Simulation of Leakage current and THD Compensation in a Large PV system

Transcription

1 Simlation of Leakage crrent and THD Compensation in a Large system Mohamed Yossef Tarnini 1, abil Abdel-Karim and Khaled Chahine 3 1,, 3 Beirt Arab University, Electrical and Compter Department, Power and Machines Program, Debbieh, Lebanon. 1 Orcid D: and ResearcherD: C Abstract n the recent years the applications of solar energy and photovoltaic panels were increasing rapidly in order to minimize the consmption of traditional fels and hence decrease the overall KWh generation cost, also to minimize the CO emission that is harmfl to the environment. The application of large panels with transformer-less inverter leads to the so-called discharging crrent problems associated with natral capacitance between different panels and the grond. Moreover, the presence of the capacitance between different layers of the same panel adds more complexity to the problem. This paper focses on the performance of a modified H5 bridge for sch problems. The proposed single phase soltion minimizes the overall natral capacitance and hence attenating the impact of the harmfl leakage crrent below standard limits with a fast response and low price. Also, the proposed soltion is associated with an effective filter in order to increase the effectiveness of sch systems. The simlation of ncontrolled and flly controlled with sitable filter was performed in order to validate the proposed method. Keywords: Leakage crrents; panels; renewable energy sorces and THD. TRODUCTO When a system is connected to the grid, safety standards shold be met dring operation for reliability, power qality and protection. Transformer-less photovoltaic inverters (T) are increasing rapidly in the world markets de to their higher efficiency, small size, lower cost and lighter weight compared to their conterpart transformer based inverters [1]. evertheless, the elimination of transformers leads to a direct galvanic connection between the panels and the grid, ths creating a leakage crrent de to the formation of parasitic capacitance between the layers of panels and the grond system. The THD noise problem also increases de to elimination of the isolation represented by traditional transformers or de to the formation of leakage capacitance crrents []. High-freqency crrents can case often electromagnetic noise that may affect the operation of some sensitive electronic devices. The higher the freqency of the distribtion transformer, the poorer the voltage qality across the eqivalent impedances of the grid. Finally, both problems of leakage crrent and THD lead to serios safety isses, especially for the applications when the leakage crrent or THD exceeds the standard limits [3]. This paper, shows the simlation work of nprotected H4 and protected H5 single phase transformer-less models for a grid connected system with a sitable filter to stdy the behavior of the leakage crrents for both cases and to show the validity of this method in minimizing the leakage crrent to meet VDE German standard and A8-014 EEE which limits the leakage crrent to 30-mA. The proposed method deals with an inverter design implementation to redce the leakage crrent by means of decopling (or isolating) the AC from DC side dring zero states via establishing proper switch configrations. n general, to garantee the safety of operation and protection of the overall installation, and to prevent harmfl effects reslting from the flow of leakage crrent, the load THD crrent and the leakage crrent shold be limited to vales set by the international standards sch as German and EEE standards [6]. MATHEMATCAL MODEL OF UPROTECTED SYSTEM Fig. 1 shows a typical single phase transformer-less system. The dangeros effect of the leakage crrent traveling to the grid is represented by the amont of RMS vale and by the THD. The electric eqivalent model of Fig. 1 is shown in Fig. for analysis prpose. n this application, the THD is limited to 5% as per EEE standard and is given by: THD n n (1) max fndamin tal 860

2 DC AC Filte V G AO DM CM (5) And the leakage crrent can be calclated as: C P L (Leakage) Figre 1: Leakage grond crrent path in a transformer-less inverter n order to simplify the analysis, the above circit is converted into its common mode (CM) and deferential mode (DM) models. n Fig., Z G is the grond impedance, Z P and Z are the line and netral impedances of the electrical line between the medim voltage/ low voltage transformer and the Point of connection of the system respectively, whereas L P and L are the decopling indctors between the grid and the inverter. C is the eqivalent parasitic capacitances to grond of the system, V A0 and V B0 are the voltages generated by the front-end inverter, the Differential-Mode crrent DM, and the common-mode crrent CM are generated by the photovoltaic inverter. CM 1 sc Z G SC (s) s C L P L [ Sbstitte (4) and (5) in (6) to get (7) CM Where voltage. V (s) LP L L AO P BO (s) ] (6) SC VTCM (s) (7) 1 sc Z (s) s C L L TCM P P G P LPL and V TCM is the total common-mode L L L LP (s) CM(s) DM (s) (8) (L L ) P For most applications, the vale of L is approximately eqal to L P or they can be eqated by sing an additional indctance to cancel the effect of differential mode voltage DM. n this case, the crrent is directly affected by the common mode voltage CM and given by: C 1 C O DC AC Filter A + MPPT B CM L P CM L Z P V G Z Z G dvcm CM C( ) dt (9) According to (9) if the CMV, CM, is constant then the vale of the leakage crrent is decreased [5] Filter A L P Figre : Single phase transformer-less system The grid impedance Z P and Z have a tendency to limit the leakage crrent, therefore the critical case or the maximm vale of the leakage occrs when Z P = Z = 0. n this case, a simplified version of Fig. is depicted by Fig. 3 where the common mode voltage VCM and differential mode voltage VDM can be expressed as [7] CM DM AO AO BO () (3) AB A B DM CM (4) + MPPT O B L C Figre 3: Common mode model of the transformer-less system Moreover, from eq. (7), (8) and (9) the nprotected system is simlated sing Matlab/Simlink in order to show the behavior of sch system with the maximm power point tracking (MPPT) for different irradiation levels and to measre the leakage crrent de to the presence of the parasitic capacitance. Here, the MPPT algorithm is based on the incremental condctance method [8] Z G V G CM 8603

