 Open Access
 Total Downloads : 61
 Authors : Dr. S. K. Sinha , Arjun Verma , Dr. A. S. Pandey
 Paper ID : IJERTV6IS120038
 Volume & Issue : Volume 06, Issue 12 (December 2017)
 DOI : http://dx.doi.org/10.17577/IJERTV6IS120038
 Published (First Online): 26122017
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
QuasiZSource Inverter for Photovoltaic Energy Conversion System
Arjun Verma
Department of Electrical engineering Kamla Nehru Institute of Technology (KNIT)
Sultanpur, Uttar Pradesh
Dr. A. S. Pandey, Dr. S. K. Sinha
Department of Electrical engineering Kamla Nehru Institute of Technology (KNIT)
Sultanpur, Uttar Pradesh
Abstract This paper represents the QuasiZSource inverter for photovoltaic energy conversion system. QuasiZSource Inverter (QZSI) is an enhancement to ZSource Inverter (ZSI). The QZSI inherits all the advantages of the ZSI, which can realize buck/boost, inversion and power conditioning in a single stage with improved reliability. In addition, the proposed QZSI has the unique advantages of lower component ratings and constant dc current from the source. The QZSI features a wide range of voltage gain which is suitable for applications in photovoltaic (PV) systems, due to the fact that the PV cells output varies widely with temperature and solar irradiation. MATLAB / SIMULINK model of both the circuit topology (QZSI and ZSI) with different loading conditions are presented. Maximum Boost control technique is employed here. Theoretical analysis of voltage boost, control methods and a system design guide for the QZSI in PV systems are investigated in this paper. A comparative analysis between ZSI and QZSI is given in the end
Keywords QuasiZSource inverter, ZSource inverter, boosting ability, modulation index, photovoltaic (PV) system, maximum boost control .

INTRODUCTION
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 or module and the interface converter system; the second is the variability of the output (diurnal and seasonal) of the PV cells. A PV cells voltage varies widely with temperature and irradiation, but the traditional Voltage Source Inverter (VSI) cannot deal with this wide range without overrating 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 [13].
The ZSource Inverter (ZSI) has been reported suitable for residential PV system because of the capability of voltage boost and inversion in a single stage. Recently, four new topologies, the quasiZSource Inverters (QZSI), have been derived from the original ZSI. This project analyzes one voltage fed topology of these four in detail and applies it to PV
power generation systems. By using the new quasiZSource topology, the inverter draws a constant current from the PV array and is capable of handling a wide input voltage range. It also features lower component ratings and reduced source stress compared to the traditional ZSI. It is demonstrated from the theoretical analysis and simulation results that the proposed QZSI can realize voltage buck or boost and dcac inversion in a single stage with high reliability and efficiency, which makes it well suited for PV power systems [59].

CIRCUIT ANALYSIS OF THE QUASIZSOURCE INVERTER

QuasiZSource Inverter Circuit
Figs. 1a and 1b show the traditional voltage fed ZSI [4] 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 nonshootthrough states (i.e. the six active states and two conventional zero states of the traditional VSI) and the shootthrough state (i.e. both switches in at least one phase conduct simultaneously).
In the nonshootthrough 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. 2a and 2b. The shootthrough 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 shootthrough state. This network will effectively protect the circuit from damage when the shootthrough occurs and by using the shootthough state, the (quasi) Zsource network boosts the dclink voltage. The major differences between the ZSI and QZSI are (1) the QZSI draws a continuous constant dc current from the source
Figure 1a. Voltage fed Zsource inverter
Fig.4.5.which is a representation of the system during the interval of the shootthrough states To, one can get;
VL1 VC 2 Vin ,VL2 VC1
(3)
VPN
0,Vdiode
VC1 VC 2
(4)
Figure 1b. Voltage fed QuasiZsource inverter
At steady state, the average voltage of the inductors over one switching cycle is zero. From (1), (3), one has
VL1
v L1 T0 (VL 2 VIN ) T1 (VIN VC1 ) 0
T
VL 2
V L 2 VC1T0 (VC 2 )T1 0
T
VC1
T1 T1 T0
Vin
VC 2
T0 T1 T0
Vin
(5)
Figure 2a. Equivalent circuit of the QZSI in nonshootthrough states
From (2), (4) and (5), the peak dclink voltage across the inverter bridge is
V pn Vc1
VC 2
T
T1 To
Vin
1
1 2To / T
Vin BVin
(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)
Figure 2b. Equivalent circuit of the QZSI in shootthrough states
while the ZSI draws a 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 wellsuited for PV power conditioning systems.

