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4 B. Supercpacitors pack sizing and modeling We have chosen that the supercapacitors, added as high power storage devices, must supply the full power P load = P N during t = 1 s, the delivered energy is then estimated to be around 4.8 MJ. The energy E sc stored at the voltage U sc of the SC s pack voltage can be expressed as follows: N p_sc 2 Esc CeqU sc CscU (1) sc 2 2 N s_sc where C eq is the equivalent capacity of the SC s pack, and N s_sc are the number of parallel branches and the number of SC s series connections, respectively, and C sc is the SC capacitance. The supercapacitors considered in this paper are the Maxwell/BCAP3 type, rated 3 F, 2.7 V. having the parameters given in Table 1. TABLE I PARAMETERS OF THE BCAP 3 SUPERCAPACITORS l It is usually allowed to utilize 75 % of the energy stored in the SC s pack by discharging the pack from its rated voltage U M = 3 V (about 8% of the DC-bus voltage ) to the half of that value equal to U m = 15 V. Every elementary SC in then discharged from an initial voltage V sci = 2.7 V to a final voltage V scf = 1.35 V. Further, the internal losses in the SC s may be taken into account through the efficiency coefficient k =.9 . The energy extracted from the SC s can then be written as: PN Δt k ( CeqU M CeqU m ) (2) 2 2 This leads, by considering an energy of 4.8 MJ, to a SC s pack having an equivalent capacitance C eq = 158 F. Referring to the initial voltage of the SC s pack, we obtain N s_sc = 112 and = 6. To optimize the supercapacitors combination with the battery in UPS applications, we need to establish a model to describe the supercapacitor behavior during fast charge and discharge cycles. We consider the equivalent electric circuit with two RC branches proposed by ,  as shown in figure 2.
6 - v sc and i sc are the elementary supercapacitor voltage and current respectively. The voltage v 2 in the secondary capacity C 2 is given by: v2 i2dt ( v1 v2 ) dt C C R Let Q 2 the instantaneous charge of C 2, we have: i dt Q2 2 (4) (5) The current i 1 going in the main capacitor C 1 is expressed as: i i (6) 1 sc i 2 On the other hand, i 1 is expressed in terms of the instantaneous charge Q 1 and C 1 as: dv1 dq1 dv1 i1 C1 (C Cv.v 1 ) (7) dt dt dt where the charge Q 1 is: 1 2 Q1 Cv1 Cv.v 1 (8) 2 The one supercapacitor element is supposed initially to be fully charged at V sci = v 1i = v 2i = 2.7 V. The initial values are then: Q i C v (9) 2 2 2i 1 2 Q1 i Cv1 i Cv. v1 i (1) 2 The equation (1) leads to the inverse relationship between v 1 and Q 1 that represents the function f(u) in Fig. 3 and it is given by: 2 C C 2CvQ1 1 (11) Cv v In Fig. 4 we represent the experimental results compared to the simulation carried out using Matlab/Simulink for the charge/discharge test at constant current 25 A and -25 A. We observe that the SC s model is in very good agreement with the experimental results.
7 Icharge / Idischarge (A) Voltage (V) 3 2 Experimental Simulation Fig. 4. Comparison between simulation and experimental results for a supercapacitor charge and discharge cycle Finally, we verify that the sized supercapacitors pack fulfills the UPS specifications set before. In fact, the instantaneous energy W sc stored in the main branch of an elementary supercapacitor is given by: W v dq (12) sc 1 1 By neglecting the energy dissipated in R 1 and referring to Eq. (7), we obtain the energy W sc stored in one SC element: as follows: Wsc ( Cvsc Cvvsc ) (13) 2 3 The total energy stored in the SC s pack is then given by: Esc Ns_sc N p_sc ( Cvsc Cvvsc ) (14) 2 3 The SC s voltage element drops from an initial value V sci = 2.7 V to a final value V scf = 1.35 V, the difference between initial energy E sci and final energy E scf of the SC s pack multiplied by the efficiency coefficient k is equal to 5.6 MJ. This means that the SC s pack sizing is in good concordance with the energy P N t = 4.8 MJ specified previously. C. Battery sizing and modeling For the studied 5 kva UPS, the battery chosen by the UPS manufacturer is made of 2226 elementary lead-acid batteries (type VISION rated 7Ah 12V CP127), the number of series and parallel connections are N s_bat = 42 and N p_bat = 53, respectively. We notice that for a full power P N = 48 kw extracted from the battery during the ten-minute autonomous operation, each elementary battery is discharged at P bat = 216 W. Since each battery is made of 6 cells, each cell is then discharged at P cell = 36 W. Referring to the one cell discharge table of the 7Ah 12V battery (cf. table II, line 1, column 2), the end point voltage of every cell is about 1.6 V, which means that the total battery end voltage is almost equal to 4 V. In conclusion, the battery voltage drops during a constant power discharge at P N = 48 kw from 555 V (2.2 V per cell) to 4 V. As the DC-bus voltage is equal to 4 V, the battery is connected to the DC-bus via a buck DC/DC converter (cf. Fig. 1).
