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K.K.Saravananet al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945 Research Paper DESIGN AND INVESTIGATION OF GRID CONN...
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K.K.Saravananet al., International Journal of Advanced Engineering Technology

E-ISSN 0976-3945

Research Paper

DESIGN AND INVESTIGATION OF GRID CONNECTED CURRENT SOURCE INVERTER FOR PHOTOVOLTAIC SYSTEM K.K.Saravanan1, Dr. N. Stalin2, Dr.T.SreeRenga Raja3 Address for Correspondence 1 Assistant Professor, Department of Electrical and Electronics Engineering, University College of Engineering, Panruti, Tamilnadu, India. 2 Assistant Professor, Department of Petrochemical Technology, Anna University-BIT Campus, Tiruchirappalli, Tamilnadu, India. 3 Associate Professor, Department of Electrical and Electronics Engineering, Anna University-BIT Campus, Tiruchirappalli,Tamilnadu, India. ABSTRACT A high efficiency and operating life of grid feeding solar photovoltaic (PV) inverters are demanded. Because of reduced dclink capacitor requirement, current source inverter (CSI) offers higher reliability than the voltage source based solar inverter. Nonetheless, conventional three-phase pulse width modulated (PWM) current source based solar inverter injects high earth leakage current into the grid. In order to suppress this current, an isolation transformer can be used. Use of this transformer increases the cost and size, and decreases overall efficiency. To address the previously stated impediments, a modified CSI is proposed in this paper. The proposed inverter suppresses the earth leakage current without using an isolation transformer, thereby increasing the efficiency and reducing cost as compared to conventional current source based solar inverters. A mathematical model of the system is derived based on which controller for the operation of the inverter is designed. The viability of the plan is verified through detailed simulation study. KEYWORDS-Pulse Width Modulated (PWM),Insulated Gate Bipolar Transistor (IGBT) PhotoVoltaic, Current Source Inverter, Voltage Source Inverter.

1. INTRODUCTION In grid sustaining solar PV system, electricity generated by solar PV is fed to the grid without storage. The major components of grid sustaining solar PV system are solar PV panels, inverters and other equalization of system. Life of solar PV panels is around 20 years. However, inverters are less reliable and normally have 3–7 years of life. It is responsible for about 60–70% of the total failures. This value is required to be higher for “string” (low/medium power) solar PV inverters, because of presentation to high ambient temperature. Considering the previously stated reasons, alternatives to conventional aluminium electrolytic capacitors (AEC) are investigated.Derated AECs are capable of operating at high temperatures, in this way enhancing reliability of the system. Solid film type ac capacitors also offer high reliability as compared to conventional AEC. However, size and cost of these choices are higher than those of AEC.Existing solar (string) inverters are based on voltage source inverter (VSI) topology [6]–[8]. Considering the high current ripple supplied/ consumed by the dc-link capacitor in VSI, large value of capacitance is required to limit the voltage ripple across solar PV terminals. Consequently, options proposedabove may not be economically suitable. Alternatively, in current source inverter (CSI) topology, dc-link inductor limits the ripple in dc current to a low value [2].In this manner, current supplied/ consumed by the capacitor has less ripple due to additional current smoothing action of the dc-link inductor. Consequently, CSI requires smaller dc-link capacitor, thereby facilitating the utilizationof highly reliable capacitor. In this manner, current source based solar inverters will have longer operational life as compared to voltage source based solar inverters. In addition to high reliability, solar inverter should also have high efficiency of power conversion. Typically, solar PV string voltages are small; in this way, extra dc-dc boost converter or ac transformer is utilized along with VSI. However, CSI has integrated boost functionality and therefore does not require an Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/280-284

