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BULL ET IN OF THE G EO RG IAN NATI ONAL ACADEM Y O F SCI ENCE S, vol.
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*Shabestar Branch, Islamic Azad University, Shabestar, Iran
JamshidRoshdi@yahoo.com
**Department of Electronic, Shabestar Branch, Islamic Azad University, Shabestar, Iran
Pouladi.Jaber@gmail.com
ABSTRACT.
In this paper, a new type of single-phase grid-connected photovoltaic system through Z- source inverter along with an automatic output voltage controller is introduced. Such system has a buck and boost converters which are being sequentially aligned to provide Z - source inverter with the required input constant voltage. Also, to minimize the output voltage ripple and to conduct filtering, the harmonics related to capacitor parallel with the output were intended which will eventually lead to an almost sinusoidal voltage and with a relative range approximately equal to the amplitude of the grid voltage. The results of simulation with MATLAB / Simulink software can be observed. © 2015 Bull. Georg. Natl.Acad. Sci.
Keywords : solar light energy, Z - source inverter, buck converter, boost converter, solar cell
INTRODUCTION
Photovoltaic (PV) energy sits among the resources that provide part of the required electric energy for human and in future. The highest share of this type of energy in the future will be for systems connected to the grid. Currently, European countries have adopted policies supporting the development of grid-connected systems in their countries [1]. By increasing the solar energy application, various technologies have been developed to connect these systems to the grid. Extracting maximum power from photovoltaic arrays and delivering appropriate energy to the grid are the important objectives of grid-connected Photovoltaic
(PV) systems. Figure 1 is one of the most common structures used to connect the Photovoltaic (PV) array to the grid. Such structure is consisted of a DC-DC boost converter and a three-phase inverter. In this structure, the inverter provides the requirements for establishing the connection to the grid and the boost converter provides the ground for extracting the maximum power, by controlling the DC bus in the array output [2].
Figure 1: The common structure of a photovoltaic system [4]
Since the solar energy is available without investing any costs, most of the photovoltaic system cost is attributed to the installation cost, consisting of the cost of solar modules, interface converters between the grid and the arrays. Thanks to the advancement in solar cell technology which has decreased the module price [3].Recently, a new inverter known as impedance source inverter is proposed for voltage boost [4]. The impedance source inverter uses LC impedance network for coupling the power source and the inverter circuit and provides the possibility of voltage boost through applying short-circuit of inverter basis.
This structure works when the intensity of the light is low and the voltage generated by the panel is low and can apply power to the system only when the voltage generated by panel is lower than the voltage required for the inverter and cannot reduce the voltage and always panel voltage should be less than the voltage required for the inverter until the used boost convertor provides the required voltage of the inverter by increasing the panel voltage. In the previous papers, the Photovoltaic (PV) connection has been investigated in [5] and [6] using the impedance source convertor to the single-phase network (grid) and also in reference [2], the application of impedance source inverter is proposed for the connection of photovoltaic system to the three-phase network.
Figure (2) also shows a structure in which the boost convertor is removed without exerting any interruption for system objectives such as extraction of the maximum array power and requirements for the connection to the grid which has some problems. In addition, in order to avoid short-circuits in the inverter bases in conventional inverters, the time delay is used causing output current damage, while no time delay is needed in the impedance source inverter. Moreover, regarding the fact that the shortcircuit incidence in the inverter bases makes no problem, the system reliability will be higher and has less susceptibility to electromagnetic noise. Accordingly, the proposed photovoltaic system is studied with MPPT control and reactive power control.
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396 V.K.Limouni, S. A.Gholamian
Using additional reactive power controller, we can continuously exploit photovoltaic (PV) system in both capacitor and inductance modes. Therefore, the system injects active power and reactive power control in day mode considering MPPT and also controls the reactive power in night mode [7]. The structure has an MPPT controller which has a high maintenance cost and mechanical amortization requiring repair and maintenance which is not cost effective.
Figure 2: Photovoltaic structure with the (MPPT) controller [2]
Some researchers have been conducted on the simple and rudimentary z-source inverter as a single phase convertor in PV systems and some articles have been written, as well [8, 9 and 10]. Z-source inverter-like model inverter has also been investigated in the papers [11, 12] and has shown the connectivity of PV system to network through this inverter. Comparing the inverter models mentioned, it can be achieved that the z-source inverter has some advantages over the inverter-like one.
In [13], PV connection is done by z-source inverter that both controllers are designed depending on the network modes.
Since the PV array output voltage widely varies depending on the light and the environment temperature, the system or the convertor must have the ability to increase voltage for receiving a constant AC output voltage and delivering it to the network.
