improved trans z-source inverter with continuous input current...

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ISSN 2348–2370

Vol.06,Issue.06,

September-2014,

Pages:465-472

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Improved Trans Z-Source Inverter with Continuous Input Current and

Boost Inversion Capability for Renewable Energy Resources E. TEJASWINI

1, T. PARMESHWAR

2, P. NAGESWARARAO

3

1PG Scholar, Dept of EEE, Vidya Jyothi Institute of Technology, Hyderabad, India, Email: [email protected]. 2Asst Prof, Dept of EEE, Vidya Jyothi Institute of Technology, Hyderabad, India, Email: [email protected].

3Assoc Prof, Dept of EEE, Vidya Jyothi Institute of Technology, Hyderabad, India.

Abstract: Energy resources and their utilization are a prominent issue of this century. The problems of natural source

depletion, environmental effects and the rising demand for new energy have been fervently discussed in recent years. Some

modified impedance source networks are proposed from time to time for increasing the output voltage gain. One such

impedance network is Improved Trans Z Source Inverter which has high boost voltage that improves upon the conventional

inverters. The proposed Improved Trans Z Source Inverter maintains all the main features of Z Source Inverter with the added

advantages like increased voltage gain, reduced voltage stress, continuous input current, boost inversion capability, higher

modulation index, lower current ripple etc. This is a novel concept of high step up inverter based on the transformer to improve

the input current profile. The Improved Trans Z Source Inverter can suppress resonant current at startup, which might destroy

the devices. Keeping in view the above advantages, the proposed concept has topologies that suit solar cell and fuel cell

applications and can be extended towards the implementation and analysis with PV cells and fuel cells which account for

renewable energy resources and a brief comparison between PV and Fuel cells is also implemented in this paper.

Keywords: Improved Trans Z Source Inverter.

I. INTRODUCTION

The rapidly increasing environmental degradation

across the globe is posing a major challenge to develop

commercially feasible alternative sources of electrical

energy generation. Energy resources and their utilization

are a prominent issue of this century. The problems of

natural source depletion, environmental effects and the

rising demand for new energy have been fervently

discussed in recent years. It is believed that the distributed

generation market will be more than $30 billion by the year

2015. Due to environmental concerns, more effort is now

being put into clean and distributed power like geothermal,

wind power, fuel cells, and photovoltaic (PV) that directly

uses the energy from the sun to generate electricity. Thus, a

huge research effort is being conducted worldwide to come

up with a solution in developing an environmentally

benign and long-term sustainable solution in electric power

generation. The major players in renewable energy

generation are photovoltaic (PV), wind farms, fuel cell,

and biomass. These distributed power generation sources

are widely accepted for micro grid applications.

The worldwide grid-connected PV system grows at a

rate of 25% every year. As the energy from the sun is free,

the major cost of photovoltaic generation is the installation

cost, which is mainly composed of the costs of solar

modules and the interface converter system, also called the

power conditioning system (PCS). A fuel cell combines

hydrogen and oxygen to produce electricity, heat, and

water. Fuel cells are often compared to batteries. Both

convert the energy produced by a chemical reaction into

usable electric power.

Fig1. PV Array with Dc to Dc Converter.

Fig2. Fuel cell with Dc to Dc Converter.

With the development of solar cell and fuel cell

technologies, the price of their modules has dropped

dramatically. To lower the cost of the PCSs has become a

E. TEJASWINI, T. PARMESHWAR, P. NAGESWARARAO

International Journal of Advanced Technology and Innovative Research

Volume. 06, IssueNo.06, September-2014, Pages: 465-472

very urgent issue of grid connected PV systems. However,

the reliability of the micro grid relies upon the interfacing

power converter. Thus the proper power regulation from

the interfacing power converter will ensure a stable and

reliable micro grid system. Fig 1 and fig 2 show pv array

and fuel cell with pcs integration to the grid.

