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Abstract —This paper proposes a residential solar power

system using Z–source inverter. The Z–source inverter employs a

unique impedance network couple with the solar cell and inverter

main circuit. The operating principle, control strategy and

characteristics of the system are described. The new and

traditional system configurations are compared in this paper.

Analysis and simulation results are presented to demonstrate thenovelty of features.

Index Terms — Photovoltaic cell, Z – source inverter, MPPT.

I. I NTRODUCTION

HE oil, coal and gas price is rising and global warming

becoming more severe due to environmental pollution.

Therefore, countries are now looking for alternative energy

sources to partially replace fossil fuels. Due to environmental

concerns, more efforts are now being put into green power

sources like solar power, wind power, geothermal, fuel cell

etc. Solar cells or photovoltaic (PV) cells directly use theenergy from the sun to generate electricity.

As the energy from the sun is free, the major cost of PV

generation is the installation cost. The installation cost mainly

consists of the costs of solar modules and the converterinterface i.e. the power conditioning system (PCS). With thedevelopment of PV technology, the price of solar modules hasdropped dramatically. A recent worldwide survey shows thatin the last three years, the retail price of solar modules hasdropped by 16.95%. However, at the same time, the prices forthe solar power system almost remain the same. Furthermore,compared with converters used in drive systems, the prices forthe converters used in solar power systems are still up to 50%

higher [1]. Reducing the cost of these systems has become amajor issue in deploying residential solar power systems.

Solar power system converts the DC output from PV cell to

230V, 50Hz AC output for residential use in India. This paper

proposes a Z-source inverter based solar power system for

residential use. By utilizing the Z-source inverter, the number

of switching components and the total volume of the system

can be minimized. Thus, the overall cost of the system is

minimized.

II. SOLAR POWER SYSTEM FOR R ESIDENTIAL USE

A. Basic Requirements

In order to transfer the energy from solar PV array into

residential circuits, the converter systems must fulfill the

following requirements:

(a) To convert the D.C voltage to A.C voltage.(b) To boost the voltage if the PV array voltage is less than the

utility voltage.

(c) To insure maximum power utilization from the PV array.

B. Traditional Converter

Fig. 1 DC to AC with step-up transformer in traditional PV

system

Fig. 2 DC to AC with DC–DC boost in traditional PV

system

Fig. 1 and Fig. 2 show the two most commonly used converter

system configurations in practice. In the system shown in

Fig.1 a transformer at line frequency is utilized to boost the

voltage after the DC–AC inverter. Usually, a line frequency

Residential Solar Power Systems using

Z – Source Inverter

Amitava Das,1

Debasish Lahiri,2

A. K. Dhakar 2

1 Electrical Engg. Dept, Techno India, Kolkata, India, 2 Applied Electronics & Instrumentation Engg. Dept.,

Techno India, Kolkata, India, 3 Electrical Engg. Dept., National Institute of Technology, Durgapur, India.

T

Authorized licensed use limited to: Panimalar Institute of Technology. Downloaded on December 15, 2009 at 00:24 from IEEE Xplore. Restrictions apply.

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transformer is associated with huge size, loud acoustic noise

and high cost. In addition, the inverter has to be oversized to

cope with the wide PV array voltage change. The KVA rating

of the inverter is doubled if the PV voltage varies at a 1: 2

range. So in order to eliminate the transformer and to

minimize the KVA rating requirement of the inverter, in many

applications, a high frequency DC-DC converter is used to

boost the voltage to a constant value as shown in Fig. 2.

Unfortunately, the switch in the DC–DC converter is veryexpensive and also reduces the system efficiency [2], [3].

III. PROPOSED I NVERTER SYSTEM

In the proposed residential solar power system shown in Fig.3,

a Z-source inverter [5] is utilized to realize inversion and

boost function in one single stage. Unlike the traditional

voltage source or current source inverters, the Z-source

inverter employs a unique impedance network with split

inductors L1 and L2 and capacitors C1 and C2 connected in X

shape. With the impedance network, the Z-source inverter can

advantageously use the shoot through states to boost thevoltage. Furthermore, with the ability to handle the shoot

through state, the inverter system becomes more reliable [5],

[6]. The inductors and capacitors in the Z-source are bothenergy storage devices, so that their value can be optimallydesigned to ensure small size and low cost.