9 Battery voltage (V) - it = idt = actual battery charge (Ah), - A = exponential zone amplitude (V), - B = exponential zone time constant inverse (Ah -1 ), - R bat = internal resistance (Ω), - i = battery current (A), - i * = filtered current at low frequency (A). The main feature of this model is that only three points of the discharge curve are required to obtain all the parameters. The points are the fully charged voltage (V full ), the end of the exponential zone (V exp, Q exp ) and the end of the nominal zone (V nom, Q nom ), respectively. We suppose that the battery is discharged at the nominal power P N = 48 kw, its voltage drops from 555 V to 4 V. The total battery current rises from 865 A to approximately 12 A. So the current extracted from every element varies between 16 A and 23 A. For an operating power range between.2p N and.9p N, the current varies from 4.5 A to 2 A. The discharge curves used to perform the battery model are chosen in this discharge current range. Fig. 6 shows the simulation results superimposed on the discharge curves for currents I d = 4.56 A, I d = 7 A and I d = 21 A. We notice that the battery model elaborated with Matlab/Simulink draws with good accuracy the battery behavior at steady state discharge and at variable discharge currents datasheet Experimental curve simulation Simulation 9 8 I d = 21A I d = 7A I d = 4.56A Discharge time (mn) Fig. 6. Experimental and simulated battery discharge curves III. DESIGN OF BATTERY-SUPERCAPACITORS COMBINATION A. Combination without control system Firstly, the battery and the supercapacitors have been combined in parallel without control system as shown in Fig. 7. The DC/DC and the AC/DC converters are supposed to be ideal without losses. The DC bus voltage V DC is equal to 4 V. I bat i load U bat I sc DC DC V DC 4V DC AC load Fig. 7. Parallel combination between SC s and battery The supercapacitors are configured as such to have approximately the same total number calculated in the previous section (672 cells) and to ensure initial voltage almost equal to 56 V (OCV battery voltage). We have then N s_sc = 25 and = 4. Fig. 8 represents the evolution of the U bat voltage versus time during the 1 minutes backup time. Note that U bat drops from an initial value of 56 V to a final value of 48 V.
12 P bat t Pload[1 exp( )] P T au IV (15) P The SC s power is then given by: sc P P (16) load bat 2- The constant T au characterizes the dynamic of the low-pass filter. It is adjustable and can act on the discharge time of the SC s especially on the smoothing degree of the peak power applied to the battery. 3- At time t 2, P load < P bat, the battery supplies a full load power and the low-pass filter operation is canceled. 4- At time t 3, the load power P load is less than a load power limit P Lm, the battery supplies both the load and the SC s pack with energy intended to recharge the supercapacitors. 5- At time t 4, the SC s voltage reaches its maximal tolerated value, the deterioration of supercapacitors is avoided by stopping the recharge process. 6- At time t 5, a new power demand occurs and the low-pass filter is applied again, the SC s pack is now fully charged and has sufficient energy to meet again the power impulses. 7- At time t 6, the SC s are discharged at a rate of 75 % and their voltage has reached its minimum value of 15 V. The SC s pack is stopped and the full power is transferred to the battery. The flowchart in Fig. 12 summarizes the principle of power distribution between the battery and the supercapacitors. no P load < P bat yes yes U sc > U m no no P load < P Lm yes P bat = P load.[1-exp(-t/t au )]+P IV P IV : initial value of P bat P sc = P load - P bat U sc = U m P bat = P load P sc = P bat = P load P sc = Fig. 12. Chart of the SC s/battery combination system no U sc = U M P bat = P load P sc = U sc < U M yes P bat = P recharge P sc = P load - P bat IV. RESULTS OF THE SC S/BATTERY COMBINATION We examined at the first step the combination of the supercapacitors with the battery during the UPS backup time of 1 minutes. A load power profile, rich in harmonics, represented in Fig. 13 is considered. This profile has been provided by APC by Schneider Electric and that is typical for and IT load. It consists of a repetitive 1 seconds cycle during all the backup time T = 6 s.