II.TRENDS IN Z-SOURCE INVERTER

The use of renewable-energy generating systems has

recently increased dramatically due to the exhaustion of

fossil fuels and their impact on the environment. The

unregulated output power of renewable energy sources

should be regulated through inverters to satisfy the

conditions for connection to the grid. A Z-source inverter

(ZSI) has been proposed to overcome the disadvantages of

the conventional scheme with a unique impedance

network. A ZSI can buck or boost the input voltage using

the shoot-through state and the modulation index in a

single stage. Since no dead time is needed, the output

voltage is free from voltage distortion. Due to these

advantages, the ZSI has been applied to single stage

conversion applications, such as PV systems, fuel cell

systems and ac motor drive systems.

Fig3. QZSI with continuous input current.

The quasi-Z-source inverter (QZSI) is similar to the ZSI

presented above, but has several advantages including, in

various combinations (see fig 3); lower component ratings,

reduced source stress, reduced component count and

simplified control strategies. The extended boost Z-source

inverters add inductors, capacitors, and diodes to the Z

impendence network in order to produce a high dc-link

voltage for the main power circuit from a very low input dc

voltage. A combination of the Z-source inverter and

switched-inductor structure, called the switched-inductor

Z-source inverter, provides strong step-up inversion to

overcome the boost limitation of the classical Z-source

inverter. In order to overcome the inconvenience of inrush

current suppression at startup of the switched-inductor Z-

source inverter, a switched-inductor quasi- Z-source

inverter is proposed in which provides continuous input

current, reduced passive component count, reduced voltage

stress on the capacitors, lower shoot-through current, and

lower current stress on inductors and diodes, in comparison

to the switched-inductor Z-source inverter for the same

input and output voltages.

The trans-Z source/-quasi-Z-source inverter is extended

to various structures in cascade topologies and parallel

operations for high power conversion system. In order to

improve the input current profile, an inductor–capacitor–

capacitor–transformer Z-source inverter (LCCT-ZSI) in

used one more inductor and one more capacitor in

comparison with the trans-Z-source/-quasi- Z-source

inverters. All topologies suit solar cell and fuel cell

applications, since they require high voltage gain in order

to match the source voltage to the line voltage. The input

dc current is discontinuous in the trans-Z-source inverter

and has a high ripple in the trans-quasi-Z-source inverter,

thus requiring a decoupling capacitor bank or an LC input

filter at the front end to eliminate current discontinuity and

protect the energy source. As shown in fig 4, the trans-

quasi-Z-source inverter can suppress the resonant current

at startup while the trans-Z-source inverter cannot, and the

resulting voltage and current spike can destroy the devices.

The startup resonant problem of the trans-Z-source inverter

occurs because a huge resonant current flows to the diode,

transformer windings, capacitor and body diode of the

insulated gate bipolar transistors (IGBTs).

Fig4. Trans z source inverter with input filter.

III. IMPROVED TRANS Z SOURCE INVERTER

The converter shown in section I need to provide high

voltage gain for higher load demand. Therefore some

modified impedance source networks are proposed from

time to time for increasing the output voltage gain. One

such impedance network is Improved Trans Z Source

Inverter which has high boost voltage that improves upon

the conventional inverters. The proposed Improved Trans

Z Source Inverter maintains all the main features of Z

Source Inverter with the added advantages like increased

voltage gain, reduced voltage stress, continuous input

current, boost inversion capability, higher modulation

index, lower current ripple etc. This is a novel concept of

high step up inverter based on the transformer to improve

the input current profile. The Improved Trans Z Source

Inverter can suppress resonant current at startup, which

might destroy the devices (see fig 5).

Improved Trans Z-Source Inverter with Continuous Input Current and Boost Inversion Capability for Renewable

Energy Resources



Fig 5 Improved trans z-source inverter.

Fig6. Basic block diagram for integration of impedance

network with renewable energy resource.

Keeping in view the above advantages, the proposed

concept has topologies that suit solar cell and fuel cell

applications and can be extended towards the

implementation and analysis with PV cells and fuel cells

which account for renewable energy resources and a brief

comparison between pv and fuel cells is also implemented.

