Design of Single-Stage Transformer less Grid Connected Photovoltaic System

Transcription

1 Design of Single-Stage Transformer less Grid Connected Photovoltaic System Prabhakar Kumar Pranav Department of Electrical Engineering, G. H. Raisoni Institute of Engineering & Technology, Wagholi, Pune, India. Abstract Now a days, renewable of energy sources have attracted the attention of researchers and investors. The photovoltaic system is the best example of the energy renewable, because of its suitability in different applications like as distributed generation, satellite systems, and transportation. In this paper, a single-phase, single-stage current source inverter based photovoltaic system for grid connection is presents. The proposed system uses single-stage transformer which has the maximum power and interfacing the photovoltaic array to the grid. The maximum power point of photovoltaic system is maintained with a fuzzy logic controller. A proportional-resonant controller is used to control the current injected into the grid. To improve the power quality and system efficiency, a double-tuned parallel resonant circuit is used to attenuate the second- and fourth- order harmonics at the inverter dc side. A modified carrier based modulation technique for the current source inverter is proposed to magnetize the dclink inductor by shorting one of the bridge converter legs after every active switching cycle. Simulation and practical results validate and confirm the dynamic performance and power quality of the proposed system. Index Terms Current source inverter (CSI), gridconnected, maximum power point tracking (MPPT), photovoltaic (PV), etc. I. INTRODUCTION Now a days lots of energy crisis and environmental issues occurs around us, therefore the renewable of energy sources have attracted the attention of different researchers and investors. The photovoltaic system is considered to be a most promising technology among the all available renewable energy sources. The photovoltaic system is suitable for distributed generation, transportation system and satellite systems. Generally, for single and three phase photovoltaic system applications single and two-stage gridconnected systems are used. In a single-stage gridconnected photovoltaic system uses a single dc/ac power inverter to interface the photovoltaic system to the grid and to track the Maximum Power Point Mrs. P. N. Korde Department of Electrical Engineering, G. H. Raisoni Institute of Engineering & Technology, Wagholi, Pune, India. (MPP). In this topology, photovoltaic maximum power is delivered into the grid with high efficiency, low cost and small size. In two-stage grid-connected photovoltaic system uses two conversion stages: first, a dc/dc converter for boosting and conditioning the photovoltaic output voltage and tracking the MPP, and second, a dc/ac inverter for interfacing the PV system to the grid. In this topology, a high-voltage PV array is not essential, because of the dc voltage boosting stage. However, this two-stage technique suffers from reduced efficiency, higher cost, and larger size. Due to the above maintained limitations of two stage grid-connected photovoltaic system, a single-stage inverters are used in low voltage applications. The conventional voltage source inverters are most commonly used as interface unit in grid-connected photovoltaic system technology because of its simplicity and availability. The voltage buck properties of the voltage source inverters can be satisfied by using a bulky transformer which provides a high dc voltage. Moreover, an electrolytic capacitor, which presents a critical point of failure, is also needed [1]. Several multilevel inverters have been proposed later on to improve the ac-side waveform quality, to reduce the electrical stress on the power switches, and to reduce the power losses due to a high switching frequency. In this paper, a single-stage single-phase gridconnected photovoltaic system which is based on a current source inverter (CSI) is present. The CSI is used in this system because of the dc input current of CSI is continuous and it is important for a photovoltaic application. The CSI increases the reliability of system by replacing the shunt input of electrolytic capacitor with a series input inductor. Also the CSI voltage boosting capability allows a low-voltage photovoltaic array to be grid interface without the need of a transformer or an additional boost stage [2]. To demonstrate the effectiveness and robustness of the proposed system, computer-aided 67 Prabhakar Kumar Pranav, Mrs. P. N. Korde

2 simulation and practical results are used to validate the system. To demonstrate the effectiveness and robustness of the proposed system, computer-aided simulation and practical results are used to validate the system. Organization of this paper is in the following way section II reviews the development of system, the different proposed methods used in this paper are presents ii this section. In section III the simulation of the proposed system in MATLAB and the experimental performance results are presents. And finally section IV concludes this paper. II. DEVELOPMENT OF SYSTEM This section reports the different methods or components used while developing this system. The experimental setup consists of an agilest modular solar array simulator to emulate photovoltaic system operation. The sun is biggest and never ending source of energy which radiates about 3.8 x MW of electromagnetic energy into the space. But only two percent of it is utilized around the world. Hence it necessitates us to find the way to harness energy which is available in nature in free of cost. Solar Supply Double Tuned Resonant Filter PIC Microcontroller To View CRO Dc-Ac Inverter Driver Circuit given below. A. Double-Tuned Resonant Filter In a single-stage grid-connected system, the photovoltaic system utilizes a single conversion unit (dc/ac power inverter) to track the maximum power point (MPP) and interface the PV system to the grid. In such a topology, photovoltaic maximum power is delivered into the grid with high efficiency, small size, and low cost. Two-stage grid-connected photovoltaic system utilizes two conversion stages: a dc/dc converter for boosting and conditioning the PV output voltage and tracking the MPP, and a dc/ac inverter for interfacing the photovoltaic system to the grid. This two-stage technique suffers from reduced efficiency, higher cost, and larger size. From the aforementioned drawbacks of existing grid connected photovoltaic systems, it is apparent that the efficiency and footprint of the two-stage gridconnected system are not attractive [3]. In a single-phase current source inverter (CSI), the pulsating instantaneous power of the system generates even harmonics in the dc-link current. These even harmonics affects in different photovoltaic system applications and it also reduces the lifetime of photovoltaic system. To reduce the necessary dc-link inductance, a parallel resonant circuit tuned to the second-order harmonic is employed in series with the dc-link inductor. The filter is capable of smoothing the dclink current by using relatively small inductances. Even though the impact of the second-order harmonic is significant in the dc-link current, the fourth-order harmonic can also affect the dc-link current, especially when the CSI operates at high modulation indices [4]. Therefore, in an attempt to improve the parallel resonant circuit, this paper proposes a double-tuned parallel resonant circuit tuned at the second- and fourth-order harmonics, as shown in Fig. 2. 5V 12v DC Power Supply Fig. 1. The Proposed Block Diagram. The baove fig.1. shows the block diagram of proposed system. The detailed description of this is Fig. 2. The Double-Tuned Resonant Filter. 68 Prabhakar Kumar Pranav, Mrs. P. N. Korde

3 In order to tune the resonant filter to the desired harmonic frequencies, the impedance of C1 and the total impedance of L1, L2, and C2 should have equal values of opposite sign. For simplicity, assume component resistances are small, and thus can be neglected in the calculation (1) From the equation (1), the capacitances are represented by the following equations: (2) where C1 and C2 are the resonant filter capacitances, L1 and L2 are the resonant filter inductances, ZC1 is C1 impedance, Zt is the total impedance of L1, L2, and C2 and ω is the angular frequency of the secondor fourth-orders harmonics [5]. Select the inductance values, which are capable of allowing the maximum di/dt at rated current. Figure 3 shows the impedance versus frequency measurement for the doubled-tuned parallel resonant circuit. The filter is capable of eliminating both the second- and fourth-order harmonics. Fig. 3. The double-tuned resonant filter response. Two-stage grid-connected photovoltaic system utilizes two conversion stages: a dc/dc converter for boosting and conditioning the photovoltaic output voltage and tracking the MPP, and a dc/ac inverter for interfacing the photovoltaic system to the grid [6]. This two-stage technique suffers from reduced efficiency, higher cost, and larger size. From the aforementioned drawbacks of existing grid connected photovoltaic systems, it is apparent that the efficiency and footprint of the two-stage gridconnected system are not attractive. B. Power Supply Since all electronics circuits work only with low DC voltage, a power supply unit is needed to provide the approximate voltage supply. This unit consists of transformer, receiver, filter and regulator. AC voltage typically 230 V is connected to a step down transformer. The output of transformer is given to the bridge rectifier and then a simple capacitor filter to produce a DC voltage initially filters it. This DC voltage is given to regulator, which gives a constant DC voltage. C. Rectifier: Rectifier to be used is bridge rectifier. It is now available in a single entity. It is IR BR Here IR stands for International Rectifier that is the company manufacturing the product. BR stands for the bridge rectifier.6 stands for its rating that is 600V,6A.Rectifier is used for converting AC into pulsating DC. Here instead of using the DC source such as battery we are using the rectifier because those sources have less life time. D. Filter Filtering should be done in order to reduce the harmonics and ripples. For this purpose we use capacitors for the filtering. They are rated at 100 V. Here output voltage from rectifier is 100V. The capacitors are used in two arms. They share this voltage equally. The capacitors are therefore rated at 1000 µf/100v. Each of the capacitors share 50V.The capacitors are electrolytic in nature. E. Regulator Regulator IC units contains the circuitry for reference source, comparator amplifier, control device and overload protection in a single IC. Although the internal construction of the IC is somewhat different from that described one, the external operation is the same. IC units provide regulation of either a fixed positive voltage, a fixed negative voltage or an adjustable set voltage. A power supply can be built using a transformer 69 Prabhakar Kumar Pranav, Mrs. P. N. Korde

