Vector Control of Fuel Cell Based Grid Connected Inverter

Transcription

1 Vector Control of Fuel Cell Base Gri Connecte Inverter Praboha Kumar Rath 1, an Kanhu Charan Bhuyan 2 1 Lecturer, Department of Electrical Engineering, College of Engineering an Technology, Bhubaneswar, Oisha, Inia 2 Asst. Professor, Department of Instrumentation an Electronics, College of Engineering an Technology, Bhubaneswar, Oisha, Inia praboharath85@gmail.com; kanhu2006@gmail.com Corresponing Author Receive 29 January 2018; Accepte 26 June 2018; Publication 19 July 2018 Abstract Electrical energy consumption increases ay by ay so we shoul fin alternative means to generate electrical energy to meet the eman in aition to existing conventional generation facilities. For this istribute generation (DG) will efinitely going to take a key role in the energy supply. Presently various technologies are available as DG, they are micro-turbines, photovoltaic cells system, fuel cells system an win energy systems. Out of these technologies fuel cell have becoming more popular ue to its characteristics like cleanliness, portability an suitability for electricity an heat generation. In this paper, moel of Soli Oxie Fuel Cell (SOFC) is presente. As fuel cells operate at lower voltages so there is a nee of DC-DC boost converter to boost. Then the booste voltage inverte by using DC-AC converter known as gri-connecte inverter in orer to inter connect to the utility gri. The control strategy of this moel is vector control (VC) metho. VC metho is use for the generation of DC-AC converter switch pulse. MATLAB/SIMULINK is use to valiate the moeling an simulation of fuel cell generation an power conitioning unit. Journal of Green Engineering, Vol. 8 3, River Publishers oi: /jge This is an Open Access publication. c 2018 the Author(s). All rights reserve.

2 202 P. K. Rath an K. C. Bhuyan Keywors: Fuel Cell (FC), Vector Control (VC), Boost Converter, Distribute Generation (DG), Shunt Filter. 1 Introuction Now-a-ays throughout the worl the use of electrical energy is much more in comparison electrical energy use in last few ecaes. Prouction of electrical energy mainly epens on fossil fuel. So to meet the eman of electrical power we are consuming huge amount of fossil fuel which has limite existence in nature an also inject harmful gases to the environment resulting in green house effect. Therefore to meet the electrical energy eman we shoul use the renewable sources of energy like win energy, solar energy, biomass power etc [1, 4]. Notable progress in electric market eregulation an new rules in terms of environmental pollution leas to use of istribute generation. There is a significant rise in use of renewable sources of energy ue to its clean power generation property an also ue to awareness among the people about the harmfulness of conventional sources of energy an ue to shortage of power generation. The use of renewable sources of energy leas to healthy environment an meeting the energy eman in a better way an researchers are working very har to fulfill the above requirements [2]. A large portion of renewable energy comes from solar power an win power. The biggest emerit of these sources of energy is there variable nature. Solar power is only available uring ay time an win tens to blow intermittently. As storing of electrical energy is a ifficult task so excess renewable energy prouce cannot be store for future use. So to overcome the emerits of solar power an win power fuel cell is use to generate electrical energy [17]. A fuel cell is a evice which is use to convert store chemical energy to electrical energy. An electrochemical process, which is an efficient process, is take place for conversion of fuel to electrical energy. There are many avantages of fuel cells over win power an solar power generation. They are: high efficiency at any loa, fuel cell can be place any site in istribution network (but solar power an win power generation unit cannot), lower maintenance an longer life, zero greenhouse gas emission [5]. The motive of this paper is to propose an present the experimental results of a gri-connecte three phase FC system using vector control scheme. In this paper FC is use to prouce electrical power. As the generate power is at low voltage level by using boost converter electrical power at higher voltage

