THREE PHASE INVERTER USING COUPLED INDUCTOR FOR GRID CONNECTED PHOTOVOLTAIC SYSTEM

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1 THREE PHASE INVERTER USING COUPLED INDUCTOR FOR GRID CONNECTED PHOTOVOLTAIC SYSTEM G.KANIMOZHI.ME.,Mrs.S.RAKKAMMAL.ME., Mail Mail M.A.M COLLEGE OF ENGINEERING, M.A.M COLLEGE OF ENGINEERING, SIRUGANUR,SIRUGANUR, TRICHY.TRICHY. Abstract This letter presents a modulation technique for themodified coupled-inductor single-stage boost inverter (CL- SSBI)-based grid-connected photovoltaic (PV) system. This technique can reduce the system leakage current in a great deal and can meet the VDE standard. To maintain the advantages of the impedance network, only a diode is added in the front of the original topology, to block the leakage current loop during the active vectors and open-zero vectors. On the other hand, the near-state pulse width modulation (NSPWM) technique is applied with one-leg shoot-through zero vectors in order to reduce the leakage current through the conduction path in the duration of changing from and to open-zero vectors. Simultaneously, the leakage current caused by other transitions can also be reduced due to the fact that the magnitude of common-mode voltages is reduced. Simulation re-sults of the transformerless PV system are presented in two cases: modified CL-SSBI modulated by maximum constant boost (MCB) control method and NSPWM. Experimental results for both CL-SSBI topology modulated by the MCB control method and mod-ified CL-SSBI topology modulated by NSPWM are also obtained to verify the accurateness of theoretical and simulation models. Index Terms Leakage current, photovoltaic (PV) power system, shoot-through zero vector, single-stage boost inverter, width modulation. I. INTRODUCTION Thetransformerless photovoltaic (PV) power system has been attracting more and more attention for its lower cost,smaller volume, as well as higher efficiency, compared to the ones with transformer [1] [15]. One of the technical challenges is the safety issue of the leakage current caused by the common-mode voltages (CMV), conducting in the loop with parasitic capacitors between the solar panel and the ground. For single-stage boost inverter transformerless PV systems, such as the Z-source inverter [7]-based systems, the modulation strategy is carefully designed to maintain the constant CMV to reduce the leakage current. But the OPWM or EPWM method uses only odd or even active vectors to synthesize the output reference voltage, leading to only 57.7% of the maximum magnitude compared to SVPWM, and also to worsen harmonic distortion of the output waveforms. A coupled inductor single-stage boost inverter (CL-SSBI) is proposed in [16], which introduced an impedance network, including coupled inductor in the front-end of the inverter bridge. The structure is simple, while LCD can be viewed as a snubber. The converter uses shoot-through zero vectors [17] to store and transfer energy within the unique impedance network, to step up the bus voltage. Turns ratio of the coupled inductor within the impedance network can also be designed to improve the boost gain. So the ac output voltage can be regulated in a wide range and can be stepped up to a higher value. Higher power loss and lower efficiency would be unavoidable if higher boost gain is required, which is the disadvantage of inverters of this type. As shoot-through zero vectors evenly distributed among the three phase legs during a switching period [17], the equivalent switching frequency viewed from the impedance network can be six times the switching frequency of the inverter bridge, which will greatly reduce the power density and cost of the inverter. This letter presents the method to reduce the leakage current of the transformerless grid-connected PV system based on CL- SSBI. A diode is added in the front of the topology to block the leakage current loop when in the active