Figure 3 shows an old complex system where figure (a) is a single-phase system which is formed from a DC/DC convertor and is used to increase the voltage in a constant frequency of a convertor and this action reduces system power and increases the noise in the system. If the PV system voltage is high enough, no convertor is needed. When the system follows this topology, it can increase the voltage at the minimum input voltage and can be connected to the network. So in order to eliminate the convertor and minimize the losses, these DC/DC inverters are applied to increase the voltage. Figure 2 shows a structure which has a relatively easy design and construction and has a lower size and weight due to the elimination of the convertor. But the switches used in the convertors of such a structure increase the cost and reduce the efficiency of the z-source systems which have been studied and discussed in papers [14, 28].
Figure 3: Two different structures for the PV connection to the grid
(a) Single- phase system, (b) Two-phase system
According to the circuit shown in Figure 4 in the paper [29], boosting operations, controlling current network, and maximum power point tracking (MPPT) being used in single-phase systems are conducted by a two-phase system in this circuit. Here, to increase the voltage, the z - source voltage is chosen greater than the input voltage which leads to the tension increase in the z source capacitor output voltage. In the z-source inverter topology (zsi) presented in the papers [4.20, 21] which has the advantages of reducing the capacitor voltage and permanent input current, the PV system is developed when placed in this topology and discussed in the paper [30]. Zsi can be considered as a single-phase structure; therefore, PV system with control strategy with zsi is similar to one-phase inverter or two-phase structure [31 and 32]. In [33], a system is suggested based on z - source series inverter (szsi), where unlike other topologies, the system size, weight, and cost are reduced.
Figure 4: The PV system connection based on z-source-like inverter
In the proposed paper, a special type of structure has been suggested that does not need the definite l ow voltage in the panel and its increase and stabilization by the boost convertor; according to the structure of figure (1) proposed in the paper [2], in the proposed method, due to the sequential presence of the two boost and buck convertors, if the voltage is
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A photovoltaic system connection to single…. 397 high in the panel output, constant and control voltage will be applied to the inverter in the two convertors combination output. On the other hand, owing to the application of this method, there is no need to use the MPPT - which is costly and has mechanical amortization. According to the structure of figure (2), mentioned in article [2], and also due to the absence of the convertors, it will not have other disadvantages related to it. In general, it can be said that the proposed structure has partly solved the disadvantages mentioned in the structures of the articles studied.
THE OVERALL STRUCTURE OF THE PROPOSED SYSTEM IN THE PAPER
In the structure shown in Figure (5), after the filtration and the removal of high frequency harmonics and voltage ripple, the generated voltage in the panel which is a DC voltage is applied to a DC / DC convertor being totally formed from two convertors of buck and boost which are placed sequentially. The converted voltage that is unequal to the panel voltage (less or more than the panel voltage) is filtered again by a capacitor and applied to a Z - source inverter. Based on the results of paper [34] and depending on the selected values for the elements in inverter output, an AC voltage with two times larger range than that of DC voltage applied to the inverter input is generated and after filtering the high-frequency harmonics in the output, the produced voltage is directed towards the AC load or to connect to the network. The impact of filtering the capacitor is simulated and investigated in the output. In general, the output waveform and the impact of each section of the circuit are shown in the part related to the simulation and results.
Figure 5: The connection of the proposed system structure of the voltage resulted from solar panel to the grid
THE EQUIVALENT CIRCUIT, (PV) CELL AND SOLAR PANEL
The PV equivalent circuit is shown in Figure 6. In the PV array equivalent circuit, the current I served as a function of the PV array voltage being investigated in paper [35].
Figure 6: photovoltaic (PV) cell equivalent circuit [35]
Given the fact that each solar cell produced voltage is approximately 0.8 V which is unacceptable and non-functional in the network, to increase the output voltage of the panels, the cells are set aside in series. To receive the required output voltage, maximize the losses of buck and boost converters, and to receive the maximum 310 V output (220 V single-phase motor), the voltage of 155 is assigned for the panel output and the number of cell series is determined according to this voltage. Also, to increase the panel current rate, a series of cells with a total voltage of 155 V are set aside in parallel with regards to the required and demanded current to create the demanded current that is connectable to the network. Finally, the solar panel with a voltage of
155 V (196 cells) and high current rate will be obtained based on the need. The circuit, the output voltage, and the panel are shown in figures 13, 14, 15, and 16.
BUCK AND BOOST CONVERTORS, THE REASON FOR THEIR BEING UTILIZED AND SEQUENCED
1. Buck Convertor
In a buck convertor whose circuit is shown in figure (7), the average output voltage is lower than the input voltage.
