Microgrids of commercial buildings: strategies to manage mode transfer from grid connected to islanded mode

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1 Unversty of Wollongong Research Onlne Faculty of Engneerng and Informaton Scences Papers: Part A Faculty of Engneerng and Informaton Scences 214 Mcrogrds of commercal buldngs: strateges to manage mode transfer from grd connected to slanded mode Lasantha Meegahapola Unversty of Wollongong, lasantha.meegahapola@rmt.edu.au Duane A. Robnson Unversty of Wollongong, duane@uow.edu.au Ashsh Agalgaonkar Unversty of Wollongong, ashsh@uow.edu.au Sarath Perera Unversty of Wollongong, sarath@uow.edu.au Phlp Cufo Unversty of Wollongong, cufo@uow.edu.au Publcaton Detals L. Gunaruwan. Meegahapola, D. Robnson, A. P. Agalgaonkar, S. Perera & P. Cufo, "Mcrogrds of commercal buldngs: strateges to manage mode transfer from grd connected to slanded mode," IEEE Transactons on Sustanable Energy, vol. 5, (4) pp , 214. Research Onlne s the open access nsttutonal repostory for the Unversty of Wollongong. For further nformaton contact the UOW Lbrary: researchpubs@uow.edu.au

2 Mcrogrds of commercal buldngs: strateges to manage mode transfer from grd connected to slanded mode Abstract Mcrogrd systems located wthn commercal premses are becomng ncreasngly popular and ther dynamc behavor s stll uncharted terrtory n modern power networks. Improved understandng n desgn and operaton s requred for the electrcty utlty and buldng servces desgn sectors. Ths paper evaluates the desgn requrements for a commercal buldng mcrogrd system to facltate seamless mode transton consderng an actual commercal buldng mcrogrd system. A dynamc smulaton model of the proposed mcrogrd system s establshed (utlzng DIgSILENT Power Factory) to ad the development of plannng and operatonal phlosophy for the practcal system. An economc operatonal crteron s developed for the mcrogrd to ncorporate selectve mode transton n dfferent tme ntervals and demand scenaros. In addton, a multdroop control strategy has been developed to mtgate voltage and frequency varatons durng mode transton. Dfferent system condtons consderng varablty n load and generaton are analyzed to examne the responses of assocated mcrogrd network parameters (.e., voltage and frequency) wth the proposed mode transton strategy durng planned and unplanned slandng condtons. It has been demonstrated that despte havng a rgorous mode transton strategy, control of certan loads such as drect onlne (DOL) and varablespeeddrve (VSD) drven motor loads s vtal for ensurng seamless modetranston, n partcular for unplanned slandng condtons. Keywords connected, manage, slanded, mode, transfer, buldngs, grd, commercal, strateges, mcrogrds Dscplnes Engneerng Scence and Technology Studes Publcaton Detals L. Gunaruwan. Meegahapola, D. Robnson, A. P. Agalgaonkar, S. Perera & P. Cufo, "Mcrogrds of commercal buldngs: strateges to manage mode transfer from grd connected to slanded mode," IEEE Transactons on Sustanable Energy, vol. 5, (4) pp , 214. Ths journal artcle s avalable at Research Onlne:

3 Mcrogrds of Commercal Buldngs: Strateges to Manage Mode Transfer from Grd Connected to Islanded Mode L.G. Meegahapola, Member, IEEE, D. Robnson, A.P. Agalgaonkar, Senor Member, IEEE, S. Perera, Senor Member, IEEE, P. Cufo, Senor Member, IEEE 1 Abstract Mcrogrd systems located wthn commercal premses are becomng ncreasngly popular and ther dynamc behavour s stll uncharted terrtory n modern power networks. Improved understandng n desgn and operaton s requred for the electrcty utlty and buldng servces desgn sectors. Ths paper evaluates the desgn requrements for a commercal buldng mcrogrd system to facltate seamless mode transton consderng an actual commercal buldng mcrogrd system. A dynamc smulaton model of the proposed mcrogrd system s establshed (utlzng DIgSILENT Power Factory) to ad the development of plannng and operatonal phlosophy for the practcal system. An economc operatonal crteron s developed for the mcrogrd to ncorporate selectve mode transton n dfferent tme ntervals and demand scenaros. In addton, a multdroop control strategy has been developed to mtgate voltage and frequency varatons durng mode transton. Dfferent system condtons consderng varablty n load and generaton are analysed to examne the responses of assocated mcrogrd