IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 7, NO. 3, JULY

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1 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 7, NO. 3, JULY Efficient Single Phase Transformerless Inverter for Grid-Tied PVG System With Reactive Power Control Monirul Islam, Nadia Afrin, and Saad Mekhilef, Senior Member, IEEE Abstract There has been an increasing interest in transformerless inverter for grid-tied photovoltaic (PV) system due to low cost, high efficiency, light weight, etc. Therefore, many transformerless topologies have been proposed and verified with real power injection only. Recently, almost every international regulation has imposed that a definite amount of reactive power should be handled by the grid-tied PV inverter. According to the standard VDE- AR-N 4105, grid-tied PV inverter of power rating below 3.68KVA, should attain power factor (PF) from 0.95 leading to 0.95 lagging. In this paper, a new high efficiency transformerless topology is proposed for grid-tied PV system with reactive power control. The new topology structure and detail operation principle with reactive power flow is described. The high frequency commonmode (CM) model and the control of the proposed topology are analyzed. The inherent circuit structure of the proposed topology does not lead itself to the reverse recovery issues even when inject reactive power which allow utilizing MOSFET switches to boost the overall efficiency. The CM voltage is kept constant at midpoint of dc input voltage, results low leakage current. Finally, to validate the proposed topology, a 1 kw laboratory prototype is built and tested. The experimental results show that the proposed topology can inject reactive power into the utility grid without any additional current distortion and leakage current. The maximum efficiency and European efficiency of the proposed topology are measured and found to be 98.54% and 98.29%, respectively. Index Terms Common mode, converter, high efficiency, leakage current, reactive power, transformerless. I. INTRODUCTION R ECENTLY, the photovoltaic power generation system has been focused as one of the most significant energy sources due to the rising concern about global warming, and the increase of electrical power consumption [1], [2]. In addition, the PV module has no moving parts, which have made it very robust, long lifetime and low maintenance device. Though the PV module is still expensive, but due to the large-scale manufacturing it has become increasingly cheaper in the last few years. It has been reported in [3] that the milestone of Manuscript received October 19, 2015; revised January 15, 2016; accepted February 21, Date of publication March 17, 2016; date of current version June 16, This work was supported in part by the Ministry of Higher Education (MoHE), Malaysia under Grant UM.C/HIR/MOHE/ENG/24 and in part by the University of Malaya under Grant RP015D-13AET. Paper no. TSTE M. Islam and S. Mekhilef are with Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia ( monir04eee@yahoo.com; saad@um.edu.my). N. Afrin is with the Department of Electrical and Electronic Engineering, Pabna University of Science and Technology, Pabna 6600, Bangladesh ( nadia.afrin89@gmail.com). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TSTE GW installed PV power all over the world was achieved at the end of 2012 and increased to 140GW at the end of 2013, and the majority were grid connected as shown in Fig. 1 [3]. Therefore, a prediction has been made in [4] that the future gridtied PV system will play an important role in the regulation of the conventional power system. In general, PV power generation system includes solar arrays and power conversion unit [5], [6]. In most countries and areas, a line-frequency transformer or a high-frequency transformer has been utilized in the grid-tied PV system to create a galvanic isolation between the PV module and the grid. However, the use of line-frequency or high-frequency transformer makes the entire system bulky, costly, and less efficient. In contrast, transformerless PV inverter system has been drawing more attention for its low cost, high efficiency, small size and light weight [7] [9]. The exclusion of transformer, and hence its isolation capability, has to be considered carefully due to the issues raised from no galvanic isolation between the PV module and the grid. In this case, because of the parasitic capacitance between the PV module and the ground, the fluctuating CM voltage that depends on the topology structure and switching scheme can cause of a capacitive leakage current [10]. The existence of leakage current increases grid current harmonics and system losses, deteriorates the electromagnetic compatibility and, more significantly, lead to a safety threat [6], [11]. Another important issue of grid-tied transformerless PV inverter is the ability of injecting reactive power into the utility grid. Recently almost every international regulation has imposed that a definite amount of reactive power should be handled by the grid-tied PV inverter. This is due to the problems of grid voltage instability. According to the standard VDE-AR- N 4105, grid-tied PV inverter of power rating below 3.68kVA, should attain PF from 0.95 leading to 0.95 lagging [12]. In order to solve the problem of leakage current, many dc-ac transformerless topologies have been proposed based on the full-bridge (FB) inverter [7], [8], [11], [13] [15] and verified with real power injection only. Most of the inverter topologies described in literature and commercially available show the European efficiency in the range of 96% 98% [16]. Therefore, to boost the efficiency, some of the transformerless topologies have been implemented with MOSFET switches because of its low switching and conduction losses [7], [13], [17] [20]. However, due to the low reverse recovery issues of MOSFET s anti-parallel diode, the risk of device failure exist in the MOSFET based phase-leg. The most attractive transformerless topology is the Highly Efficient and Reliable Inverter Concept (HERIC) topology which is shown in Fig. 2(a). Two switches and two diodes are IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See for more information.

