DC Bus Voltage Regulator for Renewable Energy Based Microgrid Application

Transcription

1 Vol:7, No:2, 23 DC Bus Voltage Regulator for Reewable Eergy Based Microgrid Applicatio Bakari M. M. Mwiyiwiwa Iteratioal Sciece Idex, Electrical ad Computer Egieerig Vol:7, No:2, 23 waset.org/ublicatio/99967 Abstract Reewable Eergy based microgrids are beig cosidered to provide electricity for the expadig eergy demad i the grid distributio etwork ad grid isolated areas. The techical challeges associated with the operatio ad cotrols are immese. Electricity geeratio by Reewable Eergy Sources is of stochastic ature such that there is a demad for regulatio of voltage output i order to satisfy the stadard loads requiremets. I a reewable eergy based microgrid, the eergy sources give stochastically variable magitude AC or DC voltages. AC voltage regulatio of micro ad mii sources pose practical challeges as well as ubearable costs. It is therefore practically ad ecoomically viable to covert the voltage outputs from stochastic AC ad DC voltage sources to costat DC voltage to satisfy various DC loads icludig iverters which ultimately feed AC loads. This paper presets results obtaied from SEIC coverter based DC bus voltage regulator as a case study for reewable eergy microgrid applicatio. Real-Time Simulatio results show that upo appropriate choice of cotroller parameters for cotrol of the SEIC coverter, the output DC bus voltage ca be kept costat regardless of wide rage of voltage variatios of the source. This feature is particularly importat i the situatio that multiple reewable sources are to be itegrated to supply a microgrid uder mai grid itegratio or isolated modes of operatio. Keywords DC Voltage Regulator, microgrid, multisource, Reewable Eergy, SEIC Coverter. W I. INTRODUCTION ORLDWIDE, about.5 billio people live without access to electricity. Without a cocerted effort, the umber of people deied from electricity supply is likely to sustai or eve icrease []. Extesio of Natioal Grids is ofte prohibited by high cost ad o feasible i isolated rural areas [], [2]. Solutios to the limitatios of rural eergy access aroud the world require the use of both cetralised ad decetralised power systems [], [2]. I 2, the Iteratioal Eergy Agecy estimated that, to achieve uiversal access to electricity, 7% of the rural areas that curretly lack access will eed to be coected usig mii-grid or off-grid decetralised solutios [3]. Mii-grids ad other decetralised solutios may be more attractive tha large, cetralised solutios i rural areas for the followig reasos [3]-[6]. Worldwide, i the effort to meet the curret ad ever growig eergy demad there is a icreasig adoptio of distributed geeratio (DG) i the form of Reewable Eergy Sources (RES) that form miigrid ad/or microgrids [7]-[9]. I this directio, may issues related to ecoomics, electrical system optimizatio ad log-term viability have bee focused ad researched. The cotrol of Bakari M. M. Mwiyiwiwa is with the Departmet of Electrical Egieerig, College of Egieerig ad Techology, Uiversity of Dar es Salaam, Tazaia ( bakari_mwiyiwiwa@yahoo.com). dc-dc voltage regulator ad iverter iterfaced microgrid etwork that combies diversity of RES is the structure tackled by this research work. This research focuses o itegratio ad cotrol of RES microgrid where, architecture ad cotroller for dc-dc coverter ad dc-ac iverter iterfaced microgrid is evisaged. This particular focus o reewable eergy research has bee motivated by uavailability of electricity i grid isolated areas (maily rural areas) while there are various electrificatio optios from locally available ad plety diversity of uexploited RES. The proposed microgrid architecture exteds the traditioal itegratio use of solar ad wid geeratio systems to iclude other diversity of RES such as solar, mii/micro hydro geeratio, biofuel geeratio, biomass, biogas ad oceaic (waves ad tides) geeratio istead