network parameters (.e. voltage and frequency) wth the proposed mode transton strategy durng planned and unplanned slandng condtons. It has demonstrated that despte havng a rgorous mode transton strategy, control of certan loads such as drect onlne (DOL) and varable speeddrve (VSD) drven motor loads s vtal for ensurng seamless modetranston, n partcular for unplanned slandng condtons. Index Terms Commercal buldng mcrogrds, slanded mode, mode transton, solarphotovoltac (PV), varablespeed drve (VSD). M I. INTRODUCTION ICROGRIDS are expected to become an ntegral part of future electrcty networks as they offer network support capabltes by ns of multple dstrbuted energy resources and exhbt capabltes of selfsuffcent standalone operaton. There are number of mcrogrd systems, wth dfferng characterstcs, whch are currently operatonal around the world [1], [2]. Typcally, mcrogrd systems comprse of one or more renewable and dstrbuted energy resources such as solar photovoltac systems, wnd and gas turbnes, controllable and noncontrollable loads, and energy storage systems. Mcrogrds, f desgned properly, are capable of operatng n ether grd connected or slanded mode. In most cases, the generatng resources of a mcrogrd are nterfaced through nertaless power electronc converters, whch may L.G. Meegahapola, A. Agalgaonkar, S. Perera, and P. Cufo are wth the Endeavour Energy Power Qualty & Relablty Centre at the Unversty of Wollongong, NSW, 2522, Australa. (emal: lasantha@uow.edu.au, ashsh@uow.edu.au, sarath@uow.edu.au, cufo@uow.edu.au). D.A. Robnson s wth the Sustanable Buldngs Research Centre at the Unversty of Wollongong, NSW, 2522, Australa (emal: duane@uow.edu.au). sgnfcantly affect the dynamcs and stablty of the mcrogrd especally durng mode transton,.e. from grdconnected to slanded mode or vce versa. Mcrogrds can be mplemented at varous dstrbuton voltage levels, and typcally appear at downstream locatons wthn a network, dependng upon ther applcaton. In practce, mcrogrd systems are mplemented ether n the form of laboratory scale prototypes [3][5] or to serve remote consumers n a network [6][8]. In recent years, mcrogrds have ganed popularty to cater for demand management and economc requrements of commercal buldng nstallatons, thereby supplementng the conventonal grd. The schedulng problem of buldng energy supples s consdered n [9] wth the practcal background of a low energy buldng facltated by a mcrogrd. The man objectve s to mnmze the overall cost for ensurng economc operaton of a buldng over a tmeframe whle satsfyng the energy balance and complcated operatng constrants of ndvdual energy supply equpment and devces n the mcrogrd. A cost optmsaton scheme amed at reducng the fuel consumpton rate of the mcrogrd system, comprsng of recprocatng gas engnes, a combned heat and power plant, a photovoltac array and a wnd generator, whle constranng t to meet the electrcal and thermal energy demand and provde a certan mnmum reserve power s proposed n [1]. An analytcal technque to evaluate the relablty for customers n a mcrogrd s proposed n [11], whle the development of a novel energy management system based on the applcaton of neural networks s proposed n [12] to determne hourly dspatch of generators wthn the mcrogrd for mnmzng the global energy costs. An teratve procedure based on bfurcaton theory s proposed n [13] to examne the mcrogrd stablty by assessng the mpacts of droopng characterstcs nt for frequency and voltage regulaton and prmary reserve schedulng. Feld tests conducted on a laboratory scale mcrogrd are reported n [14] whch demonstrate stable system behavour at crtcal operatng ponts, the flexblty of control modes, and the ablty of the system to ensure mode transton from slanded mode to grd connected mode n an autonomous manner. The management of mode transton strateges has been nvestgated n [15], [16] gvng due consderaton to operatonal characterstcs of mcrogrds. In [15], authors have adopted an nternal voltage controller and a droop based power sharng scheme for seamless mode transton, whle the voltage based modfed droop control method has been proposed n [16] for the mode transton from slanded to grd connected mode. Most of these studes have manly focused on controllng the mcrogrd sources durng