3 MATLAB/SMULK BLOCK OF THE UPROTECTED SYSTEM n order to demonstrate the harmfl effect of the leakage crrent, a Matlab/Simlink block is simlated for a 100-KW array connected to a tility grid throgh a single-phase transformer-less inverter as shown in Fig. 4. The sorce array is made p from 66 parallel strings where each string has 5 series-connected modles, at constant temperatre of 5 0 c. The modle characteristics are shown in Fig. 5. The main components of the block are: 1- Boost Converter to set the voltage otpt from the sorce for MPPT prpose. - Transformer-less nverter that acts as a variable part in or simlation, The circit is simlated for a single-phase ncontrolled bridge with PWM generator. Then with modified H5 bridge with its filter to show the validity of the proposed system in redcing the leakage crrent. 3- Load and tility grid: The otpt of the iinverter is connected to the tility grid. The grid is modeled sing a typical pole-monted transformer and an ideal AC sorce of 14.4 kv-rms. 4- Parasitic Capacitance: The circit is modeled by arrays connected to two capacitors Cp throgh the grond as shown in Fig. 4. The otpt Power and voltage of array for a given irradiance with MPPT are kw, 73.5 V at 1000 W/m and 5 0 C irradiance. Accordingly, the otpt power becomes arond 5-kW when the irradiance is decreased to 50W/m. Figre 4: Matlab/Simlink Model of Transformer-less nverter system 8604

4 Figre 5: _V & P_V characteristics for array Fig. 5 shows the power and the voltage of the system at constant temperatre and different vales of irradiance. According to Table (1), the vale of the leakage is exceeding the acceptable vales limits by all standards. it s high vale is harmfl for the system and hman being. Fig. 7 shows the variation of the voltage and crrent of the grid verss time. The harmonic contents is acceptable and the THD is arond 7% indicating that the filter after the inverter (4 switches only) in Fig. 4 is working correctly. Table 1: leakage crrent Unprotected bridge nverter Capacitance CP in μf Leakage crrent g (A) rradiance from 50W/m to Constant temperatre at 5 0 c -System Power variation from 0 to 5kW then -System voltage variation verss time Figre 6: rradiance power, temperatre, otpt power and voltage Table 1 illstrates the direct effect between Cp and the RMS vale of leakage crrent measred at the grid. As C P increases the leakage crrent increases too. n this simlation, we se stray capacitances each of vale Cp = 1nF. Figre 7: Variation of grid voltage and crrent verss time V. MODEL AD SMULATO OF H5 SWTCHG A. General switching sb-circt model techniqe This techniqe has several advantages; it offers low DC bs voltage reqirements, low leakage crrent, and high efficiency characteristics. Moreover, the dead time is not 8605

5 reqired as the switches on a DC bs short-circiting path are not in condction state in the same PWM cycle [4]. Fig. 9 shows the energized switches and its corresponding eqivalent circit. Figre 8: Eqivalent circits dring energized switching B. PWM Switching Configration At Cp=1nF the leakage crrent oscillates arond positive and negative 1.5-A with an RMS vale of 0.95-A that case a serios problem to the circit. Figre 9: Swiching configration of the 5 switches n this stdy, we propose a modified H5 bridge inverter for the leakage crrent soltion. The H5 system is bildp of 5 switches and the switching scheme plays the main role in leakage crrent soltion in addition to the effective proposed filter as shown in Fig. 10 We set S1 to open mode in the positive cycle while S4 is closed, and S3 is open in the negative half cycle while S is operating as shown in Fig. 8 S5 is operating with S4 in positive cycle mode and with S in negative half cycle mode, ctting off the circit at zero operating mode to ensre decopling of the circit The switches in H5 apply the switching configration shown in Fig. 9 C. Filter Circit Figre 10: Overall system with filter For the simlation two coils are sed instead of one coil with two capacitors as shown in Fig. 4. The Magnetic copling will split the filter coil into two coils in parallel with same nmber of trns bt in an anti-phase connection for a better filtering process. 8606

6 D. Simlation Reslt of Protected System Figre 11: Switching configration of H5 bridg Fig. 11 shows the otpt reslts of the proposed system. The switching gate control circit achieving the switching configration is derived from reference and carrier signal, as shown in Fig. 1 leakage crrent is not present since the elimination conditions are met. Frthermore, the switching voltage of all commtating switches is half of the inpt DC voltage and the switching losses are redced greatly. The high efficiency and convenient thermal design are achieved thanks to the decopling of two additional switches S5 and S6. Moreover, by adopting the doble-freqency SPWM, the higher freqency and lower crrent ripples are achieved. Conseqently, the higher qality and lower THD of the gridconnected crrent are obtained, or the smaller filter indctors are employed and the copper losses and core losses are redced accordingly. Finally, a large scale simlation system was bilt, to show the validity of the modified system. COCLUSO Figre 1: Gate control switching circit An improved grid-connected inverter topology for transformer-less systems is presented in this paper. The nipolar SPWM and doble-freqency SPWM control strategies are both implemented with three-level otpt in the inverter. With this configration, the common-mode REFERECES [1] Arajo S., V, Zacharias P, and Mallwitz R., (010). Highly Efficient Single-Phase Transformerless nverter for Grid-Connected Photovoltaic Systems, EEE Transaction on ndstrial Electronics, Vol. 57, no. 9, pp [] Azri M. and Rahim. A., (011). Design Analysis of Low-Pass Passive Filter in Single-Phase Grid- Connected Transformerless nverter, EEE First Conference on Clean Energy and Technology CET, pp [3] Barater D., Bticchi G, Crinto A. S., Franceschini and Lerenzani E (009), A new proposal for grond leakage crrent redction in Transformerless Grid-Connected Converters for Photovoltaic Plants. ndstrial 8607

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