A New Method for Dynamic Control of a Hybrid System Consisting of Fuel Cell and Battery

Transcription

1 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August A New Method fo Dynamic Contol of a Hybid System Consisting of Fuel Cell and Battey Behooz Ghahemani *1, Saed Abazai, Ghazanfa Shahgholian 3 1,,3 Depatment of Electical Engineeing, 1,3 Najaf Abad Banch, Islamic Azad Univesity Shahekod Univesity 1 Isfahan, Shahekod, 3 Isfahan Ian *1 ghahemani@sel.iaun.ac.i; saedabazai@yahoo.com; 3 shahgholian@iaun.ac.i Abstact A new contol method was poposed in the cuent pape fo contolling hybid system consisting of fuel cell and battey. In the poposed contol system, a method though which active powe of fuel cell is supplied with a elatively high time constant was chosen contibuting to enhancement of fuel cell life and maintenance of its teminal in the ange of the nominal voltage. Futhemoe, ensemble of fuel cell and invete wee used to supply instantaneous eactive powe as well; which is possible by using an independent invete togethe with fuel cell, and as such,the setting will espond quickly and pefectly capable to instantaneously supply the eactive powe wheneve equied fo the system (voltage eduction o symmetical 3 phase shot cicuit). An ensemble consisting of battey and invete has also been consideed fo supplying the instantaneous active powe, which is connected to the cicuit only at ealy times of change in the powe demanded by the system. Its geneation powe declines afte a couple of seconds simultaneously with incease in powe of fuel cell until the demanded active powe is completely supplied by the fuel cell ensemble. Keywods Dynamic Model; Dynamic Contol; Hybid System; Fuel Cell; Battey Intoduction Fuel cells among the powe geneation souces without sound pollution featue high efficiency and eliability and low contamination. In addition, thei flexible design is esponsive to many demands; which makes its extensive application in the futue industy. On the othe hand, powe geneation in the fuel cells is a chemical pocess in which amount of geneated active powe is contolled by egulating amount of input hydogen gas and ai. This causes a delay in powe geneation pocess, and hence, a hybid powe geneation system coupled with fuel cell is mostly employed in the dynamic studies aimed at impoving dynamic pefomance of fuel cell and compensating the delayed pefomance. A battey and invete togethe with fuel cell have been used in the pesent pape to impove pefomance of fuel cell. Battey and invete will geneate instantaneous powe (though contolling the fequency and voltage) in the initial moments of powe vaiation in the system, and then the geneated active powe of fuel cell will incease using an appopiate contol system and with a suitable time constant while the active powe geneation of battey and invete will popotionally decease. The whole demanded active powe will be supplied by the fuel cell afte a couple of seconds. This contolling method leads to pope powe geneation pefomance of fuel cell followed by enhancement of fuel cell life and maintenance of its teminal voltage in the ange of nominal voltage. The espective hybid system also has a vey quick dynamic esponse which can impove the dynamic stability in the netwoks lacking suitable dynamic pefomance. Geneated eactive powe of fuel cell vaies with a vey low time constant but does not bing about a limitation fo the fuel cell because the geneated eactive powe neve causes abupt changes in the amount of geneated powe in the fuel cell. This leads to impovement of geneation capacity of eactive powe compaed to the fome methods and will pomote voltage stability in voltage dop moments o in shot cicuits which cause voltage eduction. As such, fuel cell system and its invete will have the ability to independently geneate instantaneous eactive powe; which is one of the advantages of the poposed method in compaison with fome models which deployed single invete system. Additionally, the geneated active powe of fuel cell is completely govened by contolling its invete and its vaiation value is pefectly contolled at any time. In efeences, a hybid system consisting 71