Fig. 3 Proposed inverter for residential solar power system

As compared to the systems shown in Fig. 1 and Fig. 2, theproposed system does not need a bulky transformer or a DC---DC converter to boost the voltage in the circuit. Thus both sizeand cost are minimized. As there is no dead time, the controlaccuracy and THD value can also be improved. Furthermore,the split-phase Z---source inverter naturally inherits all the

advantages of the split-phase six-switch inverter. Thus, the220V AC output filter can be optimized.The PV DC voltage needs to be at least 340---680 V to producea 220 V split-phase power. Therefore, 1200-V IGBTs areneeded in the system. For a 1 kW PV system, a 2 kW inverteris needed to cope with the voltage change. On the other hand,using the proposed system of Fig. 3, the PV voltage can bedesigned to be 225---450 V, which can be inverted to 220 Vsplit-phase power by the Z-source inverter using 600V IGBTs.In addition, the required KVA rating of the Z -source inverterremains the same of 1 kW for a 1 kW PV system. Therefore,

by utilizing the Z-source inverter, the volume, the cost as wellas the number of active switching devices are minimized.

Because of the single stage operation, the efficiency of the

system can be greatly improved. In summary, the proposed

system:

1) Has only one stage to realize inversion, boost, andmaximum power tracking.

2) Has the minimized number of switching devices.3) Needs no dead time.4) Can have shoot through state in the inverter.5) Inherits all the advantages of the six switch inverter system.

IV. PRINCIPLE OF OPERATION AND CONTROL

Fig. 4 control scheme for six – switch inverter

As shown in Fig. 4, when the triangular waveforms are greater

than the maximum value or lower than the minimum value of

the three reference waveforms, all upper three switches or all

bottom three switches are turned on respectively, as indicated

in the shadowed area. During these periods of time, the output

voltage of the inverter is zero, i.e., zero states. For the Z–

source inverter, the basic idea of control is to turn zero states

into shoot through states and keep the active switching states

unchanged, thus we can maintain the sinusoidal output and at

the same time achieve voltage boost from the shoot through of

the DC link [5],[6],[11].

Fig. 5 shows the boost control method of the split-phase Z-source inverter. Phase legs ‘‘b’’ and ‘‘c’’ are controlled bySPWM to synthesize AC output and the shoot-throughcommand, and is used to boost voltage as desired. The controlof these two phase legs is similar to the simple boost controlproposed in [5] and [6]. Two straight lines are used to controlthe shoot through duty ratio. When the carrier is greater thanthe upper straight line, phase leg ‘‘b’’ goes to shoot throughstate, whereas phase leg ‘‘c’’ goes to shoot through state whenthe lower straight line is greater than the carrier. Phase ‘‘a’’ inthe inverter is switching at 50% duty cycle without shootthrough. The switching frequency of phase ‘‘a’’ is the

frequency of the carrier. By doing this, each phase leg only


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shoots through once during one carrier cycle, the equivalentswitching frequency can be doubled for the output filter.

Fig. 5 Boost control topology

As described in [5] and [6], the inductors can be woundedaround the same core and have the same inductance and thecapacitors can also have the same capacitance. Therelationship between the output AC voltage and input DCvoltage is found as:

0 . / 2 pvV MBV = (1)

Where V0 is the peak output voltage, V pv is the PV array

voltage, and M is the modulation index.