2 1206 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 7, NO. 3, JULY 2016 Fig. 1. Share of grid-connected and standalone PV installation [3]. added in the ac side of FB topology to decouple the PV module from the grid during the freewheeling period [18]. As seen in Fig. 2(b), the topology which has been proposed in [7] replaces the two switches freewheeling branch with one bi-directional switch and four diodes called H-bridge zero voltage rectifier (HB-ZVR). Also another diode (D5) is added to clamp the CM voltage at the half of dc input voltage for better eliminating the leakage current. Yu et al. proposed H6-type topology in [13], where two switches and two diodes are added in the dc side to decouple the PV module from the grid which is presented in Fig. 2(c). All of the switches of these three topologies can be implemented using MOSFET switches when inject real power. Fig. 2(d) shows another explicit transformerless topology proposed in [21] called H5 topology which is made up by adding an extra switch in the dc side of the FB inverter. An optimized H5 (oh5) topology has been presented in [14], which added a clamping branch consisting of a switch and a capacitor divider with the H5 topology as depicted in Fig. 2(e). Gonzalez et al. proposed another H6 topology in [15] called full-bridge with dc bypass (FB-DCBP) topology. It employs two switches in the dc side and a bi-directional clamping branch consisting of two diodes and a capacitor divider as shown in Fig. 2(f). In the case of real power injection, three switches (S2, S4 & S5) of Fig. 2(d) & (e) and two switches (S5 & S6) of Fig. 2(f) can be implemented using MOSFET switches. However, if a phase difference is occurred between the output voltage and current, body-diodes of the MOSFET switches of all the topologies presented in Fig. 2 will be activated. As results, reliability of the system will be reduced. In Fig. 3(a), Bin et al. proposed a topology with high reliability and efficiency (HRE) [16]. This topology splits the ac sides into two independent parts in the positive and negative half cycle of the grid current if compared with HERIC topology. The reported maximum and California energy commission (CEC) efficiencies of the HRE inverter on a 5 kw prototype circuit with 20 khz switching frequency were 99.3% and 99%, respectively. Another high efficiency inverter called dualparallel-buck converter is shown in Fig. 3(b), consisting of four CoolMOS switches and four SiC diodes [17]. The indicated peak efficiency and European efficiency of dual-parallel-buck converter on a 4.5 kw prototype circuit with 345V input voltage and 16 khz operating frequency were 99% and 98.8%, respectively. The main drawback of this topology is that the grid will be short circuit if no dead time is present between the switches S3 and S4, through which the grid is directly connected. The main advantage of these two topologies is that when a phase difference will be occurred between the output voltage and current, the diodes D1 & D2 or D3 & D4 will be activated depending on the positive or negative half cycle. As a result, these topologies may be implemented with MOSFET switches even when inject reactive power. However, both of these topologies have not been yet verified from the view of reactive power control capability. According to the above discussion, it can be seen that most of the topologies cannot be implemented using MOSFET switches when inject reactive power. And the topologies presented in Fig. 3 have not been verified yet with reactive power injection. Therefore, the main focus of this paper is to propose a new topology that can be implemented using MOSFET switches with high reliability, efficiency, and low leakage current even when inject reactive power. In section II, the circuit structure and the operating principle of the proposed topology is presented. Next the CM characteristic of the proposed topology is presented in section III. Later the control strategy with reactive power control is investigated in section IV. After that, the theoretical analysis is initially verified in the MATLAB/Simulink software environment and the results are given in section V. Finally, the proposed topology is validated with a laboratory prototype of rated 1 kw/ 50 Hz for active and reactive power injection and the experimental results is shown in section