performance analysis of z -source inverter fed … · voltage. pwm technique which is used as to...

International Journal On Engineering Technology and Sciences – IJETS™

ISSN (P): 2349-3968, ISSN (O): 2349-3976

Volume 2 Issue 2, February 2015

77

PERFORMANCE ANALYSIS OF Z-SOURCE INVERTER FED INDUCTION MOTOR DRIVE USING FUZZY LOGIC CONTROLLER

C.Pitchai (Assistant Professor) P.A.Prassath (Assistant Professor) Department of Electrical & Electronics Engineering Department of Electrical & Electronics Engineering A.S.L.Pauls College of Engineering and Technology A.S.L.Pauls College of Engineering and Technology

Coimbatore, India Coimbatore, India [email protected] [email protected]

ABSTRACT

This paper presents z-source inverter fed induction motor drive using fuzzy logic controller. The Z-source

inverter is a recently invented a new power conversion concept mainly developed for fuel cell vehicular applications.

The Z source inverter system can boost the given input voltage by controlling the boost factor, to obtain the maximum

voltage. PWM technique which is used as to given the gating pulse to the inverter switches. The four-switch inverter,

having a lower number of insulated gate bipolar transistors (IGBTs), has been studied for the possibility of reducing the

inverter cost. These inverters use a unique impedance network coupled between the power source and inverter circuit, to

provide both voltage buck and boost properties, which cannot be achieved with conventional voltage source and current

source inverters. It has single stage power conversion, high performance, minimal component count, increased

efficiency, improved power factor and reduced cost. The obtained AC voltage must be pure sinusoidal but it can’t

obtained because the harmonic content are highly present. Higher order harmonics are eliminated by with the help of

filters. Here impedance network act as a filter to reduce the lower order harmonics. This paper describes the design of

Fuzzy logic controller for Z-source inverter.

1.INTRODUCTION

Inverters are the dc to ac converters. The input dc

supply is either in the form of voltage or current is

converted in to variable output ac voltage. The output ac

voltage can be controlled by varying input dc supply or

by varying the gain of the inverter. In the late nineties,

Fang Zheng Peng popularized the concept of the Z-

Source Converters, which employ a unique impedance

network (or circuit) to couple the Converter with the

main circuit and then fed to the power source. They

provide unique features that cannot be obtained in the

traditional voltage-source and current-source converters

which use capacitor and inductor, respectively. The

conceptual and theoretical barriers and limitations of the

traditional voltage-source converter and current-source

converter are overcome by the Z-source converter

providing a novel power conversion concept that can be

applied to all dc-to-ac, ac to-dc, ac-to-ac, and dc-to-dc

power conversion.

Fig. 1 Proposed Block Diagram

The four-switch inverter topology is attractive

cost wise when it is compared with conventional six-

switch voltage source inverters. In the four-switch

inverter, one motor terminal is connected to the center

tap of the dc-link capacitors so that is utilizes two less

insulated gate bipolar transistor (IGBT) switches.

However, four-switch inverters are known to have

several disadvantages compared to normal six-switch

inverters: the voltage utilization factor is halved

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compared to the six-switch inverter.Fig.1 shows the

proposed block diagram. Specifically, the peak phase

voltage of the four-switch inverter is , while that of six-

switch inverters is . On the other hand, capacitor center

tap voltage is fluctuating, and it destroys the balance

among the motor phase currents. The reason for this is

that current flow through a capacitor either increases or

decreases the voltage steadily for each half cycle.

Therefore, the voltage fluctuation increases as the load

torque becomes higher or the frequency becomes lower.

The unbalanced motor current leads to an inverter failure

and torque pulsation.

2. INVERTER

An inverter is an electric device that converts DC

to AC, the converted AC can be at any required voltage

and frequency with the use of switching device and

control circuits. Solid state inverters have no moving

parts and are used in a wide range and application, from

small switching power supplies in computers, to large

electric utility high voltage dc applications that transport

bulk power. Inverters are commonly used to supply AC

power from DC sources such as solar panel or batteries.

Inverters are used in various applications such as

induction motor drives, UPS, standby power supplies,

induction heating etc. Normally they are used for high

power applications.

A) Principle

The output voltage waveform of the inverter can be

square wave, quasi-square wave or low distorted sine

wave. The output voltage can be controlled (i.e.

adjustable) with the help of drives of the switches.