3 Vector Control of Fuel Cell Base Gri Connecte Inverter 203 level is prouce. The propose three phase gri-connecte FC system can operate either in gri connecte or stan-alone moe [3]. The paper is organize as follows: Section 2 provies an introuction to the FC system. A single line iagram of the propose system is provie with etaile explanation of each element associate with the system. Section 3 provies the operating principle of propose system. Section 4 presents the vector control metho for gri connecte inverter. Section 5 provies the simulation results of the propose system. The steaystate conition is establishe an operational performance an stability of the system is stuie. 2 Components of The System The single line iagram as shown in Figure 1 is the representation of a FC connecte to gri through inverter system. The system consists of many equipment such as FC, DC-DC boost converter, DC-AC converter i.e. inverter, filter, control unit an gri. In the following subsection the escription of some important components of the propose system an their function given. 2.1 Fuel Cell A FC is an electrochemical cell in which the chemical energy from a fuel converte into electricity through an electrochemical reaction of hyrogen fuel with oxygen or another oxiizing agent. FC may be explaine as source of electric power which never becomes ea as long as hyrogen an oxygen are supplie. The hyrogen is supplie irectly or inirectly prouce by reformer from fuels such as natural gas, alcohols, or gasoline. Figure 1 Block iagram of fuel cell base gri connecte inverter.

4 204 P. K. Rath an K. C. Bhuyan There are various type of FCs are there an as we focus on efficiency, low emissions, low cost soli oxie fuel cell is best among them. The principal components of SOFC inclue an anoe gas flow fiel, an anoe gas iffusion layer, an anoe catalyst layer, a cathoe catalyst layer, a cathoe gas iffusion layer, an a cathoe gas flow fiel. Oxygen is supplie to cathoe gas flow fiel an it iffuses through the gas iffusion layer an infuses into cathoe catalyst layer an fuel cell membrane. At the same time, hyrogen gas is supplie to the anoe gas flow fiel, where it to enters into fuel cell through anoe gas iffusion layer an catalyst layer. A chemical reaction takes place between hyrogen an oxygen, ue to which there is transfer of H + ion through the fuel cell from anoe sie to the cathoe sie an proucing water as waste prouct [7]. The electrochemical reactions within the SOFC can be written as, Anoe sie H 2 2 H 2 +2e (1) Cathoe sie 2H + +2e O H 2O (2) Over all reaction H O 2 H 2 O + Heat (3) In practice, the ieal reversible electromotive force of the SOFC is reuce by various potential losses within the fuel cell. The terminal voltage V FC of the fuel cell is V FC = V o V act V ohm V conc (4) Here V o is open circuit voltage, V act is activation loss, V ohm is Ohmic loss an V conc is concentration loss. When fuel cell circuit is open, the reversal potential is given by the equation { ( V o = N o E o + RT ln P )} H 2 PO (5) 2F P H2 O N o is number of cells in stack, E o is stanar reversible cell potential (1.2 V), R is universal gas constant (8314 J/(k mol K), T is absolute temperature (1273 K), F is faraay s constant (96487 C/mol), P H2, P o an P H2 O are partial atmospheric pressure of H 2, O 2 an H 2 O. The polarisation curve of SOFC provies the fuel cell output voltage as a function of the current ensity in steay state.

5 Vector Control of Fuel Cell Base Gri Connecte Inverter 205 Figure 2 V-I characteristic of a single FC. The activation losses at the anoe an cathoe sie are given by V act = [ξ 1 + ξ 2 Tξ 3 T ln (c O2 )+ξ 4 T ln (i FC )] (6) ζ represents the parametric coefficients for cell moel. At the vapour-liqui interface, the issolve oxygen concentration (mol/cm 3 ) is obtaine from Henry s law as. c O2 = P O2 ( e T ) (7) The Ohmic voltage loss prouce within the fuel cell as a result of the electrical resistance of the SOFC material (ceramic material) is given by. V ohm = ( T j ) j (8) Meanwhile, the concentration polarisation loss cause by the inability of the oxygen an hyrogen gases to iffuse at a sufficient spee through the porous components of the cell is given by. ( V con = B ln 1 J ) (9) J max B is the work status constant of the fuel cell an J max is the maximum cell current ensity (Acm 2 ). For a fuel cell stack containing Nnumber of fuel cells, the output voltage V stack an the power P stack are represente as V stack = NV FC (10) P stack = V stack I FC (11)