2 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August 013 of fuel cell, capacito, battey, and an invete has been used; powe geneation is contolled by the invete and no contol is imposed on the active powe geneated by the fuel cell. Powe Systems Stability Geneally, stability signifies tendency of a powe system to have a steady and ideal pefomance in a point; and is divided into two geneal categoies: Roto angle stability, the capability of inteconnected synchonous machines in a powe system to emain in synchonic state with each othe and voltage stability, the ability of powe system to maintain an acceptable steady state voltage in its all machines eithe in nomal pefomance conditions o afte the occuence of petubation.based on the petubation intensity, stability is divided into thee goups: 1 Steady state stability (vey small petubation) Dynamic stability (small petubation) 3 Tansient stability (vey lage petubation) Dynamic Stability of Powe Systems Ability of system to maintain the new conditions afte occuence of fluctuation caused by a low amplitude petubation is efeed to as dynamic stability. These petubations might occu due to small changes of chage o geneation and can be alleviated though designing an appopiate and stable contol system. Dynamic Model of Fuel Cell Numeous dynamic models have been extacted and appliedto simulate fuel cell in the powe system. In the cuent pape, a model fo fuel cell has been simulated in DIGSILENT softwae extacted fom the simulated models of the efeence papes. The fuel cell (FC) systems have seveal othe deficiencies such as cold stating and output voltage fluctuation. Due to the low tempeatue of FC at the beginning of a FC stating, moe time is equied to poduce the desied powe. If the FC system is foced to delive the powe to a heavy load duing this peiod o duing step load, it could be damaged. To solve these poblems, a seconday enegy souce such as an electochemical battey o Supe Capacito system need to be connected to the FC system to poduce powe duing tansient states. A numbe of liteatues have studied the modeling, contol, and pefomance of FC systems. Wang and Nehi discussed the modeling, contol and fault handling of PEMFC(Poton Exchange Membane Fuel Cell) system with active and eactive powe demand. Lee and Wang investigated small signal stability analysis of an autonomous hybid enewable enegy. In thei pape, a simple model of FC system was used. S. Obaa poposed a new model of FC system based on expeimental eseach as fist ode lead lag and implemented a method to contol solid oxide fuel cell (SOFC) system in distibuted powe geneation application. Padulles et al. intoduced a simulation model of an SOFC powe plant. In thei model, electochemical and themal pocesses wee simulated as fist ode lead lag tansfe function. Li et al. contolled SOFC powe plant and investigated the SOFC dynamic behavios unde a gid connected condition. Moeove, diffeent pevious woks have aleady investigated the active and eactive powe flow contol of FC systems. Unfotunately, in the contol stategy design, the effect and behavio of FC system utilization facto unde distubance wee neglected. Howeve, in this pape, the dynamic model of FC system has been modified by consideing the effect of utilization facto to opeate in optimal value to enhance FC system pefomance and lifetime. Chemical Reactions in Fuel Cell As a static powe geneation system, fuel cell geneates DC electical powe via combination of fuel and oxygen in an electochemical eaction. Fuel supply unit, powe section, and powe condition unit ae thee main constituent pats of a fuel cell. the SOFC is in a completely solid state without any liquid components and opeating tempeatues ae about C. This high tempeatue allows SOFC system to have intenal efoming capability. The chemical eactions taking place in fuel cell fo powe geneation ae as below: At anode: H O H O e At cathode: (1 ) O e O (1) Oveall: H (1 ) O H O Fuel consumption is contolled by the atio of the cuent eacting fuel and cuent input fuel of the cell though the following fomula: in O qh q H q H U in in () qh qh 7