The modulation index is defined as:

peak

tri

V M

V = (2)

B is called Boost factor, and is defined as:

0

1

1 2

BT

T

=

−

(3)

Where T0 is the total shoot through period per carrier cycle,

and T is carrier cycle. In the simple control method [5], [6],

the amplitude of the two straight lines is the peak of modulation waveform, therefore the relationship betweenmodulation index, M and the shoot --- through ratio T

0 /T can

be found as :

0 1T

M T

= − (4)

Substituting Eqn. (4) into Eqn. (3) we have:

1

2 1 B =

−

(5)

From Eqn. (1) & Eqn. (5) the output voltage is determined as:

02(2 1)

pv

M V V

M =

−

(6)

The capacitor voltage is:

0

1 20

11

21 2

C C pv pv

T BT V V V V

T

T

−+

= = =

−

(7)

When the PV output voltage value is high enough to producethe required AC voltage, the shoot through state is no longerneeded, i.e., T = 0 and B = 1. Under this condition, the

relationship between the inverter peak output voltage and thePV output voltage can be calculated by

0 . / 2 pvV M V = (8)

From Eqn. (7), by controlling the shoot through duty cycle and

modulation index a desirable AC output voltage can be

obtained.

The shoot-through states can be created by shorting both legs‘‘b’’ and ‘‘c,’’ or all the three legs simultaneously during anygiven shoot through state according to the two straight lines.For all these shoot-through cases, the resulted boost effect andoutput voltage waveforms remain the same. In the proposedcontrol scheme, only one phase leg is used to create shootthrough at any time, thus minimizing the switching frequency.

V. MAXIMUM POWER POINT TRACKING STRATEGY

Solar PV cell has nonlinear characteristics. Its output voltageand power change according to temperature and irradiation.Fig. 6 shows the typical characteristics for a PV model.

Fig. 6 Typical P --- I & P --- V characteristics of PV cell

For a specified temperature and irradiation, the intersection of the load line with the PV voltage-current characteristic, is the

operating point. Since the proposed Z -source based solarpower system is directly connected to the residential use, thesolar power system is controlled to transfer maximum powerfrom the PV array to the residential circuits continuously.Because of nonlinear characteristic of PV modules, themaximum power can not be achieved by directly connectingthe PV models. Tracking of the MPP must be used toeffectively get the maximum output power. Several researchesare reported on MPPT-featured inverters [7]---[10]. So far, theperturbation and observation method, the incrementalconductance method and the hill climbing method, etc. are


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used for MPP tracking. In this paper, a simple power feedback method is used by the Z --- source based PCS to achieve MPPTas shown in Fig. 7.

Fig. 7 Block diagram of MPPT control.

The power can be measured and used as feedback. The voltageof the PV modules is regulated to an optimal point, whichpresents the maximum power. This special point is tracked to

satisfy the relationship of 0dP dV

= . Based on this strategy, the

maximum power point can always be tracked [10].

VI. SIMULATION R ESULTS

Usually the PV output voltage changes with temperature,while the PV output current changes with irradiation.

Fig. 8 V --- I characteristics under different condition

With constant irradiation specified, the PV output powerdecreases when the temperature power increases when theirradiation increases. One of the functions of the system is toextract the maximum power out of the photovoltaic at anygiven temperature and irradiation. Based on the curves shownin Fig. 8, simulations are performed to prove the conceptproposed in this paper. The simulation setup with thefollowing parameters:L

1= L

2= 1mH.

C1

= C2

= 10,000 µFThe switching frequency = 10 KHz.The filter parameters are:

L = 1mHC = 120µF.The simulations are performed in following two cases:Case -- I: V

pv= 240V.

For the case --- I , input voltage, Vpv

is 240V to achieve 220Vsplit phase output voltage the modulation index can becalculated from inverse of Eqn. 6 .

0

0

2 2 2 220

0.624 4 2 220 240 pv

V

M V V

× ×= = =

− × × −

(9)

The Boost factor is:-

1 14.16

2 1 2 0.62 1 B = = =

− × −

(10)

The shoot – through duty cycle can be calculated as:

0

0

1

1 0.62 0.38

T M

T

T

T

= −

= − =

(11)

The Z --- source capacitor voltage should be

01 2

0

1 /1 2 /

1 0.38240 620

1 2 0.38

C C pvT T V V V T T

V

−= =

−

−= × =

− ×

(12)

Fig. 9 shows the simulation result of case – I. Figs. 9(a) & 9(b)

Fig. 9(a) Input voltage from PV array.

show the simulation waveform of input voltage from PV arrayand the Z --- source capacitor voltage. The capacitor voltage isclose to 620V. Figs 10(a) & 10(b) show the simulation

waveform of output voltage and output current respectively.The output voltage is desired split phase 220V rms and theoutput current is nearly 25A. The output power of the invertermay be calculated around 8800 Watt. Thus the maximumpower output from the Solar cell array is realized. Thesimulation results are consistent with the theoreticalcalculations.