VI. At last, the conclusion of this study is drawn in section VII. II. PROPOSED TOPOLOGY AND OPERATING PRINCIPLE A. Structure of the Proposed Topology Fig. 4 shows the proposed transformerless inverter topologies consisting of six MOSFET switches (S1-S6) and six diodes (D1-D6). L 1A, L 1B, L 2A, L 2B, L 1g, L 2g and C o make up the LCL type filter connected to the grid. V PV and C dc represent the input dc voltage and dc link capacitor. The proposed topology is derived from the topology presented in Fig. 2(c) to overcome the low reverse-recovery issues of MOSFETs body-diode when injects reactive power into the utility grid. Therefore, the proposed topology can be implemented with MOSFET switches without reliability and efficiency penalty. The proposed topology can also employ unipolar-spwm with three-level output voltage. B. Operating Principle of the Proposed Topology The switching pattern of the proposed topology is shown in Fig. 5, where G1, G2, G3, G4, G5, and G6 represent the gate drive signals of the switches S1, S2, S3, S4, S5, and S6, respectively. The operation principle of the proposed topology within a grid period is divided into four regions as shown in Fig. 5. Due to the symmetry of the operation of the positive and negative half cycle of grid current, here only positive half cycle explanation is given. However, the circuit diagram for negative half cycle operation is depicted in Fig. 6. Region I: In this region, both the grid current and voltage are positive. During the period within this region, S2 is always on,

3 ISLAM et al.: EFFICIENT SINGLE PHASE TRANSFORMERLESS INVERTER FOR GRID-TIED PVG SYSTEM 1207 Fig. 2. Some existing transformerless topologies for grid-tied PV system using MOSFETs as main power switches: (a) HERIC topology proposed in [18], (b) topology proposed in [7] (c) topology proposed in [13] (d) H5 topology proposed in [21] (e) topology proposed in [14] (f) topology proposed in [15]. Fig. 3. High efficiency transformerless topology: (a) topology proposed in [16] (b) topology proposed in [17]. Fig. 4. (a) Circuit structure of the proposed transformerless topology for gridtied PV system (b) circuit structure with coupled inductor. while S1 & S3 synchronously and S5 complementary commutate with switching frequency. There are always two states that generate the output voltage of +V PV and 0. State 1(t0:t1): At t = t0, the switches S1 & S3 are turnedon and the inductor current increases through grid as shown in Fig. 6(a). In this state, the voltages V 1N and V 2N can be defined as: V 1N =+V PV and V 2N =0, thus the inverter output voltage V 12 =(V 1N V 2N )=+V PV. State 2(t1:t2): When the switches S1 and S3 are turned-off, the inductor current freewheels through S2 and D5. In this state, V 1N falls and V 2N rises until their values are equal. Therefore, the voltages V 1N and V 2N becomes: V 1N = V PV /2 and V 2N = V PV /2 and the inverter output voltage V 12 =0. Region II: In this region, the inverter output voltage is negative, but the current remains positive. During the period of this region, S5 is always on, while S4 & S6 synchronously and S2 Fig. 5. Switching pattern of the proposed topology with reactive power flow. complementary commutate with switching frequency. There are also two states that generate the output voltage of V PV and 0. State 3(t3:t4): In this state, the switches S4 and S6 are turned-on and the filter inductors are demagnetized. Since the inverter output voltage is negative and the current remains

4 1208 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 7, NO. 3, JULY 2016 Fig. 7. Equivalent CM model of the proposed topology. Fig. 6. The operating principle of the proposed topology: (a) state 1 (b) state 2 (c) state 3 (d) state 4 (e) state 5 (f) state 6. positive; therefore, the inductor current is forced to freewheel through the diode D1 and D2, and decreases rapidly for enduring the reverse voltage as shown in Fig. 6(c). The voltages V 1N and V 2N can be defined as: V 1N =0and V 2N =+V PV, thus the inverter output voltage V 12 =(V 1N V 2N )= V PV. State 4(t4:t5): At t = t4, the switches S4 and S6 are turnedoff and S2 is turned-on. Therefore, the inductor current flows through S2 and D5 like as state 2 (Fig. 6(b) can be referred as equivalent circuit). This state is called as energy storage mode. The voltages V 1N and V 2N could