Fig 6 Block diagram shows the Implementation of

Improved Trans Z Source Inverter for Renewable Energy

Resources

IV. PV CIRCUIT IMPLEMENTATION

There are several power converter topologies employed

in PV systems; however, they differ by several

characteristics: two stage or single-stage, with transformer

or transformer less, and with a two-level or multilevel

inverter. Single-stage inverters are becoming more

attractive in comparison to two-stage models due to their

compactness, low cost, and reliability. However, the

conventional inverter has to be oversized to cope with the

wide PV array voltage changes because a PV panel

presents low output voltage with a wide range of variation

based on irradiation and temperature, usually at a range of

1 : 2. To interface the low voltage output of an inverter to

the grid, a bulky low-frequency transformer is necessary at

the cost of a large size, decrease in efficiency, loud

acoustic noise, and high cost. The two-stage inverter

applies a boost dc/dc converter instead of a transformer in

order to minimize the required KVA rating of the inverter

and boost the wide range of voltage to a constant desired

value. Unfortunately, the switch in the dc/dc converter

becomes the cost and efficiency killer of the system.

Transformer less topologies especially deserves attention

because of their higher efficiency, smaller size and weight,

and a lower price for the PV system. The Z-source inverter

(ZSI), as a single-stage power converter with a step-

up/down function, allows a wide range of PV voltages, and

has been reported in applications in PV systems (see fig 7).

Fig7. Improved Trans z source inverter with source as

PV Module.

A. Operating Modes

Fig8. (a) equivalent circuit (b) shoot through state (c)

non shoot through state.

It can handle the PV dc voltage variation in a wide range

without overrating the inverter, as well as implement

voltage boost and inversion simultaneously in a single

power conversion stage, thus minimizing system cost and

reducing component count and cost, and improving the

reliability. Recently proposed qZSIs have some new

attractive advantages that are more suitable for application




in PV systems. This will make the PV system much

simpler and lower its cost because the qZSI: 1) draws a

constant current from the PV panel, thus no need for extra

filtering capacitors. 2) Features lower component

(capacitor) rating; and 3) reduces switching ripples to the

PV panels.

The improved inverter has extra shoot-through zero

states in addition to the traditional six active and two zero

states in a classical Z-source inverter. For the purpose of

analysis, the operating states are simplified into shoot-

through and nonshoot-through states. Fig.8 (a) shows the

equivalent circuits of the improved trans-Zsource inverter

with pv module implementation. In the shoot-through state,

as shown in Fig. 8(b), the inverter side is shorted by both

the upper and lower switching devices of any phase leg.

During the shoot-through state, the diode D is OFF. We

thus obtain the following:

vL1 = VC1 (1)

vL2 = nvL1 = nVC1 (2)

vL3 = Vpv + VC2. (3)

In the nonshoot-through state, as shown in Fig.8(c), the

improved inverter has six active states and two zero states

of the inverter main circuit. During the nonshoot-through

state, D is on. The corresponding voltages across the

primary and secondary windings of the transformers in this

state are vL1 non and vL2 non. We obtain

vL1 non + vL2 non = −VC2 (4)

vL3 = Vpv − VC1 − vL2 non (5)

vPN = VC1 − vL1 non. (6)

Applying the volt-second balance principle toL1and L2, (1)

and (2) yield

vL1 non =−D VC1/(1 – D) (7)

vL2 non =−nD VC1/(1 – D) (8)

Substituting (7) and (8) into (4), we have

VC2 =(1 + n)D VC1/(1 – D) (9)

Applying the volt-second balance principle to L3, (3), (5),

and (9) yield

VC1 =(1 – D) Vpv/(1 − (2 + n)D) (10)

VC2 =(1 + n)D Vpv /1 − (2 + n)D (11)

The peak dc-link voltage across the inverter’s main

circuit is expressed in (6) and can be rewritten as

VPN =1/(1 − (2 + n)D) (12)

Vpv = BVdc. (13)

The boost factor of the proposed inverter B is defined by

B =1/(1 − (2 + n)D)=1/(1 − (2 + n)T0/T). (14)

From (14), when n = 0, B = 1/(1 − 2D), the improved

inverter becomes the classical Z-source inverter. When n ≥

1, the boost ability of the improved trans-Z-source inverter

is higher than that of the trans-Z-source, trans-quasi-Z-

source, and classical Z-source inverters. On the other hand,

the improved inverter uses a smaller shoot-through duty

cycle at the same boost factor in comparison with the

conventional trans-Z-source/-quasi-Z source inverters.

