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2 Stablty Assessment of a Small Islanded Power Network w Hgh Penetraton of Renewable Energy Dmple Verma Member, Engneers Australa WA, Australa Dr Sumedha Rajakaruna Department of Electrcal and Computer Engneerng Curtn Unversty Bentley, WA, Australa Dr Tm Johnson Jacobs Engneerng roup Per, WA, Australa Abstract Electrcty generaton usng renewable energy sources especally wnd and solar photovoltac s ncreasng rapdly and replacng fossl generaton. The mpact s more sgnfcant n small slanded networks whch hstorcally have reled on desel engnes for generaton. Snce wnd and photovoltac generaton s ntermttent and unpredctable, t becomes dffcult to schedule and manage a small network w varyng load demand. Therefore, conventonal generaton or some knd of energy storage s requred to mantan e balance between total network generaton and load demand. Ths paper presents a case study of a small slanded electrcal network n Western Australa w varous combnatons of wnd turbnes, photovoltac modules and desel generators. It studes er mpact on voltage and frequency stablty of e network w a vew to maxmze e penetraton of renewable energy. Index Terms Renewable energy sources, wnd energy, solar energy, Photovoltac system, Stablty analyss I. INTRODUCTION Most slanded and remote power networks around e world are dependent on fossl fuels, typcally desel ol, for power generaton. These networks are exposed to desel fuel prce volatlty and hgh operaton and mantenance costs ncludng fuel transportaton and bulk storage. Renewable energy sources such as wnd and solar photovoltac (PV) are clean, affordable, readly avalable, sustanable and can supplement generators n bo grd connected and off grd resdental and commercal applcatons. Hybrd energy systems ntegrate ese renewable energy technologes w desel generators (Ds), nverters and batteres to provde grd qualty power n remote areas not connected to a utlty grd. An exstng small power system of Laverton n Western Australa s used as a test case to nvestgate bo voltage and frequency stablty w e maxmum penetraton of wnd and solar PV generaton. A revew of lterature n s area s presented n secton II. The proposed meod for load-generaton analyss and dynamc studes are presented n secton III and secton IV respectvely. Secton V presents a case study of small slanded network n Western Australa. The concluson of e proposed meod s presented n secton VI. II. LITERATURE REVIEW The stablty problem w hgh penetraton of renewable energy n e power network s a known ssue and several researches have been conducted n s area. Ma H. Nguyen, et.al. [1] and Tareq Azz et.al. [2] focused on voltage and small sgnal stablty of a power system w e ncrease n e penetraton level of renewable energy and dverse power generaton portfolos. The study shows at ncrease n wnd energy dspatch, caused by e ncrease n wnd speed, reduces e dampng rato. However, ncrease n penetraton level of wnd turbnes w permanent magnet generators slghtly mproves e dampng of nter-area mode. The paper suggests 20% to 30% renewable energy (n MW) penetraton n e test system as a preferred penetraton, whch provdes reasonable loadablty and optmal grd losses. R.H. Kemsley et.al. [3] demonstrates e ntegraton of hydro generaton w e nverter systems to acheve very low carbon electrcty supply for communtes and outlne e ssues assocated w mplementng such systems. The ssues dentfed n e paper are as below: Use of multple renewable energy sources helps to accommodate seasonal output varaton and mnmse fuel consumpton. System w multple generaton sources requre more mantenance an networks suppled from a grd or from a sngle generator Battery energy storage s a key component of e systems. However, cost and space requrement often lmt battery storage; Meetng statutory network operatng standards n small power systems s achevable but may requre careful and nnovatve desgn Frequency control w hgh penetraton of renewables s dscussed n a paper presented by Peter et.al [4]. The study shows at e frequency support deterorates by ntegraton of

3 renewables n e generaton mx but can be mproved rough addtonal controls such as energy storage system, de-loadng of wnd turbne / solar PV, knetc energy support from e wnd turbne and load-sde management. The control of an nerta-less grd s also descrbed n s paper. The paper conclude at e effect of e penetraton of renewable s most notceable durng low load stuatons as e use of renewable generaton wll cause e deactvaton of tradtonal power plants and consequently lower e overall grd nerta. The paper by Nayar [5] presents case studes of mcro-grd dstrbuted generaton systems usng wnd turbnes, photovoltac modules and detals on how an nnovatve varable speed desel generator can be ntegrated nto such systems. The study ndcates at e newly developed varable speed desel generator system s expected to provde good opportunty to showcase hgh penetraton of renewable energes usng state-of-e-art wnd turbnes and photovoltac modules. Neven et.al.