Photovoltaic (PV) power generation is becoming more promising since the introduction of the thin film PV technology due to its lower cost, excellent high temperature performance, low weight, flexibility, and glass-free easy installation. However, there are still two primary factors limiting the widespread application of PV power systems. The first is the cost of the solar cell/module and the interface converter system; the second is the variability of the output (diurnal and seasonal) of the PV cells. A PV cell‘s voltage varies widely with temperature and irradiation, but the traditional voltage source inverter (VSI) cannot deal with this wide range without over-rating of the inverter, because the VSI is a buck converter whose input dc voltage must be greater than the peak ac output voltage. Because of this, a transformer and/or a dc/dc converter is usually used in PV applications, in order to cope with the range of the PV voltage, reduce inverter ratings, and produce a desired voltage for the load or connection to the utility. This leads to a higher component count and low efficiency, which opposes the goal of cost reduction.

Merits of qZSI over ZSI: 1. The two capacitors in ZSI sustain the same high voltage; while the voltage on capacitor

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2. The ZSI has discontinuous input current in the boost mode; while the input current of the qZSI is continuous due to the input inductor L1, which will significantly reduce input stress. 3. For qZSI, there is a common dc rail between the source and inverter, which is easier to assemble and causes less EMI problems. 4. ZSI has more harmonics in the output than qZSI.

Figs. a and b shows the traditional voltage fed ZSI and the proposed voltage fed qZSI, respectively. In the same manner as the traditional ZSI, the qZSI has two types of operational states at the dc side: the non shoot-through states (i.e. the six active states and two conventional zero states of the traditional VSI) and the shoot-through state (i.e. both switches in at least one phase conduct simultaneously). In the non-shoot-through states, the inverter bridge viewed from the dc side is equivalent to a current source. The equivalent circuits of the two states are as shown in Figs. c and d. The shoot-through state is forbidden in the traditional VSI, because it will cause a short circuit of the voltage source and damage the devices. With the qZSI and ZSI, the unique LC and diode network connected to the inverter bridge modify the operation of the circuit, allowing the shoot-through state. This network will effectively protect the circuit from damage when the shoot-through occurs and by using the shoot-though state, the (quasi-) Z-source network boosts the dc-link voltage. The major differences between the ZSI and qZSI are discontinuous current and (2) The voltage on capacitor C2 is greatly reduced. The continuous and constant dc current drawn from the source with this qZSI make this system especially well suited for PV power conditioning systems.

All the voltages as well as the currents are defined in Figs c and d and the polarities are shown with arrows. Assuming that during one switching cycle, T, the interval of the shoot through state is To; the interval of non-shoot-through states is T1; thus one has T= To + T1 and the shoot-through duty ratio, D= To/T. From Fig. c. which is a representation of the inverter during the interval of the non-shoot through states, T1 , one can get vL1 = Vin – VC1, vL2 = -VC2, and (1) VPN = VC1 – vL2 = VC1 + VC2 Vdiode =0 (2) From Fig d. which is a representation of the system during the interval of the shoot-through states, To, one can get vL1 = VC2 + Vin, vL2 = VC1, and (3)

O. B. Heddurshetti1*, H. R. Zinage2, P. M. Murari3 6

At steady state, the average voltage of the inductors over one switching cycle is zero. From (1), (3), one has Thus

(5)

From (2), (4) and (5), the peak dc-link voltage across the inverter bridge is

(6)

where B is the boost factor of the qZSI. This is also the peak voltage across the diode. The average current of the inductors L1, L2 can be calculated by the system power rating P IL1 = IL2 = Iin = P/Vin (7) According to Kirchhoff‘s current law and (7), we also can get that

IC1 = IC2 = IPN – IL1 ID = 2IL1 – IPN (8)

In summary, the voltage and current stress of the qZSI are shown in Table 1. The stress on the ZSI is shown as well for comparison, where (1) M is the modulation index; vln is the ac peak phase voltage; P is the system power rating. (2) m = T1/(T1 – To); n = To/(T1 – To); thus m>1; m – n=1. (3) B = T/ (T1 – To), thus m + n = B, 1

III. CONTROL METHODS

If the inverter is operated entirely in the non-shoot-through states (Fig. c) the diode will conduct and the voltage on capacitor C1 will be equal to the input voltage while the voltage on capacitor C2 will be zero. Therefore, vPN = Vin and the qZSI acts as a traditional VSI:

(9)

For SPWM 0 ≤ M ≤ 1; and for SVPWM 0 ≤ M ≤ 2/√3. Thus when D = 0, Vln always less than Vin/√3 and this is called the buck conversion mode of the qZSI. By keeping the six active states unchanged and replacing part or all of the two conventional zero states with shoot through states, one can boost by a factor of B , the value of which is related to the shoot-through duty ratio, as shown in (6). This is called the boost conversion mode of the qZSI. The peak ac voltage becomes

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(10)

All the boost control methods that have been explored for the traditional ZSI (i.e. simple boost, maximum boost, and maximum constant boost) can be utilized for qZSI control in the same manner. Generally speaking, the voltage gain of the qZSI is G=vln/0vpn.=MB, whereas the voltage stress across the inverter bridge is BVin . In order to maximize the voltage gain and minimize the voltage stress on the inverter bridge, one needs to decrease the boost factor B and increase the modulation index M as much as possible. In the proposed PV power generation system, in order to lower the voltage stress on the inverter bridge and keep a high voltage gain, the maximum constant boost control with third harmonic injection was chosen as the control method. Fig. e. shows the sketch map. At (1/6) third harmonic injection, the maximum modulation index M = (2/√3) can be achieved. The shoot-through states are introduced into the switching cycle when the carrier is either greater than VP or less than VN, which is evenly spread in each switching cycle. Thus the qZSI network doesn‘t involve low-frequency ripples. In this case, the shoot-through duty ratio is

(11)

The boost factor is

(12)

And the voltage gain equals

(13)

The peak ac phase voltage can be calculated

(14)

As per the theoretical calculation DC voltage, VO=400 V Output AC voltage (Phase voltage)

O. B. Heddurshetti1*, H. R. Zinage2, P. M. Murari3 6

Output AC voltage (Line voltage), Vac=170*1.73=294.1 V

By simulating the above circuit in fig. f. using MATlab simulink at modulation index, M=0.85 and B=1. output voltage waveform 316V is obtained. The magnitude of the output voltage by theoretical calculation is 294V which is nearly equal to simulation results.

DC voltage, VO=400 V Output AC voltage (Phase voltage) Vac=1*1.366*400/2=273.2V Output AC voltage (Line voltage), Vac=273.2*1.73=472.636V

By simulating the same circuit using MATlab simulink at modulation index, M=1, B=1.366, the output voltage of 421V is obtained. The magnitude of the output voltage by theoretical calculation is 273.2V which is nearly equal to simulation results and observed that the voltage is increased by increasing the modulation index, M and boost factor B.

As per the theoretical calculation, DC voltage [1], VO=400 V Output AC voltage (Phase voltage) Vac=0.85*2.118*400/2=360.06V Output AC voltage (Line voltage), Vac=360.06*1.73=622.90V

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the output voltage of 440V is obtained. The magnitude of the output voltage by theoretical calculation is 360.06V which is nearly equal to simulation results.

The proposed prototype for qZSI is fabricated and tested. The hardware implementation of the microcontroller based qZSI system used for photovoltaic power generation system is as shown in fig.k. The output waveform is shown in fig. l. A solar cell is connected at the input of an inverter which produces a voltage of 18V DC. This voltage is given to the inverter. The inverter produces a constant ripple free 24V output. By observing the results above, it can be seen that the implementation of quasi Z-source inverter for PV generation is verified experimentally. Also, it can be seen that the output voltage of quasi Z-source inverter can be varied over a wide range. That is, the output voltage of quasi Z-source inverter may be either greater than or less than the input voltage. The qZSI inherits all the advantages of the ZSI which can be interchangeable.The power conditioning in a single stage with improved reliability. Unique advantage of qZSI is lower component rating and constant dc current from source. The qZSI has continuous input current, reduced source stress. The quasi Z-source inverter (qZSI) is suitable for residential PV system.

