buck/boost DC/DC converter

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1 NAOSTE: Nagasaki University's Ac Title Author(s) Citation Control of power leveling unit with buck/boost DC/DC converter Hamasaki, Shin-ichi; Mukai, Ryosuke Renewable Energy Research an Appli nternational Conference on, Articl ssue Date 212 URL Right EEE. Personal use of this m EEE must be obtaine for all other incluing reprinting/republishing t promotional purposes, creating new reistribution to servers or lists, this work in other works. This ocument is ownloae

2 Control of Power Leveling Unit with Super Capacitor using Biirectional Buck/boost DC/DC Converter Shin-ichi Hamasaki, Ryosuke Mukai an Mineo Tsuji Division of Electrical Engineering an Computer Science, Nagasaki University 1-14 Bunkyo-machi, Nagasaki, , Japan Abstract As a istribute generation system increases, a stable power supply becomes ifficult. Thus control of power leveling (PL) unit is require to maintain the balance of power flow for irregular power generation. The unit is require to respon to change of voltage an biirectional power flow. So the biirectional buck/boost DC/DC converter is applie for the control of PL unit in this research. The PL unit with Super Capacitor (SC) is able to absorb change of power, an it is examine whether the stable power supply is possible. The output current of PL unit is controlle so as to keep power balance an DC bus voltage. The effectiveness of the propose control is prove in simulation. Keywors-component; Biirectional buck/boost DC/DC converter, power leveling, super capacitor, smart gri system. NTRODUCTON n recent years, as exhaustion of fossil fuel, global warming are focuse, renewable energy sources such as photo voltaic (P) an win force keep attention. New power supply systems, such as a smart gri using renewable power as istribute generation system are spreaing. However, since the output of renewable power sources have fluctuation, stable power can not be supplie to the AC system. For this reason, problems of reverse power flow an voltage optimization occur. Therefore, power change is absorbe an the system to equalize is neee [1]-[3]. A super capacitor (SC) has avantages of no pollution an no environmental effects, an SC is low inner resistance, large capacity, an long life compare with the seconary battery. n this research, the control system of PL unit using the biirectional buck/boost DC/DC converter[2] connecte to SC is propose. The biirectional buck/boost DC/DC converter can work four-quarant operation, which is positive an negative voltages an currents. Therefore, the current is able to flow for charge to SC an ischarge from SC. Even if SC is maintaining the high voltage or the voltage became extremely small, it is possible for the PL unit to work. nverter of the AC system can control the active power an reactive power inepenently using the -q transform base on the phase of the AC gri voltage phase. Power supply to the loa is properly ajuste by the inverter. Effective of the propose system is verifie by simulation. vw q vv u Fig.1 Circuit structure. vu Fig.2 Definition of q axis. q q q Fig.3 ector iagram of q axis.. CRCUT STRUCTURE AND A CONTROL METHOD A. Circuit analysis an a control metho on AC bus Fig.1 shows a circuit structure of istribute generation system in this research. The -q component is efine in Fig.2 an calculate by the -q transformation in (1). 2 2 sin sin( ) sin( ) vu u u u v v q (1) cos u cos( u ) cos( u ) v w 3 3 Then, three phase voltages are efine in (2)-(4).

3 2 vu sin u 3 (2) 2 2 vv sin( u ) 3 3 (3) 2 2 vw sin( u ) 3 3 (4) (2)-(4) assigne to (1), the following equations are obtaine. (5) q (6) From the relation of the vector in Fig.3, the power is calculate. jq jq q q j q q (7) The active power P an reactive power Q are provie as follows. P (8) q q Q q q (9) (5), an (6) assign to (8), an (9), the following equations are obtaine. P (1) Q q (11) f is constant, the active power P is etermine by the active current an the reactive power Q is etermine by the reactive current q. n other wors, in orer to control the active power an reactive power inepenently, the active current an reactive current q shoul be controlle respectively. Fig.4 shows a block iagram of the inverter control an Fig.5 shows a block iagram of PL unit control. The control is performe combining two controls, which are the active an reactive power control for AC sie by the inverter, an input an output power control by the PL unit. The rate of the output power supplying to the loa in AC sie can be controlle by the inverter. n Fig.1, the inverter operates as a gri connecte inverter installe between AC bus an DC bus. LC filters are connecte to the output terminal of the PL system an the inverter in orer to suppress the switching ripple. The boost chopper is connecte to the P cell for regulation of P power. The inverter control is performe by block iagram in Fig.4. The -q component converting from the output voltage an currents of the istribute generation system etermine the active component an reactive component. Using them, power flow can be controlle by P control. The control law becomes (12) an (13). Then an q are reference voltage calculate from line-to-line voltage on AC bus. t t Kp G G Ki G G t (12) q q Kpq Gq Gq Kiq Gq Gq t (13) An the references of v u, v v an v w are calculate by 2-phase G Gq G q q Gq Fig.4 Block iagram of inverter control. c G e o pv e u Fig.5 Block iagram of power leveling unit control. v u v v v w to 3-phase transformation from the references an q.the inverter is controlle by PWM with the reference values. B. Operation of biirectional buck/boost DC/DC converter PL system connecte with SC via the biirectional buck/boost DC/DC converter interconnects to the DC system. PL unit has a role of ajustment of supply an eman of power as shown in Fig.6. f the power of P system is larger than the power to sen to AC bus, current of PL unit flows to charge. f the power of P system is smaller, current of PL unit flows to ischarge. The biirectional buck/boost DC/DC converter operates as a four-quarant chopper of voltage an current. Therefore the current is able to flow negative to charge to SC or positive to ischarge from SC. Even if SC is maintaining the high voltage or the voltage became extremely small, the PL unit is able to work. When the converter operates as buck chopper, the current path is illustrate in Fig.7. Then ON signal is always input to sw3 an OFF signal is always input to sw4. An PWM control is performe on sw1 an sw2. Lsc SC (sw1: ON) (14) Lsc (sw2: ON) (15) SC From above, PWM uty ratio of the converter as buck chopper ( sc < ) is erive as (16). SC Lsc (16)