of fossil fuels to icrease power desity ad maitaiig reliability, sustaiability ad also protect the eviromet. As part of cotiuous research, this paper presets a desig example of DC bus voltage regulator which is madatory for itegratio of diverse evisaged electricity sources with stochastic outputs. The DC bus voltage regulators i this case are used to maitai a DC bus voltage at costat value ad therefore used as a poit of commo couplig of several available reewable eergy sources. The DC voltage commo bus is proposed i this paper due to its simplicity to cotrol as compared to AC voltage commo bus which eeds complex cotrol aspects to achieve sychroisatio ad stable operatio. II. ROOSED MICROGRID SYSTEM WITH MULTILE RENEWABLE ENERGY SOURCES The geeral cofiguratio of the proposed microgrid system cosists of several reewable eergy sources whose geeratio is assumed to be stochastic i ature. The emphasis here is that takig a typical situatio where oe or two or eve three of the available reewable sources caot reliably supply the coected load cotiuously. I order to ehace power supply reliability usig reewable eergy based microgrid, there is a eed to itegrate may ad diversified reewable eergy sources which are available ad easily developed i the area of cosideratio. Fig. shows the proposed system of a iverter iterfaced microgrid which is supplied by several reewable eergy sources iterfaced usig a evisaged DC Voltage busbar. As it ca be see i Fig., the microgrid is cofigured to supply DC loads ad AC loads. I additio, the microgrid ca also exchage power with the available earby Mai Grid (Natioal Grid) i order to further ehace reliability of the system. With the proposed microgrid cofiguratio show i Fig., whe failures occur i the mai grid system, the Iteratioal Scholarly ad Scietific Research & Iovatio 7(2) scholar.waset.org/ /99967

2 microgrid is automatically trasferred to isolated isladig operatio to supply the coected local load. Seamless trasitio betwee the itercoected to the isladig mode of operatio is crucial to uiterrupted cotiuity of supply to the local load of the microgrid. I Fig., the et dc bus power ( dc ) is determied by the sum of all coected microsources to the respective bus. The accumulatio ivolves power from all available sources ad the storage system ( s ). The total power at the DC busbar for available eergy sources ad storage system is give as, dc = ( dc ( i)) ± s i= Vol:7, No:2, 23 () where, dc (i) is the i th DG coected to the regulated Voltage DC busbar. The DC bus power remais after cosumptio by some local DC loads, is coverted to AC by the iverter to feed load S at a stadard AC load voltage, V. The apparet power at the iverter output is give as S = + with load agle of ø, the active ad reactive power of the load are respectively give i (3) ad (4). = Q = jq S ( Cos φ ) S ( Si φ ) (2) (3) (4) Iteratioal Sciece Idex, Electrical ad Computer Egieerig Vol:7, No:2, 23 waset.org/ublicatio/99967 I (2) through (4) both resistaces ad reactaces are take ito cosideratio durig aalysig ad tuig various cotroller parameters for the microgrid i cotrast to previous desigs where resistace is usually eglected. Maitaiig stability ad power quality i the isladig mode of operatio requires the developmet of sophisticated cotrol strategies ad eed to iclude both geeratio ad demad sides. I a good quality power etwork there must be a match betwee supply ad demad at ay particular poit of time, where power balace i (5) has to be applied. S, i S, i L, j R i= i= j= j= m + Q = + + QL, j + Q Fig. Cofiguratio of the multisource microgrid [8] m X (5) where ad m represets umber of micro-sources ad loads coected at the microgrid respectively, S, i, L, j ad R represet supplied ad cosumed active power, active power due to load ad etwork resistive losses. O the other had, Q S, i, QL, j ad QX represet supplied ad cosumed reactive power due to coected load ad etwork reactace. The proposed overall cotrol strategy is aticipated to improve the stability ad power quality of isolated microgrids ad grid distributio based microgrid with greater peetratio of DG at remote locatios. A total voltage harmoic distortio (THD) of ot greater tha 5% as set by IEEE stadard is also a target i this research work. Iteratioal Scholarly ad Scietific Research & Iovatio 7(2) scholar.waset.org/ /99967

