1

Chapter-1

INTRODUCTION

1.1 Introduction

The conventional method of electric vehicle propulsion involves the use of fuel directly.

The most dominant method for motor vehicle propulsion is internal combustion engine

which uses the direct combustion of a fuel to produce output power. With the ever-

growing number of vehicles and various harmful gaseous emissions by conventional

engine, the living quality of environment is getting degraded day by day and hence

comes the need of an alternative method of electric vehicle propulsion. Use of electric

power directly for traction purposes is a cleaner and cheaper approach in this direction. It

can reduce the gaseous emissions by about 90% as compared to conventional methods.

An electric vehicle (EV), also referred to as an electric drive vehicle, uses one or more

electric motors for propulsion. These vehicles would also enable meeting the demands

for electrical power due to the increasing use of the electronic features to improve

vehicle performance, fuel economy, emissions, passenger comfort, and safety.

Electric vehicles need a fast-response motor, inverter, and control system and must

be able to operate in adverse environmental conditions. Furthermore, the integration

of actuators with power electronics not only improves the overall system

reliability but also reduces the cost, size, etc. In addition to power electronics, the

technology of the electric motor plays a major role in the vehicle’s dynamics and the

type of power converter for controlling the vehicle operating characteristics.

The traditional converter configurations for hybrid electric vehicles (HEV) traction

drive VSI and CSI. The Z-source inverter is one of quite new ideas designated to

renewable energy system, mainly fuel cell and photovoltaic. In the Z-source inverter,

a special Z-network is introduced and shoot-through states may be used in similar

manner as in Current Source Inverter.

ZSI employs a unique impedance network (or circuit) to couple the converter main

circuit to the power source, load, or another converter, for providing unique

2

features that cannot be observed in the traditional voltage and current source

inverters where a capacitor and inductor are used respectively. The Z-source

converter overcomes the conceptual and theoretical barriers and limitations of the

traditional voltage source and current source inverters and provides a novel power

conversion concept. Here is the comparison of Z-source inverter and traditional

converters:

Current source inverter

Voltage source inverter

Z-source inverter

1. As inductor is used in the DC link, the source impedance is high. It acts as a Constant current source.

As capacitor is used in the d.c link, it acts as a low impedance voltage source.

As capacitor and inductor is used in the d.c link, it acts as a constant high impedance voltage source.

2. This is used in only buck or boost operation of inverter.

This is also used in only a buck or boost operation of inverter.

This is used in both buck and boost operation of inverter.

3.Power loss should be high because of filter

Power loss is high Power loss should be low

4. Lower efficiency because of high power loss

Efficiency should be low because of power loss high

Higher efficiency because of less power loss

5. A current source inverter is capable of withstanding short circuit across any two of its output terminals. Hence momentary short circuit on load and mis-firing of switches are acceptable.

A VSI is more dangerous situation as the parallel capacitor feeds more powering to the fault

In ZSI mis-firing of the switches sometimes are also acceptable.

Table 1.1 comparison of VSI, CSI, and ZSI

Hence, from above comparison of Z-source inverter and traditional converters, it can

be deduced that Z-source inverter can be employed efficiently in electric vehicle and

can be used for high power vehicles as well.

3

Chapter-2

Z-SOURCE INVERTER AND ITS CONTROL METHODS

In this chapter, circuit and operating principle of the Z-source inverter is described with

the help of schematic diagrams. The circuit analysis of inverter is presented by making

some simplifying assumptions.

2.1 Z-Source Inverter:

To overcome the above problems of the traditional V-source and I-source converters,

this paper presents an impedance-source power converter (abbreviated as Z-source

converter) and its control method for implementing dc-to-ac, ac-to-dc, ac-to-ac, and dc-

to-dc power conversion.

Fig 2.1 Basic topology.

