A New LCL-Filter With In-Series Parallel Resonant ... - Semantic Scholar

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 61, NO. 9, SEPTEMBER 2014

A New LCL-Filter With In-Series Parallel Resonant Circuit for Single-Phase Grid-Tied Inverter Weimin Wu, Yunjie Sun, Zhe Lin, Tianhao Tang, Senior Member, IEEE, Frede Blaabjerg, Fellow, IEEE, and Henry Shu-Hung Chung, Senior Member, IEEE

Abstract—When designing a higher order output power filter for a transformerless grid-tied converter using pulsewidth modulation (PWM), the requirements on the leakage current, the electromagnetic interference (EMI) noise, and the harmonics of the grid-injected current should be addressed. For an LCL-filter-based single-phase grid-tied full-bridge inverter system, it is possible to decrease the total inductance as well as the size and the cost, if the harmonic currents around the switching frequency can be fully suppressed. In this paper, the grid-injected current harmonics and the conducted EMI noise are investigated for the conventional LCL-filter-based system. Based on this, a modified LCL-filter topology using an extra parallel Lr Cr resonant circuit is proposed to reduce the total inductance value, without increasing the capacitive reactive power. The validity is verified through the experiments on a 500-W 110-V/50-Hz prototype. Index Terms—Common-mode noise, differential-mode noise, electromagnetic interference (EMI), harmonic current, LCLfilter, parallel LC resonant circuit, stiff grid.

I. I NTRODUCTION

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ITH the increasingly grown utilization of renewable energy, the grid-tied-inverter-based distributed power generation system has been widely adopted [1]. For the consideration of cost, the transformerless and the high-order filterbased system becomes more and more popular [2]–[8]. During the design of the higher order power filter for a transformerless grid-tied converter using pulsewidth modulation (PWM), the requirements on the leakage current, the electromagnetic interference (EMI) noise, and the harmonics of the grid-injected current, such as given by IEC Std.62109-1-2007 [9], CSIPR 11 [10], and IEEE 1547.2-2008 [11], should be addressed. In order to reduce the leakage current as well as the commonmode EMI noise, a circuit structure of the conventional Manuscript received July 15, 2013; revised October 14, 2013; accepted November 4, 2013. Date of publication December 5, 2013; date of current version March 21, 2014. This work was supported in part by the Shanghai Municipal Education Commission under Award 13ZZ125, in part by the Shanghai Natural Science Foundation under Award 12ZR1412400, and in part by the Research Grants Council of the Hong Kong Special Administrative Region, China, under Project CityU 112711. W. Wu, Y. Sun, Z. Lin, and T. Tang are with the Research Institute of Electronic Automation, Shanghai Maritime University, Shanghai 201306, China (e-mail: [email protected]). F. Blaabjerg is with the Department of Energy Technology, Aalborg University, 9220 Aalborg East, Denmark (e-mail: [email protected]). H. S.-H. Chung is with the Research Institute of Electronic Automation, Shanghai Maritime University, Shanghai 201306, China, and also with the Center for Smart Energy Conversion and Utilization Research, City University of Hong Kong, Kowloon, Hong Kong (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2013.2293703

Fig. 1. LCL-filter-based single-phase grid-tied inverter system with the reduced leakage current.

LCL-filter-based single-phase grid-tied inverter shown in Fig. 1, where an Lr Cr resonant circuit is not included, was proposed in [12]. According to IEEE 1547.2-2008 for a grid-tied inverter system, if the short-circuit current of the power system is lower than 20 times the nominal grid-side fundamental current of the inverter, then each harmonic current higher than the 35th should be less than 0.3% of the rated fundamental current. In [12], the requirement on the harmonic currents has not been taken into account, and its usage may be limited. In [13], it was revealed that for a single-phase grid-tied inverter, the harmonic currents around the switching frequency could be fully suppressed and that the grid-side inductance of the LCL-filter could be significantly reduced. Hence, an LLCLfilter was proposed instead of an LCL-filter. However, more EMI measures should be addressed for the LLCL-filter-based system compared with the LCL-filter-based system. In this paper, the influence of the grid-side inductance on the harmonic currents and on the EMI noise is first investigated. Then, a novel LCL-filter structure with an in-series parallel resonant circuit is proposed. The relationship between the reactive power and the total saved inductance on the grid inductor is analyzed. The attenuation on the harmonic currents around the switching frequency affected by the parameter variation of the parallel resonant circuit is also considered. Finally, the EMI noise and the harmonic current spectrum of the proposed system are measured to verify the theoretical analysis. II. H ARMONIC C URRENTS AND EMI N OISE FOR A C ONVENTIONAL LCL-F ILTER -BASED S YSTEM In order to show the principle, a single-phase grid-tied inverter with two different kinds of LCL-filter (without or with the Lr Cr parallel resonant circuit) is given in Fig. 1. The parameters of the objective for analysis and experiment are listed