6 206 P. K. Rath an K. C. Bhuyan Figure 3 P-I characteristics of fuel cell. I FC is fuel cell current. The result obtaine from the simulation for power verses current ensity of fuel cell moel is shown in Figure 3. The fuel cell efficiency can be expresse as η = μ f V FC 1.48 (12) The result obtaine for fuel cell efficiency from the simulation is shown in Figure DC-DC Boost Converter The process that changes one DC voltage to a ifferent DC voltage is calle DC-DC conversion. A boost converter is a DC-DC power converter that steps up the input voltage to a higher output (loa) voltage [8]. It is a converter which consists of two semiconuctors (a ioe an a transistor) an one energy storage element inuctor. To reuce voltage ripple, filters mae of capacitors are normally ae to such a converter s output (loa-sie filter). Figure 4 Efficiency-current characteristics of fuel cell.

7 Vector Control of Fuel Cell Base Gri Connecte Inverter 207 For DC-DC boost converter the relationship between input voltage an output voltage is V FC = V FC(DC) (13) 1 Here V FC is the output voltage of boost converter, V FC(DC) is the input voltage to boost converter an is the uty cycle. 2.3 DC-AC Converter A DC-AC converter or inverter is an electronic evice or circuit that converts irect current (DC) to alternating current (AC). The input voltage, output voltage an frequency, an overall power hanling epen on the esign of the specific evice. The inverter oes not prouce any power; the power is provie by the DC source. For esigning of inverter circuit fully controlle power electronic switches are use so that we on t extra commutation circuit for switching off of the switches. To control the switching action of switches pulse with moulation technique is use [6]. 2.4 Shunt Filter Three-phase harmonic filters are shunt filter which are use in power system to reuce the istortion in voltage an for power factor correction as they provie reactive power at funamental frequency. Due to power electronic converters, which are non linear in nature, harmonic currents or harmonic voltages, are injecte into power system an the orer of harmonics high as the PWM switching frequency of the semi-conuctor switches (like MOSFETs, IGBTs) is high. Shunt filters reuce istortion by iverting harmonic currents in less impeance paths. Harmonic filters are esigne to be capacitive at funamental frequency, so that they coul prouce reactive power require by converters. To get an acceptable istortion, many units of filters of ifferent types are connecte in parallel. High-pass filters are use to filter out the high-orer harmonics. A special type of high-pass filter, the C-type high-pass filter, is use to provie reactive power an avoi parallel resonances. It also allows filtering low orer harmonics (such as 3r), while keeping zero losses at funamental frequency. 3 Control Scheme of Gri Connecte Inverter The control system of the propose gri connecte fuel cell system is comprise of a faster vector controller. Where, the vector controller is complete

8 208 P. K. Rath an K. C. Bhuyan Figure 5 Vector control system of VSC-HVDC system. by aitional controllers which provie the references for the vector controller. Thus, the vector controller is the inner loop an aitional controller is the outer loop. In this paper, the aitional controllers will be referre to as the outer controllers. The outer controllers inclue the DC voltage controller, the reactive power controller or the frequency controller. Even if both current an voltage control schemes are possible, current control is generally preferre for its better ynamic characteristics an inherent over-current limitation capabilities. When current control is use, gri current an inverter output currents are measure an compare with reference signals; the current errors are use as inputs to the PWM moulator, which then provies the require switching signals. The AC filters connecte to the system behave as pure capacitors at funamental frequencies. Hence in the mathematical moel presente here, the filter resistances an inuctances can be neglecte. The voltage across the transformer, the current to the filter an the voltage across the source impeance can be obtaine in three phase instantaneous form as follows: The voltage across source impeance is V ac (t) abc V s (t) abc = R s i s (t) abc + L s t i s(t) abc (14) The current through the filters is The voltage across the transformer is i s (t) abc i t (t) abc = C f t V S(t) abc (15) V s (t) abc V c (t) abc = R T i t (t) abc + L T t i t(t) abc (16)