The pulse width modulation (PWM) techniques are

most commonly used to control the output voltage of

inverters. Such inverters are called PWM inverters.

The output voltage of the inverter contains harmonics

whenever it is non-sinusoidal. These harmonics can

be reduced by using proper control schemes.

The inverters can be classified as voltage source

inverters or current source inverters. When input DC

voltage remains constant, then it is called Voltage

Source Inverter (VSI) or Voltage Fed Inverter (VFI).

When input current is maintained constant, then it is

called Current Source Inverter (CSI) or Current Fed

Inverter (CFI). Sometimes, the DC input voltage to

the inverter is controlled to adjust the output. Such

inverters are called variable DC link inverters.

B) Z Source Inverters

Z source inverter operated with the combination

of VSI(Voltage Source Inverter) and the CSI(Current

Source Inverter). Normally the traditional inverters

convert the DC voltage in to AC voltage.

Fig 2.Basic circuit of Z source inverter

Z source inverter not like the traditional inverter

it is buck or boost the voltage at the maximum level. The

impedance network is connected which is used to boost

the voltage to maximum level and it act as a filter. The

structure is as shown in Figure 2. Z-source inverters are

single-stage electronic power converters which have both

voltage-buck and boost capabilities. A Z-source inverter

is proposed, which can operate at wide range load (even

no-load) with small inductor, eliminating the possibility

of the dc-link voltage drops, and simplifying the inductor

and controller design. The Z-source inverter is a buck–

boost inverter that has a wide range of obtainable

voltage. The traditional V- and I-source inverters cannot

provide such feature. This shoot-through zero state is

forbidden in the traditional V-source inverter, because it

would cause a shootthrough. The shoot-through zero

state, which can be generated by seven different ways:

shoot-through via any one phase leg, combinations of

any two phase legs, and all three phase legs. The Z-

source network makes the shoot-through zero state and


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non shoot through switching state is possible. This shoot-

through zero state provides the unique buck-boost feature

to the inverter.

C) Inductor selection

During traditional operation mode, when there is

no shoot-through, the capacitor voltage is always equal to

the input voltage; therefore, there is no voltage across the

inductor and only a pure dc current going through the

inductors. The purpose of the inductor is to limit the

current ripple through the devices during boost mode

with shoot-through. The average current through the

inductor is

IL=P/Vin

During shoot-though, the inductor current increases

linearly, and the voltage across the inductor is equal to

the voltage across the capacitor; during non-shoot-

through modes (six active modes and the two traditional

zero modes), the inductor current decreases linearly and

the voltage across the inductor is the difference between

the input voltage and the capacitor voltage.

L1=L2=fsw Vc/∆Il D) Capacitor Selection

The purpose of the capacitor is to absorb the

current ripple and maintain a fairly constant voltage so as

to keep the output voltage sinusoidal. During shoot-

through, the capacitor charges the inductors, and the

current through the capacitor equals the current through

the inductor. Therefore, the voltage ripple across the

capacitor can be roughly calculated by

C1=C2=Iav Fsw/∆Vl

3. Z-SOURCE INVERTER

The new impedance-source power inverter has been

recently invented, eliminates all problems of the

traditional V-source and I-source inverters. It is being

used in ac/dc power conversion applications. Fig.3

shows the general Z-source converter structure. The

power source can be either voltage source or current

source.

Fig. 3 Z-Source inverter structure

The Z-source inverter consists of a unique impedance

network which couple the converter main circuit to the

power source, load, or other converter, for providing

unique features that cannot be observed in the traditional

VSI and CSI inverters. The impedance network consists

of two inductors and two capacitors connected to each

other as shown in the figure forms the second order filter

network. The values of both inductor and both capacitor

are equal. The two inductors can be two separate

inductors or two inductors inductively coupled to each

other on a single core. For size and cost reduction film

capacitors of desired value and voltage rating can be

selected.

4. PWM TECHNIQUE

The introduction and wide acceptance of ZSI as

an alternative for traditional voltage source and current

source inverters (VSI/CSI), the modified switching

schemes from the traditional schemes has reached the

point where the further improvements in firing the

switches and inserting the shoot through states bring

crucial benefits. In addition to the four active switching

states for the VSI, ZSI has seven shoot-through zero

states, when the positive and negative switches of a same

phase leg are simultaneously switched on. This shoot-

through state is harmful in VSI/CSI and can result short

circuiting and damaging of entire application.ZSI is

suitable for the applications with unstable power supply

such as fuel cell, wind power, photovoltaic etc. Same

pulse width modulation (PWM) logics and methods of

VSIs can be adapted to a switch a ZSI with slight

modifications. The distribution of the shoot-through in

the switching waveforms of the traditional PWM concept

is the key factor to control the ZSI. The DC link voltage


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high performance (diagonal capacitor voltage),

controllable range of ac output voltage, voltage stress

across the switching devices and harmonic profile of the

ac output parameters are purely based [9].