4 3 Phase Power (kw, kvar) 3 Phase Power (kw, kvar) 2 mode transton and have placed less emphass on the mpact of load dynamcs wthn the mcrogrd. Moreover, mcrogrds predomnantly embedded wth ntermttent renewable energy resources pose sgnfcant challenges n terms of managng the mode transfer. The mcrogrd system presented n ths study s derved from the prospectve scheme desgned for the Sustanable Buldngs Research Centre (SBRC) at the Unversty of Wollongong, Australa. The proposed system s specfcally nt to contrbute to the net zero energy target of the SBRC and mprove practcal understandng of mcrogrd operaton, especally for commercal buldng scale nstallatons. The generatng resources n the proposed mcrogrd nclude renewable resources such as rooftop solarpv and smallscale wnd generators, and lthumon batteres for energy storage to meet the statc and dynamc loads of the buldng. The proposed mcrogrd s expected to operate n ether grdconnected mode or slanded mode n order to maxmze the energy harvestng from renewable energy sources, ensure economcally feasble operaton, and practcally demonstrate varous mcrogrd research actvtes undertaken at the unversty. A model of the proposed SBRC mcrogrd has been developed usng DIgSILENT Power Factory n order to evaluate the developed control technques. A transtonal strategy has been proposed for seamless operaton of the mcrogrd under a commercal buldng envronment. Tme doman smulaton studes have been conducted n DIgSILENT Power Factory for examnng the system performance under dfferent modes of operaton. The mode transton affects operatonal characterstcs of the mcrogrd and qualty of power beng delvered to the loads are examned through studes. It s envsaged that the crtcal components of the mcrogrd such as generators, energy storage systems, and loads requre specal desgn consderatons to effectvely manage mode transton. The paper s organzed as follows: Secton II provdes a descrpton of the proposed SBRC mcrogrd system, ncludng some commentary on the desgn consderatons; Secton III descrbes the SRBC mcrogrd model used for the dynamc smulatons; Secton IV outlnes the conceptual framework for the mode transton strategy proposed for commercal scale mcrogrds; Secton V presents an evaluaton of the dynamc performance of the modeled system; Secton VI provdes some commentary on addtonal consderatons for commercal scale mcrogrds; and Secton VI concludes the work presented n ths paper. II. SBRC MICROGRID DESIGN CONSIDERATIONS Ths secton presents the desgn consderatons of the SBRC mcrogrd. A specal emphass has been placed on the generaton system, energy storage system, and capactor bank szng. To assst n the desgn process and examne the prospectve SBRC load profle, feld surements of voltage, current, and actve and reactve power were obtaned from another commercal buldng located at the Unversty of Wollongong, whch contaned smlarly confgured electrcal loads and load types. The sured data has been scaled to match the desgn detals assocated wth the SBRC faclty; however ramp rates related to equpment operaton, power factor, etc. reman unchanged. In addton, characterstcs of the other dynamc loads are also consdered durng the modelng study. The desgn phlosophy of the SBRC mcrogrd system s prmarly based on the net zero energy target of the SBRC buldng as part of the faclty s certfcaton requrements [17]. The net zero energy target s acheved through approprate szng of the mcrogrd s man generaton unt, whch s made up of several solarpv systems. A. Actve and Reactve Power Characterstcs The load profle of the SBRC s proposed to closely match that of the smlar buldng for whch a surement campagn was undertaken. Fg. 1 llustrates the projected real and reactve power demand of the SBRC faclty and mcrogrd Sat Sun Mon Tue Wed Thu Fr Sat Day of Week (Day) (a) Hour of the day (b) Fg.1: Projected actve and reactve power for SBRC; (a) weekly profle, (b) typcal weekday profle. Table I: Load composton durng peak load Load Type Actve Power Computer equpment Other (Lab equpment) Lghtng DOL motor load VSD motor load Total Reactve Power (kvar) Real Power (kw) Reactve Power (kvar) Real Power (kw) kw 3 kw 25 kw 87.9 kw 21.4 kw kw Table I llustrates the load composton durng peak load perod. Approxmately 5% of the nstalled loads are dynamc loads such as motors; hence t s a sgnfcantly challengng especally under ssue for modetranston for the renewable mcrogrd under study.