4 connected to the AC supply line to step the AC voltage to the desired amplitude. It is then rectified filtered with the capacitor and finally regulating the DC voltage using an IC regulator. The regulators can be selected for operation with load currents from hundreds of milli amperes to tones of amperes, corresponding to power rating from milli watts to tens of watts. F. Single Phase Inverter Single phase Inverter are designed by using Mosfets. IGBT can be used but it is of high cost. Same is the case with special devices such as GTO, SITH, IGT etc. SCR can also be used but it has commutation problem. Also it requires commutating circuit which is complex in nature [8]. BJT has the problem of second breakdown. Hence we have the MOSFET as optimal device for this application. G. MOSFET MOSFET used is IRF P250.It s voltage rating is 250V,current rating is 20A.It has following advantages: Extremely high dv/dt capability Very low intrinsic capacitances Gate charge is minimized. Fast switching is possible. Ease of paralleling with other MOSFET. H. PIC Microcontroller 16F 877A PIC is used to generate square pulses to switch the MOSFETs. For this purpose in this project PIC 16F 877A is used. It is a 40 pin device. It has following features: It has 5 ports. It can handle 15 interrupts. It has 8 A/D input channels It has parallel slave port No of instructions is 35 only. Fig.4. Simulation Circuit of Proposed System. Fig.5. Simulation Circuit Of Photovoltaic Panel. III. MAT LAB SIMULATION AND EXPERIMENTAL RESULTS This section shows the MATLAB simulation and the experimental results of proposed system. MATLAB is a software package for computation in engineering, science, and applied mathematics. It offers a powerful programming language, excellent graphics, and a wide range of expert knowledge. To validate the performance of the proposed system, simulations are performed using MATLAB/Simulink. The results of the proposed system under normal weather conditions. Fig.4. shows the Simulation circuit of proposed system and fig. 5. shows the simulation circuit of photovoltaic panel. 70 Prabhakar Kumar Pranav, Mrs. P. N. Korde

5 Fig.6. CSI with a large inductance at M = 0. Fig.8. CSI with a large inductance at M = 0.9 Fig.7. CSI with a resonant filter at M = 0.5 Fig.9. CSI with a resonant filter at M = 0.9 Fig.10. Experimental results of the proposed grid connected system. 71 Prabhakar Kumar Pranav, Mrs. P. N. Korde

6 Fig. 6 shows the input dc current and the output ac voltage and current of the CSI with a large inductance at M = 0. Fig. 7 shows the input dc current and the output ac voltage and current of the CSI CSI with a resonant filter at M = 0.5. Fig. 8 shows the input dc current and the output ac voltage and current of the CSI with a large inductance at M = 0.9. And fig. 9 shows the input dc current and the output ac voltage and current of the CSI with a resonant filter at M = 0.9. The results of the proposed grid connected CSI are shown in Fig. 10. The optimum photovoltaic current is attained in a relatively short time and has a small steady-state oscillation. Also, the CSI successfully injects the photovoltaic current into the grid with low total harmonics distortion. IV. CONCLUSION A single-stage single-phase grid-connected PV system using a CSI has been under study that can meet the grid requirements without using a high dc voltage or a bulky transformer. The control structure of the proposed system consists of MPPT, a current loop, and a voltage loop to improve system performance during normal and varying weather conditions. Since the system consists of a singlestage, the photovoltaic power is delivered to the grid with high efficiency, low cost, and small footprint. A modified carrier-based modulation technique has been proposed to provide a short circuit current path on the dc side to magnetize the inductor after every conduction mode. Moreover, a double-tuned resonant filter has been proposed to suppress the second- and fourth-order harmonics on the dc side with relatively small inductance. The THD of the grid- injected current was 1.5% in the simulation and around 2% practically. The feasibility and effectiveness of the proposed system has been successfully evaluated with various simulation studies and practical implementation REFERENCES [1] Bader N. Alajmi, Khaled H. Ahmed, Single Phase Transformer-Less Grid Connected Pv System. IEEE Transactions On Power Electronics, Vol. 28, No. 6, June 2013 [2] R.Janakiraman and V.Karthikeyan, Single Phase Transformer-Less Grid Connected Pv System International Journal of Emerging Technology in Computer Science & Electronics (IJETCSE) ISSN: Volume 8 Issue 1 APRIL [3] Marcelo Gradella Villalva, Jonas Rafael Gazoli, and Ernesto Ruppert Filho, Comprehensive Approach to Modeling and Simulation of Photovoltaic Arrays IEEE Transactions On Power Electronics, Vol. 24, No. 5, May [4] Fei Wang, Student Member, IEEE, Jorge L. Duarte, Member, IEEE, and Marcel A. M. Hendrix, Member, IEEE, Grid-Interfacing Converter Systems With Enhanced Voltage Quality for Microgrid Application Concept and Implementation Ieee Transactions On Power Electronics, Vol. 26, No. 12, December 2011 [5] Jason R. Wells, Member, Patrick L. Chapman, Senior Member, IEEE,Philip T. Krein, Fellow, IEEE, and Brett M. Nee, Student Member, IEEE, Modulation-Based Harmonic Elimination IEEE Transactions On Power Electronics, Vol. 22, No. 1, January [6] Salim Abouda, Frederic Nollet, Design, Simulation and Voltage Control of Standalone Photovoltaic System Based MPPT: Application to a Pumping system International Journal Of Renewable Energy Research. 72 Prabhakar Kumar Pranav, Mrs. P. N. Korde