vectors and open-zero vectors. In addition, the near-state PWM (NSPWM) technique is used with one-leg shoot-through zero vectors to reduce the leakage current caused in the transient states of changing from and to open-zero vectors. And the leakage current caused by other transitions can also be reduced due to the fact that the magnitude of CMVs is reduced. Note that the leakage current can be reduced effectively without lowering the maximum magnitude of the output reference voltage, for the modulation index of NSPWM stays in the high modulation section. II. PROPOSED TRANSFORMERLESS GRID-CONNECTED PV SYSTEM BASED ON CL-SSBI The modified CL-SSBI is shown in Fig. 1. Only a diode is added in the front of the topology compared to the original structure, to block the leakage current loop during the active vectors and open-zero vectors, of which the CMV v C M is defined as [4] v C M = 3 v +v +v a N bn cn. (1)

2 mode voltages (vc M) and vol tages (vp n, vn n ) of C L-S SBI and linear area for N SPWM contro l is mi π 3 3, π 2 3 = TABLE II SIMULATION AND EXPERIMENTAL PARAMETERS OF CL-SSBI AND CL-SSBI-D TRANSFORMERLESS PV SYSTEM Fig. 1.Transformerless grid-connected PV system based on CL-SSBI with an additional diode. TABLE I COMMON-MODE VOLTAGESv CM, VOLTAGESv N n ANDv P n IN DIFFERENT SPACE VECTORS (a) FOR CL-SSBI-D (b) FOR CL-SSBI In section A1, V 1,V 2,V 0, and V 7 are used to synthesize the output reference voltage and V shoot is inserted in open-zero vectors to realize the boost characteristics. Fig. 2(b) and (c) illustrates that the magnitude of CMV of CL-SSBI-D is lower than that of CL-SSBI, which indicates that the magnitude of the leakage current can be also reduced. III. MODULATION TECHNIQUE TO REDUCE LEAKAGE CURRENT The CMV of CL-SSBI-D changes in a maximum step value when the active vectors V o dd convert to open-zero vector V 0, and changes in a relatively high step value when the open-zero vector V 0 convert to shoot-through zero vectors V shoot, as shown in Fig. 2(c), which will induce high spikes in the leakage current due to the parasitic capacitor path. Therefore, open-zero vectors are the key to be considered to reduce the magnitude of the leakage current. The voltage between positive P or negative N solar panel and One possible technique is the NSPWM control, which omits grounded neutral n can be expressed as the open-zero vectors and employs three adjacent voltage vecv +v +v a N bn cn tors to synthesize the output reference voltage. V shoot can still v N n = 3 = v C M (2) be inserted to boost the output voltage. The utilized voltage vectors are changed every 60 throughout the space, as shown in v P n =v P N +v N n =v P N v C M. (3) Fig. 3(a). Compared to the MCB control method [see Fig. 2(a)], Because shoot-through of the inverter bridge becomes a the sections rotate 30 in clockwise. Moreover, only one-leg normal operation state, the possible switching states in- shoot-through vectors are used in order to reduce switching clude six active vectors (V 1 V 6 ), two open-zero vectors events, and are changed every 120 to assure equal current stress (V 0, V 7 ), and seven shoot-through zero vectors including one- of each leg during shoot-through zero vectors, that is V a for through (V a, V b, V c ), two-legs shoot 30 to 150, V b for 150 to 180 and 180 shoot leg shoot to 90, and through (V ab, V ac shoot shoot shoot V b shoot, V b c ) and three-legs shoot through for 90 to 30. ab c shoot shoot shoot shoot (V shoot ). For all the odd active vectors (V 1, V 3, V 5 ), all the The voltage utilization level can be indicated by the modulaeven active vectors (V 2,V 4,V 6 ), all the open-zero vectors tion index m i (m i =V m /(2V b /π), where V m is the magnitude (V 0, V 7 ), and all the shoot-through zero vectors, the common- of the reference voltage vector). Modulation index within the CL-SSBI with an additional diode (CL-SSBI-D) can be derived [0.61, 