Fig. 2.1 shows the general Z-source converter structure proposed. It employs a unique

impedance network (or circuit) to couple the converter main circuit to the power

source, load, or another converter, for providing unique features that cannot be

observed in the traditional V- and I-source converters where a capacitor and inductor

are used, respectively. The Z-source converter overcomes the above-mentioned

conceptual and theoretical barriers and limitations of the traditional V-source

converter and I-source converter and provides a novel power conversion concept. In

Fig. 2.2, a two-port network that consists of a split-inductor L1 and L2 and capacitors

and connected in X shape is employed to provide an impedance source (Z-source)

coupling the converter (or inverter) to the dc source, load, or another converter. The

4

dc source/or load can be either a voltage or a current source/or load. Therefore, the

dc source can be a battery, diode rectifier, thyristor converter, fuel cell, an inductor, a

capacitor, or a combination of those. Switches used in the converter can be a

combination of switching devices and diodes this paper focuses on an application

example of the Z-source converter: a Z-source inverter for dc-ac power conversion

needed for fuel-cell applications.

.

Fig. 2.2 ZSI for fuel cell applications.

Fig. 2.2 shows a Z-source inverter for such fuel-cell applications, which can directly

produce an ac voltage greater and less than the fuel-cell voltage. A diode in series with

the fuel cell is usually needed for preventing reverse current flow.

The unique feature of the Z-source inverter is that the output ac voltage can be any

value between zero and infinity regardless of the fuel-cell voltage. That is, 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.

2.2 Operation, And Control:

To describe the operating principle and control of the Z-source inverter in Fig. 2.1, let

us briefly examine the Z-source inverter structure. In Fig. 2.1, the three-phase Z-

source inverter bridge has nine permissible switching states (vectors) unlike the

traditional three-phase voltage source inverter that has eight. The traditional three-

phase voltage source inverter has six active vectors when the dc voltage is impressed

across the load and two zero vectors when the load terminals are shorted through

either the lower or upper three devices, respectively.

5

However, the three-phase Z-source inverter bridge has one extra zero state (or vector)

when the load terminals are shorted through both the upper and lower devices of

any one phase leg (i.e., both devices are gated on), any two phase legs, or all three

phase legs. This shoot-through zero state (or vector) is forbidden in the traditional

voltage source inverter, because it would cause a shoot-through. We call this third

zero state (vector) as the shoot- through zero state (or vector), 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 possible. This shoot-

through zero state provides the unique buck-boost feature to the inverter.

Fig. 2.3 shows the equivalent circuit of the Z-source inverter shown in Fig.2.1 when

viewed from the dc link. The inverter bridge is equivalent to a short circuit when the

inverter bridge is in the shoot-through zero state, as shown in Fig. 2.4, whereas the

inverter bridge becomes an equivalent current source as shown in Fig. 2.5 when in

one of the six active states. Note that the inverter bridge can be also represented by

a current source with zero value (i.e., an open circuit) when it is in one of the two

traditional zero states. Therefore, Fig. 2.5 shows the equivalent circuit of the Z-source

inverter viewed from the dc link when the inverter bridge is in one of the eight non

shoot-through switching states.

Fig. 2.3 Equivalent circuit of the Z-source inverter viewed from the dc link

6

Fig. 2.4 Equivalent circuit of the Z-source inverter viewed from the dc link When the inverter bridge is in the shoot-through zero

state

Fig. 2.5 Equivalent circuit of the Z-source inverter viewed from the dc link when the

inverter bridge is in one of the eight non shoot-through switching states All the traditional pulse width-modulation (PWM) schemes can be used to control the Z-

source inverter and their theoretical input–output relationships still hold. In every

switching cycle, the two non shoot-through zero states are used along with two

adjacent active states to synthesize the desired voltage. When the dc voltage is high

enough to generate the desired ac voltage, the traditional PWM of Fig. 2.6 is used.

While the dc voltage is not enough to directly generate a desired output voltage, a

modified PWM with shoot-through zero states will be used to boost voltage.

