Bidirectional Boost/Buck Quadratic Converter for Distributed Generation Systems with Electrochemical Storage Systems

Transcription

1 Bdrectonal Boost/Buck Quadratc onverter for Dstrbuted Generaton Systems wth Electrochemcal Storage Systems D. Foto ESTSetubal, Polytechnc Insttute of Setúbal Setúbal, Portugal A. ordero ISEL Polytechnc Insttute of Lsboa Lsboa, Portugal Abstract The ncreasng number of dstrbuted generaton systems usng renewable and non-conventonal energy sources show the trend of future generaton systems. Most of these systems requre power electronc converters as an nterface between the D voltage buses and electrochemcal storage systems. Such storage systems, lke batteres or supercapactors, usually need bdrectonal D-D converters to allow ther charge or dscharge accordng wth necessary operaton condtons. In ths paper, a non-solated bdrectonal Buck-Boost converter wth hgh voltage gan for electrochemcal storage devces used n dstrbuted generaton systems s presented. To acheve hgh voltage gan ratos, the proposed topology presents quadratc characterstcs n both step-down (Buck) and step-up (Boost) operaton modes. In addton to the wde converson range, t presents contnuous nput and output current, reduced chargng/dschargng rpple and smple control crcutry. All these features allow the energy exchange smoothly and contnuously resultng n a longer durablty of storage devces. The prncple of the operaton of the proposed converter n both operaton modes, as well as ther theoretcal analyss wll be dscussed. The performance of ths bdrectonal power converter s confrmed through smulaton and expermental results. Keywords bdrectonal D/D converter, Boost and Buck quadratc converter, dstrbuted generaton systems. I. INTRODUTION The nvestment n renewable and sustanable energy sources s contnuously ncreasng. Accordng wth data avalable from 2015 the global new nvestment n renewable energy and fuels clmbed to a record USD bllon [1]. Furthermore, a sgnfcant nterest over the last years about losses over long transmsson and dstrbuton lnes, reducton of nstallaton costs and voltage regulaton problems have encouraged the development of dstrbuted generaton systems based on photovoltac panels (PV), fuel cells (F), wnd turbne, mcro-turbnes, etc., allowng small-scale generaton located near to the customers rather than remote locatons [2, 3]. Also remarkable s the ncreasng number of other applcatons (telecommuncatons systems, hybrd electrc vehcles, avaton, unnterruptble power supples, etc.) usng non-conventonal energy sources [4-8]. Undesrably the floatng nature of most renewable energy sources makes them unsutable for standalone operaton as the sole power source. For nstance, hydrogen and hydrocarbon fuel cells contan hgh electrochemcal energy but usually requre back-up power at startng condtons [9, 10]. The most common soluton to overcome ths problem s to use energy storage devces such as batteres or supercapactor n addton to the renewable energy source to compensate for these fluctuatons and mantan a smooth and contnuous power flow to the load [11]. Bdrectonal D-D converter plays an mportant role n such applcatons, allowng energy exchange

2 between storage devces and the rest of the system. Besdes the adaptaton to dfferent voltage levels these converters wll also control the charge or dscharge of storage devces accordng wth requred operatng condton. Several bdrectonal D-D converters based on solated and non-solated topologes have been presented n lterature for ths purpose [12-17]. Most of solated topologes need a transformer and a hgh number of swtchng devces whch ncreases the cost and the swtchng losses, n addton to more complcated control schemes [18-21]. Non-solated topologes are manly based on the conventonal Buck-Boost and uk confguratons whch are farly smple and easy to control [22-25]. However, n many applcatons the hgh gan requrements are such that the conventonal D-D converter operates wth extremely duty-cycle operaton whch may result n reverse-recovery problem and causes not only low effcency but also creates electromagnetc nterference (EMI) problems [13]. To extend the voltage gan rato, several D-D converters wth magnetc couplng technques have been proposed, notwthstandng some dsadvantages such as pulsatng bus voltage reman and complex control schemes [26, 27]. In ths paper, a non-solated bdrectonal Buck-Boost quadratc converter s proposed. Ths soluton s sutable for dstrbuted generaton systems where conventonal converters are nadequate for hgh-frequency applcatons and where the specfed range of nput voltages and the specfed range of output voltages call for an extremely large range of converson ratos. Ths converter wth quadratc D converson rato offer a sgnfcantly wder converson range, contnuous nput and output current, reduced chargng/dschargng rpple and also smple control crcutry. II. BIDIRETIONAL BOOST/BUK QUADRATI ONVERTER One of the most known topologes wth wde voltage range s the Boost quadratc converter. As can be seen by Fg. 1, ths topology s characterzed by the use of two nductors, one ntermedate capactor, three dodes and a sngle power swtch. Fg. 1. lasscal quadratc Boost converter. In order to overcome the lmtaton of the undrectonal power flow of the Boost quadratc converter t s proposed a new bdrectonal power converter. Fg. 2 shows the power topology of ths converter. As presented n ths fgure, the power crcut does not requre more passve components (nductors and capactor) than the requred by the classcal Boost quadratc converter. It s also characterzed by a statc voltage gan wth a quadratc functon for both Boost and Buck operatng modes. Fg. 2. Proposed bdrectonal Boost/Buck quadratc converter.

