COMPARATIVE ANALYSIS OF TRANSFORMER

BASED Z SOURCE INVERTER (TZSI) AND

IMPEDANCE SOURCE INVERTER (ZSI) Dr.A.Radhika1,Dr.P.Maruthupandi2,Dr.J.Karthika3

1Assistant Professor, Department of EEE, Sri Krishna College of Engineering and

Technology, Tamil Nadu, India. 2Assistant Professor, Department of Electrical Engineering, Government College of

Technology, Tamil Nadu, India. 3Assistant Professor, Department of EEE, Sri Krishna College of Engineering and

Technology, Tamil Nadu, India.

Abstract The performance analysis of Transformer based Z source Inverter (TZSI) and

Impedance Source Inverter (ZSI) is examined in this paper. The simple boost control

technique is incorporated to control both inverters. The efficient way of single stage

power conversion is possible with these two inverters. However ZSI suffers from

high voltage stress, large shoot through period requirement and low voltage gain

during high modulation index conditions. In order to overcome these drawbacks,

TZSI has been developed to achieve high voltage gain with high modulation index

condition with small shoot through period. The high boosted voltage is achieved by

adjusting the turn’s ratio of transformer and duty ratio of the inverter. In this paper

comparative analysis has been made between ZSI and TZSI using MATLAB

Simulink software.

Keywords: Z Source Inverter, Trans Z Source Inverter, shoot through

period, Simple boost Control and PWM inverter.

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1. Introduction

Traditional inverters like Voltage-Source Inverters (VSIs) and Current-

Source Inverters (CSIs) have their own boundaries and issues. In VSIs: 1) the

acquirable ac output voltage cannot be beyond the dc source voltage. Hence a dc–dc

boost converter is essential to get higher ac output voltage. 2) Insertion of dead time

is mandatory to prevent the shoot-through between upper and lower switches of

each phase leg. On the other hand, it influences waveform distortion. In CSIs: 1)

their ac output voltage cannot be inferior than the dc input voltage. 2) Overlap time

between phase legs is essential to avoid the open circuit of all the switching devices.

The ZSI [1] and the quasi-Z-source inverters (qZSI) [2], [3], can defeat the

abovementioned problems. They utilize the shoot-through period of the inverter to

boost voltage in the VSIs and buck the voltage in the CSIs. Hence, buck–boost

operation is obtained with a single-stage power conversion. They also enhance the

resistance of the inverters to the EMI noise [4], which may create misgating and

shoot-through to wipe out the conventional VSIs and CSIs. The ZSI can have

infinite voltage boost gain theoretically. The voltage gain is higher for the smaller

modulation index. Applications like grid-connected photovoltaic (PV) generation,

fuel cell power conversion and wind energy conversion a low dc source voltage has to

be boosted to a advisable ac output voltage. A high voltage stress imposed on the

inverter during small modulation index. Several types of pulse width modulation

(PWM) methods [5], [6] have been built-up to obtain voltage gain as much high as

possible. Hence, restraining the voltage stress across the switching devices.

The maximum boost control PWM method [5] achieves the maximum voltage

gain by turning all the zero to shoot-through zero states in the traditional VSIs.

However, it produces low-frequency ripples in the output voltage. Therefore, the

constant boost control method [6] has been developed to eliminate low frequency

ripples. Here the requirement of L and C in the z source network is reduced with

slight scarification of voltage gain. In recent times, some source networks were

implemented in [7]–[10] for the rise of output voltage gain. One of the most popular

modified ZSI is TZSI [7]. It increases voltage gain with minimum component usage.

Two transformer windings and one capacitor of impedance network is utilized in the

form of T shape in the TZSI. The diode is replaced with a bidirectional conducting,

unidirectional blocking switch [11] to have bidirectional operation. The trans-Z-

source inverters succeed to their exceptional features, and it can be controlled by

the PWM methods which are all applicable to the Z-source inverters. In this paper

simple boost control method is implemented for both inverters and comparative

analysis made between them with the help of simulation results. The simulation is

done in MATLAB/Simulink platform.