Circuit Analysis
All the voltages as well as the currents are defined in Figs 2a, 2b and the polarities are shown with arrows. Assuming that
According to Kirchhoffs current law and (7), we also can get that
IC1 =IC2 =IPNIL1 ID=2L1IPN (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

M is the modulation index; Vin is the ac peak phase voltage; P is the system power rating.
during one switching cycle, T , the interval of the shoot through state is To; the interval of nonshootthrough states is T1; thus one has T= To+T1and the shootthrough duty ratio,
(2)
m T1 T1 T0
n To T1 T0
thus m>1; mn=1;
D= To/T. From Fig.3.6 which is a representation of the
inverter during the interval of the nonshoot through states T1, one can get;

B= T/(T1T0) thus m+n =B, 1<m<B
From Table 1. we can find that the QZSI inherits all the advantages of the ZSI. It can buck or boost a voltage with
And
VL1 Vin VC1 ,VL2 VC1
(1)
a given boost factor. It is able to handle a shoot through state, and therefore it is more reliabl than the traditional VSI. It is
PN
V VC1 VL 2 VC1 VC 2 .Vdiode 0
(2)
unnecessary to add a dead band into control schemes, which reduces the output distortion.
Table 1 Voltage and average current of the QZSI and ZSI network

The two capacitors in ZSI sustain the same high voltage; while the voltage on capacitor C2 in QZSI is lower, which requires lower capacitor rating;

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;
maximum M and the minimum voltage stress across the inverter bridge with the same voltage gain. However, it has the drawback of lowfrequency ripples on the passive components of the QZSI, which requires a larger volume and weight and higher cost inductor and capacitor in the QZSI network.
The simple boost control has evenly spread shootthrough states, thus it doesnt involve lowfrequency ripples associated with output frequency; but its voltage stress is the largest with a given G.
The maximum constant boost control [7] [8] [10] makes a compromise of the two mentioned boost control methods. 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. 4 shows the sketch map. At (1/6) third harmonic injection, the maximum modulation index M = (2 / 3) can be achieved.
The shootthrough 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 doesnt involve lowfrequency ripples.
In this case, the shootthrough duty ratio is;

For the QZSI, there is a common dc rail between the source and inverter, which is easier to assemble and causes less EMI problems.




CONTROL METHODS
A. Buck/Boost Conversion Method
D T0
T
The boost factor is
1
1 3 M
2
1
(9)
If the inverter is operated entirely in the nonshootthrough states (Fig. 2a) the diode will conduct and the voltage on capacitor C1 will be equal to the input voltage while the
B 1 2D
3M 1
(10)
voltage on capacitor C2 will be zero. Therefore, VPN = Vin and the QZSI acts as a traditional VSI:
Thus when D = 0, v1n is 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 shootthrough states, one can boost PN v by a factor of B , the value of which is related to the shootthrough duty ratio, as shown in (6).This is called the boost conversion mode of the QZSI.
B. Boost Control Methods
All the boost control methods that have been explored for the traditional ZSI (i.e. simple boost, maximum boost, maximum constant boost) [57] can be utilized for QZSI control in the same manner. Generally speaking, the voltage gain of the
And the voltage gain equals
G MB
M
M 1
(11)
QZSI is G =
v1n
/ 0.5 vPN
= MB, whereas the voltage stress
Figure 3. Sketch map of constant boost control for QZSI
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.
The peak ac phase voltage can be calculated as
Fig. 3 shows the voltage gain versus the modulation index of these three boost control methods. All have significantly higher gain than traditional PWM methods. Among these three boost control methods, the maximum boost control makes the most use of the conventional zero states, so it has the
v1n
Vin G
2 2
MVin
3M 2
(12)
The above switching cycle describe the switching pattern of the leg devices which is suitable for shoot through as well as nonshootthrough states. These are the pulsing pattern which are given to the phase leg devices like IGBTs or MOSFETs.