3 In order to analyze the proposed bdrectonal dc/dc converter wll be assumed steady state operaton, contnuous conducton mode (M) and deal components. As descrbed, the proposed power converter allows for two dstnct modes of operaton: dschargng and chargng of the storage system. III. OPERATION OF THE POWER ONVERTER In the dschargng mode the converter operates as a Boost converter, transferrng the energy from the storage system to the load. In ths mode two of the transstors (T 1 and T 4 ) are always n the OFF state and the transstor T 3 s always n the ON state. Ths mode s also characterzed by two equvalent crcuts durng one swtchng cycle (Ts), as descrbed below: Frst equvalent crcut (Fg. 3 a)): Ths crcut s obtaned when the transstor T 2 s n the ON state (durng the tme nterval of δts). In ths state the nductors L 1 and L 2 wll be charged snce the currents n these nductors wll ncrease lnearly. The energy stored n the capactor s transferred to the nductor L 2. Second equvalent crcut (Fg. 3 b)): Ths crcut s related wth the transstor T 2 n OFF state (durng the tme nterval of (1- δt s ). Ths state s characterzed by the dscharge of the nductors L 1 and L 2. Durng ths tme nterval the currents n these nductors wll decrease lnearly. The energy that was stored n the prevous state s now transferred to the capactor and to the load: a) b) Fg. 3. Equvalent crcuts durng one swtchng perod for the dscharge mode of the storage system. From the analyss of these two operatng stages s possble to obtan the voltage gan of ths converter n dscharge mode and n M. Thus, consderng that the average voltages n the nductors n the two operatng stages are equal, the followng relatonshps are obtaned δ V + ( 1 δ)(v V ) =0 (1) δ V + ( 1 δ)(v V ) =0 (2) o From the partal voltage gans that are obtaned from the prevous equatons, the nput to output voltage gan n ths dscharge mode s gven by:

4 V V o 1 = 2 (3) ( 1 δ) The operaton of the converter n chargng mode s characterzed by the energy transfer from the load to the storage energy system. In ths mode of operaton the converter operates as a Buck converter. As n the prevous operaton mode n ths case only a sngle transstor controls the output voltage through commutaton. Thus, the transstors T 2 and T 3 are always n the OFF state. The transstor T4 could be always n the ON state or can commutate at the same tme wth transstor T 1 (addtonal swtchng losses). From the analyss of ths operatng n M s possble to verfy that durng one swtchng cycle there are two equvalent crcuts, as followng descrbed: Frst equvalent crcut (Fg. 4 a)): Ths frst equvalent crcut s related wth the on-tme (tme nterval of δts) of transstor T 1. Durng ths tme perod the currents n nductors L 1 and L 2 wll ncrease n absolute value. apactor wll dscharge snce ts energy wll be transferred to nductor L 1. Second equvalent crcut (Fg. 4 b)): The obtaned crcut assocated to the complementary tme (tme nterval of (1-δTs) s related wth the transstor T 1 n OFF state. In ths state the currents n the nductors L 1 and L 2 wll decrease n absolute value. The energy that was stored n those nductors wll be transferred to the capactor and to the load (storage system). a) b) Fg. 4. Equvalent crcuts durng one swtchng perod for the charge mode of the storage system The relatonshp between the output and nput voltage of the converter n dscharge mode n M can be obtaned from the analyss of the equvalent crcuts presented n Fg. 4. Thus, accordng to these crcuts and consderng that the sums of the average voltages n the nductors durng one swtchng cycle are equal to zero, the next relatonshps are obtaned: δ ( V V ) ( 1 δ) V =0 (4) δ ( V V ) + ( 1 δ) V =0 (5) o

