An Improved Transformerless Grid Connected Photovoltaic Inverter with Common Mode Leakage Current Elimination

Transcription

1 n Improved Transformerless Grid nnected hotovoltaic Inverter with mmon Mode Leakage Current Elimination Monirul Islam*, Saad Mekhilef*, Fadi M. lbatsh* *ower Electronics and Renewable Energy Research Laboratory (ERL), Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia. Keywords: mmon Mode Voltage; nverter; Grid connected; Junction Capacitance; Leakage Current; V; Transformerless. bstract Transformerless V inverters are most preferred for grid connected photovoltaic (V) generation system due to higher efficiency and lower cost. However, to meet the safety regulations, the leakage current, which deteriorates the power quality and generates EMI of transformerless V inverter, have to be addressed carefully. In order to eliminate the leakage current, an improved topology based on H5 topology is proposed in this paper. The operation principle, common mode (CM) characteristics, and the impact of junction capacitance of the switches are analyzed in detail. Three-level output voltage has been achieved in the proposed improved inverter by implementing unipolar SWM with wide input voltage range. Furthermore, the European Union (EU) efficiency is improved by replacing the IGTs with MOSFETs. The proposed improved inverter topology has been simulated by MT/Simulink software to validate the accuracy of the theoretical explanations. Finally, a 1kw prototype has been built and tested. Experimental results show 98.2% maximum efficiency and 97.76% EU efficiency on the 1kw prototype circuit with 16 khz switching frequency. 1. Introduction Energy crisis is one of the most crucial problems in recent time and renewable-energy sources play an important role in the attempt to address these problems. mong the renewableenergy sources, the photovoltaic (V s) is one of the most upto-date techniques and third (after hydro and wind power) most important renewable-energy sources in terms of globally installed capacity. ased on the newest report on installed V power, the milestone of 100GW V system was achieved at the end of 2012, and the majority were grid-connected [1, 2]. single-phase converter is used in low power grid-connected applications, but its design embeds generally a line-frequency transformer or a high-frequency transformer that adjusts the converter dc voltage and separates the V arrays from the grid [3, 4]. ecause of size, weight and price in favour of high-frequency transformers, the aptitude is to remove the line frequency transformers when designing the new converter. Furthermore, the existence of the high-frequency transformer desires several power stages, as a result, reducing cost and increasing efficiency will be a challenging task [5]. nsequently, the transformerless grid connected V inverters are manifested to offer the benefits of lower cost, higher efficiency, smaller size, and weight. However, a galvanic connection is existed between the power grid and the V module due to the omission of transformer. Therefore, fluctuating CM voltage is generated between the V module and the ground, as a result, leakage current flows through the loop consisting of the parasitic capacitors, the filter inductors, the bridge, and the utility grid [6-8]. This CM leakage current increases the grid current harmonics and system losses and also creates a strong conducted and radiated electromagnetic interference. nsequently, if anyone touches the V module who is connected to the ground will be conducted by the capacitive current [9-11]. Many topologies have been investigated to minimize the CM leakage current and improve the efficiency of the transformerless grid-connected V inverters, which can be separated into two groups: (1) half-bridge inverter topology, (2) full-bridge inverter topology. The half-bridge inverter topology eliminates the fluctuating CM voltage and produces almost zero leakage current. The main drawback of the halfbridge topology is the necessity of high input voltage (700VDC) corresponds to 230VC application [12]. On the other hand, in the full-bridge inverter, this required input voltage is only 350 VDC for the same application. ut the main disadvantage of the full-bridge inverter is that it can employ only bipolar sinusoidal pulse width modulation (SWM) with two-level output. nsequently, high ripples in the output current are produced and the efficiency of the entire system is decreased. To overcome these problems, many advanced topologies have been