International Jornal of Engineering Research and Development e-issn: 78-067X, p-issn: 78-800X, www.ijerd.com Volme 10, Isse (Febrary 014), PP.106-111 Common-Mode eakage Crrent Eliminated Photovoltaic Grid- Connected Power System for Domestic Distribtion jith shokan 1,os Mathew Jos, enny Cherian 3 1 MTech stdent, power electronics, Mar thanasis College of engineering, Kothamangalam,3 Professor, Mar thanasis College of engineering, Kothamangalam bstract:- Improved single-phase inverter topology is sed to avoid the common-mode leakage crrent in the transformerless PV grid-connected system. The condition of eliminating common-mode leakage crrent in both the nipolar sinsoidal plse width modlation (SPWM) and the doble freqency SPWM control method can be applied to implement C otpt in the presented inverter. y decopling of two additional switches connected to the dc side the efficiency and convenient thermal design are improved. esides, the higher freqency and lower crrent ripples are obtained by adopting the doble-freqency SPWM, and ths the total harmonic distortion of the grid-connected crrent are redced to 7%. It is sed for low ac power application. Working of the proposed circit and verification by simlation reslts are discssed in this paper. Simlation is done in MT. Keywords:- Common-mode leakage crrent, photovoltaic (PV) system, sinsoidal plse width modlation (SPWM) strategy, transformer less inverter, bck. I. INTRODUCTION The need for a cleaner environment and the continos increase in energy needs makes decentralized renewable energy prodction more and more important. This continosly-increasing energy consmption overloads the distribtion grids as well as the power stations, therefore having a negative impact on power availability, secrity and qality. One of the soltions for overcoming this is the Distribted Generation (DG) system. DG systems sing renewable energy sorces like solar PV system is still mch more expensive than traditional ones, de to the high manfactring costs of PV panels, bt the energy that drives them -the light from the sn- is free, available almost everywhere and will still be present for millions of years, advantages of PV technology is that it has no moving parts. Therefore, the hardware is very robst; it has a long lifetime and low maintenance reqirements [1]. common mode leakage crrent flows throgh parasitic capacitor between the PV arrays. To avoid the common-mode leakage crrent, a high voltage DC is converted by bck DC/DC and connected fll bridge improved inverter with nipolar SPWM []-[6]. high voltage inpt approximately, 400V for 0V ac application. power electronic converter ses semicondctor devices to transform power from one form to another form. bck converter is a specific type of dc-dc power electronic converter whose goal is to efficiently step down DC voltage to a lower level with minimal ripple. Typically the bck converter employs feedback to reglate the otpt voltage in the presence of load changes. This improvement in performance over voltage dividers and reglators comes at the cost of additional components and complexity. In the remainder of this handot, we will examine the characteristics of the bck chopper and derive relationships and tools necessary to properly specify the components reqired to implement a desired design. II. CONDITION OF EIMINTING COMMON-MODE EKGE CURRENT Withot an isolated transformer in the PV grid-connected power systems, there is a galvanic connection between the grid and the PV array, which may form a common-mode resonant circit and indce the common-mode leakage crrent. The common-mode voltage can be defined as the average of the sm of voltages between the otpts and the common reference. In this case, the common reference is taken to be the negative terminal of the PV 106
Common-Mode eakage Crrent Eliminated Photovoltaic Grid-Connected Power System for... (1) Fig. 1: The simplified eqivalent model of the common-mode resonant circit The differential-mode voltage is defined as the difference between the two voltages. () The simplified eqivalent model of the common-mode resonant circit has been derived in as shown in Fig.1, where C dc is the parasitic capacitor, and are the filter indctors, I is the common-mode leakage crrent. nd, an eqivalent common-mode voltage U e is defined by, (3) dm It is clear that the common-mode leakage crrent I is excited by the defined eqivalent common-mode voltage e. Therefore, the condition of eliminating common-mode leakage crrent is drawn that the eqivalent common-mode voltage e mst be kept a constant as follows, 107 Constant (4) In the half-bridge inverter family, one of the filter indctors and is commonly zero. Therefore, the condition of eliminating common-mode leakage crrent is accordingly met that, e e e constant ( = 0) (5) constant ( = 0) (6) Similarly, in the fll-bridge inverter family, the filter indctors and are commonly selected with the same vale. s a reslt, the condition of eliminating common-mode leakage crrent is met that [1], cem dm constant ( = ) (7) III. IMPROVED INVERTER TOPOOGY D OPERTION MODES Fig.. shows the improved grid-connected inverter topology, which can meet the condition of eliminating common-mode leakage crrent. In this topology, two additional switches S 5 and S 6 are symmetrically added to the conventional fll-bridge inverter, and the nipolar SPWM and doble-freqency SPWM strategies with three-level otpt can be achieved.. UNIPOR SPWM STRTEGY ike the fll-bridge inverter with nipolar SPWM, the improved inverter has one phase leg inclding S 1 and S operating at the grid freqency, and another phase leg inclding S 3 and S 4 commtating at the
