ECAI 29 - International Conference Second Edition Electronics, Computers and Artificial Intelligence 3-5 July, 29, Piteşti, ROMÂNIA Efficiency of Current Ripple Passive Filtering Techniques for Inverter System Supplied by a Fuel Cell: Modeling and Simulations OPROESCU Mihai BIZON Nicu SOFRON Emil University of Pitesti Str. Tg. din Vale, No.1 moproescu@yahoo.com, nicubizon@yahoo.com Keywords: current ripple, fuel cell, passive filtering Abstract In this work are analysed pasive filtering techniques for reducing the inverter input current ripple effect. A large signal model of inverter system is used to evaluate the current ripple. Three inverter systems types are simulated and the obtained results are shown and commented. The current harmonics level is dependent by filtering capacitance value and the harmonics number is given by the used switching techniques: rectangular full-wave, PWM modify sine and PWM pure sine, respectively. Both passive filtering structures are analyzed: structure with passive filter on the low DC voltage bus and structure with passive filter on the high DC voltage bus, respectively. Also, the input current and output voltage power spectrums are simulated for different load power levels in order to validate the used model. 1. INTRODUCTION Typical structure of the system inverter is presented in figure 1 [1-3]. In order to increase the inverter efficiency we should to minimize the stages of energy conversion [4-7]. A possible structure is presented in figure 2, where a fuell cell he was directly connected to invertor, and the system of energy storage has been eliminated. Connecting structure for energy conversion using an inverter topology system of this type is given in Matlab diagram of figure 3. This structure was used to analyze the propagation of inverter current ripple towards the DC energy source, the Fuel Cell with Proton Exchange Membrane (PEMFC). Fig. 1. Structure typical inverter system interface with the DC-DC with fuel cell Fig. 2. Inverter system powered directly from the fuel cell
62 Mihai OPROESCU, Nicu BIZON, Emil SOFRON Fig. 3. Matlab diagram of inverter system analysis Figure 4 shows how appears the current ripple in the inverter system and then this is propagated to the PEMFC. Vf c Rs Rcin Modelul original Convertor C.C. / C.C. Lf Rcdc Iload Cin Cdc a) Original model Modelul de curent continu Rs 1:n Lf Rcin Rcdc Fig. 4. Propagation of current ripple in inverter system There are need limitation of these currents ripple in the inverter, and from this kind of operation it will obtain an efficient and safe operation of the DC power source the PEMFC stack [8-15]. Vf c Cin b) AC model Rs Modelul riplului de curent alternativ Rdc Lf Cdc Iload Rcin Rcdc 2. MODELING OF INVERTER SYSTEM FOR EVALUATE OF CURRENT RIPPLE Vf c Cin Cdc Iload Starting from the inverter system proposed in [5], we extract the current ripple model of AC that we used in the simulations (OrCAD 1.5.) c) Current ripple model Fig. 5. Modeling inverter system for studying the propagation of current ripple
Efficiency of current ripple passive filtering techniques for inverter system supplied by a fuel cell: modeling and simulations 63 Simulation results for different values of the input filtering capacitor, C1, are presented in table 1 using the diagram below (figure 6). 24 V1 C in [F] R1.28 R3.3 C1 1u R2.1 L1 84m Fig. 6. Current ripple model of AC R4 2 C2 22u The harmonic amplitude of current [ma] Harmonics [Hz] 1 3 5 7 15,1 364,295 11,61 64,678 47,84 21,598,6 363,188 14,259 55,618 36,18 1,915 1 358,665 94,721 45,693 27,855 7,2716 3 312,563 53,64 21,5 11,829 3,2614 6 232,428 3,772 12,196 7,1776 2,522 I2 IOFF = 3 FREQ = 1 IAMPL = 1 Table 1. The low frequency harmonics amplitude for PEMFC power supply according to the input capacity. C dc [mf] The harmonic amplitude of current [ma] Harmonics [Hz] 1 3 5 7 15 76,24 84,31 49,694 36,836 17,269 74,823 84,515 49,613 36,829 17,288 5 274,86 84,343 49,673 36,813 17,39 74,734 84,315 49,672 36,81 17,31 15 274,751 84,314 49,669 36,812 17,312 22 274,794 84,285 49,672 36,813 17,312 Tabel 2. The low frequency harmonic amplitude for PEMFC power supply according to capacity of intermediate DC bus, C1=3F. C dc [mf] The harmonic amplitude of current [ma] Harmonics [Hz] 1 3 5 7 15 1 339,58 98,626 5,76 33,196 11,389 1 337,281 98,54 5,728 33,16 11,397 5 337,199 98,297 5,727 33,149 11,41 1 337,118 98,265 5,727 33,15 11,398 15 337,163 98,248 5,722 33,149 11,4 22 337,184 98,233 5,727 33,149 11,4 Table 3 The low frequency harmonic amplitude for PEMFC power supply according to capacity of intermediate DC bus, C1=6F. Nivelul amplitudinii armonicelor de curent de alimentare (ma) 4 35 3 25 2 15 1 5 1 Hz 3 Hz 5 Hz 7 Hz 15 Hz Frecventa Fig. 7. The harmonic amplitude of low frequency for power supply according to the input capacity C1=,1mF C1=,6mF C1=1mF C1=3mF C1=6mF Fig. 8. Normalized PEMFC current ripple according to input filtering capacity (see also [5]) New technologies to achieve higher capacities (such as making ultracapacitors) allow obtaining a reduced size for passive filters.