[6] ndcates at due to hgh energy costs n e small slands prove to be excellent test beds for e ntroducton of renewable energy technologes. The paper descrbes e H2RES model for optmsaton of ntegraton of hydrogen usage w ntermttent renewable energy sources on e solated sland. The H2RES model ncludes reversble hydro and batteres as storage technologes w e renewable energy sources. As noted above, several papers descrbe varous ssues w hgh renewable penetraton n a small slanded network. Ths paper has consdered a transent stablty studes for a practcal power systems network to dentfy e maxmum possble renewable energy penetraton on a small slanded network n Western Australa. III. LOAD-ENERATION ANALYSIS The load-generaton analyss provdes a gudelne to e optmal nstalled capacty for e renewable energy sources. The gudelne presented n s paper ncludes generaton from wnd energy source and solar PV. The frst step n s analyss s to dentfy network load demand for a partcular network. The network load demand vares w e tme and generally be avalable for a year n e form of hstorcal records. In certan cases, e demand can also be predcted for e future study years based on e hstorcal load data. To meet e load demand, e nstalled capacty of generaton needs to be hgher an e maxmum load at any pont of tme. In case of renewable generaton, e generaton at any pont of tme depends on e avalable energy sources (wnd and solar) at at pont of tme and e nstalled capacty of e renewable sources. A typcal daly load-generaton curve for an nstalled renewable generaton s shown n Fg.1. As shown n e Fg.1, e total renewable energy generaton s hgher an e total load demand durng 06:00 hrs to 09:00 hrs and for rest of e s, total renewable generaton was lower an e total load demand. Ths shows a requrement of energy storage for 06:00 to 09:00 s and conventonal energy generaton to be n servce for most of e day. Power Fgure 1. Daly load-generaton curve The nstalled capacty of each renewable source can be optmsed by load-generaton analyss. The load-generaton analyss ams to reduce e desel energy roughout a year w a dfferent mx of solar and wnd energy. The optmal wnd and solar generaton can be derved usng e below equaton, whch s a mnmzaton of desel generaton: Mnmse Where; L = Loadat D W = Deselgeneraton at = Wnd generaton at = Solar PV generatonat ( L ), = functon of wnd nstalledcapactyand wnd speed at S 8760 = 1 D 8760 = One Day: Load-Renewable eneraton Hours PV en WT MW (4x0.1=0.4 MW) LoadMW = 1 (1) = functon of solar nstalledcapactyandsolar radaton at W S A set of curves w combnaton of wnd and solar nstalled capacty can be plotted to dentfy e requred mx of generaton whch can present a maxmum dsplacement of desel generaton n a load year. IV. DYNAMIC STUDIES The load-generaton provdes a maxmum possble renewable generaton n e network. The network stablty w ese generaton mx needs to evaluate aganst e network operatng crtera. To dentfy e network stablty for dfferent network dsturbance, e system dynamc model needs to be developed n a power system smulaton software program. The dynamc models for desel generators are to nclude an automatc voltage regulator w excter and a governor system. A sutable dynamc model for e solar PV nverter and wnd turbne s requred. These dynamc models are to be ncluded n e system smulaton model. The network load s

4 to be represented w an approprate voltage and frequency dependent load models. The network dynamc stablty can be assessed n terms of system frequency and bus voltages. The network frequency s requred to reman w e specfed range durng and after a dsturbance. Smlarly, e voltage at each customer end also requred to reman wn e stablty lmt. enerally, ese lmts are specfed n e respectve network techncal codes for a stable network operaton. Several network dsturbances are requred to be smulated on e developed network. A typcal set of network dsturbances are lsted below: Loss of a porton of e wnd turbne generaton (WT) due to wnd varablty Loss of a porton of solar PV output due to movng clouds Sudden trppng of a sngle wnd turbne or solar PV nverter due to nternal or external fault Sudden trppng of a desel generatng unt Network faults For each of e above dsturbance, e bus frequency and voltage s to be montored and system stabltys to be assessed accordngly. The complete network modelng, smulaton of above mentoned dsturbance events and montorng of e above mentoned varables can be done usng any sutable power system smulaton software. The modelng software s requred to have features for dynamc modelng of renewable energy sources. V. CASE STUDY To dentfy e network stablty w e maxmum penetraton of renewable energy sources n an electrc network of Laverton was used n s