Common-Mode eakage Crrent Eliminated Photovoltaic Grid-Connected Power System for... switching freqency. Two additional switches S 5 and S 6 commtate alternately at the grid freqency and the switching freqency to achieve the dc-decopling states. ccordingly, for operation modes that generate the voltage states of +U dc, 0, U dc. MODE 1: when S 4 and S 5 are ON, U = +U dc and the indctor crrent increases throgh the switches S 5, S 1, S 4, and S 6. The common-mode voltage is Fig. : Improved inverter topology 1 1 dc dc 0 (8) MODE : when S 4 and S 5 are trned OFF, the voltage U falls and U rises ntil their vales are eqal, and the antiparallel diode of S 3 condcts. Therefore, U = 0V and the indctor crrent decreases throgh the switch S 1 and the antiparallel diode of S 3. The common-mode voltage changes into 1 1 dc dc dc (9) MODE 3: when S 3 and S 6 are ON, U = U dc and the indctor crrent increases reversely throgh the switches S 5, S 3, S, and S 6. The common-mode voltage becomes 1 1 dc 0 108 dc (10) MODE 4: when S 3 and S 6 are trned OFF, the voltage U rises and U falls ntil their vales are eqal, and the antiparallel diode of S 4 condcts. Similar as to Mode, U = 0V and the indctor crrent decreases throgh the switch S and the antiparallel diode of S 4. The common-mode voltage also keeps U dc /. 1 1 dc dc dc (11) In ideal wave form, in the positive half cycle, S 1 and S 6 are always ON. S 4 and S 5 commtate at the switching freqency with the same commtation orders. S and S 3, respectively, commtate complementarily to S 1 and S 4. ccordingly, Mode 1 and Mode continosly rotate to generate +U dc and zero states and modlate the otpt voltage. ikewise, in the negative half cycle, Mode 3 and Mode 4 continosly rotate to generate U dc and zero states as a reslt of the symmetrical modlation [1]. IV. SIMUINK MODE In fig 3(a) shows the main circit of system. This is a sbsystem and one otpt scope, block, 1 st one is UCK and another one is inverting circit, and otpt signal is connected into scope. The otpt shows voltage across point and crrent throgh parasitic capacitors. Fig 3(b) represent UCK converter, its convert 400V to 00V, DC-DC converter. C is sed for filter. Fig 3(c) shows the inverting circit, it gives inpt from otpt of the bck convert. Parasitic capacitor connected across the inpt voltage and checks the crrent throgh it. This is common mode leakage crrent. This crrent is almost eqal to zero. Fig 3(d) shows the internal circit of inverter switches S 1 -S 6 is connected. This is converting DC-C, from 00V dc to 00 ac. For working IGT, gate signal is applied by SPWM.
Common-Mode eakage Crrent Eliminated Photovoltaic Grid-Connected Power System for... (a) (b) (c) (d) Fig. 3: Simlink of (a) main model (b) bck circit (c) main circit (d) inverter circit 109
Common-Mode eakage Crrent Eliminated Photovoltaic Grid-Connected Power System for... V. SIMUTION RESUTS D YSIS For simlation, following components are sed: f =4mH, f1 =4mH, C dc =75nF V dc=400v, grid freqency, f g =50 Hz; switch freqency, f s = 0 khz y sing these components the inpt voltage is converted into ac sorce. Here DC is stepdowned by sing bck and converter. In bck converter, an C filter with = 47µH, C=47µF. high voltage DC is convert into low voltage DC V b =00VC. This voltage is converted into grid voltage, Ug =00 V ac; (a) (b) (c) (d) (e) (f) (g) (h) Fig. 4: Simlated waveforms of (a) switch S 1, (b) switch S (c) switch S 3 (d) switch S 4 (e) switch S 5, (f) switch S 6 (g) V ot, (h) I. 110
Common-Mode eakage Crrent Eliminated Photovoltaic Grid-Connected Power System for... VI. CONCUSION This paper presents an improved grid-connected inverter topology for transformerless PV systems. The nipolar SPWM control method is implemented, which can garantee not to generate the common-mode leakage crrent becase the condition of eliminating common-mode leakage crrent is met completely. Moreover, the switching voltages of all commtating switches are half of the inpt dc voltage and the switching losses are redced greatly. The high efficiency and convenient thermal design are achieved by the decopling of two additional switches S5 and S6. It is sed low ac power application. Working of the proposed circit and verification by simlation reslts are discssed in this paper. Simlation is done in MT. REFERENCES [1]. o Yang, Wha i, Ynjie G, Wenfeng Ci, and Xiangning Improved Transformerless Inverter With Common-Mode eakage Crrent Elimination for a Photovoltaic Grid-Connected Power System IEEE Transactions On Power Electronics, Vol. 7, No., Febrary 01 []. S.. Kjaer, J. K. Pedersen, and F. laabjerg, review of single-phase grid-connected inverters for photovoltaic modles, IEEE Trans. Ind. ppl., vol. 41, no. 5, pp. 19 1306, Sep./Oct. 005 [3]. Q. i and P.Wolfs, review of the single phase photovoltaic modle integrated converter topologies with three different DC link configrations, IEEE Trans. Power Electron., vol. 3, no. 3, pp. 130 1333, May 008. [4]. H. Xiao and S. Xie, eakage crrent analytical model and application in single-phase transformerless photovoltaic grid-connected inverter, IEEE Trans. Electromagn. Compat., vol. 5, no. 4, pp. 90 913, Nov. 010 [5]. T. Kerekes, R. Teodoresc, and M. iserre, Common mode voltage incase of transformerless PV inverters connected to the grid, in Proc. IEEE Int. Symp. Ind. Electron., Jn./Jl. 008, pp. 390 395 [6]. R. Gonzalez, J. opez, P. Sanchis, and. Marroyo, Transformerless inverter for single-phase photovoltaic systems, IEEE Trans. Power Electron., vol., no., pp. 693 697, Mar. 007 111