9 Vector Control of Fuel Cell Base Gri Connecte Inverter 209 The above equations in can be converte into αβ frame as follows: t i s(t) αβ = R s i s (t) αβ + 1 {V ac (t) αβ V s (t) αβ } (17) L s L s t V s(t) αβ = 1 i s (t) αβ 1 i t (t) αβ (18) C f C f t i t(t) αβ = R T i t (t) αβ + 1 {V s (t) αβ V C (t) αβ } (19) L T L T By using the transformation angle Ø erive from the phase locke loop (PLL), the above equations are further transferre to the synchronously rotating q- reference frame, using Park s transformation, as follows: t i s(t) q = R s i s (t) q jωi s (t) q + 1 {V ac (t) q V s (t) q } (20) L s L s t V s(t) q = 1 i s (t) q 1 i t (t) q jωv s (t) q (21) C f C f t i t(t) q = R T i t (t) q jωi t (t) q + 1 {V s (t) q V C (t) q } (22) L T L T the q-current component through the transformer can be given by, t i t(t) = ωi t (t) q + V s(t) V C (t) R T i t (t) (23) L T L T t i t(t) q = ωi t (t) + V s(t) q V C (t) q R T i t (t) q (24) L T L T The propose simulation moel consists of reactive power controllers at inverter sie. The outer controller generates the reference values of the q-current components for inner current controllers. The inner controller gains are higher when compare to the outer controller to ensure the stability of the complete system. A reactive power controller is obtaine from equation i q = 2 Q ref (25) 3 V s Here, Q ref is reference reactive power. For accurate control of the reactive power, in combination with a feeback loop an open loop is use. i q = 2 ( Q ref + K 1 + K ) 2 (Q ref Q actual ) (26) 3 V s s Where, K 1 an K 2 are the proportional an integral gains respectively of the reactive power controller.

10 210 P. K. Rath an K. C. Bhuyan Figure 6 DC voltage control. 3.1 DC Voltage Control at Inverter Sie The inverter controller controls the DC link voltage of the system. DC voltage controller consists of outer control loop (Figure 6) where the reference DC voltage (V c ) is compare with actual DC link voltage (V c ) an error is fe to the PI-controller which generates the output in the form of reference current (I ). This reference current is compare to the actual -current component (I ) of the AC system in inner current control loop (Figure 6). The output is in the form of voltage which is then compare to -component of system voltage (V ) an cross-coupling term to get the output in the form of voltage (V ). 3.2 Reactive Power Control at VSC 2 The reactive power flowing between inverter an loa system is controlle by reactive power controller at inverter. The reactive power controller consists of outer an inner current control loops as well. In the outer current control loop, reactive power reference (Q ) is compare with the measure system reactive power (Q). The error is than passe through the PI-controller which gives output of reference current (I q ). This q-component of the current controls the reactive power flow in the system so it is use as a reference current for the inner current controller an compare with q-component of measure AC current (Iq). The error (E Iq) gives voltage after passing through PI- controller. This voltage is then compare to the q-component of voltage (Vq) an cross coupling term to get the controller output voltage (Vq ).

11 Vector Control of Fuel Cell Base Gri Connecte Inverter 211 Figure 7 Reactive power control. Figure 8 Inner controller loop. 4 Simulation Results The fuel cell unit is connecte to a DC-DC boost converter to get the require voltage level to connect it to the utility gri via the DC-AC inverter whose switching action is controlle by vector control metho. The fuel cell connecte to gri system is moele in MATLAB/Simulink R2010a an Simulations are also performe to verify the simplifie moel. Some assumptions are taken in this system: The switching loss of the power electronics switches is zero. The three phase transmission system is ieal. The reactive power is zero. In the Simulink moel of the propose system its control system use sinusoial PWM metho of frequency 1350 Hz, the moel is simulate with a time