study. Laverton s a town located n Western Australa and has a maxmum load demand of 1.2 MW n 2013 and expected to observe a maxmum load of 1.9 MW n The smplfed network dagram of s network s shown Fg. 2 To assess e possblty of maxmum renewable penetraton n e network, e PV solar output was calculated for e entre year (for each ). The PV generaton profle was consdered, based on e past records from one of e solar farm [7] n Western Australa. The daly power generaton record for e past one year was provded by Independent Market Operator (IMO) [8] for s study. The record provdes a suffcent reflecton of solar radaton pattern ncludng a change n solar PV generaton due to daly cloud movements. To get e wnd generaton n e Laverton, wnd speed records were collected from e Bureau of Metrology [9]. These records were en extrapolated for ly records. The extrapolaton was based on e lnear relaton between e two gven ponts. An optmsaton of solar PV and wnd generaton was performed w varous combnatons of solar PV and wnd generaton nstalled capactes. For each of e nstalled renewable generaton, energy dsplaced from desel generaton was calculated.the energy dsplaced for each of e solar PV and wnd generaton scenaros s presented n Fg. 3 and Fg. 4. As shown n ese fgures, e maxmum desel generaton energy s dsplaced n 2013 when e system ncludes 0.8 MW PV solar plus+ 0.4 MW wnd nstalled capacty. In 2020 e maxmum dsplacement occurs w 1.3 MW PV solar plus 0.7 MW wnd nstalled capacty. POWER STATION Fgure 2. Sngle lne dagram of Laverton electrc network Desel eneraton Energy Dspaced (MWh) Bus B Bus A PV=1.1 WELD WINDARRA BERIA PV=1.0 PV=0.9 PV=0.8 PV=0.7 Fgure 3. Desel generaton energy dsplaced vs. renewable generaton penetraton for 2013 load scenaro Fgure 4. Desel generaton energy dsplaced vs. renewable generaton penetraton for 2020 load scenaron PV= PV=0.5 Installed Wnd eneraton PV=0.4 PV=0.3 Maxmum desel generaton energy dsplaced PV=0.2 PV=0.1 PV=0

5 The network dynamc model was developed based on e standard IEEE dynamc models and lbrary models avalable n power systems smulaton software DIgSILENT PowerFactory [10]. These models ncludes desel generator controls (automatc voltage regulator and governor), wnd turbne and solar PV nverter models. The study cases n e PowerFactory model were setup for e 2013 and 2020 network load scenaros along w dfferent level of renewable generaton. The cases as setup n e PowerFactory model are shown n TABLE I. Case Id Load TABLE I. D 0.0 MW (1 D On) 0.2 MW (2 D On) 0.4 MW (2 D On) 0.4 MW (2 D On) 0.8 MW (4 D On) 0.0 MW (1 D On) 0.35 MW (2 D On) 0.7 MW (3 D On) 0.6 MW (3 D On) 1.2 MW (5 D On) DYNAMIC STUDY CASES D Spnnng Reserve WT Solar PV Total Renewable Energy (% of total generaton) The lst of dsturbances and er descrpton s lsted TABLE II. Event Id Event-1 TABLE II. Event Descrpton DYNAMIC STUDY EVENTS Network load ncreased by 10%over a second: Ths event s to demonstrate e system stablty for e sudden ncrease n e load, also observed n e load curve analyss. A load ramp event was appled n PowerFactory whch ramps e load by 10% n 1.0 seconds. Event-2 PV solar generaton at one locaton reduced by 50%: PV solar generaton s mpacted by movng clouds. To demonstrate e mpact of movng clouds, e generaton at one locaton s reduced to 50% of ts ntal value. In PowerFactory, s event was smulated as reducton n modelled sources dc current. Event-3 Event-4 Trppng of one wnd turbne out of four: It s expected at any of e wnd turbnes n e network can trp due to any dsturbance n e network or n e wnd turbne generator system. To model s event n PowerFactory, a wnd turbne crcut breaker was turned off. One desel generator trpped: Ths event s smlar to e above event. One of e desel generators whch were n servce was turned off to smulate s event. To assess e system stablty, system frequency and bus voltages were recorded durng e smulatons. Followng stablty lmts were consdered for stable operaton of e network: As per e utlty techncal rules, e system can operate wout load sheddng for a frequency down to 49.0 Hz. Therefore, a mnmum frequency of 49.0 Hz was used as e stablty crtera. After a dsturbance n e network, f e system voltage does not recover to wn 0.9 to 1.1 pu en t wll be consdered as unstable operaton. Each of e above mentoned study scenaros were tested for e lsted events as above. The summary of system mnmum frequences recorded durng e smulaton s shown n TABLE III. Those events w frequency reductons below 49.0 Hz are grey shaded. The system voltage n all e studed cases recovers to e nomnal operatng voltage range and does not mpose any nstablty ssues. However, n 2020 study case e reactve power requrement was hgher an e avalable reactve generaton sources. To meet e addtonal reactve power, a 125 kvar capactor at each feeder was connected. TABLE III. FREQUENCY FOR THE SIMULATED EVENTS Case Id Event-1 Event-2 