5 The nput voltage gan of ths converter n dscharge mode wll therefore be gven by the partal voltage gans that can be obtaned from the prevous equatons, as follows: V V o 2 = δ (6) IV. SIMULATION RESULTS The operaton modes of the proposed bdrectonal Boost/Buck quadratc converter were verfed through numercal smulatons. These smulatons were performed usng the Matlab/Smulnk program. The converter operaton was verfed at V = 24V (battery voltage), V o = 200V, L 1 = 1 mh, L 2 = 2 mh = 150 µf and swtchng frequency f s = 14 khz. In the dscharge mode t s consdered the voltage of the storage as the nput and the V o as the output voltage. Fgures 5, 6 and 7 show the obtaned smulated results for the converter, operatng n dschargng mode (quadratc Boost) and M. These results were obtaned for a duty cycle of 0,66. In Fg. 5 s possble to verfy the nput voltage (1), capactor voltage (2) and output voltage (3). The hgh gan of the power converter s confrmed by ths result. Fgs. 6 and 7 show the nductor currents L1 and L2 and the voltage at the termnals of transstors T 1 and T 2. From these fgures s possble to confrm the contnuous nductor current operaton. The rpple current n both nductors are also smlar n magntude. Fg. 5. Dschargng mode. Smulaton waveforms of the: 1 nput voltage V, 2 - capactor voltage V c and 3 - output voltage V o. Fg. 6. Dschargng mode. Smulaton waveforms of the transstor T 1 voltage (V T1) and nductor L 1 current ( L1).

6 Fg. 7. Dschargng mode. Smulaton waveforms of the transstor T 2 voltage (V T2) and nductor L 2 current ( L2). The operaton n charged mode (quadratc Buck) was also confrmed through the smulatons. Fgs 8-10 show the obtaned smulated results n ths mode of operaton for a duty cycle of 0,35. The nput voltage (1), capactor voltage (2) and output voltage (3) are presented n Fg. 8. As expected, n ths case the output voltage (storage system) s much lower than the nput voltage. The nductor currents L1 and L2 and the voltage at the termnals of transstors T 1 (V T1 ) and T 2 (V T2 ) are presented n Fgs. 9 and 10. From these fgures s possble to confrm the contnuous nductor current. Even n ths mode the rpple current n both nductors are stll smlar n magntude. Fg. 8. hargng mode. Smulaton waveforms of the: 1 nput voltage V o, 2 - capactor voltage V c and 3 - output voltage V. Fg. 9. hargng mode. Smulaton waveforms of the transstor T 1 (V T1) voltage and nductor L 1 current ( L1).

7 Fg. 10. hargng mode. Smulaton waveforms of the transstor T 2 voltage (V T2) and nductor L 2 current ( L2). V. EXPERIMENTAL RESULTS A prototype of the proposed converter was bult to valdate the theoretcal consderatons and obtaned smulaton results. The crcut parameters of the expermental prototype are the same as those used for smulaton. As made for the smulaton the prototype was verfed n the dschargng mode (V =24V V o =200V) and chargng mode (V o =200V V =24V) power flow. The waveforms obtaned from the expermental prototype n dschargng power flow mode (quadratc Boost) are presented n Fgs. 11, 12 and 13. All these results were obtaned for a duty cycle of 0,67. From Fg. 11 s possble to confrm that the nput voltage (1), capactor voltage (2) and output voltage (3) are n agreement wth the expected. Ths fgure also confrms the hgh gan of the power converter. Fgs. 12 and 13 show the nductor currents L1 and L2 and the voltage at the termnals of transstors T 1 (V T1 ) and T 2 (V T2 ). Through these fgures s possble to confrm the expected contnuous nductor current operaton. They also show that the rpple current n both nductors are also smlar n magntude. Fg. 11. Dschargng mode. Expermental waveforms of the: h1 nput voltage V, h2 - capactor voltage V c and h3 - output voltage V o. Fg. 12. Dschargng mode. Expermental waveforms of the: h1 transstor T 1 voltage (V T1) and h4 nductor L 1 current ( L1).