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WU et al.: LCL-FILTER WITH PARALLEL RESONANT CIRCUIT FOR SINGLE-PHASE GRID-TIED INVERTER

TABLE I PARAMETERS OF THE F ILTERS FOR A NALYSIS AND E XPERIMENT

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quency can be fully suppressed with some measure, the gridside inductance may be saved, whereas the EMI noise around 150 kHz can still satisfy the requirement given by the CISPR Class A standard. III. P ROPOSED M ODIFIED LCL-F ILTER

Fig. 2. Measured harmonics spectrum of the grid-injected current for the conventional LCL-filter-based system.

A new structure of the LCL-filter-based system is proposed, as also shown in Fig. 1, where the Lr Cr resonant circuit is included. Compared with the conventional LCL-filter-based system, the extra circuits that are resonant at the switching frequency are inserted to be connected with the grid inductor of L2 , achieving a large enough grid impedance around the switching frequency and forcing the related harmonic currents to inject into the loop of Cf . In principle, the design of L2 depends on the requirements of the EMI noise and the harmonic currents around the double switching frequency instead of the switching frequency, resulting in a reduced grid inductance. The transfer function of the grid-injected current versus the output voltage for the proposed LCL-filter-based inverter can be derived as ig (s) Gui →ig (s) = ui (s) s=jω ZC (s) = Z1 (s)Z2 (s)+Z1 (s)ZC (s)+Z2 (s)ZC (s) s=jω where the transfer functions of Z1 (s), Z2 (s), and ZC (s) are ZC (s) = 1/sCf Z1 (s) = sL1 + R1 Z2 (s) = s(L2 + Lg ) +

Fig. 3.

Measured EMI noise for the conventional LCL-filter-based system.

in Table I, where R1 , R2 , and Rr are the equivalent resistors of L1 , L2 , and Lr , respectively. The inverter works under the condition that the power ratio is 500 W, the grid voltage is 50 Hz/110 V, the current ripple of L1 is 30%, the reactive power is about 1.5%, discontinuous unipolar modulation [14] is adopted, and the switching frequency is 20 kHz. Figs. 2 and 3 show the measured harmonic spectra of the grid-injected current and the EMI noise for the conventional LCL-filter-based system. It can be seen in Fig. 2 that the attenuation of the harmonic current around the switching frequency (20 kHz) is about 51 dB, which is very close to the upper limit of 0.3% given by IEEE 1547.2-2008, whereas the attenuation of the harmonic currents around the double switching frequency (40 kHz) are far lower than the upper limit. It can be seen in Fig. 3 that around the frequency of 150 kHz, the measured EMI noise still has a margin of 10 dB from the limitation given by the CISPR Class A standard [10]. From the given analysis, it can be concluded that for a conventional LCL-filter-based grid-tied inverter system, if the harmonic current around the switching frequency is subject to IEEE 1547.2-2008 and the reactive power is expected to be small, the grid-side inductance is difficult to be reduced. However, if the harmonic current around the switching fre-