3 Iteratioal Sciece Idex, Electrical ad Computer Egieerig Vol:7, No:2, 23 waset.org/ublicatio/99967 Vol:7, No:2, 23 REGULATION USING DC-DC CONVERTERS III. REVIEW OF THE METHODOLOGIES FOR DC VOLTAGE Various types of DC-DC coverters have bee researched ad ca be readily used for DC Voltage regulatio [], []. The well kow DC-DC coverters are Buck, Boost, Buck-Boot ad Cuk Coverters. Sigle- Eded rimary Iductor Coverter (SEIC) which has emerged i recet years is icreasigly gaiig popularity [2]-[4]. The Buck-Boost ad Cuk coverters have similar characteristics that the output voltage ca be higher or lower tha the iput voltage. Also these two types of coverters reverse the voltage polarity betwee iput ad output. However, SEIC coverter ca boost ad buck the iput voltage ad it ca also maitai the polarity betwee iput ad output. Maiteace of voltage polarity betwee iput ad output is particularly of iterest i reewable eergy applicatios such as battery chargig. Fig. 2 shows a circuit diagram of SEIC coverter. The details of aalysis, modelig ad duty cycle cotrol are well covered i [2]-[4]. The goverig operatioal relatioships betwee iputs ad outputs, assumig lossless system, are give by (6) through (8): V I i = out (6) D = (7) D o V i D = (8) D o I i where i is iput power, out is output power, V i is iput voltage ad V o is output voltage. Furthermore, D is operatig duty cycle, I i is iput curret ad I o is output curret. IV. DC BUS VOLTAGE REGULATION DESIGN EXAMLE USING SEIC DC-DC CONVERTER As a desig example of a dc bus voltage regulator, SEIC coverter has bee used i this paper. The choice of SEIC coverter has o particular major basis except for its two features amely buck-boost characteristics ad oreversal of voltage polarity betwee iput ad output. I this desig example the followig assumptios were made. (i) The DC voltage bus which is coupled with storage system is able to absorb all the eergy produced by the reewable sources to the load ad storage system. (ii) The DC voltage bus is able to supply the eergy required by the coected load. It is therefore the fuctio of the SEIC coverter to feed the DC voltage bus with costat voltage as set by the DC bus. Fig. 2 depicts power circuit diagram of a SEIC coverter. The coverter parameters are listed i the appedix. Fig. 2 Circuit Diagram of SEIC Coverter The SEIC coverter used i the desig example is rated at 4V output regulated DC voltage ad up to A output (load) curret which amout to 4kW maximum output power. The circuit parameters used i off-lie ad Real-Time Simulatios are show i the appedix. MATLAB/SIMULINK Sim ower System versio 7. was used to model the power circuit of the SEIC coverter [5]. The operatioal characteristics of the SEIC coverter have bee well covered i the literature [2]-[4]. The mai purpose of this research work is to ivestigate the capability of the DC bus voltage regulator to regulate the dc voltage fed to the DC busbar irrespective of voltage variatio output of the reewable eergy sources such as Solar ad Wid. Fig. 3 shows a feedback cotrol block diagram of the SEIC coverter fed by varyig DC voltage. I Fig. 3, a I cotroller was used with suitably chose parameters. It should be oted that, at this stage there was o attempt to aalytically optimise the cotrol parameters of the regulator. The cotrol parameters of the regulator are give i the appedix. Reewable Eergy Source V ref L C Voltage Regulator V ac AC-DC Coverter V dc SEIC Coverter Fig. 3 DC voltage regulator based o SEIC coverter V. MOFF-LINE