4 obtaine through LPF of sc. Here sco is approximate to sc. The following equations are erive from Fig.5. (a) Charge current. Fig.6 The role of PL unit. (b) Discharge current. G (2) o Epvpv (21) sc o c (22) t c Kpc Kic t (23) (a) Charge current. Fig.7 Buck chopper operation. (b) Discharge current. (a) Charge current. (b) Discharge current. Fig.8 Boost chopper operation. When the converter operates as boost chopper, the current path is illustrate in Fig.8. Then ON signal is always input to sw1 an OFF signal is always input to sw2. An PWM control is performe on sw3 an sw4. Lsc SC (sw3: ON) (17) Lsc (sw4: ON) (18) From above, PWM uty ratio of the converter as boost chopper ( sc > ) is erive as (19). Lsc (19) sc C. Control metho of PL unit A reference sc of the PL unit is calculate from the active component current G an the output current pv of a P cell. Further, in orer to keep the DC bus voltage constant, feeback control of the is carrie out. Output current is controlle by a current regulator base on iscrete time system. Fig.5 is a block iagram when the biirectional buck/boost DC/DC converter performs a boost operation as seen from DC bus. sc is calculate from the uty ratio of switching an the current of L c when the biirectional buck/boost DC/DC converter performe a buck operation as seen from DC bus. sco is (2) an (21) are obtaine from the law of conservation of power. (22) is obtaine from the Kirchhoff s current law. n (23), DC bus voltage can be regulate by the P control. n aition, a igital control in sampling perio T S is introuce for a current regulator of the PL unit. ik ( 1) ik ( ) L in k out k (24) TS When the converter operates as buck chopper, replacing i(k + 1) = i, a optimal uty ratio can be obtaine by (16) an (24). i L (25) T in sc sc sc S When the converter operates as boost chopper, a optimal uty ratio can be obtaine by (19) an (24). i L (26) T in sc sc sc S sc sc The converter is controlle with the uty ratios. The real value can follow the reference value for every cycle T by this control. D. Power control base on the main AC bus A current reference G of inverter output to AC bus shown in Fig.4 can be given arbitrary value irectly. Then excessive reverse power flow may give a negative influence to the power quality. n aition, when the reference value is less than power of the loa, main power line will supply the power to the loa. Therefore if power of istribute generation system is supplie as much as possible, the cost is low. The loa current can be represente by the following equation. L S G (27) To feeback the main circuit current flowing into the loa, the reference current of the inverter output is calculate by the following equation. (28) G S G n this control metho, if the power is provie from only istribute generation system, the main power line oes not sen out the power. Therefore the power of istribute generation system is able to take full avantage.

