparison Table of THD and Torque ripple for general and proposed DTC.

Type of multilevel inverter with NVC Type of controller Torque ripple value (peak to peak) Flux ripple value (Peak to Peak) VTHD (%) ITHD (%)

No Load Load No +ve TL Load -veTL No +ve TL Load -veTL

Symmetrical 3 level Normal hysteresis bands 12 9 0.01 25.68 19.03 18.69 12.28 3.33 2.21

Asymmetrical 27 Level Multiband hysteresis 7 7.5 0.01 20.21 17.23 22.34 2.53 3.19 2.22

Asymmetrical 27 level Proposed multiband hysteresis 6.7 7 0.01 16.94 16.65 22.63 2.09 2.64 2.41

Time (Sec) ■

Figure 19 Identification of error combinations from voltage vectors 28-52 for sector I.

Table 8, observation of —ve TL THD values is very low at 3 Level Inverter based Control strategy, because this type of load condition injects currents in reverse direction and these currents are affected at very low state in 3 level inverter and more for Multilevel depends on its range of levels. Generally these —ve TL conditions are avoided by applying mechanical braking or regenerative strategies almost in every application.

4. Conclusion

This manuscript dealt with a comparison study of a Direct Torque Controlled Induction Motor Drive. Proposed Modified Multiband Hysteresis Controllers have been compared with existing system, in order to find an optimal arrangement with low torque and speed ripples and also to improve output voltage quality with sinusoidal current without filter action.

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Rohilh Balaji Jonnala received the B.Tech.

degree in Electrical and Electronics Engi-vs neering, in 2008, and the M.Tech. degree in Power Electronics and Electric Drives, in 2010 from the Jawaharlal Nehru Technological University Hyderabad, AP, India. He served as an Assistant Professor in MIST & GVVIT Engg. Colleges from 2010 to 2012. He is currently working towards the Ph.D. degree in Electrical and Electronics Engineering in the Department of Electrical and Electronics Engineering, University college of engineering Kakinada, Jawaharlal Nehru Technological University Kakinada, Kakinada, AP, India. His research interests include Electric Motor Drives and the application of Multilevel Inverters.

Ch Sai Babu received the B. E. degree in Electrical and Electronics Engineering, in 1983 from AU College of Engineering Waltair, and the M.Tech. degree in Electrical Machines and Industrial Drives, in 1986 from the Regional Engineering College Warangal, and Ph. D. degree in Reliability Studies of HVDC Converters, in 1996 from Jawaharlal Nehru Technological University Hyderabad, AP, India. He is currently a Professor and Director of Evaluation, Jawaharlal Nehru Technological University Kakinada, Kakinada, AP, India. His research interests include Power Electronics, Power Semi-conductor controlled electric drives, Resonant Converters, Multilevel Converters, Flexible AC Transmission Systems (FACTS), Power Quality and Solar PV Cell Technologies. He published over 200 scientific papers in international journals and conferences.