3 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August Equation of any ideal gas like hydogen and oxygen can be independently witten as below: P V n RT (3) an By deivation with espect to time fom the above elation: dp H dt RT RT ( q in V an H 0 qh q q H H V an Output mola of each gas (hydogen) is popotional to patial pessue inside the canal. q P o K M an K Using equations (4) and (5), patial pessue of hydogen can be witten as: dph RT in ( q H K ) H P H q dt V (6) an Using equation (), equation (6) can be ewitten as: dph RT qh ( K H P H q ) H dt V an U (7) τh is defined as below: van H RTK (8) Patial pessue of hydogen is deived by taking Laplace tansfom fom Equation (7) as follows: 1/ K 1 P H q ( 1) 1 s H U (9) Accoding to fundamental electochemical equations, ) (4) (5) eacting mola cuent of hydogen can be calculated via the following elation: N I 0 fc q KI (10) H F fc Using NERNST equation and Ohm s law, output voltage can be witten as: 0.5 RT ph p O V N 0 ( E0 [ Ln( )]) I fc F p (11) O Figue 1 illustates dynamic model of fuel cell. The paametes and values of fuel cell dynamic model used in the cuent pape ae shown in table 1. TABLE 1. VALUES AND PARAMETERS OF FUEL CELL CONTROL SYSTEM AND DYNAMINC MODEL Model Paametes Value K1 Ibase 1000 K K/Uopt.74e 6 K3 1/tau_f 0. K4 *K_.33e 6 K5 1/K_H K6 1/tau_H K7 1/K_HO K8 1/tau_HO K9 1/_HO K10 K_ 1.16e 6 K11 1/K_O K1 1/tau_O K13 R*T//F K14 N0 450 K15 1/Vbase FIG. 1 DYNAMIC MODEL OF THE FUEL CELL UNDER STUDY 73

4 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August 013 Contol and Powe System Modeling of the Fuel Cell in DigSilent Softwae With futhe cuent extacted fom the fuel cell, a voltage dop occus on the fuel cell teminal due to pesence of intenal esistances. This voltage dop compensated by contolling patial pessue of the fuel causes a dynamic behavio in the fuel cell unde load change conditions. This dynamic behavio is modeled with contol system of fuel cell whose input and output espectively ae intensity of the cuent extacted fom the fuel cell teminal and the signal epesenting value of additional geneated voltage in the dynamic conditions. This signal can be conveted into an effective voltage on the output teminal of fuel cell by applying it to a dependent voltage souce connected in seies. Geneal scheme of the fuel cell contol system can be obseved in Figue. FIG. GENERAL SCHEME OF CONTROL SYSTEM OF THE FUEL CELL The modeled fuel cell is composed of two powe and contol systems. In the powe system of fuel cell, standad ideal diving foce (E0) was modeled using a DC voltage souce and additional geneated voltage in dynamic conditions was epesented by anothe voltage souce connected in seies with the fome voltage supply; and intenal esistance of souces was modeled by an intenal esistance inside the connected voltage souce. Geneal scheme of powe system of the fuel cell can be obseved in Figue 3. Values of fuel cell contol system paametes ae listed in table 1. Block Fame Diagam of the Fuel Cell in DSL (Digsilent Simulation Language) Block fame diagam elated to fuel cell fo connection between cuent measuement device and contol and powe systems of fuel cells is demonstated in figue 4. FIG. 4 BLOCK FRAME DIAGRAM OF FUEL CELL The Poposed Contol Method FIG. 3 GENERAL SCHEME OF POWER SYSTEM OF THE FUEL CELL DC/AC invetes ae used to connect DC powe geneated in fuel cell to AC powe distibution system; 74

5 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August which enables contol of active and eactive powe geneation. As a esult, coefficient of geneated powe can be adjusted in the case of using convetes with switches of mandatoy commutation type (e.g. IGBT). Fuel cell is connected to the AC netwok by a PWM invete, and similaly, a battey togethe with anothe PWM invete is connected paallel to the fuel cellinvete ensemble. These two systems togethe undetake the duty to supply the equied powe. Battey and invete ensemble act as the instantaneous powe supplie, and in the event of vaiation in the demanded powe, immediately egulate thei deliveed powe popotional to these vaiations. They keep supplying the powe chage if no othe supply souce is available in the system. Yet, immediately following injection of powe supply fom anothe souce, this ensemble adjusts its deliveed powe commensuate with amount of the injected powe. Fuel cell and invete ensemble is used as powe geneation system which