12 212 P. K. Rath an K. C. Bhuyan Table 1 Parameters of fuel cell base gri connecte inverter system S.L. Parameters Value 1 Inverter Output Frequency (f) 50 Hz 2 Filter Inuctance (L) H 3 DC link voltage 180 V 4 Switching Frequency of PWM 1350 Hz 5 Loa Resistance (RLoa) 100 Ohms 6 Filter Rating 490 VAR of 7.406μs. By using a small time step it is possible to observe the system performance in etail uring transient state an steay state conitions. The Table 1 presents the system parameters an its ratings. From the following Figure 9 it is clear that the output voltage of FC unit after the DC-DC boost converter which is the DC link voltage is settle to 188 V after 0.12 sec from starting. The following Figure 10 shows the output three phase voltage of the DC-AC converter, from this it is clear that the voltage profile settle to its steay state value 0.57 pu at 0.12 sec. The following Figure 11 shows the output three phase current waveform of the inverter unit, from this it is clear that the current settle to its steay state value at 0.1 sec. From above voltage an current waveforms it is clear that both contain many istortions so we couln t connect the inverter output to gri irectly. We shoul filter out the istortion from the voltage an current. The following Figure 12 shows the three phase voltage after the filtration process an from this it is clear that we get three phase sinusoial output voltage with less istortion. Figure 9 Fuel cell output voltage (V).

13 Vector Control of Fuel Cell Base Gri Connecte Inverter 213 Figure 10 Inverter output voltage (pu). Figure 11 Inverter output current (pu). Figure 12 Inverter output voltage (pu) after filter.

14 214 P. K. Rath an K. C. Bhuyan The FFT analysis gives the THD content of voltage is 3.43% which is shown in following Figure 13. The following Figure 14 shows the three phases current after the filter an from this it is clear that we get three phase sinusoial output current with less harmonic content. The FFT analysis gives the THD content of current is 3.8% which is shown in following Figure 15. Figure 13 FFT analysis of voltage. Figure 14 Inverter output current (pu) after Filter. Figure 15 FFT analysis of current.

15 5 Conclusion Vector Control of Fuel Cell Base Gri Connecte Inverter 215 A moel of the soli oxie fuel cell (SOFC) connecte to gri through inverter was evelope in MATLAB 2010a.ADC-DC boost converter with close loop control feeback system has been built to increase the voltage level so that it coul be connecte to gri. A three phase DC-AC inverter has been esigne an connecte between the SOFC-loa systems. Vector control technique along with sinusoial pulse with moulation metho is use for the switching of inverter effectively. The characteristics waveforms of voltage an current for the propose system have been obtaine an from the waveforms it is cleare that the parameters have faster response time, lower overshoot an less oscillation. The phase-phase voltage measure from the system an its THD is stuie an it is foun that harmonic content is very less. This esigne system coul be use as a istribute generation system. As the system base on fuel cell so there is no ba effect on the environment that s why it coul be use wiely to meet the power eman efficiently. References [1] Uzunoglu, M., an Alam, M. S. (2008). Moeling an analysis of an FC/UC hybri vehicular power system using a novel-wavelet-base loa sharing algorithm. IEEE Transactions on Energy Conversion, 23(1), [2] Panigrahi, A., an. Bhuyan, K. C. (2017). Fuzzy Logic Base Maximum Power Point Tracking Algorithm for Photovoltaic Power Generation System, RP Journal Publication 22. [3] Raghavenran, S., Babu, B. C., an Piegari, L. (2015). Analysis, esign an experimental valiation of moifie simple soft switching DC-DC boost converter. International Journal of Emerging Electric Power Systems, 16(4), [4] Rath, M. P. K., an charan Bhuyan, K. (2016). Moeling, Control an Steay State Analysis of Back To Back VSC HVDC System. International Journal of Engineering Research an Science (IJOER), 2(3), [5] Kakac, S., Pramuanjaroenkij, A., an Zhou, X. Y. (2007). A review of numerical moeling of soli oxie fuel cells. International journal of hyrogen energy, 32(7), [6] Anerson, B. R., Xu, L., an K. Wong, T. G., Topology for VSC Transmission, AC-DC Power Transmission, IEEE conference.