19 elim /cost (% per Meuro) Tau=1s Tau=2s Tau=5s Tau=1s (b) Fig. 19. Elimination rate of peak power versus T au and In conclusion, by comparing the evolution of all these indicators, we notice a trade-off between the peak power elimination rate and the other performance indicators. In fact, unlike the RMS current-gain, the gain in battery energy losses and the global efficiency, elim increases for decreasing filter constant for a given number of parallel branches. However, the evolution of the performance parameters is in good concordance while divided by the cost of the SC s pack. The maximum point is obtained for T au = 1 s and = 6. If we look at figures 16 (b), 17 (b) and 18 (b) showing the first three parameters G_I RMS, W losses and global returned to the cost, we notice that for T au = 1 s and T au = 2 s, and for = 8, the system performance are almost equal. On the other side, taking elim as the primary basic indicator, we have a clear advantage for T au = 1 s. In fact, we show in the next paragraph that by planning SC s recharge through the battery, T au = 2 s becomes better than T au = 1 s for = 8 since 1 % of the power inrushes are eliminated and the three other performance parameters are improved. B. Combination with SC s recharging In this section, the influence of the SC s recharging via the battery on the performance parameters is analyzed. The system is designed to extract from the battery a power used simultaneously to supply the load and to recharge the SC s pack whenever the load power P load drops below a power limit denoted P Lm. The power load limit is set to P Lm = 8 kw and the power supplied by the battery is fixed at two levels. A first recharge type consists in delivering 8 kw by the battery split between the load and the SC s (I bat_charge = 2 A at V DC = 4 V). In the second recharge type, the battery supplies 16 kw (I bat_charge = 4 A at V DC = 4 V). It should be noted that the elimination rate as defined in Eq. (19) is minimally affected by the recharge of the SC s via the battery, since only rising power demand applied to the battery is considered. We set the filter constant at T au = 2 s and we planned SC s recharge through the battery in order to inspect if the performance indicators can exceed those obtained for T au = 1 s. In Fig. 2 (a), (b), (c), (d), (e), (f), (g) and (h), we show the results of this investigation for T au = 2 s and between 6 and 15 branches.
21 Indicator in (%) Indicator in (%) We notice a clear improvement of all performance parameters for increasing I bat_charge. This improvement is limited for between 6 and 8 branches and more noticeable for I bat_charge = 4 A. The best configuration is then explored. It consists in finding a compromise between: - Minimum of parallel branches for cost considerations, - Maximum constant filter T au for a better damping of power surges inflicted to the battery, - Maximum gain in battery power losses, - Maximum elimination rate of peak power. We consider that the last criterion consisting in the potential gain in peak power is the most important one since it traduces peak stresses relief on battery and therefore the positive influence on the system performance , , . We demonstrate in Fig. 21 that SC s recharge via the battery for I bat_charge = 4 A and = 8 makes the system performances better for T au = 2 s than for T au = 1 s. Moreover, T au = 2 s leads to better smoothing of the power extracted from the battery and implies better stresses reduction. The total cost of the SC s pack is then about 4 k Tau = 1 s Tau = 2 s without recharge Gain 1 Gain 2 3 elim 4 W losses I RMS (a) Tau = 1 s Tau = 2 s with recharge Gain 1 Gain 2 3 elim 4 W losses I RMS (b) Fig. 21. Comparison between system performances for T au = 1 s and T au = 2 s ( = 8) Fig. 22 shows the P load, P bat and P sc profiles at the end of the backup time of 6 s for the optimal combination (T au = 2s, = 8 and I bat_charge = 4 A). We notice that the SC s overcome all the peak power during T = 1 min. thanks to the recharging operation through the battery.
22 Power (W) Power (W) 1 x 15-1 P bat (a) P load and P bat P load 2 x 15 1 P sc P load (b) P load and P sc Fig. 22. Power waveforms with intermediary SC s recharging V. SIMULATION OF THE GLOBAL SYSTEM The system controlling the combination between the SC s and the battery in the 5 kva rated UPS is designed so that the SC s supply the full load power during short failures of the power source lasting up 1 seconds. Furthermore, the reappearing intermittent grid permits to recharge the SC s between successive outages. The SC s pack is then ready to supply the transient power load during the backup operation in order to reduce the battery stresses. Fig. 23 illustrates the power waveforms of P load, P bat and P sc for the UPS operation during three 1 seconds brownouts and for the optimal combination previously found. At time t = 8 s, the system is switched to the 1 minutes autonomous operation. The full power is supplied by the SC s during the first intermittent functioning with fast charging through the power source. On the other side, during the backup time, only power impulses are covered by the SC s which are recharged through the battery.