proposed in the literature [13-21] shown in Fig. 1, which are based on the full-bridge topology. These topologies can achieve three-level output voltage by employing the unipolar SWM and also can keep constant CM voltage during all operational modes. The main two issues for the transformerless V inverter are; (1) the inverter should not have any leakage current and (2) achieve higher efficiency over wide load ranges. In order to obtain these two main issues, an improved single phase transformerless grid-tied V inverter topology is proposed in this paper. The main features of the proposed improved inverter are: (1) the efficiency of the inverter is improved by replacing the IGTs with MOSFETs because of low conduction and switching losses of the supper MOSFETs (2) analysis of the influence of switches junction capacitance 1

2 Vpv V D1 D2 Vpv V S6 Vpv V 1 2 S6 D7 D8 (c) Vpv V 1 S6 (d) Vpv V (e) Vpv V S6 (f) 1 2 Fig. 1. Some existing transformer-less topologies for grid-tied V inverter Topology proposed in [15] HERIC topology proposed in [19] (c) H6 topology proposed in [18] (d) Topology proposed in [20] (e) H5 topology proposed in [17] (f) oh5 topology proposed in [13] and disconnection of V module from the grid at the freewheeling mode ensure minimized ground leakage current, (3) dead time is not required at both high-frequency switching commutation and grid zero-crossing instant which improves the output power quality and increases the efficiency. The efficiency of the improved inverter topology, H6 topology and HERIC topology are measured and compared. This paper is prepared as follows: The proposed improved converter structure and operation principle with the unipolar SWM control scheme is investigated in section 2. The impact of switches junction capacitance is discussed in section 3. Experimental and simulation results are presented in section 4, and section 5 concludes the paper. Vpv V 2. Improved inverter topology and modulation 2.1 Structure of the converter In order to de-couple the converter from the grid in the freewheeling mode, an extra MOSFET switch is added into the conventional full H-ridge topology and the two lower high frequency IGT switches of two phase legs are replaced by MOSFET switches in this paper which is shown in Fig. 2. L, L, and C o constructs the LCL type filter, coupled to the grid. This improved topology can achieve the three-level output voltage with unipolar SWM. 2.2 Operation principle analysis Grid-tied photovoltaic system generally operates at unity power factor. Fig. 2 shows the waveform of the gating signals of the improved inverter. The additional switch commutates at the switching frequency to ensure the dc decoupling states. In the positive half-cycle of grid current, is always on and & commutate at the switching frequency with the identical commutation order to create +V dc and zero Fig. 2. Improved transformerless grid-tied V inverter proposed circuit configuration gating signals with unity power factor. state. Likewise, in the negative half-cycle, is always on and & commutate at the switching frequency. The freewheeling current flows through and body diode of in the positive half-cycle while and body diode of in in the negative half-cycle. nsequently, four operational modes are proposed that produce the output voltage states of +V dc, 0, and -V dc. Fig. 3 shows the operational principles of the proposed inverter. 2

3 Vpv V (c) Vpv V Fig. 3. Equivalent circuit of four operational modes; ctive and freewheeling modes in the positive half cycle of grid current; (c) ctive and (d) freewheeling modes in the negative half cycle of grid current. Mode I: When and are turned-on, the inductor current i L, flowing through, and, is increased. In this mode, V = +V dc and the CM voltage is 1 1 V V V V V V 0 cm 2 2 V (1) 2 Mode II: Fig. 3 shows the freewheeling path when and are turned-off. In this mode, V falls and V rises until their values are equal. Therefore, V = 0 and the inductor current decreases through and the body diode of. So the CM voltages can be defined as 1 1V V V V V V V cm 2V V (2) Mode III: This mode starts when and are turned-on and the inductor current increases reversely through, and. Therefore, the output voltage V = -V V and the CM voltage becomes 1 1 V 0 V V V V V cm 2 2 V (3) 2 Mode IV: In the negative half-cycle of grid current, freewheeling mode starts when and are turned-off which is shown in Fig. 3(d). In this mode, V falls and V rises until their values are equal. Therefore, V = 0 and the inductor current decreases