4.1Types Of PWM Techniques

There are number of control methods have been

presented in recent years that include sinusoidal pulse

that include

1. Sinusoidal Pulse Width Modulation Techniques

2. Modified Space Vector Modulation Techniques.

The various PWM control algorithms are

1. Simple Boost Control (SBC)

2. Maximum Boost Control (MBC)

3. Maximum Constant Boost Control (MCBC)

4. Traditional Space Vector Modulation

(TSVPWM)

5. Modified Space Vector Modulation

(MSVPWM)[16]

A Z-source inverter is proposed, which can operate at

wide range load (even no-load) with small inductor,

eliminating the possibility of the dc-link voltage drops,

and simplifying the inductor and controller design. The

Z-source inverter is a buck–boost inverter that has a wide

range of obtainable voltage. The traditional V- and I-

source inverters cannot provide such feature. This shoot-

through zero state is forbidden in the traditional V-source

inverter, because it would cause a shoot through. The

shoot-through zero state, which can be generated by

seven different ways: shoot-through via any one phase

leg, combinations of any two phase legs, and all three

phase legs. The Z-source network makes the shoot-

through zero state and non shoot through switching state

is possible. This shoot-through zero state provides the

unique buck-boost feature to the inverter.

5. FUZZY LOGIC CONTROLLER

The Fuzzy Logic Controller (FLC) requires that

each control variables which define the control surface be

expressed in fuzzy set notations using linguistic labels.

The Fuzzy logic controller is appropriate for nonlinear

control because it does not use complex mathematical

equation. Fuzzy controller is a non-linear controller that

does not require a precise mathematical model for its

design [12]. In essence, fuzzy controller is a linguistic-

based controller that tries to emulate the way a human

thinks in solving a particular problem. The basic fuzzy

logic control system is composed of a set of input

membership functions, a rule-based controller, and a

defuzzification process.

The fuzzy logic input uses member functions to

determine the fuzzy value of the input. There can be any

number of inputs to a fuzzy system and each one of these

inputs can have several membership functions. The set of

membership functions for each input can be manipulated

to add weight to different inputs. The output also has a

set of membership functions. These membership

functions define the possible responses and outputs of

the system [15]. The fuzzy inference engine is the heart

of the fuzzy logic control system. It is a rule based

controller that uses If-Then statements to relate the input

to the desired output [13].

The fuzzy inputs are combined based on these

rules and the degree of membership in each function set.

The output membership functions are then manipulated

based on the controller for each rule. All of the output

member functions are then combined into one aggregate

topology. The defuzzification process then chooses the

desired finite output from this aggregate fuzzy set. There

are several ways to do this such as weighted averages,

centroids, or bisectors. This produces the desired result

for the output. FLC is the combination of various

different processes which are shown above in the Fig.4.It

means a fuzzy logic controller comprises of numbers of

methods [14] which are described below in stepwise

form. Here the processes are explained in general format

as explained above are described in detail below:

Fig.4 Structure of fuzzy logic system


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A. Fuzzification

Fuzzy logic uses linguistic variables instead of

numerical variables. In a control system, error between

reference signal and output signal can be assigned as (for

example) Negative Big (NB), Negative Medium (NM),

Negative Small (NS), Zero (ZE), Positive small (PS),

Positive Medium (PM), Positive Big (PB). The triangular

membership function is used for fuzzifications. The

process of fuzzification converts numerical variable (real

number) to a linguistic variable (fuzzy number).

B. Rule Elevator

Conventional controllers like PI and PID have

control gains which are numerical values. Fuzzy logic

controller uses linguistic variables instead of the

numerical values. The linguistic variables of error signal

taken as input (en) and output is represented as in the

form of degree of membership functions.

C. Defuzzification

The rules of fuzzy logic generate demanded

output in a linguistic variable, according to real world

requirements. Linguistic variables have to be transformed

to crisp output. The choices available for defuzzification

are numerous. So far the choice of strategy is a

compromise between accuracy and computational

intensity.

