high gain single-stage inverter for photovoltaic ac modules
TRANSCRIPT
High Gain Single-Stage Inverter for Photovoltaic AC Modules
Omar Abdel-Rahim, IEEE Student Member, Mohamed Orabi, IEEE Senior Member and Mahrous E. Ahmed, IEEE Member
APEARC, South Valley University, Aswan City 81542, Egypt
Abstract- in this paper, a high gain single-stage buck-boost grid-connected system is proposed. The proposed system consists of a high voltage gain - high efficiency - switched inductor buck-boost converter (SIBBC) and a folded cascade H-bridge inverter. The proposed converter switch is derived with a sinusoidal modulation, so that the converter output voltage is a rectified sine wave which requires only a folded cascade as a second stage. Therefore, H-bridge is used to fold converter output voltage at fundamental frequency that eliminates the switching loss. The proposed system is used to connect the PV module to the grid with achieving maximum Power Point Tracking (MPPT) control; AC module. The converter operates in DCM to inject a sinusoidal current into the grid with unity power factor. The proposed dc-ac system has some advantages such as low cost, small size and simple control. In addition, the grid connection, MPP, and unity power factor controls are executed through only one switch, the converter switch. A prototype has been built and tested for validation. Some selected simulation and experimental results have been provided.
Keywords — Single-Stage, Low-Switching-losses, High-Gain, SIBBC, H-bridge, DCM, MPPT.
I. INTRODUCTION
In recent years, the substantial increase of research and development work in the area of photovoltaic (PV) systems have made the PV power generators a feasible alternative energy resource that complements other energy sources in hybrid energy systems. The trend of fast increase of the PV energy use is related to the increasing efficiency of solar cells as well as the improvements of manufacturing technology of solar panels. The PV generators can either be grid connected (operate in distributed generation systems) or can operate in stand-alone systems [1].
A photovoltaic panel is a device that, through the photoelectric effect, converts luminous energy into electric energy. Despite the electric energy is available in the terminals of the panels in the same instant that the light reaches it, most of the electric equipment of standard use cannot directly be connected. This because the panel generated current is continuous (DC) and at low voltage (generally between 12 and 68 volts, depending on the technology used in the panel construction) and the majority of the equipment operates at alternating current (AC), at higher voltages [2]. This brings the need for power interface or as called power conditioning inverter.
H-Bridge inverter, shown in Fig. 1 is used to convert dc power into ac power, there are many pulse width modulation (PWM) techniques used to control the inverter switches. If all switches operate at fundamental grid frequency, the output of the inverter will contain low order harmonics so that large output filter is required to remove them. PWM techniques were introduced in [3] to provide high output quality with low filter size; this is done by operating the inverter at high switching frequency. It is worth noting that these approaches suffer from high switching losses which reduce the efficiency of the inverter. H-Bridge Inverter is a bucking-mode converter that requires an input voltage of greater than the designed output voltage. In this case, a boost converter is required to be employed before the inverter to boost the PV voltage into the required level as shown in Fig. 2 [4].
Inverters may be classified as single stage and two stage configurations. Two stages inverters consist of two cascaded stages. The first stage is a boost dc-dc converter and the second one is an H-bridge inverter. Single stage inverters should do both functions of boosting the input voltage and converting it into ac voltage. Single stage inverters have some advantages over two stage inverters; such as low cost and compact size. On the other hand, it suffers from low gain and low efficiency compared to two stage inverters [5]-[8].
Figure 1: H-bridge Inverter
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Figure 2: Two stages H-Bridge inverter [4].
Figure 3: The proposed dc-ac system.
Typically, the grid-connected PV systems may be required
to either buck or boost the voltage levels depending on the available PV array voltage. Usually, grid-connected PV systems involve multiple power stages, with a dedicated DC–DC converter stage for MPPT and voltage level transformation. The disadvantage with multi-stage systems is that they have a relatively lower efficiency, larger size and higher cost. Therefore the modern day trend is derived towards single-stage grid-connected configurations because of their small size, low cost, high efficiency and high reliability. Clearly, the single-stage philosophy cannot afford a dedicated DC–DC converter stage for MPPT. Therefore, in single-stage grid-connected (SSGC) PV systems, the sole power stage must achieve MPPT, boosting or bucking (if required) and inversion together.
The proposed inverter in this paper consists of switched inductor buck-boost converter with PWM modulation followed by a 50Hz H-bridge inverter; all switches of the H-bridge inverter are switched at fundamental grid frequency to reduce switching losses and so improving the efficiency of the
inverter. Buck-boost converter switch is modulated using sine wave at high switching to reduce the size of the output filter. The converter operates in the DCM to inject a naturally sinusoidal current into the grid with unity power factor.
The paper is organized in the following way. Section II presents analysis and operation of the proposed system and its principle of operation. Section II presents maximum power point tracking technique. Section IV summarizes simulation results of the proposed system. Section V summarizes some experimental results of the proposed system.