be: V 1N = V PV /2 and V 2N = V PV /2, and thus the inverter output voltage, V 12 =0. Fig. 8. Simplified CM model at switching frequency for positive half cycle. III. HIGH FREQUENCY CM MODEL OF THE PROPOSED TOPOLOGY FOR LEAKAGE CURRENT ANALYSIS The PV module generates an electrically chargeable surface area which faces a grounded frame. In case of such configuration, a capacitance is formed between the PV module and the ground. Since this capacitance occurs as an undesirable side effect, it is referred as parasitic capacitance. Due to the loss of galvanic separation between the PV module and the grid, a CM resonant circuit can be created. An alternating CM voltage that depends on the topology structure and control scheme, can electrify the resonant circuit and may lead to higher ground leakage current [6], [16], [22]. In order to analyze the CM characteristics, an equivalent circuit of the proposed topology as shown in Fig. 7 can be drawn, where V 1N, V 2N, V 3N and V 4N are the controlled voltage source connected to the negative terminal N, L CM and C CM are the CM inductor and capacitor, C PVg is the parasitic capacitance, and Z g is the grid impedance. During the positive half-cycle, the switches S4 and S6 are always off. As a result, the controlled voltage sources V 3N and V 4N are zero and can be removed. According to the definition of common-mode and differential-mode voltage: V CM = 1 2 (V 1N + V 2N ) (1) V DM = V 1N V 2N (2) Solving (1) and (2), V 1N and V 2N can be expressed as follows: Fig. 9. Simplified single loop CM model. V 1N = V CM V DM (3) V 2N = V CM 1 2 V DM (4) In order to illustrate the CM model at switching frequency, equation (3) and (4) could be replaced for the bridge-leg in Fig. 7. The grid is a low frequency (50 60 Hz) voltage source; thus the impact of grid on the leakage current can be neglected [23]. The DM capacitor C o can also be removed since it has no effect on the leakage current. Consequently, the simplified high frequency CM model of the proposed topology for positive half-cycle could be drawn as Fig. 8. Finally, the simplified single loop CM model of the proposed topology for positive half cycle is derived in Fig. 9. From Fig. 9, the following equation of the total CM voltage can easily be derived as: V tcm = V CM + V DM 2 L 2 L 1 L 2 + L 1 (5) where V tcm represent total CM voltage, and L 1 = L 1A + L 1g and L 2 = L 1B + L 2g. In the proposed inverter if L 1A = L 1B and L 1g = L 2g for a well-designed circuit with symmetrically structured magnetics [16], equation (5) can be rewritten as

5 ISLAM et al.: EFFICIENT SINGLE PHASE TRANSFORMERLESS INVERTER FOR GRID-TIED PVG SYSTEM 1209 follows: V tcm = V CM = 1 2 (V 1N + V 2N ) (6) According to the operation principle of the proposed topology presented in section II (B), the total CM voltages can be calculated for each state of positive half cycle operation as follows: State1 : V tcm = 1 2 (V 1N + V 2N )= 1 2 (V PV +0)= 1 2 V PV (7) State2 : V tcm = 1 2 (V 1N + V 2N ) = 1 2 (1/2V PV +1/2V PV )= 1 2 V PV (8) Fig. 10. Control diagram of the proposed topology. State3 : V tcm = 1 2 (V 1N + V 2N )= 1 2 (0 + V PV)= 1 2 V PV (9) State4 : V tcm = 1 2 (V 1N + V 2N )= 1 2 (1/2V PV +1/2V PV ) = 1 2 V PV (10) It is clear from equations (7)-(10) that the total CM voltage for the proposed topology during positive half cycle operation is kept constant at V PV /2. Likewise, the total CM voltage for the negative half cycle operation can be calculated and found to be constant at V PV /2 due to the symmetry of operation for the positive and negative half cycle of grid current. The only difference is the activation of different power devices. Therefore, it can be concluded that the total CM voltage during the whole grid cycle is kept constant, reducing ground leakage current. IV. CONTROL OF THE PROPOSED TOPOLOGY The control system for the proposed topology is illustrated in Fig. 10, which contains an orthogonal signal generator (OSG) unit to calculate active and reactive power, two proportional integral (PI) controllers, a grid current controller and a SPWM generation block. Based on the OSG system, the active power P and reactive power Q for the proposed topology can be calculated by using the following equation which is shown in Fig. 11 [4], [24]: P cal =1/2[v gα i gα + v gβ i gβ ] (11) Q cal =1/2[v gβ i gα v gα i gβ ] (12) where v gα,v gβ,i gα, and i gβ represents the α and β components of grid voltage and current. Based on equation (11) and (12), the current in αβ-reference frame can be derived as follows: i gα =2(P cal v gα + Q cal v gβ ) / ( vgα 2 + vgβ 2 ) (13) i gβ =2(P cal v gβ + Q cal v gα ) / ( vgα 2 + vgβ 2 ) (14) According to the single phase P-Q theory, the grid-in current reference can be generated by regulating the averaged active and reactive power [25], [26]. Since the active and reactive power are constant in steady state, so to control them two PI Fig. 11. OSG based power calculation. controllers has been used as shown in Fig. 10. The grid reference current can be derived with the help of OSG system in the following equation [25], [26]: i g =[(P ref P cal ) G p (s) v gα +(Q ref Q cal ) G q (s) v gβ ]/ ( v 2 α + v 2 β) (15) where P ref and Q ref are the power references, G p (s) and G q (s) are the transfer function of PI based controller that can be defined as follows: G p (s) =K pp + K pi 1 / s (16) G q (s) =K qp + K qi 1 / s (17) where K pp,k pi,k qp, and K qi are the proportional and integral gain for the active and reactive power. In order to control the grid current, several existing control methods such as conventional PI controller, repetitive controller (RC), proportional resonant (PR) controller, and deadbeat (DB) controller can be adopted due to the capability of tracking reference signal without steady state error [27], [28]. Since the PR controller has better performance of tracking the reference signal if compared to the normal PI and RC controller, it is selected to control the output current of the proposed topology. The block diagram of the PR controller with harmonic current compensator is shown in Fig. 12, where G c (s), G h (s), and G d (s) are the transfer function of fundamental current controller, harmonic compensator, and inverter respectively. The transfer functions are given below [27], [29]:

6 1210 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 7, NO. 3, JULY 2016 Fig. 12. Block diagram of PR controller with harmonic compensator. TABLE I SPECIFICATION OF THE PROTOTYPE Fig. 13. CM characteristics of the proposed topology with pure real power flow. s G c (s) =K pi + K ii s 2 + ωf 2 (18) G h (s) = K ih s s 2 +(hω f ) 2 (19) h=3,5,... 1 G d (s) = (20) 1+1.5T s s where K pi and K ii are the proportional and resonant gain, ω f is the fundamental frequency, K ih is the resonant gain at the nthorder harmonic, h is the harmonic order, and T s is the sampling period. V. SIMULATION RESULTS The simulations are carried out using MTALAB/Simulink software to analyze and initially verify the theoretical analysis. The parameters are used in simulation are given in Table I. The PV module is replaced with a 400V dc voltage source and the parasitic capacitance between the PV module and the ground is emulated using a thin film capacitor of 75nF. The simulated CM characteristics of the proposed topology with pure real power and both real and reactive power flow conditions are shown Figs. 13 and 14, respectively. It can be seen that the CM voltage ((V 1N +V 2N )/2 for positive half cycle and (V 3N +V 4N )/2) for negative half cycle) for both unity power factor and other than unity power operation is kept constant at the half of dc input voltage excluding a small fluctuation during the grid zero crossing instant. However, the ground leakage current is very small and its RMS value is only 10 ma which is far lower than the limitation requirement of the German standard [30]. Figs. 15 and 16 show the dynamic results under the changes of only P ref, and both P ref and Q ref. It is clear that the grid current is changed according to the step load changes, and Fig. 14. CM characteristics of the proposed topology with real and reactive power flow. the active and reactive power controller track the reference power within four cycle of operation. As seen, the grid current and voltage has very low distortion and the leakage flows through the whole system is very less. Therefore, it can be concluded that the fast and effective response of the load changes are achieved which validate the robustness of the proposed topology with the presented control scheme. VI. EXPERIMENTAL RESULTS In order to experimentally verify the performance of the proposed topology, a 1 kw prototype is built and tested. The