VI. CONCLUSION AND FUTURE WORK

In this paper a quasi-Z-source inverter with a new topology is presented, which is derived from the traditional ZSI. The proposed qZSI inherits all the advantages of the ZSI and features its unique merits. It can realize buck/boost power conversion in a single stage with a wide range of gain that is suited well for application in PV power generation systems. Furthermore, the proposed qZSI has advantages of continuous input current, reduced source stress, and lower component ratings when compared to the traditional ZSI. Theoretical analysis, control method, and system design guide are presented in this paper. Both simulation and experimental results show that with a voltage range of 1:2 at the PV input, the qZSI can provide single phase 50 Hz, 230 Vllrms ac voltage, which verifies the theoretical analysis. A grid-connected PV power generation system is one of the most promising applications of renewable energy sources. The proposed qZSI based PV power generation system is intended as a grid connected system and transfers the maximum power from the PV array to the grid by maximum power point tracking technology. In that case, the efficiency would be improved and the cost would be reduced with the proposed one stage power conversion system. For the high power application we can use IGBT‘s in place of MOSFET. SPWM can be replaced by SVPWM. DSP processor can be used instead of microcontroller.

Y. Huang, M.S. Shen, F.Z. Peng, J.Wang, ―Z-Source inverter for residential photovoltaic systems,‖ IEEE Trans. on Power Electron., vol.21, no. 6, pp. 1776-1782, November 2006. R. Badin, Y. Huang, F.Z. Peng, H.G. Kim, ―Grid Interconnected ZSource PV System,‖ in Proc. IEEE PESC’07, Orlando, FL, June 2007, pp. 2328-2333. J. Anderson, F.Z. Peng, ―Four Quasi-Z-Source Inverters,‖ in Proc. IEEE PESC’08, Rhodes, Greece, June 2008.

O. B. Heddurshetti1*, H. R. Zinage2, P. M. Murari3 6

Appl., vol. 39, no. 2, pp. 504-510, March/April 2003. F. Z. Peng, M. Shen, and Z. Qian, ―Maximum boost control of the Zsource inverter,‖ IEEE Trans. on Power Electron., vol. 20, no. 4, pp. 833–838, Jul./Aug. 2005. M.S. Shen, J. Wang, A. Joseph, F.Z. Peng, L.M. Tolbert, D.J. Adams, ―Constant Boost Control of the Z-Source Inverter to Minimize Current Ripple and Voltage Stress,‖ IEEE Trans. on Ind. Appl., vol. 42, no. 3, pp. 770-778, May/June 2006. P. C. Loh, D. M. Vilathgamuwa, Y. S. Lai, G. T. Chua, and Y. Li, ―Pulsewidth modulation of Z-source inverters,‖ in Conf. Rec. IEEE-IAS Annu. Meeting, Seattle, WA, 2004, pp. 148–155.

Received B.E. Degree in Electrical and Electronics Engineering From Visvesvaraya Technological, University, Belgaum in 2005. Received M.Tech in Power Electronics from P.D.A College of Engineering, Gulbarga Affiliated to Visvesvaraya Technological, University, Belgaum, in 2009. He is working as an Asst. professor in Electrical and Electronics Engineering Department, at Hirasugar Institute of Technology, Nidasoshi, Karnataka. He is Life Member of Indian Society for Technical Education. His main fields of interests are Electrical and Electronics Measurement, HVDC systems, Computer Simulation of Power Electronics Circuits, ac and dc drives, High Voltage Engineering and Control Systems.

Received B.E. Degree in Electrical and Electronics Engineering From Karnataka university Dharwar in the year 1999. Received M.Tech in Power systems from Walchand College of Engineering, Sangli Affiliated to Shivaji university Kolhapur, in 2010. She is working as an Asst. professor in Electrical and Electronics Engineering Department, at Hirasugar Institute of Technology, Nidasoshi, Karnataka. She is Life Member of Indian Society for Technical Education. Her main fields of interests are Electrical power system, Harmonic analysis, Computer techniques in power system analysis.

P. M. Murari Received B.E. Degree in Electrical and Electronics Engineering From Visvesvaraya Technological, University, Belgaum in 2009. Received M.Tech in Power Systems & Power Electronics from UBDTCE Davangere, Davangere University in 2011. He is working as an Asst. professor in Electrical and Electronics Engineering Department, at Hirasugar Institute of Technology, Nidasoshi, Karnataka.

Cell.no : 9739733021

Department of Electrical & Electronics Engineering, Hirasugar Institute of Technology, Nidasoshi, India