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2. Z Source Inverter

The Z source inverters are paying more attention than the other

inverters because it defeats the drawbacks of traditional inverters. The DC voltage

source with X shaped impedance network followed by three leg inverter, forms Z

source inverter. It is shown in Figure.1.

Figure 1 Z source Inverter

Figure 2 Inductor current in ZSI

The current flow through inductor during shoot through and active period is shown

in figure2.

Shoot through mode

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Peak at t=DT

Peak to peak ripple current

(1)

Active mode

(DT)

Minimum at t=T

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Peak to peak ripple current

(2)

By volt-sec balance principle (1) = (2)

(3)

RMS value of ac output voltage of Z source inverter

(4)

Voltage gain of the ZSI

(5)

Here D = 1-M

In ZSI voltage stress across switches is equal to Vi. The gain of the inverter is high

when the modulation index is low and for large shoot through duty ratio D.

3. Trans Z Source Inverter

The Trans ZSI can able to produce high voltage gain with high

modulation index. The boost capacity is adjusted by changing the transformer turns

ratio and shoot through duty ratio. Two transformer windings and one capacitor of

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impedance network is utilized in the form of T shape in the TZSI. It is shown in

Figure.3.

Figure 3 Trans Z Source Inverter

Shoot through mode

In this mode the switches in the same leg turned on and the diode is in off condition.

Vw1=nVw2

Vw2=Vc1

Active mode

In this mode the switches are not short circuited and the diode is in on condition.

Vw1=Vdc

Vw2=Vdc-Vc

The voltage across the capacitor in TZSI

(6)

(7)

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(8)

(9)

where n is the transformer turns ratio. The Equation (9) clearly shows that the

inverter voltage gain raised by increasing n ≥ 1 or by lowering d.

4. Comparative Analysis between ZSI and TZSI

The comparison between ZSI and TZSI has been made with the load capacity of

3KW through MATLAB/simulink software. The switching frequency of the inverter

is 10KHZ. The boost factor for the both inverter is fixed to 3.33. The shoot through

period for the ZSI is set to 1.75x10-5 seconds and active period is set to 3.25x10-5

seconds. The modulation index for ZSI is set to 0.65. Therefore the duty ratio is D

for ZSI = 1-0.65=0.35.The shoot through period for the TZSI is set to 0.8x10-5

seconds and active period for TZSI is set to 4.2x10-5 seconds. The modulation index

for TZSI is set to 0.84. Therefore the duty ratio is D for TZSI = 1-0.84=0.16. The Dc

supply voltage Vdc for the both inverter is 154.5V. The simulation results of ZSI

and TZSI is shown in figure 4 and 5. Figure 6 shows the performance of TZSI and

ZSI for various modulation index conditions. The table 1 shows the comparative

results between the inverters with respect to ac line voltage, shoot through period

requirement, component size and gain of the inverter.

(a)

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(b)

Figure 4 (a) DC link voltage of ZSI (b) AC line voltage of ZSI for 154.5V DC

Supply

(a)

(b)

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Figure Error! No text of specified style in document. (a) DC link voltage of

TZSI (b) AC line voltage for 154.5V DC Supply

Table 1. Comparative Analysis between ZSI and TZSI

Inverter DC

supply

Voltage

Vdc

(V)

Modulation

Index

M

Shoot

through

period

T0

(10-5 S)

Duty

ratio

D

Boost

factor

B

Inverter

DC link

Voltage

Vi

(V)

L1

(mH)

L2

(mH)

C1

(μF)

C2

(μF)

C3

(μF)

AC

line

voltage

VLac

(V)

Gain

ZSI 154.5 0.65 1.75 0.35 3.33 515 10 10 1000 1000 - 289.3 1.87

TZSI 154.5 0.84 0.8 0.16 3.33 513 7.5 5 400 - - 373.2 2.42

Figure 6 Modulation index vs Voltage gain for ZSI and TZSI

5. Conclusion In this paper the comparative analysis has been carried out between TZSI

and ZSI for the load capacity of 3KW. The simple boost control technique is

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incorporated to control both inverters. The simulation has been done in MATLAB/

simulink platform. TZSI voltage gain value is 2.42 for the modulation index of 0.84.

It is observed that for higher modulation index the gain value is high and also the

shoot through period requirement is very less.

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