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Fig 9(b) Z --- source capacitor voltage.

Fig. 10 (a) output voltage of the inverter

Fig. 10(b) output current of the inverter.

Case -- II: Vpv

= 1000VFor the case --- II the input voltage is much higher, Vpv =1000V, the control scheme is different in compare with thecase --- I.The modulation index can be calculated as

02 2 2 220 0.621000 pv

V M V

× ×= = = (13)

As no Boost is needed for this scheme, the Boost factor B = 1

Thus the Z – source capacitor voltage is same as input PV

voltage, which is 1000V. Fig. 11(a) &11(b) shows the

simulation waveform of PV input voltage and Z – source

capacitor voltage respectively.

Fig. 11(a) PV voltage

Fig.11 (b) Z --- source capacitor voltage.

Fig 12(a) &12(b) shows the waveform of output voltage andoutput current of the solar inverter respectively.

Fig. 12(a) inverter output voltage

Fig. 12(b) inverter output current.

From the above two cases, it can be realized that thesimulation results show that at different input voltage, theproposed residential solar power system’s output voltagemaintained as 220V rms which is essential for householdrequirement in India. The output power tracked the maximumPV output power at different temperature and irradiation. Thebasic topology of the propose scheme was verified.


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VII. CONCLUSION

This paper presented a new residential solar power system

using Z – source inverter. The proposed system shows that the

boost and the inversion with maximum power tracking at a

single stage is possible, and minimize the no of switching

devices. All the possible advantages of Z – source inverter and

six – switch inverter is used and integrated together for high

efficiency residential solar energy system with minimized

volume and reliable cost. The proposed system is very promising for residential solar energy system in rural area in

India.

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IEEE 33rd Annu. Power Electron. Spec. Conf., Jun. 2002, vol. 4, no. 23---27,pp. 1995---2000.[3] T.-F. Wu, C.-H. Chang, and Y.-K. Chen, ‘‘A multi-function photovoltaicpower supply system with grid-connection and power factor correctionfeatures,’’ in Proc. IEEE 31st Annu. Power Electron. Spec. Conf., Jun. 2000,vol. 3, no. 18---23, pp. 1185---1190.

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[5] F. Z. Peng, ‘‘Z-source inverter,’’ IEEE Trans. Ind. Appl., vol. 39, no. 2, pp.504---510, Mar./Apr. 2003.

[6] F. Z. Peng, M. Shen, and Z. Qian, ‘‘Maximum boost control of the Z-source inverter,’’ in Proc. IEEE 35th Annu. Power Electron. Spec. Conf., Jun.2004, vol. 1, pp. 255---260.[7] Y. C. Kuo, T. J. Liang, and J. F. Chen, ‘‘Novel maximum-power-pointtracking controller for photovoltaic energy conversion system,’’ IEEE Trans.

Ind. Electron. , vol. 48, no. 3, pp. 594---601, Jun. 2001.[8] W. Xiao and W. G. Dunford, ‘‘A modified adaptive hill climbing mpptmethod for photovoltaic power systems,’’ in Proc. IEEE 35th Annu. Power

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[10] E. Koutroulis, K. Kalaitzakis, and N. C. Voulgaris, ‘‘Development of amicrocontroller-based, photovoltaic maximum power point tracking controlsystem,’’ IEEE Trans. Power Electron., vol. 16, no. 1, pp. 46---54, Jan. 2001.[11] M. Shen, A. Joseph, J. Wang, F. Z. Peng, and D. J. Adams, “Comparison

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