Figure 7: The Buck Convertor Circuit
In this article, considering the fact that the intensity of sunlight which is the source of voltage production, the connected panel to buck and boost converters, and the inverter of z-source is changeable in each hour of day and night, a certain kind of technique is needed to control these changes and consequently deliver a constant voltage to the z-source. Thus, a buck convertor is used first
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398 V.K.Limouni, S. A.Gholamian
in case of increased intensity of sunlight which may increase the voltage in cells and the panel. This convertor will reduce the voltage to reach the defined span in the convertor which is the range of changes in the panel. Finally, buck convertor is used for initial voltage control.Therefore, the minimum rated output voltage of the panel is defined by buck convertor. This means that the circuit may not be able to deliver output voltage if the output voltage of the panels is below this limit. The panel will be controlled electronically and let out of the circuit in order to compensate the disconnection of the panel from the circuit due to the reduced panel voltage which can be caused by decreased intensity of sunlight or other factors such as periodic repair, etc.
However, in case of high intensity of sunlight (high output), the reference voltage or called the output voltage of buck convertor can be altered to reduce the losses of buck and boost convertors. As a result, the loss of the convertors will decrease. If there is ideal and proper light, the best reference voltage for the least loss will be 155 V for the inverter input.
2. Boost Convertor
In a boost convertor the circuit of which is shown in figure (8), the output voltage is more than the input voltage.
Figure 8: the boost convertor circuit
In this article, the reason for using boost convertor was to transfer the voltage stabilized by buck convertor – received from the panels – to the inverter input which must be a constant voltage of 155 V since the output voltage of buck convertor is adjusted to be less than this voltage. Thus, this convertor is used to increase this voltage to be connected to z-source inverter input. It means that through the convertor controller, the inverter input voltage and eventually z-source inverter output will be determined and stabilized.
3. How to Sequence Buck and Boost Convertors
Since the intensity of sunlight changes during the day as a result of changes in weather, a fixed rate as the reference voltage is allocated for buck convertor. If the produced panel voltage is lower than this rate, the panel circuit will disconnect automatically.
However, if the produced voltage is within a limited range (higher than the reference voltage), the buck convertor output voltage will be the reference voltage itself. Thus, the voltage reaching boost convertor as the input voltage of the convertor will be a constant voltage – the reference voltage itself. As a result, the boost convertor must deliver its input voltage to the stabilized voltage determined by the reference voltage which is determined by boost convertor that is required by z-source inverter. Since the required applicable voltage for the next step (z-source input) is 155 V, the reference voltage of boost convertor is considered to be 155 V. It is proved and investigated in the related figures that in every input condition, the constant voltage is received from boost convertor output (by the change in buck convertor input, the output voltage of boost convertor, i.e. the delivered voltage to z-source will remain constant). To inject pulses to the switches (switching) in buck and boost convertors which can have one single switch with the potential to connect and disconnect, a generator pulse or the pwm technique both with the same outcomes must be used so that the output of the both convertors can be controlled and adjusted using the switches.
4. The Equivalent Circuit and the Function of Sequenced Buck and Boost Convertors
The combination of these two convertors shown in figure (9) receives the produced voltage in the panels, although being lower than the voltage required by the inverter, changes it to the voltage of 155 V, and applies it to the inverter input – under control of the defined voltage in buck convertor. The outputs of this set in figures (16, 17, and 18) are shown, investigated, and compared for various inputs which deliver a constant output to the circuit. In fact, in order to produce a constant voltage to be delivered to the next stage (z-source inverter), such a convertor is needed that can deliver the required constant output for each input in a nearly controlled manner.
Figure 9: the combination of buck and boost converters in a sequence
Z- INVERTER- SOURCE AND ITS EQUIVALENT CIRCUIT
This type of inverters, consistent with the equivalent circuit shown in Figure (10) and pertaining to its benefits such as less number of switches toward the three-phase and five-phase inverters and using Lc filter instead of the RC filter, is composed of two A and B converters which are completely (180 degrees) opposite the output of which is available from the capacitor. By applying a voltage to the resistor, a voltage about twice the DC voltage can be applied to the input voltage the amount of which depends on the switching speed in which the switching along with the carrier wave frequency is generated and adjusted by the generator pulse in the existing software (the switching frequency is 5Hz and the carrier wave frequency is 50Hz). Since this converter is connected to the global grid. Accordingly, the generator pulse carrier wave frequency is adjusted at 50 Hz. Also, in
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A photovoltaic system connection to single…. 399 order to get the optimum output close to the sinusoidal voltage, the circuit elements values have changed to the desired voltage when connected to the network to measure the output without a network connection, and if the desired voltage is obtained (310 V the maximum), the inductance and resistance circuits by filtering the network and inverter voltage puts output voltage in the circuit network in parallel with the network. And to pulse the switches in the inverter that can have on/off switch, a generator pulse or the SPWM technique both with the same outcomes must be used so that the inverter output can be controlled and adjusted to obtain the desired voltage.