7

Fig. 2.6 Traditional carrier-based PWM control without shoot-through zero States, where the traditional zero states (vectors) V111 and V000 are generated every

switching cycle and determined by the references

Fig. 2.7 Modified carrier-based PWM control with shoot-through zero states

That are evenly distributed among the three phase legs, while the equivalent

Active vectors are unchanged

8

Each phase leg still switches on and off once per switching cycle. Without change the

total zero-state time interval, shoot-through zero states are evenly allocated into each

phase. That is, the active states are unchanged. However, the equivalent dc-link voltage

to the inverter is boosted because of the shoot-through states. The detailed relationship

will be analyzed in the next section.

With the Z-Source inverter, there are two control freedoms: modulation index and

shoot through duty ratio.

Two control methods are presented: they are simple control and maximum boost

control.

2.2.1 SIMPLE BOOST CONTROL

The simple control uses two straight lines to control the shoot-through states, as shown in

Fig. 2.8. When the triangular waveform is greater than the upper envelope VP, or lower than

the bottom envelope VN, the circuit turns into shoot-through state. Otherwise it operates

just as traditional carrier-based PWM. This method is very straightforward however, the

resulting voltage stress across the device is relatively high because some traditional zero

states are not utilized.

Fig. 2.8 Sketch map of simple control

9

From fig 2.2, assuming that the inductorsL1 andL2 and capacitors C1and C2 have

the same inductance (L) and capacitance(C), respectively, the Z-source network

becomes symmetrical. From the symmetry and the equivalent circuits, we have,

Vc1 = Vc2 = Vc & VL1 = VL2 = VL - (2.1)

Given that the inverter bridge is in the shoot-through zero state for an interval of to,

during a switching cycle T, and from the equivalent circuit, Fig. 2.4, one has

VL = VC Vd = 2Vc VI = 0 -(2.2)

Now consider that the inverter bridge is in one of the eight non shoot through states

for an interval of T1, during the switching Cycle T. From the equivalent circuit, Fig. 2.5,

one has

VL = V0 - Vc Vd = V0 Vi =Vc – VL= 2Vc – V0 - (2.3)

Where V0 is the dc source voltage and T=T0+T1. The average voltage of the inductors

over one switching period (T) should be zero in steady state, from (2.2) and (2.3), thus,

we have

−(2.4)

- (2.5)

Similarly, the average dc-link voltage across the inverter bridge can be found as

follows:

10

The peak dc-link voltage across the inverter bridge is expressed in (2.3) and can be

rewritten as

Where

Is the boost factor resulting from the shoot-through zero state. The peak dc-link voltage

Vi is the equivalent dc-link voltage of the inverter. On the other side, the output peak

phase voltage from the inverter can be expressed as

Where M is the modulation index. Using (2.7), (2.9) can be further expressed as

For the traditional V-source PWM inverter, we have the well known relationship:

Equation (2.10) shows that the output voltage can be stepped up and down by choosing

an appropriate buck–boost factor BB.

11

Fig 2.9 Voltage gain v/s modulation index for simple boost control

BB = M.B = (0~∞). -(2.11)

From (2.1), (2.5) and (2.8), the capacitor voltage can express as

The buck–boost factor BB is determined by the modulation index M and boost factor B.

The boost factor B as expressed in (2.8) can be controlled by duty cycle (i.e., interval

ratio) of the Shoot-through zero state over the non shoot-through states of the inverter

PWM. Note that the shoot-through zero state does not affect the PWM control of the

inverter, because it equivalently produce the same zero voltage to the load terminal.

The available shoot through period is limited by the zero-state period that is determined

by the modulation index.

12

2.2.2 Maximum boost control:

To fully utilize the zero states so as to minimize the voltage stress across the device,

maximum boost control turns all traditional zero states into shoot-through state, as

shown in Fig. 2.10.

Fig. 2.10 sketch map of maximum boost control

Indeed, turning all zero states into shoot-through state can minimize the voltage stress.

However, doing so also causes a shoot-through duty ratio varying in a line cycle, which

causes inductor current ripple this, will require high inductance for low-frequency or

variable-frequency applications.