through and the body diode of, like as mode 2. The CM voltage is calculated as 1 1V V V V V V V cm 2V V (4) It is clear that, during the aforementioned four commutation modes, V cm almost remains at constant value that is V cm = 1/2V V [from (1) (4)]. Therefore, the improved inverter can keep constant CM voltage during all operational modes with unipolar SWM. Vpv V (d) Vpv V V d c C V (Vdc) S 5 C 5 C5 C2 i2 i c 5 ic5 ic2 i c 5 i 1 C 1 C 2 i c 2 (Vdc) i1 S 2 i c 3 S 1 C 3 S 3 i3 ic3 Fig. 4. Transient circuit by considering the switching from mode I to mode II Transient circuit Simplified circuit i c 4 C 4 i 2 C3 i 3 i 3 S 4 3. Impact of junction capacitance When the inverter commutates from each non-decoupling state to decoupling state, the slope of voltage V and V depends on the junction capacitance of the switches. s a result, the CM voltage is affected. The aforesaid four operational modes can be divided into two groups based on the decoupling and non-decoupling states. In the active mode (mode I and mode III), the V module and the grid are directly coupled by the output filter inductors. Therefore, the effect of switches junction capacitance on the CM voltage is insignificant in this mode. ut in the freewheeling mode (mode II and mode IV), the V module is disconnected from the grid by the additional switch and the CM voltage is influenced by the switches junction capacitance. The complexity of eliminating leakage current is increased, when the switches junction capacitance is taken into consider. There are always two phases when the inverter commutates from each non-decoupling state to decoupling state (from mode I to mode II as an example) because of the symmetry of the operation modes. hase I: n equivalent transient circuit can be drawn by considering the commutation from mode I to mode II as shown in Fig. 4, where C1-C5 symbolizes the junction capacitors of the switches -. The transient charging or discharging circuits are drawn up by the junction capacitors C2, C3, C4, and C5, when the switches and are turnedoff but the body diode of has not conducted to flow the freewheeling current. The following equations are given by applying Kirchhoff s current law i i i (5) 1 c3 c5 C4 L L (0) ic3 ic4 3

4 (Vdc) ic5 C5 C2+C5 C2+C4+C5 Vdc (0) ic2 i6 C2 ic4 C4 CV i6 Fig. 5. Simplified equivalent resonant circuit in mode II i i i i (6) 2 c4 c2 c5 i i i i (7) 3 c2 c3 c5 where i 1, i 2, and i 3 are the charging or discharging current of the circuit and i c2, i c3, i c4, and i c5 are respectively the current of C2, C3, C4 and C5. The equivalent circuit model for the transient state is given in Fig. 4, where the primary voltages are shown in the bracket. In Fig. 4, it is explicit that C4 is charged by C2 and C5 in parallel through the output filter inductors L and L. Therefore, the voltage across C4 will increase and the voltage across C2 will decrease until their values are equal. ccording to the charge conversion theory, it is found that C2C5 V V V V C2C4C5 hase II: In the meantime, the body diode of conducts to freewheel and the transient state is ended. ccording to equation (8), the equivalent resonant circuit is drawn in Fig. 5. The voltage V and V will be equal to V V /2 only if C4 = C2+C5 at the end of transient state. Therefore, the CM voltage will remain constant in mode II. On the other hand, at the end of transient state, if C4 C2+C5 the voltage V, V and V cm will not be equal to V V /2. So the condition of eliminating CM leakage current will be broken. s a result, a high-frequency leakage current will flow through the junction capacitors, the parasitic capacitors and the filter inductors as shown in Fig. 5. In the proposed unipolar SWM inverter, there are always two commutations from one of the non-decoupling modes to one of the decoupling modes. When the inverter commutates to mode IV from mode III, we can analyze same as before and finally summarized the following equations to get V = V = V V /2 when the transient state is ended. 