Figure 10: Z- inverter circuit- source [8]
To receive the AC voltage from the solar panel, after the given steps and changing the variable panel voltage to a constant voltage which is operational and functional, the voltage that is a DC voltage should be made proportional sinusoidal voltage, proportional to the voltage network and with the same frequency. Accordingly, the Z - source inverter performs the conversion.
Figure (11) shows the block circuit inverter in DC voltage status to the input and connecting output to the network and figures
(20, 19 and 21) show the inverter outputs for different DC inputs representing a sinusoidal AC voltage produced by the inverter which is twice as much as the maximum DC voltage input (controlling the circuit switching frequency using SPWM technique).
Figure 11: Inverter Block circuit in the DC voltage state to the input and its output connection to the network
THE PARALLEL CAPACITOR IMPACT WITH THE OUTPUT IN VOLTAGE RIPPLE
In the inverter circuit output, to filter and remove high frequency harmonics and smoothing the output voltage to get close to sinusoidal voltage, a capacitor parallel to the load has been placed all outputs of which will be relatively flat in the presence of capacitor, and in the absence of any capacitor in the outputs, the ripples resulted from the switching and variable switching frequency will be generated, for example, the output at the state when the input is 150V. By placing a THD in the current path, the voltage ripple in two states of with/without capacitor are shown in figures (25 and 26) and can be compared.
THE COMBINATION OF CIRCUITS AND FINAL CONCLUSIONS
Finally, by connecting the above circuits (the photovoltaic connection, sequential buck and boost converters, Z- source inverter) to each other in series and connecting to the network, for each generated input in the panel, the final sinusoidal output with a maximum single phase range of 310 V (can be connected to the network) can be generated. Figures (22, 23 and 24) show the outputs for panel changing voltage that is a DC voltage.
THE SIMULATION RESULTS
Figure 12: Photovoltaic cell equivalent circuit [18]
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400 V.K.Limouni, S. A.Gholamian
Figure 13: Solar cell output voltage waveform
Figure 14: The Solar Panel equivalent circuit
Figure 15: Panel output voltage waveform in constant voltage mode
Figure 16: The sequential buck and boost converter waveforms with 100 V input, A: Buck Input, B: Buck Output or Boost Input,
C: Boost Output
Figure 17: The sequential buck and boost converter waveforms with 150 V input, A: Buck Input, B: Buck Output or Boost Input,
C: Boost Output
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Figure 18: The sequential buck and boost converter waveforms with 200 V input, A: Buck Input, B: Buck Output or Boost Input,
C: Boost Output
Figure 19: Z - source inverter waveforms with100 V input without a network connection, A: input voltage, B: output voltage
Figure 20: Z - source inverter waveforms with150 V input without a network connection, A: input voltage, B: output voltage
Figure 21: Z - source inverter waveforms with 200 V input without a network connection, A: input voltage, B: output voltage
Figure 22: the final circuit waveform with 100 V voltage (panel output) when connected to the network. A: input, B: output
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Figure 23: the final circuit waveform with 150 V voltage (panel output) when connected to the network. A: input, B: output
Figure 24: the final circuit waveform with 200 V voltage (panel output) when connected to the network. A: input, B: output
Figure 25: voltage ripple range at capacitor presence with panel voltage of 150 V
Figure 26: voltage ripple range at capacitor absence with panel voltage of 150 V
CONCLUSION
As mentioned in the proposed structures in the article (Figure 1) before the impedance source inverter, a boost converter was used in which the generated voltage should be less than the required voltage by the inverter to increase, stabilize and deliver the panel voltage to the inverter. In the structure of figure (2), to get the maximum power and constant voltage of the panel and finally from the inverter output, the MPPT method should be used. Using such interpretation, it can be stated that the proposed method in this article will no longer need to have the produced voltage panel to be less than required voltage of the inverter or to use the MPPT method to get the constant voltage, or the maximum power and use the other proposed methods. Under any circumstances, if the panel voltage is lower or higher than the required inverter voltage, it delivers a constant voltage to the inverter by the used buck and boost converters with which we can receive the power and constantly a constant voltage from the inverter output which will surely be cost effective.
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