additional component for voltage boosting. Comparison of the efficiencies of VSI and CSI topologies is reported in [3]. In summary, CSI offers high reliability at almost the same efficiency as compared to that of voltage source based solar inverter. Another issue identified with diminishing the size of dc capacitor is the adequacy of maximum power point tracking (MPPT). As dc capacitor value is reduced in VSI, high (switching) frequency voltage ripple on the dc bus voltage increases, thereby reducing the MPPT viability [9]. However, in case of CSI, both dc-link inductor and dc-link capacitor collectively determine the ripple in dc voltage. In this manner, low value of voltage ripple can be achieved by suitable selection of dc inductor. Thus, low capacitance values in CSI do not affect the MPPT. Utilization of three-phase pulse width modulated (PWM) CSI for solar PV application is recommended in [7]–[11]. The utilization of space vector modulation technique for grid sustaining CSI is discussed in [10], while in [11], issues related to control and MPPT are discussed. Modeling of CSI for PV application is reported in [1], and the utilization of one cycle controller for this inverter is recommended in [5]. However, these inverters produce high common mode voltage, which forces common mode earth leakage current. This current should be limited to small value for human safety and EMI compatibility [6]–[8]. Standard DIN VDE 01261-1 defines the limits on the common mode earth leakage currents to be 300 mA for grid-connected solar PV systems [3]. Immediate disconnection of inverter is recommended if leakage current exceeds this limit. In this paper, issues identified to common mode voltage and earth leakage current in conventional PWM-CSI are discussed. The proposed topology and its operation are described. Performance of the proposed inverter along with its controller is predicted by simulation study, and the results are shown in simulation results.

K.K.Saravananet al., International Journal of Advanced Engineering Technology

2. THREE INVERTER

PHASE

CURRENT

E-ISSN 0976-3945

SOURCE

Figure 1. Three Phase Current Source Inverter Power inverters are devices which can convert electrical energy of DC form into that of AC. They come in all shapes and sizes, from low power functions such as powering a car radio to that of backing up a building in case of power outage. Inverters can come in many different varieties, differing in price, power, efficiency and purpose. The purpose of a DC/AC power inverter is typically to take DC power supplied by a battery, such as a 12 volt car battery, and transform it into a 120 volt AC power source operating at 60 Hz, emulating the power available at an ordinary household electrical outlet Power inverters are used today for many tasks like powering appliances in a car such as cell phones, radios and televisions. They also come in handy for consumers who own camping vehicles, boats and at construction sites where an electric grid may not be as accessible to hook into. Inverters allow the user to provide AC power in areas where only batteries can be made available, allowing portability and freeing the user of long power cords. On the market today are two different types of power inverters, modified sine wave and pure sine wave generators. These inverters differ in their outputs, providing varying levels of efficiency and distortion that can affect electronic devices in different ways. 3. CONVENTIONAL THREE-PHASE PWM-CSI Conventional three-phase PWM-CSI requires an intermediate isolation transformer to feed power to the grid. The use of this transformer reduces the efficiency of conversion by about 2–3%. Furthermore, it increases the size, cost, and weight of inverter, making it unviable for module integrated or string inverter applications. It recommends the use of a common mode choke in the dc-link. In addition, a modified PWM strategy is also suggested. However, the strategy does not completely eliminate the common mode voltage. Therefore, this topology is suitable for module integrated inverters (low power), wherein parasitic capacitance is small and therefore the leakage current. However, for string inverter application, where in various modules are connected to a single inverter, small common mode voltage can inject significant leakage current. To address the aforementioned limitations, a modified SVPWM CSI is proposed in this paper.

Figure 3.System configurations. (a) Direct grid connection. (b) Grid connection. The inverter can be either directly connected to ac grid or through an isolation transformer as shown in Fig. 3(a) and (b), respectively. In the latter case either negative or positive terminal of input dc can be connected to earth. Therefore, voltages appearing across the parasitic capacitors, Cp1 and Cp2 are dc. For instance, if negative terminal of PV is connected to earthas shown in Fig. 3(b).Voltage across Cp1 is 0 V and that across Cp2 is equal to Vdc. Therefore, no current flows through the parasitic capacitance to earth. In case of a transformer less system as shown in Fig.3(a), neither negative nor positive terminal of PV can be directly connected to earth.This is because, grid neutral is connected to earth at the substation, and potential difference between terminals of PV (either −ve or +ve terminal) and neutral depends on the conducting state of the inverter switches. Therefore, connecting PV terminals to earth will inject high earth fault currents. On the other hand, if PV terminals are not earthed, high (switching) frequency voltage appears across Cp1 and Cp2. This results in the flow of high earth leakage current. To demonstrate the generation of high frequency common mode voltage and corresponding current, a conventional SVPWM-CSI based solar PV system is used.Space vector PWM technique reported in is used to generate gating signals. Though dc-link voltage is maintained at 300 V, voltage across the PV terminals and earth has both dc and high-frequency ac components. This highfrequency ac is due to the switching action of the inverter devices. Harmonic spectrum of parasitic capacitor voltage vp2. 3.1 SIMULATION RESULTS FOR CONVENTIONAL CSI