The shoot through state repeats periodically every (π/3). Assume that the switching

frequency is much higher than the modulation frequency; the shoot-through duty ratio

over one switching cycle in the interval (π/6, π/2) can be expressed as

-(2.13)

The average duty ratio of shoot-through can be calculated by integrating (6) which

yields

13

-(2.14)

From (7), the boost factor B is obtained,

-(2.15)

With this type of control method, the voltage gain can be determined

by the modulation index M

-(2.16)

Fig. 2.11 voltage gain v/s modulation index for maximum boost control.

14

2.3 Conclusion:

Simple boost control and maximum Boost Control are discussed.

After having the brief introduction to two control methods of Z-source inverter, the

detailed simulations of Z-source inverter with simple-boost control and maximum boost

control and selection of switches and element ratings are discussed in next chapter.

15

CHAPTER-3

RATING OF COMPONENTS

3.1 SWITCHING DEVICE SELECTION

In the electric vehicle, two-level inverter will operate from a nominal DC link voltage of 1300V and drive an AC induction motor. Simulations of the steady-state operation of a two-level inverter driving an induction motor model were carried out to investigate the performance of the motor drive system. The power switching component ratings and overall inverter specifications are determined from an analysis of these simulation results.

Figure 1 presents the simulation results showing the waveforms of the inverter output line-to-line voltage and three-phase currents under steady-state operation with a motor load at the nominal fundamental frequency of 50Hz. Motor current is very close to a sinusoidal waveform.

Fig 3.1 Inverter output voltage.

16

Figure 3.2 Inverter output phase current.

As shown in Figure 1, the steady-state RMS value of the per-phase inverter output current is about 63 A when the inverter operates with a maximum motor overload at a frequency of 50Hz. To account for overload current transient conditions, the current rating of the switching device should be at least 1.5 times the value of the steady-state current. Thus the overload rating of the switching device should be at least 96 A RMS. Therefore, each switch should have at least a peak current rating of about 144 A.

The two-level inverter operates from a 1300V DC link power supply. According to the simulation results, the voltage stress experienced by each switching device is 1200V. To allow an adequate voltage margin for inductive di/dt voltage spikes, switching devices and clamping diodes with at least a blocking voltage rating of 2000V need to be selected.

The switching frequency selected for the inverter operation is 4.5 kHz. Hence the best switching device for the given specifications is an IGBT. IGBT has a current rating of up to 500 A and a voltage rating of up to 2 kV. The normal switching frequency of IGBT varies from 1 kHz to 100 kHz. The on-state resistance of IGBT is set to be 10e-3 Ω which results in a forward voltage drop of about 2.5 V.

17

3.2 Calculation of inductor and capacitor values for Z-Source Network:

The original ZSI consists of the conventional VSI with a modified dc link stage. A diode

(D) and an impedance network connected between the variable voltage energy storage

(Es) and the conventional VSI are the main differences in the power circuit.

Design method described here assumes that the capacitor voltage and inductor current

varies linearly with time. This assumption is accurate when the ripples of these variables

are small compared to their average values. Since larger ripple contents increase the

current and voltage ratings of all the devices in ZSI and also increase the harmonic

content in the output ac waveform, practical ZSI are normally designed with large

capacitors and inductors.

The general equations that describe the impedance network can be written as,

Vl = L (dIl/dt) (1)

Ic =C (dVc/dt) (2)

Where Vl and Il are the voltage across and current through any of the two inductors and

Vc and Ic are the voltage and current of any of the two capacitors. The switching

equivalent of ZSI shown in Fig. 1 clearly illustrates these variables. The diode D on its

input side has two switching states as ‘On’ and ‘Off’ and the VSI on its output side has

three switching states as 1, 2 & 3

.

18

Fig. 3 illustrates the linear waveforms of the capacitor voltage and inductor current for

one switching cycle of the dc link. The shoot-through duty ratio is denoted by ds in Fig.

3.By taking the peak ripples and average values of capacitor voltage and inductor

current as ΔVc,Vc,ΔIl and Il respectively, the ripple factors can be written as, kv= ΔVc/Vc

and ki= ΔIl/Il and indeed a re the two ‘design variables’.