7 ISLAM et al.: EFFICIENT SINGLE PHASE TRANSFORMERLESS INVERTER FOR GRID-TIED PVG SYSTEM 1211 Fig. 17. Gate drive signals. Fig. 15. Performance of the proposed topology under the changes of P ref :grid voltage v g (V), grid current i g (50 A), leakage current i leakage (A), measured active power P m (W) and reactive power Q m (VAR), reference active power P ref (W) and reactive power Q ref (VAR). Fig. 18. CM characteristics of the proposed topology. Fig. 16. Performance of the proposed topology under the changes of P ref and Q ref : grid voltage v g (V), grid current i g (50 A), leakage current i leakage (A), measured active power P m (W) and reactive power Q m (VAR), reference active power P ref (W) and reactive power Q ref (VAR). specifications of the prototype are listed in Table I. Like as simulation, the PV module is replaced with a 400V dc voltage source and the parasitic capacitance between the PV module and the ground is emulated using a thin film capacitor of 75nF. The proposed system is implemented in the dspace 1104 platform. A. Verification With Real Power Injection In this section, the proposed topology is verified with 1 kw power injection. Fig. 17 shows the experimental gate drive signals for the proposed topology. It can be seen that the switching signals are fully matched with the proposed PWM scheme, and the gate drive voltages are kept constant at the desired level. The waveforms of CM characteristics are shown in Fig. 18. It is clear that the voltages V 1N, V 2N, V 3N, and V 4N are clamped at 200V during the freewheeling period of positive and negative half cycle. As a result, the CM voltage is kept constant at 200V for the whole grid cycle except a small fluctuation during grid zero crossing instant as witnessed in Fig. 18. Consequently, the leakage current flows through the system are well reduced as shown in Fig. 19. During zero crossing instant, a small spike can be observed in Fig. 19 due to the fluctuation of CM voltage. However, the peak and RMS value of leakage current flows through this topology are measured 24 ma and 13 ma, respectively which is lower than the limitation requirement of the German standard [30]. The waveforms of the grid voltage v g and grid current i g are shown in Fig. 20. It can be seen that v g and i g are pure sinusoidal and achieved unity power factor. In Fig. 20, the current flows through the inductor L 1A, and L 2B is also shown. It is clear that L 1A conduct for only positive half cycle and L 2B conduct only for negative half cycle. It can also be seen that no overlaps have been occurred. Fig. 21 shows the dynamic response of the system when it is subject to 750W load to 1000W load step change. It can clearly be seen that fast and effective response under the changes of

8 1212 Fig. 19. Waveform of grid voltage vg, grid current ig, and leakage current ileakage. Fig. 20. Current flows through inductor L1A and L2B. IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 7, NO. 3, JULY 2016 Fig. 22. Voltage stress of the switches S1, S2, and S3. Fig. 23. CM characteristics of the proposed topology. B. Verification With Real and Reactive Power Injection Fig. 21. Experimental result of the system dynamic response. active power reference are achieved with the proposed topology. Therefore, it can be concluded that the proposed topology can inject real power into utility grid with low leakage current and low THD at output. In this section, the proposed topology is verified with real power (1000W) and reactive power (500VAR) injection. The voltage stress across the switches is presented in Fig. 22. It is very clear that voltage stress of the switches S1 and S2 are 400V while S3 ensure 200V and also no spike is noticeable. In Fig. 23, the CM characteristics are shown. It can be seen that the voltages V1N, V2N, V3N, and V4N are clamped at 200V during the freewheeling period of positive and negative half cycle. As a result, the CM voltage is kept constant at 200V for the whole grid cycle except a small fluctuation during grid zero crossing instant similar to unity power factor operation. The waveform of leakage current is shown in Fig. 24. The peak and RMS values of leakage current flows through the system are measured 13.6 ma and 26 ma respectively which are below the requirement of the German standard VDE The waveform of grid current ig and grid voltage vg for inductive power generation is shown in Fig. 25. It is noticeable that no extra distortion is occurred in grid current when inject reactive power. However, the THD of grid current is measured 1.89% for inductive power generation as shown in Fig. 26 that can meet the requirement of IEEE Std TM [31]. In Fig. 25, the current flowing through the inductor L1A and L2B is given. Like as verification with real power, it is clear that L1A