5 . SMULATON Simulation is performe to verify the propose control metho by using the circuit in Fig.1. The choppers an GBT inverter are controlle on the iscrete time system. Carrier frequency of the choppers an GBT inverter is 1kHz an sampling time of the igital control is 1s. Table shows the parameters of the circuit. Simulation is achieve on five following conitions. 1. Power fluctuation - At a certain time, the power of P system or the inverter output are change by step. 2. Loa change - At a certain time, the loa changes to ouble by step. 3. Continuous fluctuation of solar cells - The amount of power generation of P system is always varie in short time simulating a variety of conition change. 4. Switch buck-boost operation - n the PL unit, the buck-boost operation of biirectional buck/boost DC/DC converter switches between charge an ischarge. 5. Power control base on the main AC bus - The control metho introuce in -D is applie Reference value Measure value (b) Output current of the PL unit. Fig.9 shows results in case that the biirectional chopper works as a boost chopper seen from the DC bus ( sc > ). t is controlle so as to keep balance of the charge an ischarge of SC against the change of P output in Fig.9(a), (b). n Fig.9 (b), the positive current means charge to SC an the negative one means ischarge to the DC bus. The power supplie to AC sie keeps constant in Fig.9(c). The inverter can supply the output power constantly in Fig.9(c). n Fig.9(), DC bus voltage keeps constant, an the biirectional buck/boost DC/DC converter is operating properly as a boost chopper. Fig.1 shows results in case that the buck/boost chopper works as a buck chopper seen from the DC bus ( sc < ). Slightly change has occurre in the DC sie when the loa changes from 5% to 1%. Although, when the loa changes, the PL unit is able to work to absorb the fluctuation without problem. The power supplie to AC sie keeps constant in Fig.1(c). n Fig.1(), the DC bus voltage is kept constant, an the biirectional buck/boost DC/DC converter is operating properly as a buck chopper. [] (c) Active current of inverter on AC bus. TABLE. Parameters of the circuit Source voltage 1 Frequency 6 Hz R S.25 L S 218 H R G.15 L G 131 H R L 15. L L 13. mh L f.1 H C f 5. F 2 C 5 F L c 1. mh R sc.2 C sc 1. F L 1. mh E pv 8. L cf.2 mh C cf 6. F () oltages in DC bus. Fig.9 Results in case of boost chopper operation. Fig.11 shows the results in case with fluctuation of P power at the conition sc <. This power fluctuation is consiere imitating change of P conition. The fluctuation of P current o is shown in Fig.11(a). Then PL unit is fast enough to absorb minor fluctuations of the P in Fig.11(a) an(b). Therefore is constant in spite of the fluctuation of P power. Fig.11() shows voltages on the DC bus. Output voltage E pv of the P system is constant at 8 an DC bus voltage can be controlle to keep constant at 2 by the DC bus voltage control. As a result, the inverter output current

6 [A] sc [A] [A] (b) Output current of the PL unit (b) Output current of the PL unit. G [A] Measure value Reference value (c) Active current of inverter on AC bus (c) Active current of inverter on AC bus SC 1 E pv () oltages in DC bus. Fig.1 Results in case of buck chopper operation () oltages in DC bus. Fig.11 Results with P fluctuation. an the DC bus voltage is kept constant in Fig.11(c) an (). Response of the output current of the PL unit is fast an accurate. Fig.12 shows results with a switch of buck-boost operation. n this simulation, charge current sc flows. Therefore oltage of SC sc graually increases for the power charge to the capacitor. n Fig.12(c), the biirectional buck/boost DC/DC converter is switche from buck operation to boost operation between.5s-.6s. When the operation change, small fluctuation of voltage occurs. However the currents of the AC bus an the DC bus are not affecte in that perio. Fig.13 shows results with the power control base on the main AC bus. n Fig.13(c), S almost becomes zero. G an L are the same value. This means that all the power is supplie from only istribute generation system an the main system oes not sen out the power. Therefore P power is

7 15 1 o [A] 5 sc c G [A] (b) Active current of inverter on AC bus. (b) Active current of inverter on AC bus. (c) oltages in DC bus. (c) Currents on AC bus. Fig.12 Results with switch buck-boost operation. use effectively. n Fig.13(), the voltage on DC bus is kept constant. Therefore c becomes in Fig.13 (a). n DC bus, all the currents an voltages are able to be controlle stably. []. CONCLUSON n this stuy, control of PL unit with SC using the biirectional buck/boost DC/DC converter an the gri connecte inverter was propose. t is confirme that the integrity is maintaine both the power of P an the inverter output power by PL unit. The effectiveness was confirme by the simulation in various conitions such as charge or ischarge to the SC, P power fluctuation an so on. REFERENCES [1] Y. to, Z. Yang, an H. Akagi, A Control Metho of a Small-Scale DC Power System ncluing Distribute Generators, EEJ Trans. A, ol.126, No.9, 26, Japan [2] S. Funabiki, M. Yamamoto : Estimation of Biirectional Buck/boost DC/DC Converters with Electric Double-Layer Capacitors for Energy Storage Systems, EEJ Trans. A, ol.129, No.6, pp (29) () oltages in DC bus. Fig.13 Results with power control base on the AC bus. [3] H. Kakigano, T. se, et al. : DC oltage Control of the DC Micro-Gri for Super High Quality Electric Power Distribution (in Japanese), JEE Trans. nustrial Application, vol. 127, No. 8, 27, pp [4] K. Yukita, Y. Shimizu, Y. Goto, et al., Stuy of AC/DC Power Supply System with DGs using Parallel Processing Metho, The 21 nternational Power Electronics Conference (PEC), pp , 21 [5] S. Hamasaki, M. Tsuji, E. Yamaa : A Stuy on Power Flow Control for Distribute Generator with EDLC, SYMPOSUM ON POWER ELECTRONCS, ELECTRCAL DRES, AUTOMATON AND MO- TON, ol.1, pp (21)