connects to the cicuit with a delay. This is possible by applying two invetes, and accoding to efeences, the absence of contol on the output powe of fuel cell does not guaantee a safe poductivity of the fuel cell. This delayed pefomance also contibutes to enhancement of fuel cell life, and also, maintenance of DC Bus voltage in the nominal value. The system is capable of abuptly geneating eactive powe with a shot delay paallel to battey and invete ensemble. The delay is due to the absence of identical woking point to supply the eactive powe with the battey invete ensemble in the tansient state; hence, a woking point is ceated fo the netwok in tansient states. In single invete systems, amount of geneated eactive powe is less than that of the poposed hybid system and fuel cell is not used as independent geneation souce of abupt eactive powe. Additionally, single invete systems, due to having less geneated eactive powe souces than the two invete systems, exhibit lowe capability to supply eactive powe of netwok in shot cicuit cases when the voltage dops seveely. Model of Poposed Hybid System Geneal scheme of the netwok unde study is illustated in figue 5. In this netwok, fuel cell is modeled with an independent DC voltage souce and a dependent DC voltage souce connected via a DC/DC boost convete to the invete, and accodingly, its voltage inceases to nominal DC Bus voltage value of invete. This DC voltage is tansfomed by an invete into AC voltage and gets connected to AC Bus. As well, paallel to fuel cell ensemble, the battey is also connected to AC Bus using an invete and the whole setting of fuel cell, battey as hybid system, and gas tubine feed the load in island netwok. Contol Method of Fuel Cell Invete As mentioned ealie, the demanded active and eactive powes ae instantaneously supplied by battey and invete ensemble in the netwok unde study. The amount of consumed active powe load (PL), the eactive powe deliveed by battey to the load (Qb), active powe (PFC), and eactive powe (QFC) deliveed to invete fuel cell ae all sent to powe measuement Fuel Cell Battey Hybid System FIG. 5 GENERAL SCHEME OF THE NETWORK UNDER STUDY 75

6 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August 013 by the contolle. Consumption load powe is then compaed with fuel cell invete deliveed powe and an eo signal is geneated and contolle detemines values of d axis cuents by means of conventional PI contolles popotional to this eo signal. Afte passing though a low pass filte, this cuent is deliveed to the invete as efeence d axis cuent (Id). Since the vaiations load might occu in steps, intensity of efeence d axis cuent (Id) in the output of PI contolle undegoes sudden changes. If this cuent is diectly applied to the invete, an abupt DC cuent will be extacted fom the fuel cell which might be unable to suppot and the teminal might suffe fom sevee voltage decline distubing pefomance of invete as a consequence. Accodingly, these abupt changes can be completely eliminated by passing signal of efeence d axis cuent though a low pass filte and the cuent extacted fom fuel cell will always decease o incease with a gentle slope. Besides enhancement of fuel cell life, this causes the DC Bus voltage of fuel cell to pemanently emain close to the nominal voltage and invete will always have a safe pefomance in this state. The amount of eactive powe deliveed by battey is also compaed with eactive powe of invete fuel cell and the esulting eo signals ae tansmitted to a conventional PI contolle and value of Refeence d axis cuent (Id) is detemined. This cuent is given to the invete as the efeence cuent of q axis afte passing though a low pass filte with a vey small time constant. Selection of eactive powe FIG. 6 THE CONTROL SYSTEM USED IN THE HYBRID SYSTEM geneated by battey as the efeence eactive powe is due to the fact that incease in eactive powe geneated by fuel cell educes the eactive powe supplied by the battey, and also, when these two values ae equal and eactive powe of consumption load ae supplied equally, both souces will have geate capacity of eactive powe geneation compaed to the state whee one of them supplies the eactive powe of consumption load individually. Netwok fequency and the zeo passing voltage point ae given by PLL in the fom of two signals (Sin, and Cos) to the invete. Block Fame Diagam of Contol Method in DSL FIG. 7 BLOCK FRAME DIAGRAM OF CONTROL METHOD IN DSL The elationship among powe measuements, contol block, and invete of the fuel cell is shown by blockfame diagam in Figue 7. Contol System of Fuel Cell Invete Active and eactive powes of measuement tools ae 76