9 ISLAM et al.: EFFICIENT SINGLE PHASE TRANSFORMERLESS INVERTER FOR GRID-TIED PVG SYSTEM 1213 Fig. 24. Leakage current flows through the proposed topology. Fig. 27. Experimental result of the system dynamic response. Fig. 25. Current flows through inductor L1A and L2B. Fig. 28. CM voltage and leakage current of H6-type topology. power into utility grid with low leakage current and low THD at output. C. Leakage Current and Efficiency Comparison Under Unity Power Factor Operation Fig. 26. Total harmonic distortion of output voltage and current. conduct only for positive half cycle and L2B conduct only for negative half cycle. Fig. 27 shows the dynamic response of the system when it is subject to 750W and 250VAR load to 1000W and 500VAR load step change. It can clearly be seen that fast and effective response under the changes of active and reactive reference power are achieved with the proposed topology. Therefore, it can be concluded that the proposed topology can inject reactive The waveform of leakage current for the H6-type topology proposed in [13] with unity power factor is shown in Fig. 28. The RMS value of leakage current flows through this topology is measured 24 ma. In contrast, 13 ma leakage current flows through the proposed topology which is given in Fig. 19. Fig. 29 presents the efficiency comparison between the proposed and H6-type topologies. The YOKOGAWA WT1800 precision power analyzer has been used to measure the efficiency at different output power. It may be noted that the presented efficiency diagram covers the total devices losses and the filter inductor losses but it does not contain the losses for the control circuit. It can be seen that the maximum efficiency with the proposed topology is 98.54%, while H6-type topology obtain maximum efficiency of 98.60%. The European efficiency is calculated by combining several weighted factors at various output power, as expressed in equation (21) [15]: ηeu = 0.03η5% η10% η20% η30% η50% + 0.2η100% (21)

10 1214 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 7, NO. 3, JULY 2016 REFERENCES Fig. 29. Measured efficiency comparison. TABLE II PERFORMANCE COMPARISON The calculated European efficiency for the proposed and H6-type topologies is 98.29% and 98.32%, respectively. The comparison among them has been summarized in Table II to demonstrate the effectiveness of the proposed topology. It can be seen that the efficiency of these two topologies is almost same, but the leakage current flows through the proposed topology is lower than the H6-type topology. Therefore, the proposed topology has better CM characteristics while both keep almost same efficiency. In addition, the proposed topology is capable of injecting reactive power into utility grid. VII. CONCLUSION In this paper, a new high efficiency transformerless topology for grid-tied PV system is presented. The main advantages of the proposed topology can be summarized as: 1) The inherent circuit configuration of the proposed topology does not lead itself to the reverse recovery issues which allow utilizing MOSFET switches even though when inject reactive power. Therefore, without compromising the overall efficiency, proposed topology can inject reactive power into the utility grid. 2) The CM voltage is kept constant at the mid-point of dc bus voltage; as a result, low leakage current flows through the system which is lower than the H6-type topology. 