s2 L

sLr + Rr + R2 + Rg . r Cr + sRr Cr + 1

Fig. 4 shows the characteristics of the proposed LCL-filter. Fig. 4(a) shows the Bode diagrams of the grid-injected current versus the ac output voltage when the grid impedance is equal to zero. It can be seen that the harmonic currents around the switching frequency can satisfy the requirement of IEEE 1547.2-2008 easily, if the parameters are well designed. As shown in Fig. 4(b), if the grid inductance becomes large, the characteristic frequency of fr2 becomes close to the switching frequency of fs , which may result in poor attenuation on the sideband harmonic currents at the switching frequency, particularly when the parameter of Lr drifts (as seen in Fig. 5). Hence, the proposed LCL-filter structure may only fit for the stiff-grid condition, for example, under the condition that the short-circuit current of the power system is higher than 15 times the nominal grid-side fundamental current of the inverter. The parameters design procedure of the proposed LCL-filter is similar to the LLCL-filter design [13], which are described as follows: L1 is based on the current ripple ratio of 20%–30% at the rated output power; C is based on the requirement of the reactive power, which is less than 5% of the rated output power; and L2 is chosen to ensure that the grid current harmonics around the double switching frequency can meet the requirement of IEEE 1547.2-2008. During the design of the grid-side inductors, the attenuation on the harmonic currents

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Fig. 6. Relationship of the total saved inductance between the conventional LCL-filter and the proposed LCL-filter.

Fig. 4. Characteristics of the proposed LCL-filter. (a) Bode diagrams of the grid-injected current versus the ac output voltage (Lg = 0, Rg = 0). (b) Effects caused by the variation of the grid impedance (Rg = 0.2 Ω).

Fig. 5. Sensitivity analysis on the Lf parameter drifting.

around the switching frequency should be examined, while the parameter drift of Lr in a range of ±10% is considered (see Fig. 5), since the real inductance of Lr may not match the design value quite well in a real application. Further, due to the reduced grid inductance, the attenuation on the harmonics around 150 kHz should be examined; otherwise, an extra ac EMI filter has to be inserted. Under the condition that the discontinuous unipolar modulation is adopted, the modulation index is 0.9, the current ripple ratio of the inverter-side inductor is 30%, and Lr is equal to L2 ; the total saved inductance versus the reactive power is illustrated in Fig. 6. It can be seen that compared with the conventional LCL-filter, the proposed LCL-filter can save inductance as well as reduce size and cost. IV. E XPERIMENTAL R ESULTS A prototype of the single-phase full-bridge grid-tied inverter with the digital signal processing (TMS320LF2812A) con-

Fig. 7. Measured power spectrum of the grid-injected current for the proposed LCL-filter-based system under (a) the stiff-grid condition (Lg = 0.15 mH) and (b) the weak-grid condition (Lg = 1.5 mH).

troller is constructed. The experiments are evaluated and investigated under the same conditions as the theoretical analysis. Fig. 7 shows the measured power spectrum of the gridinjected current for the proposed LCL-filter-based system with the oscilloscope of TDS3012. It can be seen that under the stiff condition (Lg = 0.15 mH), the harmonic current around the switching frequency can be fully suppressed, whereas under the weak-grid condition (Lg = 1.5 mH), it cannot be fully suppressed. A spectrum analyzer (Agilent E4402) and line impedance stabilization network (LISN) (EMCO 4825) are used to measure the EMI noise. In the spectrum analyzer, the peak value of the conducted EMI voltage is tracked. Fig. 8 shows the measured grid-side conducted EMI voltage noise of the proposed LCL-filter-based systems. It can be seen that the total conducted EMI noise can meet the standards of CSIPR 11 Class A.

WU et al.: LCL-FILTER WITH PARALLEL RESONANT CIRCUIT FOR SINGLE-PHASE GRID-TIED INVERTER

Fig. 8. Measured conducted EMI noise for the proposed LCL-filter-based system.