6. SIMULATION RESULTS

Simulation is performed using

MATLAB/SIMULINK software. Simulink library files

include inbuilt models of many electrical and electronics

components and devices such as diodes, MOSFETS,

capacitors, inductors, motors, power supplies and so on.

The circuit components are connected as per design

without error, parameters of all components are

configured as per requirement and simulation is

performed. Maximum constant boost control with third

harmonic injection method is used for PWM generation

and simulation. The complete simulation diagram is

shown in the figure 5. The component values of Z-source

inverter depends on switching frequency only. These

component values are chosen as per design guidelines in

[1] and [3]. For this circuit L1 = L2 = 4mH and C1 = C2

= 1000uF. The purpose of the system is to produce

230Vrms line to line voltage. For PWM generation the

carrier frequency is set to 10 KHz and modulating

reference signal frequency is set to 50Hz. The

modulation index is 0.8 and the input DC voltage is

188V. Maximum constant boost PWM with third

harmonic injection is generated using PWM generator

and logic circuit. The PWM generator block generates

normal three phase PWM waveforms for a given carrier

frequency.

Using triangular function, comparator and adder

repeated shoot-through pulses are generated. These

shoot-through pulses are evenly spread in all the three

phase PWM waveform using OR logic function. The

detailed analysis is given below.

Fig. 5 Simulation configuration

The shoot-through duty ratio can be is calculated as T0/T

= 0.308.

The boost factor = B = 2.593

Average dc link voltage = Vdcl = 337V

Peak dc link voltage = Vdcl = 2.593 * 188 = 487V

Peak ac output voltage = Vacp = 0.8*2.593*188/2 =

194.5V

RMS ac output voltage Vac = 137.5V

Output line to line rms voltage = *137.5 = 238V

The buck-boost factor = BB = 0.8*2.593 = 2.075

The capacitor voltage = Vc = 337V


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Gain of inverter = G = 2.075

The voltage gain of inverter obtained in above

analysis is 2.075. As we increase the shoot-through time

interval (T0), the boost factor will increase and this will

increase the inverter voltage gain. Thus inverter boost

factor and voltage gain are depends on the shoot-through

time. The simulation results with the same input voltage

and carrier frequency are shown in following Figures,

which agrees well with the analysis and theoretical

results. For a traditional inverter, to obtain the output

voltage of 230Vrms with modulation index of 0.8, 486V

dc voltage is required this is undesirable since it will

require additional voltage booster circuit. Figure 10

shows input dc voltage applied to Z-source inverter is

188V.

The capacitor voltage is the average dc link voltage

remains almost constant about 337V as shown in figure

8. Thus the input voltage (188V) is boosted (337V) and

applied as dc link voltage. The peak value of this dc link

voltage appears as input voltage across the main inverter

circuit. The output dc link voltage across Inverter Bridge

appears as shown in the figure 9. The peak dc link

voltage remains almost constant about 480V. It is

observed that during shoot-through state dc link voltage

becomes zero since all devices in main inverter are

switched on simultaneously, short circuiting the dc link.

Fig. 6 Input DC voltage = 188V

Fig. 7 Capacitor voltage = 337V

Fig.8. Peak dc Link voltage across inverter Bridge = 480V

Three phase stator current waveforms and stator

voltage for a given load condition is shown in the figure

9 and 10 respectively. Stator current waveforms are

observed to be very smooth sinusoidal waveform as

compared to the traditional PWM inverter.

Fig. 9 Three phase stator current

Fig.12 shows the simulation and experimental results of

diode voltage and inductor current. The diode is reverse

biased by capacitor voltage during shoot-through when


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all the six switches are turned on, blocking the reverse

flow of current. Also, we can see that during the shoot-

through period, the capacitor voltage becomes equal to

the inductor voltage. The capacitor charges the inductor

so that the inductor current increases during this time

and releases its energy during active state

Fig. 10 line to line stator voltage

Fig. 11 Diode voltage and inductor current

Fig. 12 Speed Variation

This is the basic property of the Z-source inverter. Due to

this operational behavior, z-source inverter can boost the

output voltage to any value greater than input voltage.