QZSI DESIGN FOR PV POWER GENERATION SYSTEMS
A. Voltage and Current Rating for Switches
Coupled inductors are used in this application in order to minimize the size and weight. With identical current flow, the flux is doubled for each inductor.
= 2Ni /g = 2Ni AL (19)
Thus the inductance for each inductor is
Fig. 5 shows the proposed QZSI in the PV power generation system. It connects the PV arrays and outputs three
L1 L2
N / i 2N 2 / L
(20)
phase 50 Hz, 150 Vrms ac to resistive loads, which can be varied according to the modulation index (M) as well as boosting factor (B).
Given an input PV voltage range, the maximum required voltage gain of the QZSI can be determined by;
The AMCC250 core was selected, whose AL Value (H / N2) is 0.55 when the air gap lg = 2 l mm. So using (4.19), each inductor is designed with 17 turns. The saturation current is approximately 65 A by referencing the appropriate tables on the datasheet for the AMCC250 core.
Gmax
vi n
V / 2
1.7
(13)
The two capacitors are in series in the QZSI network when in the nonshootthrough states. These two capacitors absorb
in
With maximum constant boost control method, the potential minimum modulation index M and maximum boost factor
B can be determined by (4.10), (4.11)
G max
the current ripple and limit the voltage ripple on the inverter bridge so as to keep the output voltage sinusoidal. Assuming that the capacitance should be the same for each capacitor, the capacitance needed to limit the PN voltage ripple by rv%, e.g. 1%, can be calculated by;
M min
3G max1
1
.875
(14)
C1 C 2 2 V
IcT

V
B max
1.94
(15)
C1 C 2
(21)
3M 1
2 I L 1 To _ max 310F
The maximum voltage stress on the inverter bridge is
B.Vin.rv % 2
predicted by
vPN
= BmaxVin 388V. As a result, 600 V


SIMULATION RESULTS AND DISCUSSION
IGBTs and a 600 V diode are chosen for the proposed QZSI. According to Table 4.1, the voltage ratings of capacitors C1 and C2 are approximately 300 V and 100 V. respectively. Furthermore, with a given system power rating, e.g. 10 kW, the maximum current flow through the inductor is
I in = I L1 = I L2 = P /Vin = 50 A (16)
B. Inductor and Capacitor Selection
When the system is operating in boost conversion mode, the potential maximum interval of the shootthough T0, per switching cycle, can be calculated by
In this section the two models, created with the help of MATLAB/SIMULINK version 2010 for the purpose of DC to AC inversion from Photovoltaic generation system; ZSource Inverter for Photovoltaic Energy Conversion System and QuasiZSource Inverter for Photovoltaic energy conversion system. These two models are compared by its output waveform and THD analysis.
The simulations were done in MATLAB/SIMULINK with switching frequency fsw = 8kHz.To simplify the simulation the PV array was simulated with PV panel voltage
Vdc =12V. The impedance source network elements are; L1 = L2 = 500ÂµH and C1 = C2 = 400ÂµF. Filter inductor L 3 = L4 =2 H
To _ max
2
3M min
fs
24S
(17)
and Filter Capacitor C3 = 2 ÂµF.
Both systems are further analyzed for different loading conditions like R and RL loads. The resistive load is 5 k and
The inductors in the QZSI network will limit the current ripple
through the devices during boost conversion mode.
During shootthough, the inductor curren increases linearly. With the maximum constant boost control mode, the shoot through interval, To, is evenly split into two intervals of half the duration. Choosing an acceptable peak to peak current ripple, rc%, e.g. 20% in this application, the inductance can be calculated by;
RL load is 5 k5 H. Both the models are designed for single phase.

