Dual Input Single Phase Quasi Z Source Inverter for Integrated Photovoltaic Systems

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Introduction
Photovoltaic (PV) systems are certainly trendy these days, due to the large-scale implementation of solar-based power stations globally. Environmental disasters in particular have made people acutely aware of the negative consequences of using and depending upon fossil fuels as a species. So, tremendous steps have been and are still being taken, by the owners of the power generating stations to switch over to alternative energy sources [1,2]. Te challenges regarding the use of this cleaner, alternative source as a major source of energy remains its intermittent nature, as well as the low power generating capacity of the technology [3]. PV systems, after all, can operate only with an additional source of energy as well. Traditionally, batteries are used as a backup source for PV systems, as they can only operate, store, and feed extra power to loads when necessary [4]. Solar-based inverters are used as a power conditioning unit to ensure the consistency of output power, irrespective of variations in input power [5]. But even still, the efciency of such inverters is very low.
Another important factor that afects the efciency of a PV system is the voltage gain of the inverter and the stress on the power electronic switches used [6]. Terefore, attention towards solar inverter design and performance has gained importance too. Transformer less inverters are a major area of interest for researchers as they have some notable advantages over their counterparts, such as taking up less space, weighing less, and also increased protection [7,8]. Tere are many topologies of front-end converters and load-side inverters in the literature that have specifc pros and cons. Many researchers have paid attention to Z sources, due to their unique shoot-through capability, with which they can boost power tremendously [9][10][11]. Quasi Z source inverters have improved boosting capability as well as high voltage gain [12]. Te switched capacitor models can operate with a lower number of switches, have high boosting capabilities, and put less stress on switches, which in turn improves the efciency of the system [13]. Nowadays, hybrid systems are more popular in meeting the challenges of the intermittent nature of solar PV systems. Multi-input systems have been developed recently to integrate more than one renewable energy source together, and they are found to have a better power electronic interface and bidirectional fow of power [14]. Integration of wind and solar PV systems together has a much greater efciency in the case of grid-connected systems as well [15].
To meet the major challenge of intermittence in green energy systems, a novel dual-input quasi Z source inverter is proposed, which can operate with two diferent sources independently. Tis hybrid qZSI is designed to integrate intermittent renewable energy systems and to harvest the maximum power from the hybrid sources. A pulse width modulation (PWM) scheme based on simple boost control is adopted to control the switches, which are also interfaced with the perturb and observe (P and O) method to track the maximum available power at any instant. Te proposed inverter is compared with 3 topologies of quasi Z source inverter that were proposed earlier. Tey were also simulated and the output voltage and current waveforms were plotted. Te proposed inverter can overcome the aforementioned issues such as low output voltage, low voltage gain, high stress on switches, and low boosting capability. Te design criterion mainly focuses on the integration of two sources and their independent operation. Te inverter can operate with on-grid and of-grid systems efectively when compared to the existing topologies. A scale down model was developed to validate the performance.
Te novelty of the proposed methodology is the implementation of qZSI with reduced number of switches, increased voltage gain, and a high boosting factor which results in increased efciency of the overall system. Te system performance is evaluated based on the boosting factor and THD level.
Te segments involved in this manuscript have been structured with 7 sections. Section 2 deals with the operation of the proposed inverter under various modes using the control strategy. In Section 3, P and O method and its modelling are discussed with its fowchart and simulation results. Section 4 deals with the mathematical modelling and steady state analysis of the proposed system. Te features of three diferent topologies of qZSI that are in literature earlier are compared with the proposed system in Section 5. Simulation results and experimental verifcation are performed in Sections 6 and 7, respectively.

Proposed Dual Input Quasi Z Source Inverter
A quasi Z source inverter is a type of impedance source inverter that can produce extended boosting capability. Like a traditional Z source inverter, the quasi Z source inverter also has passive elements and diodes. Te rapid boosting of this type of inverter is achieved due to the shoot through state in which both of the switches of the same leg are turned on at the very same instant. Tere are many topologies of quasi Z source inverter, yet there is no single-phase quasi Z source inverter with dual input. Te inverter proposed in this paper operates with a dual source, so that it can feed the load continuously at times of intermittence.
Te proposed dual input qZSI is shown in Figure 1. It consists of a front end qZSI network and a switched capacitor model at the back end. Both the blocks are powered by two individual sources +V in and − V in of opposite polarity. Te quasi Z source inverter block operates to boost the power from the input source. Te inductor and capacitor network feed the extra power during the shoot through mode and produce a very high boosted voltage at the output. Te diodes operate the capacitors and inductors during the shoot through mode. Te switch S 1 connects the source to the qZSI network at the front end. D 5 is a freewheeling diode that operates after the discharge of the capacitor and maintains the output current continuously.
Te use of a switched capacitor model ensures a high boost of the input voltage and very high voltage gain during the shoot through mode of operation. Since more voltage is shared by the passive elements, the stress on the switches and the heat loss across them are reduced as well. Tis characteristic reduces the switching losses in the process. Te two sources can operate individually and can also be fed to the load resistor as both the front and back end blocks are coupled by the passive elements. Te control strategy and the associated modes of operation are discussed below.