3) PWM dead time is not required for the proposed topology that reduces the THD at the output. Finally, to demonstrate the feasibility and effectiveness of the proposed topology, a 1 kw laboratory prototype is built and tested with both real and reactive power injection. The experimental results verified the above mentioned advantages. It has shown that the proposed topology presents almost the same characteristics for both real and reactive power injection, which are very suitable for grid-tied PV system. Therefore, it can be concluded that the proposed inverter is an attractive solution for grid-tied PV system. [1] I. Patrao, E. Figueres, F. González-Espín, and G. Garcerá, Transformerless topologies for grid-connected single-phase photovoltaic inverters, Renew. Sustain. Energy Rev., vol. 15, pp , [2] M. Islam, S. Mekhilef, and M. Hasan, Single phase transformerless inverter topologies for grid-tied photovoltaic system: A review, Renew. Sustain. Energy Rev., vol. 45, pp , [3] I. PVPS, Trends in photovoltaic applications. Survey report of selected IEA countries between 1992 and 2013, International Energy Agency, St. Ursen, Switzerland, Report IEA-PVPS T1 25, [4] Y. Yang and F. Blaabjerg, Low-voltage ride-through capability of a single-stage single-phase photovoltaic system connected to the lowvoltage grid, Int. J. 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11 ISLAM et al.: EFFICIENT SINGLE PHASE TRANSFORMERLESS INVERTER FOR GRID-TIED PVG SYSTEM 1215 [23] E. Gubía, P. Sanchis, A. Ursúa, J. López, and L. Marroyo, Ground currents in single-phase transformerless photovoltaic systems, Progr. Photovoltaics Res. Appl., vol. 15, pp , [24] S. Dasgupta, S. K. Sahoo, and S. K. Panda, Single-phase inverter control techniques for interfacing renewable energy sources with microgridpart I: Parallel-connected inverter topology with active and reactive power flow control along with grid current shaping, IEEE Trans. Power Electron., vol. 26, no. 3, pp , Mar [25] Y. Yongheng, F. Blaabjerg, and W. Huai, Low-voltage ride-through of single-phase transformerless photovoltaic inverters, IEEE Trans. Ind. Appl., vol. 50, no. 3, pp , May/Jun [26] R. Bojoi, L. R. Limongi, D. Roiu, and A. Tenconi, Enhanced power quality control strategy for single-phase inverters in distributed generation systems, IEEE Trans. Power Electron., vol. 26, no. 3, pp , Mar [27] M. Monfared and S. Golestan, Control strategies for single-phase grid integration of small-scale renewable energy sources: A review, Renew. Sustain. Energy Rev., vol. 16, pp , [28] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, Overview of control and grid synchronization for distributed power generation systems, IEEE Trans. Ind. Electron., vol. 53, no. 5, pp , Oct [29] W. Yong and L. Rui, Novel high-efficiency three-level stacked-neutralpoint-clamped grid-tied inverter, IEEE Trans. Ind. Electron., vol. 60, no. 9, pp , Sep [30] Automatic Disconnection Device Between a Generator and the Public Low-Voltage Grid, Germany Standard DIN VDE 0126, [31] IEEE Standard Conformance Test Procedures for Equipment Interconnecting Distributed Resources With Electric Power Systems, IEEE Std , pp. 0_1-54, Nadia Afrin received the B.Sc. degree in engineering in electrical and electronic engineering from Rajshahi University of Engineering and Technology, Rajshahi, Bangladesh, in Currently, she is working as an Assistant Professor with the Department of Electrical and Electronic Engineering, Pabna University of Science and Technology, Pabna, Bangladesh. Her research interests include renewable energy and wireless communication. Saad Mekhilef (M 01 SM 12) received the B.Eng. degree in electrical engineering from the University of Setif, Setif, Algeria, in 1995, and the M.Eng.Sci. and Ph.D. degrees from the University of Malaya, Kuala Lumpur, Malaysia, in 1998 and 2003, respectively. He is currently a Professor with the Department of Electrical Engineering, University of Malaya. He is the author and coauthor of more than 200 publications in international journals and proceedings. He is actively involved in industrial consultancy for major corporations in the power electronics projects. His research interests include power conversion techniques, control of power converters, renewable energy, and energy efficiency. Monirul Islam received the B.Sc. degree in electrical and electronic engineering from Rajshahi University of Engineering and Technology (RUET), Rajshahi, Bangladesh, in 2009, and the M.Eng.Sc. degree in electrical engineering from the University of Malaya, Kuala Lumpur, Malaysia, He is currently working as a full-time Research Assistant with Power Electronics and Renewable Energy Research Laboratory (PEARL), University of Malaya. His research interests include power converter topologies and control for grid-tied photovoltaic application.