Since the average inductor voltage and average capacitor current over a complete

switching cycle in steady-state are zero,

Vc/Es=Il/I0= (1−dS) / (1−2dS) (3)

By substituting (7) in (5) and (6),

L = EsdsTs / (2kiI0) (4)

C=I0dSTS /(2kvES) (5)

By neglecting inverter power losses and equating real power on the dc and ac sides of

the inverter, I0 in the above equation can be written as,

19

I0=3MImcosø /4 (1-dS) -(6)

Where M is the modulation index, Im is the peak phase current on the ac side, Vm is the

peak phase voltage and φ is the power factor angle on the ac side. Substituting for I0 in

(8) and (9),

(7)

Thus, if Es, Im, φ, ds, Ts and M are known, L and C for any control strategy can be

calculated from above equations to result in a desired level of ripples.

Now, for electric vehicle purposes, following values of voltages and current are chosen:

Es= 250V.

Ts=1/9000 sec.

For a 460 V line-line voltage motor, Vm=375.58 V.

Power factor cos ø=.866.

For a motor output torque of 250 Nm and rotor speed =1165 rpm = 122 rad/sec.

Motor output power= 250*122 =30.5 kW.

Slip s= .223

Neglecting stator losses,

Motor input power = 30.5/ (1-s) =30.5/(.776) =39.3kW

Assuming same input power on both side of the inverter,

20

Inverter input power through DC link= Motor input power =39.3 kW.

Now,

Inverter input power = (2Vm/M) Il (1-2dS) =39300

I l = 157.2 A

Here, M = inverter modulation factor = (.6/1.0375)

And dS =.4024

Now since Dc link current is equal to the average inductor current,

Hence Il = 157.2 A.

From equations (7), values of L and C can be calculated for the desired values of

inductor ripple current and capacitor ripple voltage.

For ki = 5% peak-peak,

and kv= 1% peak-peak

L= 7.25 mH and C= 550 µF.

21

Chapter-4

SIMULATION MODEL AND RESULTS

In this chapter the simple boost control of Z-source inverter is simulated under constant

load torque conditions. Sine wave PWM is modified to include the shoot through

envelope to boost the capacitor voltage. Open loop voltage control is used to maintain

DC link voltage.

4.1 Simple boost control of Z-source inverter

Fig 4.1 simulation model of simple boost control of Z-source inverter

22

Fig 4.1 indicates the Z-source inverter simulink model. The power flow can be controlled

using simple L and C combination. The simulink is implemented using parameters as

shown in table 4.1.

Table 4.1 circuit parameters of z-source inverter

Parameters Values Input DC voltage 250 V

L1 , L2 7.2 mH

C1 , C2 550 µF

M(modulation index) .578

Shoot through duty ratio(dS) .4024

Table 4.2 Specifications of 3-ø induction motor

Parameters Value

Rotor type Wound rotor

Nominal power 40 HP

Line-line voltage 460 V

Stator resistance(RS) (Ω) 0.454

Stator inductance(LS) (H) 0.002915

Rotor resistance(Rr) (Ω) 0.7938

Rotor inductance(Lr) (H) 0.002915

Mutual inductance(Lm)(H) 0.1077

Inertia(kg.m^2) 0.1

Pole pairs 2

External resistance (r1 = r2)( Ω) 0.502

23

Fig 4.2 Reference signal for zero shoot through state

Calculation of height of reference signal:

Vi= B Vdc

B = T/ (T – 2 *T0)

Time T is directly proportional to peak height of the triangular wave Htri and T0 is

proportional to (Htri - h) where h is the height of reference signal of zero through state.

B = Htri /(2 *h – Htri)

This implies h = (Htri * Vdc)/Vi + Htri / 2

24

Fig 4.3 Modified PWM generator

Sub-circuit of gate pulses:

When carrier is greater than SSup or less than SSdown, relational operator 3 and relational

operator 4 gives output 1 , hence logical operators output is 1 and s1 ,s3 and s5 are 1

which results in zero-shoot through state.