8 Fg. 13. Dschargng mode. Expermental waveforms of the: h1 transstor T 2 voltage (V T2) and h4 nductor L 2 current ( L2). Expermental tests wth the laboratory prototype n chargng mode (quadratc Buck) were also performed. The waveforms that were obtaned n ths mode wth a duty cycle of 0,36 are presented n Fgs. 14, 15 and 16. The nput voltage, capactor voltage and output voltage are presented n Fg. 8. These results are smlar wth the ones presented n the prevous test. Regardng the expermental waveforms of nductor currents L1 and L2 and the voltage at the termnals of transstors T 1 and T 2 they are presented n Fgs. 14 and 15. These fgures show that the converter works n contnuous conducton mode and that the rpple current n both nductors are smlar n magntude. Fg. 14. hargng mode. Expermental waveforms of the: h1 output voltage V o, h2 - capactor voltage V c and h3 - nput voltage V. Fg. 15. hargng mode. Expermental waveforms of the: h1 transstor T 1 voltage (V T1) and h4 nductor L 1 current ( L1).

9 Fg. 16. hargng mode. Expermental waveforms of the: h1 transstor T 2 voltage (V T2) and h4 nductor L 2 current ( L2). Accordng to the fgures presented n ths secton s possble to confrm that the expermental results are very smlar wth the obtaned through numercal smulatons. VI. ONLUSIONS A bdrectonal D-D converter wth Boost/Buck characterstcs for electrochemcal storage systems was proposed n ths work. The proposed converter presents a Boost quadratc characterstc when the storage system s n dscharge mode and Buck quadratc characterstc when the referred system s n charge mode. As a result, wth ths converter s possble to obtan hgher and lower voltage gan wth reduced duty ratos. Besdes that, t presents contnuous nput and output current, reduced chargng/dschargng rpple and smple control crcutry. Due to these features, ths non-solated bdrectonal Buck-Boost converter wth hgh voltage gan s ndcated for several applcatons, such as, electrochemcal storage devces used n dstrbuted generaton systems. The performance of the proposed converter was tested and verfed through smulatons and wth a laboratoral prototype. The obtaned expermental results are n agreement wth the smulaton results, confrmng the expected results for ths topology. AKNOWLEDGMENT Ths work was supported by natonal funds through FT Fundação para a ênca e a Tecnologa, under project UID/E/50021/2013. REFERENES [1] Renewables 2016 Global Status Report, REN21 Renewable Energy Polcy Network for the 21st entury. [2] T. Nknam, A. Kavousfard, S. Tabatabae, J. Aghae, Optmal operaton management of fuel cell/wnd/photovoltac power sources connected to dstrbuton networks, Elsever, Journal of Power Sources 196 (20), pp , [3] A. Krubakaran, S. Jan, R.K. Nema, A revew on fuel cell technologes and power electronc nterface, Elsever, Renewable and Sustanable Energy Revews 13 (9), pp , [4] L. M. Tender, S. A. Gray, E. Groveman, D. A. Lowy, P. Kauffman, J. Melhado, R.. Tyce, D. Flynn, R. Petrecca, J. Dobarro, The frst demonstraton of a mcrobal fuel cell as a vable power supply: powerng a meteorologcal buoy, Elsever, Journal of Power Sources 179, pp , [5] O.. Onar, M. Uzunoglu, M. S. Alam, Modelng, control and smulaton of an autonomous wnd turbne/photovoltac/fuel cell/ultra-capactor hybrd power system, Elsever, Journal of Power Sources 185 (2), pp , [6] A. Payman, S. Perfederc, F. Mebody-Tabar, Energy management n a fuel cell/supercapactor multsource/multload electrcal hybrd system, IEEE Transactons on Power Electroncs, vol. 24 (12), pp , [7] J. Bauman, M. Kazeran, A comparatve study of fuel-cell battery, fuel-cell ultracapactor, and fuel-cell battery ultracapactor vehcles, IEEE Transactons on Vehcular Technology, vol.57 (2), pp , 2008.

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