Comparing Fig. 8 with Fig. 3, it can be seen that around the frequency of 8 MHz, the measured total EMI noise of the proposed LCL-filter-based system is improved compared with the conventional LCL-filter, which is due to the fact that the small-sized grid-side inductor has less parasitic capacitance. During the measurement of the conducted EMI noise, no extra ac EMI filter is inserted. V. C ONCLUSION This paper has analyzed and addressed the attenuation on the harmonic current and the conducted EMI noise for the proposed LCL-filter-based single-phase full-bridge grid-tied inverter system. The following can be concluded: 1) The selection of the grid-side inductor for the LCLfilter mainly depends on the requirement of the harmonic currents around the switching frequency, whereas for the proposed LCL-filter, it is subject to the harmonic currents around the double switching frequency and the EMI noise requirement around 150 kHz. 2) Compared with the conventional LCL-filter, the total inductance of the proposed LCL-filter can be reduced. 3) The proposed LCL-filter has the most effect mainly on the stiff grid, where the short-circuit current of the power system is larger than 15 times the nominal grid-side fundamental current of the inverter, due to the inserted in-series parallel resonant circuit. The experimental results also show that the proposed LCLfilter-based system has better EMI noise attenuation than the LCL-filter around the frequency of 8 MHz, which is due to the small-sized grid-side inductor with less parasitic capacitance. R EFERENCES [1] 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. 1398–1407, Oct. 2006. [2] Y. Wang and R. Li, “Novel high-efficiency three-level stacked neutral point clamped grid-tied inverter,” IEEE Trans. Ind. Electron., vol. 60, no. 9, pp. 3766–3774, Sep. 2013. [3] S. Eren, M. Pahlevaninezhad, A. Bakhshai, and P. K. Jain, “Composite nonlinear feedback control and stability analysis of a grid-connected voltage source inverter with LCL-filter,” IEEE Trans. Ind. Electron., vol. 60, no. 11, pp. 5059–5074, Nov. 2013. [4] J. Yin, S. Duan, and B. Liu, “Stability analysis of grid-connected inverter with LCL filter adopting a digital single-loop controller with inherent damping characteristic,” IEEE Trans. Ind. Electron., vol. 60, no. 2, pp. 1104–1112, Feb. 2013.

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[5] B. Yang, W. Li, Y. Gu, W. Cui, and X. He, “Improved transformerless inverter with common-mode leakage current elimination for a photovoltaic grid-connected power system,” IEEE Trans. Power Electron., vol. 27, no. 2, pp. 752–762, Feb. 2012. [6] W. Wu, Y. He, and F. Blaabjerg, “A new design method for the passive damped LCL- and LLCL-filter based single-phase grid-tied inverter,” IEEE Trans. Ind. Electron., vol. 60, no. 10, pp. 4339–4350, Oct. 2013. [7] F. Huerta, D. Pizarro, S. Cobreces, F. J. Rodriguez, C. Giron, and A. Rodriguez, “LQG servo controller for the current control of LCL gridconnected voltage-source converters,” IEEE Trans. Ind. Electron., vol. 59, no. 11, pp. 4272–4284, Nov. 2012. [8] J. R. Massing, M. Stefanello, H. A. Grundling, and H. Pinheiro, “Adaptive current control for grid-connected converters with LCL filter,” IEEE Trans. Ind. Electron., vol. 59, no. 12, pp. 4681–4693, Dec. 2012. [9] Safety of Power Converters for Use in Photovoltaic Power Systems— Part 1: General Requirement, IEC Std. 62109-1-2007, 2007. [10] “Specification for Industrial, Scientific and Medical (ISM) radiofrequency equipment electromagnetic disturbance characteristics limits and methods of measurement—Publication 11,” International Electrotechnical Commission (IEC), Geneva, Switzerland, 2004. [11] IEEE Application Guide for IEEE Std. 1547, IEEE Standard for Interconnecting Distributed Resources With Electric Power Systems, IEEE Std. 1547.2-2008, 2008. [12] D. Dong, F. Luo, D. Boroyevich, and P. Mattavelli, “Leakage current reduction in a single-phase bidirectional AC–DC full-bridge inverter,” IEEE Trans. Power Electron., vol. 27, no. 10, pp. 4281–4291, Oct. 2012. [13] W. Wu, Y. He, and F. Blaabjerg, “An LLCL-power filter for single-phase grid-tied inverter,” IEEE Trans. Power Electron., vol. 27, no. 2, pp. 782– 789, Feb. 2012. [14] D. G. Holmes and T. A. Lipo, Pulse Width Modulation for Power Converters: Principles and Practice. Hoboken, NJ, USA: Wiley, 2003.