V. FUEL CELL IMPLEMENTATION

Recently, energy management based on classical PI

controllers has been proposed. This strategy is based on the

control of the main performance parameters such as the

battery state of charge (SOC), the super capacitor voltage

or DC bus voltage using PI controllers. The knowledge of

an expert is not necessary and the PI controllers can be

easily tuned online for better tracking. The load power is

distributed in such a way to allow the fuel cell system to

provide the steady state load demand. The frequency

decoupling strategy ensures the fuel cell provide low

frequency demand while the other energy sources deal with

high frequency demand. This is achieved through the use

of low pass filters; wavelet or fast Fourier transforms

(FFT) techniques. This strategy improves the life time of

the fuel cell system as the dynamic stress on the fuel

supply system is prevented. Here, the fuel cell system

supplies a nearly constant mean load power while the other

energy sources discharge or recharge when the load power

is above or below its mean value respectively (see fig 9).

Fig9. Improved Trans z source inverter with source as

fuel cell.

A. Operating Modes

The improved inverter has extra shoot-through zero

states in addition to the traditional six active and two zero

states in a classical Z-source inverter. For the purpose of

analysis, the operating states are simplified into shoot-

through and nonshoot-through states. Fig 10 (a) shows the

equivalent circuits of the improved trans-Zsource inverter

with pv module implementation. In the shoot-through state,

as shown in Fig. 10(b), the inverter side is shorted by both

the upper and lower switching devices of any phase leg.

During the shoot-through state, the diode D is OFF. We

thus obtain the equations similar to those derived in section

IV for a voltage of Vfc for fuel cell.


Energy Resources



Fig 10 (a) equivalent circuit (b) shoot through state (c)

non shoot through state.

VI. SIMULATION RESULTS.

A. PV Module Simulation Results




Fig11. Simulation results using PV module.

B. Fuel Cell Simulation Results


Energy Resources



Fig12. Simulation results using fuel cell: output phase

voltage Va; three phase voltage and current; dc link

voltage Vpn; input current Iin; shoot through current Ish;

capacitor voltages steady state- dc link voltage; diode

voltage; input current; shoot through current.

Figure11 and 12 shows the simulation results using PV

module and fuel cell. We selected the simulation

Parameters L3 =1 mh, C1=C2 =1000 μf, Lf =1.5 mh,Cf =

10 μf, and R = 50 Ω/phase. The turn ratio of the

transformer is 2. The magnetic inductance measured from

the primary side was set to 0.737 mh. The leakage

inductance was set to 0.5 μh. The switching frequency was

10 khz, the input was 100 Vdc, And the output phase

voltage was 115 Vrms to meet the grid-tied requirement.

Constant boost control was used. When using the constant

boost control method for the improved inverter, According

to M = 0.95 to produce the output phase Voltage of 115

Vrms from the 100 V input dc voltage. Thus, we obtain D

= 0.1772, B = 3.42, G = 3.25, VPN = 342 V, VC1 = 281 V,

and VC2 = 181 V for the improved Trans-Z-source

inverter. Adding of one more inductor and one more

Capacitor makes more resonances in the improved inverter.

The conventional trans-quasi-Z-source inverter has the

same simulation results as the trans-Z-source inverter,

except for eliminating the huge resonant current at startup,

lower voltage stress on the capacitor, and a different input

current drawn from the dc source. These results can also

be enhanced by using different semiconductors in case of

pv cells and their trend over coming years is shown in fig

13 and a comparison graph between pv and fuel cell is

shown in the graph below in fig 14.

Fig13. Trend in pv cells for different semiconductors.

Fig14. A comparison between pv and fuel cell for a

given load demand in case of improved trans z source

inverter.