II. THE PROPOSED INVERTER SYSTEM
The proposed dc-ac system is shown in Fig 3. It consists of a sine-modulated buck-boost converter operates in DCM to inject current into the grid with unity power factor and to provide low filter size at the output. This switch is the main control switch that can execute the proposed control commands. The H-bridge inverter switches operate at the grid fundamental frequency to reduce the switching losses and to provide higher efficiency. It works as a folded cascade unit. During positive half cycle; switches SW2 and SW5 are always on and during negative half cycle; switches SW3 and SW4 are always on.
Buck-Boost converter inductor will be replaced by switched inductor proposed in [9]. It is worth noting that adding switched inductor introduces the advantage of high voltage gain with keeping the efficiency almost without change. The only limitation of the proposed converter is that it operates in DCM ( , that's inductor current ripple is higher than CCM operation that gives a limit to the power rating to be processed; which is the same feature of any DCM operation. However, as the target application is the AC modules where the required power to be process is in the range of hundred watts, DCM operation is the suitable choice.
When the converter operates in DCM, it has three modes of operation as shown in Fig. 4. Mode-1, Fig. 4 (b), takes place when SW1 and diodes D1 and D3 are on. In this case, the steady state equation of the converter is given by:
(1)
(2) Mode 2 occurs when diodes D2 and D4 are on and SW1 is off as shown in Fig. 4(c), (3)
(4)
Mode 3 occurs when SW1 and all diodes are off as shown in Fig. 4(d). Then, the inductor current becomes zero. 0 (5)
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Figure 4: (a) Switched inductor buck-boost converter (b) mode 1 (c) mode2
(d) mode 3.
(6)
Figure 5 shows inductor current and Fig. 6 shows diode D4 current. From steady state analysis and balance theory, the average value of inductor voltage is zero; 0 (7)
(8)
Using (7) and (8), the gain of the converter can be obtained as follow: √2 (9)
Where , D: the converter dusty cycle, Ts is the switching period, L is the converter inductor, and R is the load resistance. Equation (9) shows that the gain of the converter is higher than traditional buck boost converter by √2. Figure 7 shows a comparison between the gain of the switched inductor and the traditional buck-boost converter. It is clear that the gain of the switched inductor is higher than traditional buck boost converter, especially for higher duty cycles.
III. THE PROPOSED CONTROL
The proposed control is based on DCM operation. It requires the achievement of MPPT and unity power factor for the output current of the inverter in addition to boosting the
input voltage into its required level. Figure 8 shows the schematic of the proposed system, as shown in the figure, grid voltage is sensed to provide control signal for the H-bridge inverter to operate in synchronization with grid voltage, PV module's voltage and current are provided to the MPPT controller to generate reference signal. To provide sinusoidal modulation, grid voltage is rectified and then multiplied by the reference signal, comparing its output with adjusted sawtooth to generate converter control signal.
Figure 5: inductor current.
Figure 6: diode D4 current.
Figure 7: Comparison between Switched inductor and traditional buck-boost converter.
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Figure 8: Proposed control circuit.
1) MPPT Control Algorithm PV generation efficiency and power quality are the
fundamental issues. PV power sources are usually integrated with control algorithms that have the task of ensuring maximum power point (MPP) operation. Many algorithms have been developed for tracking the maximum power point of a solar array [10]---[12]. Most commonly used are the perturb-and-observe (P&O) algorithm [10] and the incremental conductance algorithm [11]. The main disadvantages of these algorithms include oscillations around the optimal operating point and the necessity of measuring the array current. Consequently, researchers have been focused on the improvement of maximum-power-point-tracking (MPPT) control and the reduction of total harmonic distortion (THD). It is very important to design the MPPT control operation so that the voltage ripple at the terminals of the PV module is a minimum. This can be achieved if the operation is performed without too much fluctuation. Maximum power point tracking algorithm proposed in [13] will be used in this paper. Figure 9 shows the flowchart of the used MPPT control technique where , and are the momentary voltage and current of the PV array. And are the previous voltage and current, respectively. The P term can be replaced by I IV V , making the calculation easier. The major check of this algorithm is achieved by detecting I IV V , and then D (duty) will be adjusted in order to move the operating point into the direction of maximum power point of the PV array.
The algorithm begin with checking if dV 0 or not. If dV 0 , then dI is checked. For dI 0 , D is held unchanged. For I 0 , D is decreased, while if dI 0 , D is increased. On the other hand, if dV 0, then I IV V is
should be checked. For I IV V 0 , D is held unchanged,
but if I IV V 0 , then D must be decreased and if
I IV V 0 , D must be increased. Then, the algorithm continues until we reach to maximum power point.