64 Mihai OPROESCU, Nicu BIZON, Emil SOFRON Mag (% of Fundamental) 4 3 2 1 Fig. 9. Normalized PEMFC current ripple according to capacity of the intermediate DC bus (see also [5]) 3. SIMULATION In this chapter, using a PEMFC Matlab model (shown in figure 1), the current and voltage spectra of the whole inverter system is analyzed. The current harmonics level is dependent by filtering capacitance value and the harmonics number is given by the used switching techniques: rectangular full-wave, PWM modify sine and PWM pure sine, respectively (figures 12, 13 and 14). 3 4 5 Fig. 11. Output voltage spectrum of the inverter (pure sine PWM control with constant amplitude modulation index). Mag (% of Fundamental) 2 1.5 1.5 3 4 5 Fig. 12. Output current spectrum of the inverter (pure sine PWM control with constant amplitude modulation index) 5 FFT window: 5 of 9.549 cycles of selected signal -5.2.4.6.8 Time (s) 25 Fundamental (5Hz) = 416.9, THD= 32.15% Mag (% of Fundamental) 2 15 1 5 Fig. 1. U-I and power characteristics of fuel cell used in simulation, with parameters: 12V/1kW (1V/2kW) 2 4 6 8 1 Fig. 13. Output voltage spectrum of a single inverter (modified sine PWM control with π/6 phase shift angle)
Efficiency of current ripple passive filtering techniques for inverter system supplied by a fuel cell: modeling and simulations 65 75 7 65 6 55 FFT window: 5 of 9.328 cycles of selected signal The effect of load change in the functioning of the inverter system is shown in figure 16. 5.5.6.7.8.9.1.11.12.13 Time (s) 8 DC component = 61.39, THD= 2.36% Mag (% of DC component) 6 4 2 3 4 5 Fig. 14. Spectrum of fuel cell current for an L-C filter (L_filter=,1mH and C_filter=2µF) on 4V voltage bus, and rectangular full-wave control. Output current and voltage shape for a large variation of voltage on the intermediate DC bus is shown in figure 15. A pure sine PWM switching command and control by amplitude modulation index in the feed forward voltage loop is used [16]. Fig. 16. Dynamic variables of inverter system CONCLUSION Fig. 15. Output current and voltage shape for a large variation of voltage on the intermediate DC bus. For a DC power source that supply a variable load, such as an inverter system supplied by fuel cells, is recommended to use a boost interface instead of a passive filter on low DC voltage bus. In this case, obviously we will get better filtering of the fuel cell current. The reducing of the low frequency harmonics increases the life of fuel cells. If, for reasons of efficiency it not allows to use the boost interface, we must redesign the filter on the high voltage DC bus to limit current ripple under imposed limits. Obtaining of the high DC voltage is
66 Mihai OPROESCU, Nicu BIZON, Emil SOFRON made effectively by connecting in parallel to input and series to output of several DC-DC converters with galvanic isolation (full bridge or half bridge), followed by a bridge rectifier with fast diodes. Voltage inverter is realized with a rectangular fullwave switching command or a modified sine PWM control unless it connects to the grid or supplied a load witch support high levels for frequencies harmonics. For grid connected inverter system is necessary to use a pure sine PWM switching command and control synchronized with the grid voltage. To increase the overall efficiency of inverter system the converters structure must be of ZVS type (zero voltage switching) and / or ZCS type (zero current switching). It is obviously that this increases considerably the complexity level of the command and control circuit, so the price cost increases. In that analysis by simulation (and by experiment