7 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August deliveed to contol system of fuel cell invete as input, and efeence d and q axes powes ae geneated. Figue 8 epesents this contol system. PI Contolle Low Pass Filte FIG. 10 ACTIVE POWER OF BATTERY Needed eactive powe of load FIG. 8 CONTOL SYSTEM of FUEL CELL INVERTER Simulation Results The espective system is simulated by DIGSILENT softwae, on which RMS simulation studies ae applied. The simulation esults ae subsequently pesented. To check efficiency of the poposed contol system, abupt vaiations of load active and eactive powe in diffeent times wee applied accoding to Table. Geneated powes of fuel cell and battey wee evaluated duing these vaiations in Figues 9 to 11. Results indicate that battey and invete immediately supply the powe changes in the initial instants of vaiations in the system, and afte a couple of seconds, geneation powe of fuel cell inceases slowly and geneation powe of battey deceases as the fome goes up. It must be noted that eactive powe deliveed by fuel cell is able to vay instantaneously because DC cuent of fuel cell will not vay fo changing the eactive powe of fuel cell invete. Geneated eactive powe of fuel cell & battey FIG. 11NEEDED REACTIVE POWER OF LOAD...GENERATED REACTIVE POWER OF FUEL CELL GENERATED REACTIVE POWER OF BATTERY FIG. 1 DC VOLTAGE OF FUEL CELL TERMINAL TABLE ABRUPT CHANGES OF ACTIVE AND REACTIVE POWERS OF LOAD Changes of eactive powe 50% 5% 50% 80% Changes of active powe 75% 50% 5% 50% Time (Second) Case Needed active powe of load FIG. 13 DC CURRENT of FUEL CELL Geneated active powe of fuel cell FIG. 9 NEEDED ACTIVE POWER OF LOAD; GENERATED ACTIVE POWER OF FUEL CELL FIG. 14 VOLTAGE CURVE OF AC TERMINAL CONNECTED TO THE LOAD WITHOUT HUBRID SYSTEM; WITH HYBRID SYSTEM 77

8 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August 013 REFERENCES A. Gebegegis, P. Pillay, D. Bhattachayya, R. Rengaswemy, Solid oxide fuel cell modeling, IEEE Tansactions On Industial Electonics, JANUARY 009. A.K. Saha, S. Chowdhuy, S.P. Chowdhuy, Application of solid oxide fuel cell in distibuted powe geneation, IET Renew. Powe Gene., 1 (4) (007), pp FIG. 15 FREQUENCY CURVE OF AC TERMINAL CONNECTED TO THE LOAD ----WITHOUT HUBRID SYSTE WITH HYBRID SYSTEM Using this method, vaiations of fuel cell teminal dc voltage and the dc cuent extacted fom the fuel cell emain close to the nominal voltage and cuent (Figs 1 and 13). In ode to check the effect of the poposed hybid system on the netwok, this hybid system was connected to an island netwok consisting of a load, a gas tubine and abupt vaiations of load, in which the active and eactive powe in diffeent times wee applied. Figues 14 and 15 show the cuves of voltage and fequency ac teminal to which load is connected. Simulation esults fo duation of 00 seconds ae pesented in Figues 9 to 15. CONCLUSIONS Simulation esults implied that in the poposed contol method, the espective hybid powe geneation system is capable to immediately supply the demanded powe and povide a highly favoable dynamic esponse in the case of sudden load vaiation. Futhemoe, geneation powe of fuel cell did not incease suddenly which ceated favoable pefomance conditions fo the fuel cell so that it had sufficient time to supply the needed powe and as well DC voltage of its teminal emained aound the nominal value. Consequently, an appopiate pefomance was achieved fo fuel cell invete. Also by placing this poposed hybid system in the netwok, an impovement in the contolling voltage and fequency of island netwok was achieved in compaision with the state. The absence of that system showed high pefomance of hybid system, eliminated steady state fault of govene of gas tubine and minimum deviation of nominal fequency. On the othe hand, hybid system had an influence