Weimin Wu received the Ph.D. degree from Zhejiang University, Hangzhou, China, in 2005. From July 2005 to June 2006, he was a Research Engineer with the Delta Power Electronic Center, Shanghai, China. Since July 2006, he has been a Faculty Member with Shanghai Maritime University, Shanghai, where he is currently an Associate Professor with the Department of Electrical Engineering. From September 2008 to March 2009, he was a Visiting Professor with the Center for Power Electronics Systems, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA. From November 2011 to January 2014, he was also a Visiting Professor with the Department of Energy Technology, Aalborg University, Aaalborg East, Denmark, working at the Center of Reliable Power Electronics. He has coauthored over 50 papers and holds five patents. His areas of interest include power converters for renewable energy systems, power quality, smart grid, and energy storage technology.

Yunjie Sun was born in Heilongjiang, China, in 1988. He received the B.S. and M.S. degrees from Shanghai Maritime University, Shanghai, China, in 2011 and 2013, respectively. He is currently with Action Power Electric Company, Ltd., Xi’an, China. His current research interests include digital control techniques, power quality compensators, and renewable energy generation systems.

Zhe Lin received the B.S. degree from Anhui University, Hefei, China, in 2011 and the M.S. degree from Shanghai Maritime University, Shanghai, China, in 2013. He is currently a Research Assistant with the Center for Smart Energy Conversion and Utilization Research, City University of Hong Kong, Kowloon, Hong Kong. His current research interests include control of power converters and renewable energy applications.

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Tianhao Tang (SM’01) received the B.S. and M.S. degrees in electrical engineering from Shanghai University of Technology, Shanghai, China, in 1982 and 1987, respectively, and the Ph.D. degree in electrical engineering from Shanghai University, Shanghai, in 1998. He is currently a Professor with the Department of Electrical Engineering and the Director of Electric Drives and Control Systems with Shanghai Maritime University, Shanghai. He is also the Vice Director of the Sino-French Joint Research Institute of Galileo and Maritime ITS for Safer Seas. His current research interests include power electronics and electric drive systems, renewable energy systems, and their applications in marine systems.

Frede Blaabjerg (S’86–M’88–SM’97–F’03) received the Ph.D. degree from Aalborg University, Aalborg, Denmark, in 1992. From 1987 to 1988, he was with ABB-Scandia, Randers, Denmark. In 1992, he became an Assistant Professor with Aalborg University, where he was promoted to Associate Professor in 1996 and Full Professor of power electronics and drives in 1998. His current research interests include power electronics and applications in wind turbines, photovoltaic systems, reliability, harmonics, and adjustable-speed drives. Dr. Blaabjerg has received 15 IEEE Prize Paper Awards, the IEEE PELS Distinguished Service Award in 2009, the EPE-PEMC Council Award in 2010, and the IEEE William E. Newell Power Electronics Award 2014. From 2006 to 2012, he was the Editor-in-Chief of the IEEE T RANSACTIONS ON P OWER E LECTRONICS. He was a Distinguished Lecturer of the IEEE Power Electronics Society from 2005 to 2007 and of the IEEE Industry Applications Society from 2010 to 2011.

Henry Shu-Hung Chung (M’95–SM’03) received the Ph.D. degree from Hong Kong Polytechnic University, Kowloon, Hong Kong, in 1994. Since 1995, he has been with the City University of Hong Kong, Kowloon, Hong Kong. He is currently a Professor with the Department of Electronic Engineering and the Director of the Center for Smart Energy Conversion and Utilization Research. He has authored six research book chapters and over 300 technical papers and holds 26 patents. His research interests include time- and frequency-domain analysis of power electronic circuits, switched-capacitor-based converters, random-switching techniques, control methods, digital audio amplifiers, softswitching converters, and electronic ballast design. Dr. Chung is currently the Chairman of the Technical Committee on HighPerformance and Emerging Technologies of the IEEE Power Electronics Society and an Associate Editor of the IEEE T RANSACTIONS ON P OWER E LEC TRONICS , IEEE T RANSACTIONS ON C IRCUITS AND S YSTEMS , PART I: F UNDAMENTAL T HEORY AND A PPLICATIONS, and IEEE J OURNAL OF E MERGING AND S ELECTED T OPICS IN P OWER E LECTRONICS.