VII. CONCLUSION

The proposed concept aims to improve the trans-Z-source

inverter with the following main characteristics: high boost

voltage inversion ability, continuous input current, and

resonance suppression at startup. Compared with the

conventional trans- Z-source and trans-quasi-Z-source

inverters, for the same transformer turn ratio and input and

output voltage, the improved inverter has a higher

modulation index with reduced voltage stress on the dc

link, lower current stress flow to the transformer windings

and diode, and lower input current ripple. If the modulation

index is kept fixed, the improved inverter uses a lower

transformer turn ratio to produce the same input and output

voltage compared to the conventional trans-Z-source/-




quasi-Z-source inverters. As a result, the size and weight of

the transformer in the improved inverter can be reduced.

The improved inverter application to fuel cells and

photovoltaic cells is shown in this paper, where a low input

voltage must be inverted to a high ac output voltage and a

comparison between pv and fuel cells simulation results

can be done for the case of improved trans z source

inverter.

VIII. REFERENCES

[1] F. Z. Peng, “Z-source inverter,” IEEE Trans. Ind.

Appl., vol. 39, no. 2, pp. 504–510, Mar./Apr. 2003.

[2] J. B. Liu, J. G. Hu, and L. Y. Xu, “Dynamic modeling

and analysis of Zsource converter-derivation of AC small

signal model and design-oriented analysis,” IEEE Trans.

Power Electron., vol. 22, no. 5, pp. 1786–1796, Sep. 2007.

[3] P. C. Loh, D. M. Vilathgamuwa, G. J. Gajanayake, Y.

R. Lim, and C. W. Teo, “Transient modeling and analysis

of pulse-width modulated Zsource inverter,” IEEE Trans.

Power Electron., vol. 22, no. 2, pp. 498–507, Mar. 2007.

[4] M. Shen, J. Wang, A. Joseph, F. Z. Peng, L. M.

Tolbert, and D. J. Adams, “Constant boost control of the Z-

source inverter to minimize current ripple and voltage

stress,” IEEE Trans. Ind. Appl., vol. 42, no. 3, pp. 770–

778, May/Jun. 2006.

[5] O. Ellabban, J. V. Mierlo, and P Lataire, “A DSP-based

dual-loop peak DC-link voltage control strategy of the Z-

source inverter,” IEEE Trans. Power Electron., vol. 27, no.

9, pp. 4088–4097, Sep. 2012.

[6] Y. Huang, M. Shen, F. Z. Peng, and J. Wang, “Z-source

inverter for residential photovoltaic systems,” IEEE Trans.

Power Electron., vol. 21, no. 6, pp. 1776–1782, Nov. 2006.

[7] F. Z. Peng, M. Shen, and K. Holland, “Application of

Z-source inverter for traction drive of fuel cell-battery

hybrid electric vehicles,” IEEE Trans. Power Electron.,

vol. 22, no. 3, pp. 1054–1061, May 2007.

[8] P. C. Loh, F. Gao, P. C. Tan, and F. Blaabjerg, “Three-

level ac–dc–ac Zsource converter using reduced passive

component count,” IEEE Trans. Power Electron., vol. 24,

no. 7, pp. 1671–1681, Jul. 2009.

[9] P. C. Loh, F. Gao, F. Blaabjerg, and S. W. Lim,

“Operational analysis and modulation control of three-level

Z-source inverters with enhanced output waveform

quality,” IEEE Trans. Power Electron., vol. 24, no. 7, pp.

1767–1775, Jul. 2009.

Author’s Profile:

E. Tejaswini received B-Tech

Degree in Electrical and Electronics

Engineering from Sridevi Womens

Engineering College Hyderabad,

India. She is pursuing her Post

Graduation from Vidya Jyothi

Institute of Technology, Hyderabad,

India.

T.Parameshwar received B-Tech

Degree in Electrical and Electronics

Engineering from VNR Viganan

Jyothi Institute of Technology

Hyderabad, India. He did his Post

Graduate in Electrical Power System

from Jawaharlal Institute of

Technology University Anatapuram,

India. He has 6 years of experience in teaching. Currently

he is with Vidya Jyothi Institute of Technology Hyderabad,

India, as an Assistant Professor in Electrical and

Electronics Engineering.

P. Nageswararao is an associate

professor working with Vidya Jyothi

Institute of Technology, India in the

Electrical and Electronics

Engineering. His areas of interests are

Fuzzy Logic Applications in Power

Systems, FACTS Controllers in

Power Systems.

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