ZSource Inverter for Photovoltaic energy conversion system
The ZSource Inverter for Photovoltaic energy conversion system with R and RL Load is described with the following circuit diagram.
L L
VL T
mVin
1 To max 356H
1 2 I
I L maxrc % 2
(18)
Figure 4a. Circuit Diagram of ZSource Inverter with R load
Figure 4b. Circuit Diagram of ZSource Inverter with RL load
For simplicity the two loading conditions; R load and RL load, the simulation diagram is shown only for R load. The results are given further for both loading conditions.
Figure 5. SIMULINK model of ZSource Inverter for Photovoltaic energy conversion system
(a)
(b)
(c)
Figure 6. Various simulation outcomes for ZSource Inverter for Photovoltaic energy conversion system for R load

Output voltage and current waveform (b) Source current and ZSource Capacitor voltage (c) THD analysis with FFT method for R load
(a)
Figure 7. Various simulation outcomes of ZSource Inverter for Photovoltaic energy conversion system for RL load
(a) Output voltage and current waveform (b) Source current and ZSource Capacitor voltage (c) THD analysis with FFT method for RL load


QuasiZSource Inverter for Photovoltaic energy conversion system
The ZSource Inverter for Photovoltaic energy conversion system with R and RL Load is described with the following circuit diagram.
Figure 8a. Circuit Diagram of QuasiZSource Inverter with R load
Figure 8b Circuit Diagram of QuasiZSource Inverter with RL load
Figure 9. SIMULINK model of QuasiZSource Inverter for Photovoltaic energy conversion system
(a)
(b)
(c)
Figure 10. Various simulation outcomes of QZSI for Photovoltaic energy conversion system for R load
(a) Output voltage and current waveform (b) QuasiZnetwork capacitor (C2)
Voltage and Source Current (c) THD analysis with FFT method for R load
(a)
.(b)
(c)
Figure 11. Various simulation outcomes of QZSI for Photovoltaic energy conversion system for RL load
(a) Output voltage and current waveform (b) QuasiZnetwork capacitor (C2) Voltage and Source Current (c) THD analysis with FFT method for RL load

Performance comparison of various topology
Table 2 Comparison table for both ZSI and QZSI system with resistive loading
S.No.
Output parameter
ZSource inverter
QuasiZSource inverter
1
Output Voltage
20 V (rms)
115 V (rms)
2
Output Current
4 mA(rms)
0.032 A (rms)
3
THD
9.21%
0.86%
4
Impdencesource capacitor vg
177
49 V
5
Source current
9 A
8 A
Following comments can be given from the Table 2:

QZSI has high boosting ability than ZSI.

QZSI has higher output current.

THD is excellent in QZSI.

Less Voltage Stress on ImpedanceSource capacitor in QZSI.

Less Source current hence less losses in switching devices in QZSI.
Table 3 Comparison table for both ZSI and QZSI system with RL loading
S.No.
Output parameter
ZSource inverter
QuasiZSource inverter
1
Output Voltage
21.2 V (rms)
113 V (rms)
2
Output Current
3.7 mA(rms)
0.021 A (rms)
3
THD
6.91%
0.83%
4
Impdencesource capacitor vg
178
49 V
5
Source current
8.5 A
8.2 A
Following comments can be given from the Table 3:

QZSI has high boosting ability than ZSI.

QZSI has higher output current.

THD is excellent in QZSI.

Less Voltage Stress on ImpedanceSource capacitor in QZSI.

Less Source current hence less losses in switching devices in QZSI.



CONCLUSION
This paper presents a QuasiZsource inverter for Photovoltaic energy conversion system, 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.
With the help of Table 2 and Table 3 we can conclude that QuasiZSource Inverter based topology for photovoltaic energy conversion system has following advantageous conclusion remarks;
.

Boosting ability of QZSI is high for the same modulation index and circuit component as compared to ZSI based system. Since it boosts input PV voltage 12V to 115 V in case of R load which results very good feature of this QZSI topology.

Total Harmonics distortion is very low 0.86% & 0.83% for R and RL loading simultaneously for QZSI system, given in comparison Table 2 and Table 3 which is under the acceptable limit of THD according to IEEE standards. This is an excellent merit over the basic ZSI topology. It is universally demanded that supply voltage and currents should be of pure sinusoidal in modern power quality concepts.

The impedance source capacitor voltage stress is very low (49V) as compared to the ZSI topology, this causes increase of capacitor size and rating. Higher the value of voltage stress higher will be the size and rating of capacitor.

Source current drawn by the QZSI circuit is less than ZSI for R and RL loading simultaneously, given in comparison Table 2 and Table 3 that causes less switching losses and lower ratings of components.
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