SIMULATION OF DC VOLTAGE REGULATOR The SEIC coverter power circuit ad the cotroller were modelled usig MATLAB/SIMULINK Sim ower System versio 7. [5]. Off-lie simulatios of the DC bus voltage regulator (SEIC coverter power circuit ad the cotroller) system were coducted usig discrete fixed step simulatio mode i order to imitate ad prepare for real-time simulatio ad possibly hardware implemetatio. The purpose of ruig off-lie simulatios was maily to evaluate the suitability of fixed time step used i terms of covergece of umerical computatio i the computer before ruig the simulatios i the Real-Time Digital Simulator. I this case a time step of µs was foud to be adequate i terms of computatioal accuracy of the system sigals. Extesive off-lie simulatios were coducted aimig at obtaiig system parameters which gave best possible results. The results obtaied from off-lie D V i S L2 C 2 Cotrol V o V o Iteratioal Scholarly ad Scietific Research & Iovatio 7(2) scholar.waset.org/ /99967

4 Iteratioal Sciece Idex, Electrical ad Computer Egieerig Vol:7, No:2, 23 waset.org/ublicatio/99967 simulatios were used to determie suitable system parameters for best results ad therefore are ot icluded i this paper. VI. REAL-TIME SIMULATION RESULTS OF DC VOLTAGE REGULATOR Results obtaied from off-lie simulatios guided the researcher to further implemet the system i RT-LAB Real-Time Digital Simulator from OAL-RT Techologies Icorporatio [6]. The Real-Time Digital Simulator accepts MATLAB/SIMULINK models as iput ad rus the simulatios i real time. The importace of Real-Time Simulatio is that, for well modelled system, the results obtaied ca be treated as almost the same as results obtaied from real hardware. I order to test the performace of the DC bus voltage regulator, voltages of varyig ripple magitude ad frequecy were used as test iput voltages. Fig. 4 shows Real-Time simulatio results for relatively slow varyig iput DC voltage. It ca be see that the DC voltage regulator is capable of regulatig the output DC voltage withi less tha 2% of the iteded DC bus voltage. Also, it ca be see that the DC voltage regulator is capable of regaiig voltage regulatio after step chage of the load coected to the DC voltage bus Fig. 4 Real-Time Simulatio results for slow chagig iput voltage Fig. 5 shows Real-Time simulatio results for fast varyig iput DC voltage. It is evidet that the DC voltage regulator is rugged ad capable of regulatig the output DC voltage but with high ripple amplitude. The test results i Fig. 5 show extreme situatio where the ripple of the iput DC voltage varies at a frequecy of 5Hz with peak-to-peak amplitude of 2V. I real situatio, this type of voltage variatio from reewable eergy source is ulikely to happe. I that case the evisioed DC bus voltage regulator is adequately rugged for use i real situatios. 4 Vol:7, No:2, Fig. 5 Real-Time Simulatio results for fast chagig iput voltage Fig. 6 depicts expaded time scale of DC voltage dip showig a voltage dip caused by a step load icrease. It ca be observed that the regulator is capable to regai DC voltage regulatio withi 2ms based o the preseted desig example Fig. 6 Expaded time scale showig voltage dip due to step load icrease The proposed regulator is evisaged to hadle various types of DC voltages icludig rectifier output of varyig magitude voltage from wid turbie coupled geerator. I this case, the DC voltage of the rectifier cotais high frequecy ripple of the rectifier superimposed o a slow magitude varyig voltage. This sceario is show i Fig. 7. It is evidet from Fig. 7 that the proposed regulator is capable to maitai the DC voltage withi small ripple aroud the set referece value, i this case 4V. Fig. 8 depicts expaded time scale of DC voltage dip showig a voltage dip caused by a step load icrease. It ca be observed that the regulator is capable to regai DC voltage regulatio withi the same 2ms as i Fig. 6. Iteratioal Scholarly ad Scietific Research & Iovatio 7(2) scholar.waset.org/ /99967