Control
Strategy. PWM based on simple boost control is adopted for its enhanced boosting capability of the inverter. We consider a PV system operating with PV and wind as major sources and a battery as a backup source. Te system modeled to investigate the performance of the inverter is shown in Figure 2. Tere are four carrier signals CW 1 , CW 2 , CW 3 , and CW 4 , based upon which the output voltage is regulated. Here, CW 1 is fetched from the user defned value, CW 2 is fetched from the MPPT of the PV system, CW 3 is fetched from the battery voltage, and CW 4 is fetched from the other renewable energy source.

Mode 1, Front End Source Active.
In this mode, the source in the front end is active. Te front end consists of the impedance network, in which the boosting takes place rapidly. In this mode, while the switch S 1 is on, the fow of current is through all the passive elements and the load is fed with a high voltage and current. When the switch S 2 is on, the current fows in the back end network, i.e., capacitor C 4 is  Advances in Materials Science and Engineering charged in the switched capacitor network and the load is energized. During the shoot through state, both the switches are turned on and the load is fed with a huge current while the switched capacitor is charged. Te direction of the current in this mode is shown in Figure 3.

Mode 2, Back End Source Active.
In mode 2, the back end source is made active. When switch S 1 is turned on, diode D 1 is reverse biased and buck operation is possible. When S 2 is turned on, both the networks are active and there is a boosted voltage to the load. During the shoot through state, both S 1 and S 2 are simultaneously on and a high current fows through the load. Te capacitor is charged during this mode and the current direction is kept uniform by the freewheeling diode. Te current direction in this mode of operation is shown in Figure 4.

Mode 3, Both the Sources Active.
In this mode of operation, both the sources are active and the inverter is fed with its rated supply. When switch S 1 is turned on, the Z source network is active and a wide boosting of the supply power takes place. When switch S 2 is turned on, the inverter operates in the switched capacitor mode. Buck operation is possible in this mode as well. During the shoot through mode, both the sources feed the inverter with a very high voltage, and the load is fed with a tremendously high voltage.
Here, the voltage gain is said to be very high and the inverter operates at maximum efciency. Te current direction in this mode of operation is shown in Figure 5.

Logical Diagram of the PWM Strategy.
Simple boost control is adopted in this paper and is interfaced with the perturb (P) and observe (O) algorithm. Te logical diagram is shown in Figure 6. Te four-input carrier wave (CW) feeds the gates, which generate two gate pulses. CW 1 , CW 2 , CW 3 , and CW 4 are the carrier signals which feed the inputs of the user reference value, MPP, battery, and wind power, respectively. Te switching pattern is shown in Figure 7. Te gate pulse is generated by correlating two sine waveforms with a carrier triangular wave. Whenever the magnitude of the carrier wave is more, the shoot through state is achieved and when less individual switches are turned on. When CW 1 and CW 3 are in a high state, the system operates from the energy delivered by the PV system alone and mode 1 is realized. If CW 4 is in a high state, the system exhibits mode 2 operation. When all three values are set to high, the system operates in mode 3. Te gate pulses are complementary during buck and boost modes. During the shoot through mode, both the gate pulses have the same magnitude, and during this duration, shoot through capability is achieved. Extended boost capability is also possible in this mode. It is to be noted that in the entire mode, buck,