1) mmutation from mode I to mode II: C4=C2+C5 2) mmutation from mode III to mode IV: C2=C4+C5 Therefore, the condition of junction capacitor for eliminating CM leakage current is concluded that C5=0 and C2=C4. ut for practical application, the junction capacitance of the switches cannot be zero. ccordingly, the theoretical value is amended for practical application as follows C2 = C4 >> C5 (9) (8) V Fig. 6. Waveforms of V, V, and V cm by applying unipolar SWM when the switches junction capacitances are identical Simulated waveform Experimental waveform 4. Simulation and experimental results In order to verify the performance of the proposed improved topology, a 1kWp V array is simulated by using MT/Simulink software, having the frame of panels connected to the ground with the parasitic capacitance of 75nF. lso, a 1 kw prototype has been built in our laboratory. The specifications of the prototype are listed in Table 1. The waveforms of V, V, and V cm are shown in Fig. 6 when the switches junction capacitances are identical. The condition of junction capacitance narrated in equation (9) for eliminating the CM voltage is not fulfilled here, so the CM voltage fluctuation is relatively large as shown in Fig. 6. The simulated waveform shows the results V = V = 130V at the end of transient state for each commutation of nondecoupling modes to decoupling modes, and as a result, the CM voltage is fluctuating from 130V to 200V. The experimental results are similar to the simulated results which are shown in Fig. 6. Therefore, to verify the analysis proved in equation (9), two additional capacitors whose values are much greater than the junction capacitance of, are connected in parallel to and, respectively. Depending on two constraints, first, increasing switching losses and second, minimization of CM leakage current, additional capacitors value is selected as 5000pF. s a result, the simulation results presented in Fig. 7 shows the voltages V = V = 200V at the end of the transient period. nsequently, CM voltage remains constant at 200V and the generating ground leakage current is 4

5 ig(10/div) t=5ms/div Fig. 8. Experimental waveforms of grid current i g, grid voltage V g, and leakage current i Leakage. Fig. 7. Waveforms of V, V, and V cm after adding two additional capacitors Simulated waveform Experimental waveform minimized to safe level, since V and V are fully complementary in the switching periods. The experimental results are completely matched with simulation results which are shown in Fig. 7. The measured peak value and RMS value of CM leakage current are less than 40m and 10m respectively as shown in Fig. 8, even though the practical switches junction capacitances are non-linear and challenging to be matched accurately. This peak and RMS values are lower in magnitude corresponding to the German standard VDE In Fig. 9, the inverter output voltage V has three levels as V dc, 0, -V dc. It indicates that the proposed topology employs unipolar SWM with galvanic isolation and the differential mode characteristic is excellent. The grid-connected current and voltage, shown in Fig. 9, shows that the proposed inverter can inject power to the grid with higher power factor and lower harmonic distortion. The efficiency comparison curve among the proposed, H6 and HERIC topologies with the same condition are illustrated in Fig. 10, which are measured by the fluke 1735 power logger from Fluke rporation. It is clear that the efficiency of the proposed topology is higher than the H6 topology and almost same to the HERIC topology. The maximum efficiency of the proposed inverter is measured 98.2%. The European Union (EU) efficiency of the proposed, H6 and HERIC topologies are 97.76, 97.5, and respectively, which are calculated in equation (10) (10) EU 5% 10% 20% 30% 50% 100% Fig. 9. Experimental waveforms of grid current i g, grid voltage V g, and differential voltage V. Fig. 10. Efficiency comparison curve Table 1 Specifications of the prototype Inverter arameter Value ominal Input Voltage 400VDC Grid Voltage / frequency 240V / 50Hz Rated ower 1000 W ominal C current 4.2 Switching Frequency 16kHz DC bus capacitor 470µF Filter capacitor 2.2µF Filter Inductor L, L 3mH V parasitic capacitor Cpv1, Cpv2 75nF 5 nclusion In this paper, an improved inverter topology based on the H5 topology is presented to reduce the CM leakage current. The operation modes of the topology and the impact of junction capacitance of the switches are analyzed in details, which 5