Figure 4.Output of Conventional CSI Ac phase voltage vg-a (200v/div), inverter PWM current ii-a (5A/div), and grid current ig-a(2A/div).X axis: time (10ms/div) 4. SIMULATION STUDIES F igure 2. Conventional current source based solar inverter without isolation transformer. Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/280-284

K.K.Saravananet al., International Journal of Advanced Engineering Technology

E-ISSN 0976-3945

Figure 5. Proposed Current Source Inverter Simulation Circuit Diagram initial stage. It is the output voltage after low pass 4.1 OUTPUT OF PROPOSED CURRENT secondorder filter. SOURCE INVERTER

Figure 6.Solar Panel Output Voltage Irradiance is produced by comparing the repeating sequence constant value (100) and it is given to the PVpanel. It gives the DC power to inverter.PV panel is designed by 100e6 parallel connected cells and 850 series connected cells to the producing the current and voltage respectively.

Figure 7.Inverter Output Voltage At, initial state the capacitor takes some time for charging.so the wave form has some fluctuation in

Figure8. Inverter Output Current Simulated result: Grid current (200 A/div), X-axis: Time (0.02 ms/div)

Figure 9.Common Mode Leakage Current Common mode leakage current (0.5 A/div),X-axis: time (0.02 ms/div).By reducing the action of, grid voltage affect the panel at zero crossing condition, the common mode current can be reduced below1 Amps.

Figure 10.Total Harmonic Distortion of Line Current Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/280-284

K.K.Saravananet al., International Journal of Advanced Engineering Technology

E-ISSN 0976-3945

Figure 11.Total Harmonic Distortion of Line Voltage Total Harmonic Distortion of line current is reduced where, EON,I, EOFF,I, and EOFF,D are IGBT turn on, as 4.02% and fundamental frequency is increased IGBT turn off, and diode turn off energy losses, than the conventional system respectively, at voltage Vr and current Ir. Peak value Total Harmonic Distortion of line voltage for 5 cycle of line voltage and switching frequency are is 3.92%. Low THD confirms the usability of the represented by Vline,p−p and fs, respectively. proposed circuit as grid feeding solar inverter. 5. CONCLUSION This paper recommends the use of current source 4.2 EFFICIENCY COMPARISON Efficiency of the proposed four-leg CSI is compared based solar inverters to improve the reliability as with that of conventional CSI in this section. Both compared to voltage source based inverters. It is conduction and switching losses are considered to shown that conventional CSI injects much higher evaluate efficiency.Any instant, one of the four top earth leakage current than that recommended by the switches carry idc1, and one of the four bottom standards. To address this limitation, SVPWM based switches carry idc2. Similarly, in conventional CSI, CSI can be used. Due to the connection between two one of the three top switches carry idc1, and one of the semiconductors switch midpoint of split capacitor three bottom switches carry idc2 at any time. Since dc arrangement across the PV array, undesired current link current is same in both topologies (for same flows through the C–L filter during the zero state. To power delivered), total conduction losses remain address this issue, two reverse blocking same. semiconductor switches are used as the fourth leg of CSI and it is connected with DC link capacitor. 4.3 SWITCHING LOSSES Since dc link current is same in both topologies, Following are the techniques used to eliminate the switching losses in CSI depend on the voltage flow of leakage current in the system.Introduction of appearing across the switch during turn-on and turncommon mode inductor in the dc link offers high off. The switching transition (n+ n−) → (a+ b−) in impedance to the flow of common mode leakage four-leg CSI. In case of four-leg CSI, there are two current. Split capacitor arrangement and connection transitions at this instant (n+ → a+) and (n−→b−), between fourth leg and split capacitors eliminates and the switching loss depends on phase voltages Van high-frequency component from the common mode and Vbnappearing across the switches at that instant, voltage, which in turn restricts the flow of common respectively. As Van and Vbn are positive in this mode leakage current.Addition of fourth leg avoids sector, instantaneous value of Vab is equal to the sum the possibility of undesired current flow through of instantaneous values of Van and Vbn. Therefore, neutral or any phase during the zero state. Key loss in conventional CSI for transition (a−→b−) is advantages of the proposed current source based approximately equal to the sum of losses in proposed transformer less solar inverter are small dc capacitor, CSI for transitions (n+ → a+) and (n−→b −).In case single stage conversion, and low leakage currents. A of four-leg CSI, losses are distributed over more mathematical model of the system is developed to number of devices than the proposed CSI.In proposed facilitate the design of controller. Detailed simulation CSI the fourth leg switch is connected to the studies are carried out to predict the performance of midpoint of the split capacitor it block the power the system. when reverse blocking voltage is arrived. It improves REFERENCES 1. P. P. Dash and M.Kazerani, ‘Dynamic modeling and the efficiency without the neutral wire.Loss formulas performance analysis of a grid connected current source derived for the four-leg scheme are provided below. inverter based photovoltaic system’,IEEE Transactions Total conduction losses on Sustainable Energy, vol. 2, no. 4, pp. 443– 450, 2 PC, 4−leg =3[Idc (uCE, 0 + uF,0)+Idc (rCE + rF )] 2011 2. E. Al Nabi, B. Wu, N. R. Zargari, and V. Sood Where, uCE, 0 and uF, 0 are forward voltage drops of ‘Sensorless control of CSC fed IPM machine for zero IGBT and diode, respectively. Forward resistances of and low speed operations using pulsating HFI method’, IGBT and diode are represented by rCE and rF, IEEE Transactions on Industrial Electronics, vol. 60, respectively. Total switching lossesPS, 4−leg =2Pn no. 5, pp. 1711–1723,2013. 3. B. Sahan, S. V. Araujo, C. Noding, and P. +6PphWhere,Pn is switching loss in neutral leg switch, Zacharias,‘Comparative evaluation of three phase and Pph is switching losses in phase leg switch.PS, 4−leg current source inverters for grid interfacing of = (3fs/ π) (Vline,p−pIdc )/VrIr [EON,I + EOFF,I + EOFF,D] Int J Adv Engg Tech/Vol. VII/Issue II/April-June,2016/280-284