0.907]. Therefore,m i stays in the high modulation infrom (2) and (3), as shown in Table I. dex section, leading to lower harmonic distortion of the output For convenience, supposing the turns ratio N of the coupled waveforms than the remote-state PWM (RSPWM) control [19], inductor is 2.5, shoot-through zero duty cycle D 0 is 0.17, and then boost factor B is 3, according to the bus voltage expression which include OPWM and EPWM control. Under the same circuit conditions from Section II and by of V b =Bv P N [16], and using the maximum constant boost (MCB) control method realized by space vector-based PWM using the NSPWM control, the switching pattern and CMV of CL-SSBI-D in section B1 and B2 can be obtained, as shown control [18], the switching pattern and CMV of CL-SSBI and CL-SSBI-D in section A1 [see Fig. 2(a)] can be obtained, as in Fig. 3(b) and (c), in which T sh is defined as a shoot-through period. shown in Fig. 2(b) and (c), in which T s is defined as a switching period. From Fig. 3(b) and (c), changes of CMV should result in eight spikes in the leakage currents per switching cycle, corresponding

3 Fig. 2. (a) Voltage space vectors of a three-phase inverter; switching pattern and CMV of (b) CL-SSBI in section A1, and of (c) CL-SSBI-D in section A1. Fig. 3. (a) Voltage space vectors of NSPWM definition; switching pattern and CMV of CL-SSBI-D in (b) section B1 and (c) section B2. to 1600 spikes in the leakage current per fundamental cycle (T s = 100 μs, 50 Hz grid). Nevertheless, the magnitude of the leakage current is lower than that of CL-SSBI with the MCB control method. It is important to note that leakage current occurs from CMV only in the duration of transiting from or to shootthrough zero vectors with NSPWM control, when open-zero vectors are omit-ted. And the magnitude of CMV is also reduced, which leads to lower leakage current. IV. SIMULATION AND EXPERIMENTAL RESULTS In order to validate the theoretical analysis, the simulation and experimental tests of the transformerless grid-connected PV system constructed by CL-SSBI and CL-SSBI-D are carried out, respectively. The PV frame and the neutral point of the grid are grounded. The simulation and experimental parameters are shown in Table II. Fig. 4 shows the simulation results of the grid-connected CL-SSBI system modulated by MCB control. The three-phase

4 Fig. 4. Simulation waveforms of transformerless grid-connected PV system based on CL-SSBI with MCB control: (a) grid currents; (b) CMV v CM ; (c) leakage current. Fig. 5. Simulation waveforms of transformerless grid-connected PV system based on CL-SSBI-D with MCB control: (a) grid currents; (b) CMV v CM ; (c) leakage current, and with NSPWM control: (d) grid currents; and (e) CMV v C M ; and (f) leakage current. currents as shown in Fig. 4(a) present high ripple due to the high leakage current [see Fig. 4(c)]. CMV v cm shown in Fig. 4(b) has four different levels and changes eight times. The magnitude of the leakage current is 1.5 A, and its RMS is calculated as 0.96 A, which is well above the 300 ma threshold level stated in the VDE standard [20]. Fig. 5 shows the simulation results of the CL-SSBI-D gridconnected system modulated by the MCB control method and by NSPWM, respectively. The magnitude of the leakage current is 0.45 A, and its RMS is calculated as 0.28 A of CL-SSBI-D modulated by the MCB control method. While the magnitude of the leakage current is 65 ma, and its RMS is calculated as 27 ma of CL-SSBI-D modulated by NSPWM, which is below the threshold level of VDE standard. The three-phase currents of both control methods have lower ripple than that of Fig. 4(a) due to the lower leakage current. CMV v cm of CL- SSBI-D modulated by NSPWM has three different levels which lower than that of Fig. 4(b) and Fig. 5(b), and similar to Fig. 3(b). Fig. 6 shows the experimental results for CL-SSBI-D topology modulated by MCB control with the same parameters of simulation. The shoot-through duty cycle is 0.17, and boost factor is 3 if the coupled inductor is fully coupled. The diodes D 1, D 2, D 3, and D 4 are 600 V/30 A fast-recovery diodes. Due to the proper regulation of the shoot-through zero vectors and