Perturb and Observe Method
Solar power is intermittent in nature, of course. Te level of insolation changes throughout the day. A specifc system is required to track the maximum power that can be generated at any specifc instant. Perturb and observe is a method of MPPT by which the maximum power can be tracked continuously. Te method is more accurate because of its continuous tracking mechanism and the iterations are very high. A solar cell is designed with the P and O algorithm and simulated in MATLAB/Simulink. Te modeled circuit is shown in Figure 8. Te current and voltage sensors sense the current and voltage at the output of the panel and feed them to the controller. Te controller is programmed with a base and high value within which the panel operates. Based upon these values of voltage and current, continuous tracking of maximum power is achieved and the values are recorded. Te maximum value is fed to the PWM generation circuit via CW 2 . Te controller then generates pulses according to the MPP, and the inverter operates in either buck or boost mode accordingly.
Te P and O algorithm is interfaced in the system by the fow chart shown in Figure 9. Te values of current and voltage are measured periodically. Te resultant power is stored and compared with the previously obtained power. Based upon the resultant value, an error signal is generated, which triggers CW 2 and the pulse width is modifed. R(t) is the instantaneous value of the reference achieved from the old and new values of power. Figure 10 shows the simulation results of the proposed P and O algorithm. Te variation of the duty cycle with respect to the change in the output voltage and the marginal changes of irradiance to that of the temperature is clearly depicted from the fgure.

Steady State and Mathematical Analysis
By applying Kirchof's voltage law to Figure 5, the equations for the voltage can be expressed as By applying the voltage second principle and the equations from (1) to (7), the voltage gain versus duty cycle can be obtained as    Advances in Materials Science and Engineering Substituting the equations (8) to (11) in the voltage equations and applying coupled inductor turns ratio (N 2 / N 1 � n) and coupling coefcient k, the voltage across the capacitors can be given as Ignoring leakage reactance and assuming k � 1, the voltage gain is given by In addition to the capacitors, the associated voltage stress on the switches can be found by M S � V S /V 0 is the voltage stress on the switch and M D � V D /V 0 is the voltage stress on the diode. By using Ampere second balance principle, the stress on the switches can be calculated as

Comparison Study
Quasi Z-source inverters have been discussed in the literature since 2011. Since then, there have been many new topologies of quasi Z source inverters. Yet, a topology with a dual input is very rare in the literature. In addition, no single-phase dual input qZSI had been published in the literature. Dual input can always serve in times of intermittence of renewable energy systems and aids in the efcient handling of sources with less overall stress on the switches of a system. Here, a few dual input quasi Z source inverters are compared with the proposed inverter. A dual input PV inverter was proposed in [16], which can produce two DC and one AC output. A multi-input, multi-output inverter was introduced in [17] whose dynamic response is fast. A cascaded type multilevel inverter was proposed in [18] which has a high boosting capability. A comparison in terms of the number of components, phase, and boost factors is shown in Table 2 and it is to be noted that the boost factor of the proposed inverter is high compared to its counterparts.

Simulation
Te system comprising the dual input hybrid quasi Z source inverter in Figure 2 was simulated in the MATLAB/Simulink environment. Tree diferent conditions are taken into consideration to plot the characteristics and depict the three modes of operation in this simulation.
To plot the characteristics of mode 1, the voltage from the PV alone was considered. Now, the front-end network alone was powered and the boost operation was performed. Figure 11 shows the output voltage and current waveforms for mode 1. For an input of 50 V, the system generated an output of 100 V.
Te characteristics of mode 2 were simulated by removing the PV power and activating the wind power alone. In this case, the back end network was fully powered and the output voltage was boosted. Figure 12 shows the output voltage and current waveforms for mode 2. When an input of 75 V was fed, the inverter boosted it to 150 V.
Mode 3 characteristics were plotted by activating both the sources. Now, there is a very high boost in voltage and continuous current is obtained. When the two sources were powered by 50 V and 50 V, respectively, the system fed the load with 200 V. Te output waveforms are shown in Figure 13. Te various input and output voltages are shown in Table 3.
FFT analysis of the waveform was performed in MAT-LAB to measure the total harmonic distortion (THD). Te bar chart of the magnitude of the fundamental frequency vs the harmonic order is shown in Figure 14. Te THD value was 24.15% with an LC flter connected across the load. Te low THD value is because of the lower stress and the minimal number of switches. Also, the passive elements carry the maximum voltage and the THD value is found to be considerably low.
Te proposed PWM-based simple boost controller was used to test the performance of its counterparts, and the voltage and current waveforms were obtained with the same input voltages as that of the proposed topology. Te topology proposed in [16] produced an output voltages of 40 V, 60 V, and 90 V for input voltages of 50 V, 75 V, and 100 V, respectively. Te corresponding voltage and current waveforms are shown in Figure 15.
Te output waveforms for the topology proposed in [17] are shown in Figure 16. Te output voltages are 60 V, 85 V, and 120 V for an input voltage of 50 V, 75 V, and 100 V, respectively.
Te voltage and current waveforms for the topology proposed in [18] are shown in Figure 17. Te inverter boosted an input voltage of 50 V, 75 V, and 100 V to 65 V, 100 V, and 130 V, respectively.
Te output voltages for various inputs are tabulated in Table 4 and the comparison chart is shown in Figure 17. Te proposed topology has shown a signifcant rise in the output voltages compared to its counterparts.
Te inference from the comparative chart of the voltage profle from Figure 18 is that, the topologies proposed in [16] are seen to operate in buck mode for all the three diferent input voltages. Te topology proposed in [17] is seen to operate in buck mode for an input voltage of 50 V and boost mode for an input voltage of 75 V and 100 V. However, the voltage gain is not large compared to the proposed inverter. Te topology proposed in [18] has shown a boost mode of operation for all three diferent types of input voltages. For an input voltage of 50 V, 75 V, and 100 V, the inverter gave an output voltage of 65 V, 100 V, and 130 V, respectively, with a voltage gain of 1.3.
Te proposed inverter has produced an output voltage of 100 V, 150 V, and 200 V for an input voltage of 50 V, 75 V, and 100 V, respectively. Te voltage gain is 2 for the proposed inverter which is very high and also has reduced number of switches. Te boosting factor is also very high compared to its counterparts and is more reliable for intermittent sources.