16 216 P. K. Rath an K. C. Bhuyan [7] Gebregergis, A., Pillay, P., Bhattacharyya, D., an Rengaswemy, R. (2009). Soli oxie fuel cell moeling. IEEE Transactions on Inustrial Electronics, 56(1), [8] Sahu, B., an Rincon-Mora G., (2005). A high-efficiency, ual-moe, ynamic, buck-boost power supply IC for portable applications, in International conference on embee system esign (VLSID 05). [9] Anersen, G. K., Klumpner, C., Kjaer. S. B., an Blaabjerg, F (2002). A New Green Power Inverter for Fuel Cells, in IEEE Power Electronics Specialist Conference. [10] Gopinath, R., Kim, S., Hahn, J. H., Enjeti, P. N., Yeary, M. B., an Howze, J. W. (2004). Development of a low cost fuel cell inverter system with DSP control. IEEE transactions on Power Electronics, 19(5), [11] Raju, M. N., Sreeevi, J., Meera, K. S., an Mani, R. P. (2016). Active Power Control of VSC-HVDC system, National Conference on Recent avances in control strategies for integration of Distribute Generation sources to gri an control of their power quality issues, at REVA University, Bangalore, uring 22 23r, [12] Cha, Hanju, Trung-Kien Vu, an Jae-Eon Kim. (2009). Design an control of proportional- resonant controller base photovoltaic power conitioning system, in Proc. IEEE Energy Convers. Congr. Expo., [13] Burger, B., an Engler, A. (2001). Fast signal conitioning in single phase systems, presente at the Eur. Conf. Power Electron. Appl., Graz, Austria,. [14] Itoh, J. I., an Hayashi, F. (2009). Ripple current reuction of a fuel cell for a single-phase isolate converter using a DC active filter with a center tap, IEEE Trans. Power Electron., 25(3), [15] Jang, M., an Ageliis, V. G. (2011). A minimum power processing stage fuel cell energy system base on a boost-inverter with a biirectional backup battery storage, IEEE Trans. Power Electron., 26(5), [16] Bernier, E., Hamelin, J., Agbossou, K., an Bose, T. K. (2004). Electric roun-trip efficiency of hyrogen an oxygen-base energy storage, International Journal of Hyrogen Energy. [17] Ulleberg, Ø., an Glöckner, R. (2004). Development of renewable energy/hyrogen systems: from concepts to actual emonstrations, presente at the Hyrogen an Fuel Cells Futures Conference, Perth,.

17 Vector Control of Fuel Cell Base Gri Connecte Inverter 217 [18] Logan, B. E. (2010). Scaling up microbial fuel cells an other bioelectrochemical systems. Applie microbiology an biotechnology, 85(6), [19] He, Z., Wagner, N., Minteer, S. D., an Angenent, L. T. (2006). An upflow microbial fuel cell with an interior cathoe: Assessment of the internal resistance by impeance spectroscopy, Environmental Science an Technology, 40(17), [20] Liu, H., an Logan, B. E. (2004). Electricity generation using an air-cathoe single chamber microbial fuel cell in the presence an absence of a proton exchange membrane. Environmental science an technology, 38(14), Biographies Praboha Kumar Rath receive the B.Tech. egree in Electrical Engineering from Institute of Technical Eucation & Research, Bhubaneswar, Oisha, Inia in 2008 an the M.Tech. egree in Power System & Power Electronics specialization from Institute of Technical Eucation & Research, Bhubaneswar, Oisha, Inia in He is currently working as a faculty in Electrical Engineering ept. in CET, Bhubaneswar. His research interests inclue moeling an control of renewable energy systems an applications of Power Electronics system.

18 218 P. K. Rath an K. C. Bhuyan Kanhu Charan Bhuyan, receive his B.Tech. egree in Electronics an Instrumentation Engineering from College of Engineering an Technology (Affiliate to Biju Patnaik University of Technology), Oisha, Inia in 2003 an his M.Tech. egree in Control an Automation specialization from IIT, Delhi, Inia in He receive his Ph.D. egree from NIT, Rourkela, Inia in He is currently working as an Assistant Professor at Instrumentation an Electronics epartment, College of Engineering an Technology, Bhubaneswar. His current research interests in moeling of photovoltaic cell, fuel cell an control strategies of various power converter an renewable power generation systems.