2) Unity Power Factor Control
Current injected into the grid must be sinusoidal and in phase with grid voltage, so that to achieve unity power factor DCM is utilized. For the converter operating in the DCM, the following condition must be satisfied: ∆ (10) Where is the inductor current and ∆ is the inductor current ripple, for the SIBBBC and ∆ is given by the following equations:
(11) ∆ (12) From (10), (11) and (12), the value of required converter inductor that makes the converter operates in DCM is obtained. Output capacitor of the converter should be selected small to avoid smoothing the output voltage and keeping it in sinusoidal shape. It is worth nothing that larger value of this capacitor distorts the output current. The value of the capacitor could be calculated using the following equation: ∆ (13) Where ∆ is the capacitor voltage ripple for the proposed system ∆ will be large to keep output voltage as a rectified sine wave. The proposed system has the advantages of low switches stresses as compared to two stages system. For converter switch SW1, the voltage stress is given by , where here equal to grid voltage and vary from zero to maximum value of the grid voltage, while in case of two stages system is dc value and must be greater than grid voltage. Similar, it is clear the switches in H-bridge inverter have lower stresses than two stage system. These are added advantage into the proposed system.
IV. SIMULATION RESULTS
The proposed system was simulated using PSIM software. Two PV modules of the BP485 85W PV module are used [14]. Circuit parameters are as follows: fs = 10 kHz, input capacitor Cp =10 mF, filter capacitor Cf = 1µF, switched inductors L1=L2=85 µH and output filter inductor Lf =3.5 mH. The grid voltage and frequency are 311V and 50Hz, respectively. Figure 10 shows simulation results of the grid current which is in-phase with the grid voltage and has low total harmonic distortion. Figure 11(a) shows pulses of SW1 switch and Fig.
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11(b) and (c) shows pulses of H-bridge inverter switches. Figure 12 shows the inductor current, as shown the current has a shape like a rectified sine this is due to sinusoidal modulation. Figure 13(a) shows output voltage of the converter, the output of the converter as shown is a rectified sine wave. Figure 13(b) is the grid voltage and Fig. 13(c) is the output voltage of the PV module. These results confirm the operation of the converter with a unity power factor operation. Also, MPPT can be checked as shown in Fig. 14 where the output power of the PV module is kept constant at 170W. Figure 15 shows control output signals; Fig. 15 (a) is the output of the MPPT controller. As shown in the figure, the control change the reference until it achieves maximum power but due to sinusoidal modulation output of the MPPT control is always oscillate and Fig. 15 (b) shows the modulation signal which is generated by multiplying the output of the MPPT controller by a rectified sinewave with a unity amplitude. Figure 16 and 17 show switches’ voltage stresses SW1, SW2, SW3, SW4 and SW5. As shown in these figures, all switches have lower stresses. On the other hand, it is clear that the modules operate at its maximum power. Also, the proposed inverter efficiency has been measured from simulation about 88%. This resulted high efficiency is due to small switching losses design.
Figure 9: Flowchart of the MPPT control.
Figure 10: Grid current multiplied by 50 and grid voltage.
Figure 11: Switches pulses (a) switch SW1 pulses (b) Switches SW2 and SW6 pulses (c) switches SW4 and Sw5 pulses.
Figure 12: Inductor current.
Figure 13: Converter performance (a) output voltage of the converter (b) Grid Voltage (c) PV output voltage.
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Figure 14: PV output power.
Figure 15: Control output (a) MPPT controller output (b) modulation signal.
Figure 16: Switch SW1 voltage.
Figure 17: H-bridge inverter switches voltage (a) switches SW2 and SW5
voltage (b) switches SW3 and SW4 voltage.
V. EXPERIMENTAL RESULTS
A prototype for the proposed PV system has been established in the lab. The prototype was built using three inductors two 85µH for the converter and one 3.3 mH for the filter, three MBR40250G diodes and one RURG8060 diode, two capacitors 10 mF and 1µF, H-bridge was constructed using four IRFP27N60KPBF switches while the converter switch is IXFT36N50P. The control was applied using FPGA kit to produce the PWM modulation signal for the buck-boost switch. 16f877 PIC IC where used to convert analog signals into digital signals to be the inputs for the FPGA kite. Figure 18 shows input current and output voltage of the buck-boost converter. As shown in the figure, the input current has large low frequency ripple due to sinusoidal modulation of the buck-boost converter. The output of the converter is a rectified sinewave giving the advantage of using only one stage and the need for a folded cascade H-bridge only. Figure 19 shows drain source voltage and current of the buck-boost converter which operates at 10 kHz switching frequency. Figure 20 shows the output voltage of the inverter for an 880 ohm resistive load and input voltage (PV voltage) of 18V. As shown in the figure, the shape is sinusoidal and has a small voltage ripple with minimum filter requirements. Generally, it is clear that the proposed system has the feature of high dc gain, small number of components and so low cost, and low switching loss and so high efficiency. The measured experimental efficiency was about 85%.
VI. CONCLUSION
A high gain high efficiency dc-ac system was proposed. The proposed system has advantages of low switching losses, high efficiency, low cost, small size and simple control. The proposed system has only one switch operating at high switching that can catch the maximum power from the PV module and also connect to the grid at almost unity power factor.
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Figure 18: Converter output voltage and input current.
Figure 19: converter switch SW1 drain source voltage and current.
Figure 20: inverter output voltage for 880 ohm resistive load and 18V input.
ACKNOWLEDGEMENT
The authors gratefully thank the ministry of Science, Egyptian science and technology development funds (STDF project No 346), for supporting this project.
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