Event-3 Event a b d c a b c a. Event-4 (trppng of D) for case and case s not applcable, as ese cases has only one D n servce. b. Event-3 (trppng of WT) for case and case s not applcable, as ese cases are wout WT. c. Event-2 (trppng of solar PV) for case and case s not applcable, as ese cases are wout solar PV. d. he smulaton dd not proceed as ere s not enough generaton after executng of Event-4 (trppng of D) for case For e load ncrement event (.e. Event-1), all e cases were able to meet e stablty requrements. The 10% load ncrement event presents 0.12 MW and 0.19 MW n 2013 and 2020 cases respectvely. The ncrease n load was well wn e avalable spnnng reserve and hence e frequency dd not reach below e stablty lmt of 49.0 Hz. For e reducton n PV generaton (.e. Event-2), e system was frequency unstable for Case , Case and Case

6 For e wnd turbne trp event (.e. Event-3), e system frequency was wn e requred lmt for all e cases. The system frequency for desel generator trp event (.e. Event-4) was below e requred lmt for case and all cases n 2020 except for one case All e stable cases have hgher spnnng reserve as compare to frequency unstable cases. Addtonal desel generaton wll be requred to mantan system stablty. The comparson of system frequency before and after addtonal desel generator n one of e studed case s shown n Fg. 5. Fgure 5. Comparson of system frequency before desel generator (2020-1, Event-2 and Event-4) The mnmum numbers of desel generators requred to mantan system frequency stablty for e smulated events are lsted n TABLE IV. A mnmum of two desel generators are requred n 2013 w e maxmum renewable generaton whle a mnmum of ree desel generators are requred n e 2020 load scenaro w maxmum renewablee generaton. TABLE IV. STABLE OPERATION WITH RENEWABLE ENERATION D Case Id Load Spnnng D Reserve MW (2 D On)* MW (2 D On) MW (3 D On)* MW (2 D On) MW (4 D On) MW (3 D On)* MW (3 D On)* MW (4 D On)* MW (4 D On)* MW (5 D On) 0.55 * Addtonal desel generator was swtched ON to mantan frequency stablty and after addtonal WT Solar PV The studed presents e maxmum renewable energy mx w mnmum desel generaton requred to mantan system frequency stablty n e network. A mnmum of two desel generators wll be requred to mantan frequency stablty n 2013 and a mnmum ree desel generators wll be requred to mantan frequency stablty n 2020 along w e above mentoned maxmum renewable generaton. VI. CONCLUSION A power network of Laverton n Western Australa was selected for e study. Ths network s currently suppled by desel generators and has potental to nclude renewable energy sources. The load analyss of e network suggests at for 2013 load scenaro, whch s 1.2 MW, e maxmum dsplacement of desel generaton energy occurs w 0.4 MW wnd and 0.8 MW of solar. For e 2020 load scenaro, whch 1.9 MW, e maxmum dsplacement s occurs w 0.7 MW wnd and 1.2 MW solar energy sources. The smulated results of dynamc analyss suggest at e frequency cannot be wn e requred lmt for e smulated dsturbances n some of e cases. The events whch were found to be stable were manly where e large amount of desel generaton was n servce. A furer analyss suggests at e system can be stable f addtonal desel generator/generators s/are kept n servce. The addtonal desel generators wll provde more spnnng reserve. The study concludes at e maxmum penetraton of renewable energy s lmted by system stablty whch can be mantaned by spnnng reserve n e form of desel generators. ACKNOWLEDMENT The auors wsh to express er sncere grattude to Davd Stephens, Horzon Power, WA, Australa, for hs support to e work reported n e paper. REFERENCES [1] Ma H. Nguyen, Tapan K. Saha, Mehd Eghbal, Impact of hgh level of renewable energy penetraton on nter-area oscllaton, Australasan Unversty Power Engneerng Conference, 29 September 3 October [2] Tareq Azz, SudarshanDahal, N. Mulananan, Tapan K. Saha, Impact of wdespread penetratons of renewable generaton on dstrbuton system stablty, 6 Internatonal Conference on Electrcal and Computer Engneerng, ICECE 2010, December 2010, Dhaka, Bangladesh. [3] R.H. Kemsley, P. Mcarley, S. Wade, F. Thm, Makng small hgh- works Scottsh Island penetraton renewable energy systems Experence, [4] Peter Telens, Drk Van Hertem, rd nerta and frequency control n power systems w hgh penetraton of renewables, and_frequency_control_n_power_systems_w_hg [5] Chemmangot V. Nayar, Hgh renewable energy penetraton desel generator systems, Pas to Sustanable Energy, Dr Arte Ng (Ed.), ISBN: , InTech, Avalable from:

7 [6] Neven Duc, Mara da racacarvalho, Increasng renewable energy sources n sland energy supply: case study Porto Santo, Elsever, Renewable & Sustanable Energy Revew, November [7] reenough Rver Solar Farm, [8] Independent Market Operator of Western Australa, [9] Beureu of metrology webste, w=map [10] DIgSILENT PowerFactory, power systems smulaton software,