in the second author paper [17]) we have determined the way to design an optimum filtering capacity to obtain a required PEMFC current ripple for fuel cell that supply the inverter system. ACKNOWLEDGEMENTS We thank to ANCS and CNMP for financial support in the frame of PNCDI 27-213/contract 7229.1.1.28 REFERENCES [1]. Y. Xue, L. Chang, S. Bækhoj Kjær, Topologies of Single-Phase Inverter for Small Distributed Generators :an Overview, IEEE Trans. on Power Electronics, vol. 19, no. 5, pp. 135-1313, 24. [2]. N. Bizon, M. Oproescu, Power Converters for Energy Generation Systems (in Romanian lang.: Convertoare de Putere utilizate in Sistemele de Generare a Energiei), University of Pitesti Publishing, Pitesti, ISBN 978-973-69-644-2, 27. [3]. K. Metaxiotis, N. Bizon, Intelligent Information Systems and Knowledge Management for Energy: Applications for Decision Support, Usage and Environmental Protection - Intelligent control of the Energy Generation System, IGI Global, USA, 29. [4]. N. Bizon, E. Sofron, M. Oproescu, M. Raducu, 27, Multi-stage Inverter Topologies for an Energy Generation Systems, 13th International Symposium on Modeling, Simulation and System's Identification SIMSIS 27, September 21-22, 27, Galati, Romania. [5]. C. Liu, J. S. Lai, Low Frequency Current Ripple Reduction Technique with Active Control in a Fuel Cell Power System with Inverter Load, IEEE Transactions on Power Electronics, Vol. 22, Issue 4, pp. 1453 1463, 27. [6]. N. Bizon, M. Oproescu, Research report Grant CNCSIS no. 57, Intelligent control algorithms for the Energy Generation Systems (in Romanian lang.: Algoritmi inteligenti pentru controlul eficient al unui sistem invertor alimentat de la o pila de combustie), CNCSIS, 28. [7]. N. Bizon, M. Oproescu, Research report CEEX AMCSIT no. 226, Integrated Energy Generation Systems using renewable sources (in Romanian lang.: Sistem integrat de conversie a energiei din surse regenerabile), AMCSIT, 28. [8]. S. Basu, Recent Trends in Fuel Cell Science and Technology, Springer, Berlin, 27. [9]. N. Bizon, E. Sofron, M. Raducu, M. Oproescu, Low Frequency PEMFC Current Ripple Compensation using a Controlled Buck Current Source, Int. conf. Technologies and Power Electronics (TPE8), pp. II 987-11, Pitesti, Romania. [1]. NETL - National Energy Technology Laboratory, Fuel Cell Handbook (Fifth Edition), EG&G Services Parsons, Inc. Science Applications International Corporation, 2. [11]. R.S. Gemmen, Analysis for the effect of the ripple current on fuel cell operating condition, J. Fluids Eng., vol. 125, no. 3, pp. 576-585, 23. [12]. G. Fontes, C. Turpin, S. Astier, T. Meynard, Interactions between fuel cells and power converters: Influence of current harmonics on a fuel cell stack, IEEE Trans. on Power Electronics, vol. 22, no. 2, pp. 67-678, 27. [13]. W. Shireen, R. A. Kulkarni, M. Arefeen, Analysis and minimization of input ripple current in PWM inverters for designing reliable fuel cell power systems, Journal of Power Sources 156, pp. 448 454, 26. [14]. C. Woojin, J. Gyubum, E. N. Prasad, H. W. Jo, An Experimental Evaluation of the Effects of Ripple Current Generated by the Power Conditioning Stage on a Proton Exchange Membrane Fuel Cell Stack, JMEPEG, 13, pp. 257-264, 24. [15]. N. Bizon, L. Ionescu, A. Mazare, M. Oproescu, Analyze of the Feed-Forward Control for a Pure Sine Inverter, Second International Conference on Electronics, Computers and Artificial Intelligence - ECAI 7, no. 2, pp. 71-79, 27, [16]. OPROESCU Mihai, BIZON Nicu, SOFRON Emil, Efficiency of Current Ripple Passive Filtering Techniques for Inverter System Supplied by a Fuel Cell: Experimental Results, ECAI 9.