on the pefomance voltage egulation of gas tubine and the value of voltage also stabilized in the nominal value. C. Wang, M. HashemNehi, A physically based dynamic model fo solid oxide fuel cells, IEEE Tansactions On Enegy Convesion, Vol., No. 4, Decembe 007. C. Wang, M.H. Nehi, Contol of PEM fuel cell distibuted geneation systems, IEEE Tans. Enegy Conves., 1 () (006), pp J. Padulles et al, An integated SOFC plant dynamic model fo powe systems simulation, J. Powe Souces, 86 (1) (000), pp J. Padulles et al, An integated SOFC plant dynamic model fo powe system simulation, J. Powe Souces, 61 (1996), pp K. Sedghisigachi, A. Feliachi, Dynamic and tansient analysis of powe distibution systems with fuel cells pat II: contol and stability enhancement, IEEE Tansactions on Enegy Convesion, Vol. 19, No., June 004. K. Sedghisigachi, A. Feliachi, Dynamic and tansient analysis of powe distibution systems with fuel cells pat I: fuel cell dynamic model, IEEE Tansactions On Enegy Convesion, Vol. 19, No., June 004. L. Dong Jing, W. Li, Small signal stability analysis of an autonomous hybid enewableenegy powe geneation/ enegy stoage system pat 1: time domain simulations, IEEE Tans. Enegy Conves., 3 (1) (008), pp M. Nayeipou, M.Hoseintaba, T.Niknam, A new method fo dynamic pefomance impovement of a hybid powe system by coodination of convete s contolle, Jounal of Powe Souces, pp , Elsevie 011. S. Obaa, Analysis of a fuel cell mico gid with a small scale wind tubine geneato, J. Hydogen Enegy, 3 (007), pp Y.H. Li, S. Rajakauna, S.S. Choi, Contol of solid oxide fuel cell powe plant in a gid connected system, IEEE Tans. Enegy Conves., () (007), pp

9 Intenational Jounal of Enegy and Powe (IJEP) Volume Issue 3, August List of Symbols Used in the Pape E0 F Ifc Kan Ideal standad potential Faaday s constant Fuel cell cuent Anode valve constant K Valve mola constant fo hydogen K Valve mola constant fo wate HO K Valve mola constant fo oxygen O Ka Constant (=N0/4F) M Molecula mass of hydogen n Numbe of hydogen moles in the anode channel N 0 Numbe of cells in seies in the stack in q Input fuel flow q output fuel flow o q Fuel flow that eacts R RH O R T U Ohmic loss Ratio of hydogen to oxygen Univesal gas constant Absolute tempeatue Fuel Utilization facto V an Volume of anode V Fuel cell voltage fc Response time fo hydogen flow Response time fo wate flow HO Response time fo oxygen flow O Behooz Ghahemani was bon in Busheh, Ian, on Apil 0, He eceived his BSc degee in electical engineeing fom Islamic Azad univesity, busheh Banch, Busheh, Ian, in 010. Since 010 he is a MSc student in electical engineeing at Najaf Abad Banch, Islamic Azad Univesity Isfahan, Ian. He is now a Lectue at Depatment of Electical Engineeing, Faculty of Engineeing, Busheh Banch, Islamic Azad Univesity. He has also published the books such as Electical cicuit in simple language and Micocontolle AVR & PIC, Busheh,IAU Busheh Banch, 013. His eseach inteests include Powe system dynamics, Stability, Renewable enegy and Powe system simulation. Saed Abazai was bon in Shahekod, Ian on Octobe 5, He eceived his BSc degee in electical engineeing fom Isfahan Univesity Of Technology (IUT), Isfahan, Ian, 1989 and his MSc in electical engineeing fom Fedowsi Univesity, Mashhad, Ian, 199 and his PhD in electical engineeing fom shaif univesity, Tehan,ian, 00. He is now an Associate Pofesso at Depatment of Electical Engineeing, Faculty of Engineeing, Shahekod Univesity. His teaching and eseach inteests include Powe & Industial Electonic, Electical System, Technology Development, Compute, Powe Quality and Stability. Ghazanfa Shahgholian was bon in Esfahan, Ian, on Decembe 7, He eceived his BSc degee in electical engineeing fom Isfahan Univesity of Technology (IUT), Isfahan, Ian, 199 and his the MSc and PhD in electical engineeing fom Univesity Tabiz, Tabiz, Ian, in 1994 and fom Islamic Azad Univesity, Science and Reseach Banch, Tehan, Ian, in 006, espectively. He is now an Associate Pofesso at Depatment of Electical Engineeing, Faculty of Engineeing, Najaf Abad Banch, Islamic Azad Univesity. His teaching and eseach inteests include application of contol theoy to powe system dynamics, powe electonics and powe system simulation. 79