K.K.Saravananet al., International Journal of Advanced Engineering Technology distributed and renewable energy systems’, IEEE Transactions on Industrial Electronics, vol. 26, no. 8, pp. 2304–2318,2011. 4. A. Ristow, M. Begovic, A. Pregelj, and A. Rohatgi, ‘Development of a methodology for improving photovoltaic inverter reliability’,IEEE Transactions on Industrial Electronics., vol. 55, no. 7, pp.2581– 2592,2008. 5. A. Lahyani, P. Venet, G. Grellet, and P.-J. Viverge,’Failure prediction of electrolytic capacitors during operation of a switchmode power supply’,IEEETransactions on Industrial Electronics., vol.13, no. 6, pp. 1199–1207,1998. 6. S. V. Araujo, P. Zacharias, and R.Mallwitz,‘Highly efficient single phase transformerless inverters for grid connected photovoltaic systems’,IEEETransactions on Industrial Electronics., vol. 57, no. 9, pp. 3118– 3128,2010. 7. T. Kerekes, R. Teodorescu, P. Rodriguez, G. Vazquez, and E. Aldabas, ‘A new high efficiency single phase transformer less PV inverter topology’,IEEETransactions on Industrial Electronics., vol. 58, no. 1, pp. 184–191.2011. 8. E.S.Sreeraj, K, Chatterjee, and S. Bandyopadhyay, ‘One-cycle-controlled single stage and single phasevoltagesensorless grid connected PV system’, IEEETransactions on Industrial Electronics,vol. 60, no. 3, pp. 216–1224,2013. 9. M. A. G. de Brito, L. Galotto, L. P. Sampaio, G.E.Melo, and C. A. Canesin, ‘Evaluation of the main MPPT techniques for photovoltaic applications’, IEEE Transactions on Industrial Electronics., vol. 60, no. 3, pp. 1156–1167,2013. 10. B. Mirafzal, M. Saghaleini, and A. K. Kaviani, ‘An SVPWM based switching pattern for stand alone and grid connected three phase single stage boost inverters’, IEEE Transactions on Industrial Electronics., vol. 26, no. 4, pp. 1102–1111,2011. 11. Y. Chen, K. Smedley, and J. Brouwer,‘A cost effective three phase grid connected inverter with maximum power point tracking’, inConference Record . 41st IEEE/IAS Annual Meeting.,Tampa, FL, USA, vol. 2, pp. 995– 1000,2006.

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E-ISSN 0976-3945

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