Fig 4.4 sub circuit of gate pulses with shoot through envelope

25

Explanation of Motor starting using external resistance:

The starting torque of induction motor is less than the maximum torque produced by

the induction motor. In electric vehicle application it is necessary that maximum torque

be produced at the time of starting to overcome inertial force and static friction. In an

induction motor, maximum torque is produced at the starting of the induction motor

i.e. at slip = 1 if the value of rotor resistance is equal to a specified value given by:

Rr = (Re2+X2)1/2

Now, motor is started with the above value of resistance in the form of several

resistances whose sum is equal to the above specified value of resistance. Initially, all

the resistances are connected across the rotor terminals. As the motor starts running,

these resistances are cut out in steps and as motor starts running at the rated speed, all

the resistances are cut out and equivalent rotor resistance is equal to the original value.

Here Re and X are the values found out by the thevenin’s equivalent of induction motor

circuit.

26

At the starting of motor, both switches s1 and s2 are non-conducting, hence both the

resistances r1 an r2 are connected across each phase of motor. Now, after the motor

has achieved sufficient speed, in the first step resistance r2 is disconnected by turning

on the switch s1 through manual switch. In the second step, resistance s2 is

disconnected by turning on the s2. Hence, the equivalent resistance across the rotor

terminals is the original rotor resistance.

Fig 4.5 Induction motor starting circuit

4.2 Maximum boost control

M= 0.75.

All other components and their ratings are same. Also output and input voltage are kept

same to compare the results with simple-boost control.

27

Gate drive circuit for maximum boost control:

Fig. 4.6 Gate drive circuit for maximum boost control.

28

The lower part in the diagram generates signal for zero shoot through states.

When carrier is greater than all three reference signals relational operator 5, 6, 7

gives output 1 and logical operator1 gives output 1.

If carrier is lower than all three reference signals then relational operators 5,6,7

gives output 0 , these are changed In to 1 by corresponding NOT gates and

logical operator2 gives output 1 .

If any logical operator 1 or 2’ s output is 1, output of OR, in the lower part of

diagram is 1, this will turn on all the switches (1 to 6) causing zero-shoot through

state.

4.3 Simulation results

4.3.1 Simulation results for simple boost control

The induction motor was run for a constant load torque of 250 Nm. The modulation

index of the inverter is set to M= 0.6 and shoot-through zero state index was set to

ds=0.4024. A 250 V battery was used to supply power to the z-source network. The line

voltage obtained was 460 V.

Fig 4.7 PWM signals for inverter including shoot-through envelope

29

Fig 4.8 DC link voltage (is equal to capacitor voltage).

The capacitor voltage above is equal to the DC link voltage. The first disturbance refers

to the start of simulation when the voltage develops. The second disturbance refers to

switching on of switch s2 and third disturbance refers to switching on of switch s1.

30

Fig 4.9 Inductor current.

Fig 4.10 Inverter output voltage.

31

Fig 4.11 Inverter output phase current.

Fig 4.12 Electromagnetic torque developed in motor

32

Fig 4.13 Rotor speed of induction motor.

The change in the level of speed at 0.6 sec and 1 sec is due to the switching on of the

switches s2 and s1 respectively. Since effective rotor resistance is decreased, the

torque-speed curve of the motor changes and higher speed can be obtained at the same

torque.

Voltage stress in switches:

From figure 4.14, it is clear that during conduction, average voltage stress across

switches is 1300 V i.e. the DC link voltage.

Operating conditions Average voltage stress Output voltage VL-L M= .6,VDC=250 V 1300 V 460 V

Table 4.3

33

4.3.2 Simulation results for maximum boost control

Fig 4.14 Dc link voltage for maximum boost control.

Fig 4.15 Output motor torque for maximum boost control.

34

Fig 4.16 Inductor current for maximum boost control.

Fig. 4.17 Motor speed (RPM) for maximum boost control.