24 VI. CONCLUSION In this paper, the design of a control system which optimizes the battery-supercapacitors combination in a 5 kva rated UPS has been presented. The advantage of having a hybrid energy source for the UPS has been shown. The importance of supercapacitors in peak power smoothing has been elaborated on. The SC s pack and the battery are modeled using Matlab/Simulink software and then validated. The reduction in battery stresses has been discussed. The supercapacitors overcome the power surges and reduce high power demands away from the battery during the backup time. They also ensure the whole load power during outages lasting less than 1 s. The study of some performance parameters with respect to the cost of the SC s pack has also been presented and an optimal configuration has been found for a filter constant T au = 2s, a number of SC s parallel branches = 8 and battery recharge current I bat_charge = 4 A. The cost of this system is higher today than pure battery system however it should be pointed out that supercapacitors undergo intensive development and become more and more available in small-size and low price. At the current state, the SC s pack cost is almost triple of the battery pack cost. The system we conceived would be efficient if the battery life time is enhanced at least 4 times. We are undertaking accelerated tests on lead acid batteries to observe the effect of pulsed loads and smoothed loads on battery wear-out process and reliability. Some extensions of this work are undertaken and experimental bench has been set up to carry accelerated tests on lead acid batteries. We aim to observe the effect of pulsed loads and smoothed loads on battery wear-out process and reliability in order to quantify the efficiency of the designed system. ACKNOWLEDGMENTS The authors would like to thank APC by Schneider Electric, Grenoble, France, for its technical assistance. REFERENCES  Z. Chlodnicki, W. Koczara, and N. Al-Khayat, Hybrid UPS Based on Supercapacitor Energy Storage and Adjustable Speed Generator, Electrical Power Quality and Utilisatio, Journal, vol. XIV, no. 1, 28.  A. M. Van Voorden, L. M. R Elizondo, G. C. Paap, J. Verboomen, and L. Van der Sluis, The Application of Super Capacitors to relieve Battery-storage systems in Autonomous Renewable Energy Systems, IEEE Power Tech, Lausanne, 1-5 July 27, pp  R. A. Dougal, S. Liu, and R. E. White, Power and Life Extension of Battery Ultracapacitor Hybrids, IEEE Transactions on Components and Packaging, vol. 25, no. 1, March 22.  Alfred Rufer and Philippe Barrade, A Supercapacitor-Based Energy-Storage System for Elevators With Soft Commutated Interface, IEEE Transactions on Industry Applications, vol. 38, no. 5, Sept./Oct. 22.  S. Mallika and R. S. Kuma, Reniew on Ultracapacitor-Battery Interface for Energy Management System, International Journal of Engineering and Technology, vol. 3 (1), 211, pp  F. Rafik, H. Gualous, R. Gallay, A. Crausaz, and A. Berthon, Frequency, thermal and voltage supercapacitor characterization and modeling, Journal of Power Sources, vol. 165, pp , 27.  C. M. Krishna, Managing Battery and Supercapacitor Resources for Real-Time Sporadic Workloads, IEEE Embedded Systems Letters, vol. 3, no. 1, March 211.  F. Belhachemi, S. Rael, and B. Davat, A physical based model of power electric double-layer supercapacitors, Proc. of IEEE Industry Appl. Conf., pp , 2.  M. B. Camara, B. Dakyo, and H. Gualous, Polynomial Control Method of DC/DC Converters for DC- Bus Voltage and Currents Management Battery and Supercapacitors, IEEE Transactions on Power Electronics, vol.27, no. 3, pp , March 212.
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DC-DC BIDIRECTIONAL ISOLATED CONVERTER FOR FUEL CELLS AND SUPER-CAPACITORS HYBRID SYSTEM P.Pugazhendiran 1, Mohammed Nisham 2 Department of EEE, IFET College of Engineering, Villupuram, Tamil Nadu, India.
NESSCAP ULTRACAPACITOR TECHNICAL GUIDE. NESSCAP Co., Ltd.
NESSCAP ULTRACAPACITOR TECHNICAL GUIDE 2008 NESSCAP Co., Ltd. 1 About Ultracapacitors? Enter the ultracapacitor, also known as a supercapacitor, Electric Double Layer Capacitor (EDLC), or pseudocapacitor.

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