Experimental Verification
Te system performance was experimentally verifed regarding both buck and boost operations with a scale down model shown in Figure 19, which was developed in the laboratory. Te system was powered by a battery, a PV panel, and a small wind power system. A wind power module with a capacity of 50 W was used, and a solar PV training module of 500 W was used to realize the solar PV array. A battery bank of 12 V, 3 Ah capacity was used as a backup source. Te wind speed and the PV output were varied, and the inverter was fed with diferent voltages and the output voltage was noted.
Te output voltage curve for an input voltage of 50 V fed by the PV and battery into +V dc is shown in Figure 20 and the value of the output voltage was 100 V.
Te output voltage of the wind and battery was boosted using a chopper and was fed at 75 V. Te overall voltage through − V dc was 75 V and the output voltage of the inverter was found to be 150 V. Te output voltage characteristic is shown in Figure 21.
Furthermore, both the PV and wind were energized sufciently to feed the system with a net input voltage of 100 V and the performance of the inverter was tested. Te inverter gave an output voltage of 200 V and the characteristics are shown in Figure 22.
Te values of the voltage levels during the experiment, using a laboratory prototype, are shown in Table 5. It is evident from the characteristics that the inverter adapts itself to the variation in the input voltages and boosts the voltage consistently to a minimum value of 50%.          Figure 19: Experimental setup.  variations of the input voltages obtained from solar and wind. Te THD value is also considerably low compared to the previous converters, and the reduced number of switches has contributed to increased efciency of the system.

Conclusions
Te issue of intermittence that was existing in the renewable energy power generation is overcome by the proposed, single phase dual input hybrid quasi Z source inverter with reduced number of switches and was simulated and the output waveforms were plotted by designing a real-time PVwind integrated system. Te P and O-based MPPT algorithm was modeled and the various output waveforms were plotted for the various modes of operation. Te output waveforms depict that the proposed dual input qZSI has a high voltage gain and enhanced boosting capability. Te boost factor is doubled and the efciency is increased by 50% compared to that of their counterparts due to the reduced number of switches. Te PWM technique adopted in this system was adaptable to MPPT and was able to operate in the shoot through state in all three modes of operation. FFT analysis shows a low THD value. Also, the harmonic order is lower. However, the frst harmonic is high and can be reduced by designing a suitable flter. Te results from the laboratory prototype have shown enhanced values of voltage gain. Te intentional variation in the input has shown considerable changes in the output as well as in the experimental setup. In comparison with its counterparts, the proposed inverter has proven to be very efective in terms of component count and boosting factor. In comparison with the simulation results of the existing topologies, the proposed inverter has a higher voltage gain. Terefore, the proposed inverter can be used in places of high intermittence and large load requirements.

Nomenclature
V L2 : Voltage across inductor L 2 V L1 : Voltage across inductor L 1 V C1 : Voltage across capacitor C 1 V C2 : Voltage across capacitor C 2 V C3 : Voltage across capacitor C 3 V C01 : Total voltage discharged across capacitor C 1 V C02 : Total voltage discharged across capacitor C 2 V C03 : Total voltage discharged across capacitor C 3 V C04 : Total voltage discharged across capacitor C 4 D: Boost factor G: Voltage gain M: Voltage stress V new : Instantaneous voltage I new : Instantaneous current P new : Instantaneous power.

Data Availability
Te data used to support the fndings of this study are included within the article.