35

Voltage stress for switches:

Operating conditions Average voltage stress Output voltage VL-L

M=0.75, VDC=250 V. 1100 V. 460 V

Table 4.4

4.4 Comparison of simple-boost control and maximum –boost control:

The simulation results were obtained for both simple boost control and maximum-boost

control of Z-source inverter fed induction motor. For same value of input DC voltage,

inductor and capacitor, output voltage, and same motor ratings, the following results

were obtained:

1. The rotor speed and torque of the motor was same in both the cases.

2. The DC link voltage and hence, voltage stress for devices was less for maximum-

boost control of Z-source inverter.

3. The harmonic content was slightly greater in maximum boost control as

compared to simple-boost control.

Conclusion:

In this chapter simulation and result of Z-source inverter fed induction motor drive

under constant load condition has been presented. After analyzing the current and

voltage waveforms, it is observer that Z-source inverter can boost the capacitor voltage

to a high value and reduces the size of the DC supply. Also, the feedback provided to

control the capacitor voltage, keeps the value of DC link voltage constant even if the DC

supply changes (i.e. the battery gets discharged). Hence the voltage supplied to the

battery is constant and hence, there is no disturbance in the motor performance. It was

also observed that the ripple content of the capacitor voltage and inductor current was

under specified limits.

36

Chapter-5

CONCLUSION AND SCOPE OF FUTURE WORK

5.1 CONCLUSION

The Z-Source Inverter can boost the voltages without additional stage, which increase

the system efficiency and reduce the cost. The Voltage Boost operation is possible

because of the Impedance network present in the Z-Source Inverter. The operating

principle and circuit analysis of Z-source is presented along with a method to find the

values of Z-source network inductor and capacitor for a given output and input voltage

and corresponding shoot-through state for obtaining the desired output. A method for

keeping the DC link voltage constant is also presented. Various PWM techniques are

also discussed in chapter 2 and modeling and simulation is done for simple boost control

and maximum boost control of the Z-source network in chapter 4.

The simulation result for simple-boost control and maximum boost control for

performance and efficiency of Z-source inverter shows that maximum boost control

minimizes the voltage stress on the switches for the same input and output voltage.

Also, for same voltage stress, a lower DC supply can be used in the case of maximum

boost control technique. But, steady state DC link voltage and torque ripple is more in

case of maximum boost control.

Since the present Z-source inverter is a single stage power conversion where impedance

network is used for buck and boost the voltage, and also reduces complexity of

controlling technique. The proposed inverter can be used as power conditioning

equipment for renewable energy systems.

5.2 SCOPE FOR FUTURE WORK

In this report, a Z-Source inverter fed induction motor for electric vehicle applications

was presented. Only the power flow from DC source side to motor side was taken in to

37

account. However a dc-dc converter is needed to accept a reverse power flow and to

reduce the regenerative voltage to a battery voltage.

Hence, the proposed inverter circuit can be modified to include a current fed Z-source

dc-dc converter to make the Z-source circuit bi-directional in nature.

Also, PID controller can be included for capacitor voltage control with an excellent

transient performance which enhances the rejection of disturbance, including the input

voltage ripple and load current variation, and have good ride-through for voltage-sags.

38

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for Adjustable speed drives,” IEEE Power Electronics Letters, 1(2), June 2003, pp. 33–35.

*3+ Fang. Z. Peng, “Z-source inverter for adjustable speed drives,” IEEE/PESC Aachen,

2004, pp. 249–254.

[4] Fang. Z. Peng, Miaosen Shen, and Zhaoming Qian, “Maximum boost control of the Z-

source inverter,” published in IEEE transactions on Power Electronics, 20(4), 2005.

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APPENDIX Specifications of Induction motor is used for simulation

Power rating: 40HP

Stator voltage: 460 V

Frequency: 50 HZ

Number of poles: 4

Stator resistance: 0.454 ohm/phase

Stator leakage inductance: 0.0029 H/phase

Rotor resistance: 0.79 ohm/phase

Rotor leakage inductance: 0.0029 H/phase

Mutual inductance: 107.7e-03 H

Inertia: 0.10 kg.m2