6 ensure not to produce the CM leakage current because the condition of eliminating leakage current is met completely. Moreover, the three-level output voltage by employing unipolar SWM is achieved in the proposed inverter with excellent differential mode characteristics. o dead time is required at both the WM commutation and the zero crossing instant of grid cycle. nsequently, the higher quality and lower THD of the grid-connected current are obtained. The efficiency of the inverter is measured and compared with the conventional H6 and HERIC topologies. The EU efficiency is improved by replacing IGTs with MOSFETs. The maximum efficiency and EU efficiency of the proposed inverter are measured 98.2% and 97.8%, respectively. Therefore, the proposed inverter is very suitable for gridconnected V system. cknowledgements The authors wish to acknowledge the financial support from the University of Malaya through HIR-MOHE project UM.C/HIR/MOHE/EG/24 and UMRG project R015-13ET. References [1] H. Delesposte aulino,. J. Mello Menegaz, and D. S. Lyrio Simonetti, " review of the main inverter topologies applied on the integration of renewable energy resources to the grid," in ower Electronics nference (COE),, 2011, pp [2] "hotovoltaics," in wikipedia (2013), vailable ed. [3] F. laabjerg, R. Teodorescu, M. Liserre, and. V. Timbus, "Overview of control and grid synchronization for distributed power generation systems," IEEE Transactions on Industrial Electronics, vol. 53, pp , [4] M. mjad, Z. Salam, M. Facta, and S. Mekhilef, "nalysis and Implementation of Transformerless LCL Resonant ower Supply for Ozone Generation," IEEE Transactions on ower Electronics, vol. 28, pp , [5] D. arater, G. uticchi,. S. Crinto, G. Franceschini, and E. Lorenzani, " new proposal for ground leakage current reduction in transformerless grid-connected converters for photovoltaic plants," in 35th nnual nference of IEEE Industrial Electronics (IECO '09). 2009, pp [6] I. atrao, E. Figueres, F. González-Espín, and G. Garcerá, "Transformerless topologies for grid-connected singlephase photovoltaic inverters," Renewable and Sustainable Energy Reviews, vol. 15, pp , [7] M. Lin, T. Fen, Z. Fei, J. Xinmin, and T. Yibin, "Leakage current analysis of a single-phase transformer-less V inverter connected to the grid," in IEEE International nference on Sustainable Energy Technologies, 2008, pp [8] H. Mahamudul, M. Saad, and M. Ibrahim Henk, "hotovoltaic System Modeling with Fuzzy Logic ased Maximum ower oint Tracking lgorithm," International Journal of hotoenergy, vol. 2013, [9] Z. Li, S. Kai, F. Lanlan, W. Hongfei, and X. Yan, " Family of eutral oint Clamped Full-ridge Topologies for Transformerless hotovoltaic Grid-Tied Inverters," IEEE Transactions on ower Electronics, vol. 28, pp , [10] M... Younis,.. Rahim, and S. Mekhilef, "Simulation of grid connected THIWM-three-phase inverter using SIMULIK," in IEEE Symposium on Industrial Electronics and pplications, 2011, pp [11] M. G. M. bdolrasol and S. Mekhilef, "Three phase grid connected anti-islanding controller based on distributed generation interconnection," in IEEE International nference on ower and Energy, 2010, pp [12] S.. Kjaer, J. K. edersen, and F. laabjerg, " review of single-phase grid-connected inverters for photovoltaic modules," IEEE Transactions on Industry pplications, vol. 41, pp , [13] X. Huafeng, X. Shaojun, C. Yang, and H. Ruhai, "n Optimized Transformerless hotovoltaic Grid-nnected Inverter," IEEE Transactions on Industrial Electronics, vol. 58, pp , [14] G. in, J. Dominic, L. Jih-Sheng, C. Chien-Liang, T. Laella, and C. aifeng, "High Reliability and Efficiency Single-hase Transformerless Inverter for Grid-nnected hotovoltaic Systems," IEEE Transactions on ower Electronics, vol. 28, pp , [15] S. V. raujo,. Zacharias, and R. Mallwitz, "Highly Efficient Single-hase Transformerless Inverters for Grid- nnected hotovoltaic Systems," IEEE Transactions on Industrial Electronics, vol. 57, pp , [16] Y. o, L. Wuhua, G. Yunjie, C. Wenfeng, and H. Xiangning, "Improved Transformerless Inverter With mmon-mode Leakage Current Elimination for a hotovoltaic Grid-nnected ower System," IEEE Transactions on ower Electronics, vol. 27, pp , [17] M. Victor, F. Greizer, S. remicker, and U. Hübler, "Method of converting a direct current voltage from a source of direct current voltage, more specifically from a photovoltaic source of direct current voltage, into a alternating current voltage," ed: United States atents, [18] R. Gonzalez, J. Lopez,. Sanchis, and L. Marroyo, "Transformerless Inverter for Single-hase hotovoltaic Systems," IEEE Transactions on ower Electronics, vol. 22, pp , [19] D. Schmidt, D. Siedle, and J. Ketterer, "Inverter for transforming a DC voltage into an C current or an C voltage," ed: E atent 1,369,985, [20] S. remicker, F. Greizer, and M. Victor, "Inverter, more specifically for photovoltaic plants," ed: Google atents, [21] M. K. Menshawi, M.. bdul Kadir, and S. Mekhilef, "Voltage Vector pproximation ntrol of Multistage Multilevel Inverter Using Simplified Logic Implementation," IEEE Transactions on Industrial Informatics, vol. 9, pp ,