The simulation result for speed variation of the induction

motor is shown in the following figure 12. Initially speed

of induction motor increase linearly where at that time

the motor fetches more current so as to maintain the

torque. Under steady state condition the maximum speed

of induction motor is observed to be about 157rad/s. In

terms of rpm the maximum speed is 1500rpm

7. CONCLUSION

The Z-source converter overcomes the conceptual

and theoretical barriers and limitations of the traditional

voltage-source converter and current-source converter

and provides an advanced power conversion concept.

The Z-source inverter system can produce an output

voltage greater than the dc input voltage by controlling

the shoot-through duty ratio, which is impossible for the

traditional ASD systems. In this work, described the

operating principle, analyzed the circuit characteristics,

and demonstrated its concept and superiority. Different

PWM techniques and their comparison are presented.

Maximum constant boost control method is more

advantageous PWM control method among the other

PWM control methods. Maximum constant boost with

third harmonic injection PWM control method increases

output voltage boost while minimizing voltage stresses

across switching devices. It allows over-modulation

where modulation index can be varied from 0.57 to

1.154. Z-Source inverter fed IM drive system is

simulated using Simulink software using above described

PWM method.

In future Performance Analysis of Z-Source fed

induction motor using Nero Fuzzy, genetic algorithm,

Renewable energy etc..,

8. REFERENCES

[1] Fang Zheng Peng, “Z- Source Inverter”, IEEE

Transaction on Industry Applications. 39: 2003,2.

Wuhan,China.

[2] Miaosen Shen, Jin Wang, Alan Joseph, Fang Zheng Peng,

Leon M. Tolbert, and Donald J. adams, “Constant Boost

Control of the Z-Source Inverter to Minimize Current

Ripple and Voltage Stress”, IEEE Transactions on

industry application vol. 42, no. 3, May/June 2006

[3] S. Rajakaruna, Member, IEEE and Y. R. L. Jayawickrama,

“Designing Impedance Network of Z-Source Inverters”

IEEE Transactions on industry application.

[4] G. Pandian and S. Rama Reddy, “Embedded Controlled Z

Source Inverter Fed Induction Motor Drive” IEEE

transaction on industrial application, vol.32, no.2,

May/June 2010.

[5] K. Srinivasan and Dr. S. S. Das, “Performance Analysis of

a Reduced Switch Z-Source Inverter fed IM Drives”,

Journal of Power Electronics, Vol. 12, No. 2, May/June

2010

[6] Omar Ellabban, Joeri Van Mierlo and Philippe Lataire,

“Comparison between Different PWM Control Methods


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for Different ZSource Inverter Topologies” IEEE

Transactions on industry application, May/June 2010

[7] K.Niraimathy, S.Krithiga, “A New Adjustable-Speed

Drives (ASD) System Based On High-Performance Z-

Source Inverter”, 978-1-61284-379-7/11 2011 IEEE, 2011

1st International Conference on Electrical Energy Systems

[8] Muhammad H. Rashid, “Power Electronics”, Second

Edition, Pearson Education.

[9] Sivaraman.P, A. Nirmalkumar, “Analysis of T-Source

Inverter with Various PWM Schemes” in European

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213,2012

[10] Omar Ellabban, Joeri Van Mierlo, and Philippe

Lataire ,”Experimental Study of the Shoot-Through Boost

Control Methods for the Z-Source nverter” in The department

of Electrical Engineering and Energy Technology (ETEC),

Vrije Universiteit Brussel (VUB), 1050 Brussels, Belgium

[11] Sivaraman.P, A. Nirmalkumar, “ Modelling and

Simulation of Photovoltaic Array fed T-Source Inverter”

in International Conference on Sustainable Energy and

Intelligent System ,2011

[12] Wing-Chi So, Chi K. Tse, and Yim-Shu Lee,

“Development of a Fuzzy Logic Controller for DC/DC

Converters: Design, Computer Simulation, and

Experimental Evaluation,” IEEE Trans. Power. Electron.,

vol. 11, NO. 1, pp. 24-32, 1996.

[13] Chuen Chien Lee, “Fuzzy Logic in Control Systems:

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[14] Juhng-Perng Su, “A generic stable two-input single-output

fuzzy control scheme for nonlinear systems”, Industrial

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[15] J Choi, S.W. Kwak and B. K.Kim, “Design and

Stability Analysis of Single-Input Fuzzy Logic Controller”,

IEEE Trans. Sys., Man & Cybernetics-Part B: Cybernetics,

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performance analysis of z -source inverter fed … · voltage. pwm technique which is used as to...

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