5 Fig. 6. Experimental waveforms of transformerless PV system based on CL-SSBI with MCB control: (a) bus voltage v b, voltage v N n, leakage current i le a k ; (b) phase current i a, leakage current i le a k, and its FFT analysis. design of the coupled inductor of impedance network, the dcbus voltage is stepped up to about 400 V when the input dc source equals to 150 V. The magnitude of the leakage current is 0.4 A, of which 40 ma at the point of switching frequency and 25 ma at shoot-through frequency by FFT analysis. Fig. 7 shows the experimental results for CL-SSBI-D topology modulated by NSPWM with the same parameters of simu-lation. The magnitude of the leakage current is 80 ma, of which 10 ma at the point of switching frequency and 4 ma at shoot-through frequency by FFT Analysis. The output phase current i a has low ripple and smooth waveform. V. CONCLUSION This paper has presented a transformerless grid-connected PV system based on a coupled inductor single-stage boost three phase inverter. Diode D 4 is added in the front of the topology together with D 1, to block the leakage current loop during the active vectors and open-zero vectors. The leakage current caused in the transient states of changing from and to shoot-through zero vectors is also reduced by using the NSPWM technique with oneleg shoot-through zero vectors, when open-zero vectors are omitted. Simultaneously, the leakage current caused by other transitions can be further reduced due to the magnitude reduction of the CMV. The CMVs and the caused leakage currents are compared between CL-SSBI with MCB control and CL-SSBI-D with NSPWM. According to the simulation and experimental results, the amplitude and RMS value of the leakage current can be well below the threshold level required by the VDE standards, indicating an effective leakage current reduction. REFERENCES [1] R. Gonzalez, J. Lopez, P. Sanchis, and L. Marroyo, Transformerless inverter for single-phase photovoltaic systems, IEEE Trans. Power Elec-tron., vol. 22, no. 2, pp , Mar [2] H. Xiao and S. Xie, Transformerless split-inductor neutral point clamped three-level PV grid-connected inverter, IEEE Trans. Power Electron., vol. 27, no. 4, pp , Apr [3] S. V Araujo, P. Zacharias, and R. Mallwitz, High efficiency single-phase transformerless inverters for grid-connected photovoltaic systems, IEEETrans. Ind. Electron., vol. 57, no. 9, pp , Sep [4] M. C. Cavalcanti, K. C. de Oliveira, A. M. de Farias, F. A. S. Neves, G. M. S. Azevedo, and F. Camboim, Modulation techniques to eliminate leakage currents in transformerless three-phase photovoltaic systems, IEEE Trans. Ind. Electron., vol. 57, no. 4, pp , Apr [5] J. M. Shen, Novel transformerless grid-connected power converter with negative grounding for photovoltaic generation system, IEEE Trans. Power Electron., vol. 27, no. 4, pp , Apr [6] O. Lopez, F. D. Freijedo, A. G. Yepes, P. Fernandez -Comesana, J. Malvar, R. Teodorescu, and J. Doval-Gandoy, Eliminating ground current in a transformerless photovoltaic application, IEEE Trans. Energy Convers., vol. 25, no. 1, pp , Mar [7] F. Bradaschia, M. C. Cavalcanti, P. E. P. Ferraz, F. A. S. Neves, E. C. dos Santos, Jr., and J. H. G. M. da Silva, Modulation for three-phase trans-formerless Z-source inverter to reduce leakage currents in photovoltaic systems, IEEE Trans. Ind. Electron., vol. 58, no. 12, pp , Dec [8] X. Guo, M. C. Cavalcanti, A. M. Farias, and J. M. Guerrero, Singlecarrier modulation for neutral point-clamped inverters in three-phase trans-formerless photovoltaic systems, IEEE Trans. Power Electron., vol. 28, no. 6, pp , Jun

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