5 Vol:7, No:2, 23 ower oit Trackig (MT) i some systems, the proposed DC voltage regulatio system ca be deployed i frot of the MT device. ACKNOWLEDGMENT The author is grateful for fiacial support exteded by Sida from Swede. AENDIX ower Circuit: L :.25Ω,.5H, C : mf; L 2 :.Ω, 5Mh, C 2 : 2mF. I Cotroller arameters: Kp =.25, K I = 5.. Iteratioal Sciece Idex, Electrical ad Computer Egieerig Vol:7, No:2, 23 waset.org/ublicatio/ Fig. 7 Real-Time Simulatio results for varyig magitude iput voltage with superimposed ripple Fig. 8 Expaded time scale showig voltage dip due to step load icrease for varyig magitude iput voltage with superimposed ripple VII. CONCLUSIONS This research work aimed at desigig a costat DC voltage bus which is used to iterface betwee the stochastically varyig voltages from reewable eergy sources ad the loads which ecessarily require stadard DC or AC supply. This research is importat as part of global research works o reewable eergy sources, coversio ad itegratio techologies. The methodologies used i this research work are modelig of the DC voltage regulatio system, off-lie simulatios ad the Real-Time simulatios of the model. The results obtaied from Real-Time simulatios of DC voltage regulator show that the regulator is rugged agaist variatio of iput voltage as well as step load chages. The proposed regulator is suitable for sythesis of asychroous DC busbar which ca be used to itegrate several reewable eergy sources with varyig characteristics. For the purpose of iclusio of Maximum REFERENCES [] Alliace for Rural Electrificatio (ARE) USAID, DFID A otetial Role for AMCs i romotig Gree Mii-Grids i Tazaia CASE STUDY ANNEX March 2. [2] Clea Eergy Miisterial, Roudtable 6: Mii-Grid Developmet Sessio Structure for the ublic-rivate Roudtable, 6:5-7:45, 7 April 23, New Delhi, Idia. [3] Iteratioal Eergy Agecy, Overview of Supervisory Cotrol Strategies Icludig a MATLAB/Simulik Simulatio Tool, hotovoltaic ower Systems rogramme, Report IEA-VS T- 8:22, August 22. [4] Iteratioal Eergy Agecy, Sustaiability Coditios for V Hybrid Systems: Evirometal Cosideratios, hotovoltaic ower Systems rogramme, Report IEA-VS T-3:2, March 2. [5] Sajeev okhrel, S. K. Sigal ad S. N. Sigh, Comprehesive Study of Commuity Maaged Mii Grid, Iteratioal Joural of Emergig Techology ad Advaced Egieerig Volume 3, Special Issue 3: ICERTSD 23, Feb 23, pages [6] CAMCO-UND-Climate arliamet, Mii-Grid Toolkit Field Study Report for Keya, Mozambique ad Zambia, Field Study Report. [7] W. He Grid Iteractio of DG Uits with ower Electroic Iterfaces, Master Thesis, Electrical Machies ad ower Electroics, Departmet of Electrical Egieerig, Royal Istitute of Techology, Stockholm Swede. [8] Brisa Ortiz, Off-grid ad mii grid V systems for electrificatio: advatages ad challeges, Frauhofer Istitute for Solar Eergy Systems, Accra, November 2, 22. [9] Iteratioal Eergy Agecy, World-Wide Overview of Desig ad Simulatio Tools for Hybrid V Systems, hotovoltaic ower Systems rogramme, Report IEA-VS T-:2, Jauary 2. [] N. Moha, T. M. Udelad, ad W.. Robbis, ower Electroics: Coverters, Applicatios ad Desig, Third Editio, Joh Wiley, New York, 23. [] M. H. Rashid, ower Electroics, Circuits, Devices ad Applicatios, Third Editio, retice Hall, Upper Saddle River, NJ, 24. [2] Ray Ridley, Aalyzig the SEIC coverter, Switchig ower Magazie, Nov. 26, pp [3] Jeff Fali, Desigig DC/DC coverters based o SEIC topology, Aalog Applicatios Joural-ower Maagemet, Texas Istrumets Icorporated, 28, pp [4] Biod Kumar adhi, Airudha Narai, Cotroller Desig for Sepic Coverter Usig Model Order Reductio, Iteratioal Joural of Electrical, Electroics ad Data Commuicatio, Vol., Issue- 3. [5] The Mathworks, MATLAB/SIMULINK. Versio 7.. [6] OAL-RT Techologies Ic RT-LAB, versio.4.9. Iteratioal Scholarly ad Scietific Research & Iovatio 7(2) scholar.waset.org/ /99967