5 3 Phase Reactve Power (kvar) 3 Phase Power (kva, kw) 3 B. Component Szng 1) Szng of localzed generaton (solarpv array) The annual energy demand for the SBRC faclty was determned by makng seasonal adjustments to the weekly demand curve of Fg. 1(a) and summatng t over a year. A comparson of values derved from peak load data (Table I) and estmated operatng tmes wth dversty was also undertaken. An expected total approxmated energy demand of MWh per year was establshed. To ensure net zero energy requrements the solarpv system was desgned to provde adequate supply usng mnmum monthly fgures for solar rradaton. Mnmum monthly rradaton was determned from the prevous 5years of solar data obtaned from local weather statons. The total nstalled capacty of solarpv was selected to be 147 kw p for the SBRC mcrogrd. Ths ncluded margns for factors such as panel avalablty, load estmaton, and equpment effcency. It s noted that the SBRC buldng has been desgned to carry sgnfcantly more solarpv capacty, and as such there s potental for future expanson. The wnd turbnes assocated wth the mcrogrd are treated as expermental generaton unts and thus were not ncluded n the netzero energy calculatons. Two smallscale wnd turbnes are proposed for the SBRC mcrogrd wth a nomnal ratng of 1 kw. Ther presence provdes some addtonal margn for meetng the net zero energy requrements, and also asssts n reducng requred energy storage. It s antcpated that the wnd turbnes wll provde approxmately 12 MWh per year. 2) Szng of power factor correcton (PFC) capactors To meet utlty connecton requrements, the reactve power demand of the SRBC faclty s compensated usng dynamcally swtched banks of PFC capactors. It s envsaged that szng of the PFC capactors s to be left untl actual surements of the SBRC buldng demand profles are avalable, however, for the purpose of analyss feld surements from the example buldng have been utlzed Reactve Power Demand (kvar) PFC Operaton (kvar) summarzed n Table I). Fg. 2 llustrates the proposed PFC swtchng operatons and reactve power demand for a nomnal day usng example buldng data. It s proposed that the PFC capactor control remans unaltered for both grdconnected and slanded modes. However, durng slanded mode t s necessary to provde addtonal reactve power demand from energy storage and/or localzed generaton. Ths requrement s ncluded n szng of energy storage. 3) Szng energy storage (LIon battery bank) Several strateges can be consdered when szng energy storage; peak demand reducton, energy harvestng to reduce export, mtgatng ntermttent grd outages, and standalone operaton. Each strategy has dfferng demand on energy storage szng and economc constrants. For the SBRC mcrogrd standalone operaton s desred for demonstraton and research purposes (whle stll utlzng the grd for relablty). Szng energy storage for prolonged ntentonal slandng s not requred for the SBRC, and n order to mnmze the energy storage requrement (due to fnancal constrants), ntentonal slanded operaton s lmted to sngle day demonstraton. Durng the ntentonal slanded perod t s expected to have load sheddng procedures n place. LIon batteres are the selected energy storage technology due to ther hgh performance and materal consderatons assocated wth buldng certfcaton [17]. The expected demand profle of the SBRC s plotted aganst the solarpv localzed generaton (nverter output) to assst n establshng energy storage requrements, as shown n Fg. 3. The localzed generaton s based on solarpv desgn and weather data from the prevous 5years, wth mnmum monthly average nsolaton data utlzed to provde margn n energy storage requrements. The reactve power requrement s also allowed for n battery szng. From Fg. 3 t s estmated that the energy requrement from the batteres wll vary from 6 kwh to 3 kwh dependng on avalable generaton. It s antcpated that approprate capacty wll le somewhere between these fgures Max Energy Storage Mn Energy Storage Max Demand (kva) Warmer Month Gen. (kw) Cooler Month Gen. (kw) Hour of the day Fg. 2: Reactve power demand and PFC capactor swtchng operaton. Sx segmented PFC stages are selected to match typcal nstallatons at most of the commercal buldng locatons wthn the unversty. The peak reactve power demand of the sample buldng s able to be delvered by the frst four segments of PFC capactors,.e. 4 kvar. Addtonal segments (.e. 2 kvar) are provded to delver margn for motor start requrements of expermental equpment proposed for the SBRC (but not ncluded n demand calculatons Hour of the day Fg. 3: Commercal buldng demand profle and estmated localzed generaton based on mnmum monthly average nsolaton. For prelmnary desgn 1 kwh s selected as the capacty, wth ntentonal transton to slanded mode dependent on state of charge (SOC), avalable generaton (weather forecast) and load sheddng. Extendng slanded mode operaton may n the future may be made possble by ncreasng energy storage or localzed generaton.

6 4 A. SBRC Mcrogrd Layout A schematc of the SBRC mcrogrd model s shown n Fg. 4. Wthn the SBRC buldng surng nstruments have been placed at each source and offce crcuts n order to obtan real tme surements on generaton and load. DB Water Pump F VSD DB C F HghBay DB Ventlaton Ar handlng Fans unt Lab Loads F 2 X 1 kw ~ ~ 147 kw SolarPV System Heat Pump F VSD DB C 4 V / 8 A DB Ground Floor Offce Frst Floor Offce DB ~ LIon Battery Bank 1 kah Grd ~ 1 kva 11 kv/ 433 V Dy11 PFC 6 X 1 kvar Fg. 4: Schematc of the SBRC mcrogrd. Generatng sources n the SBRC mcrogrd are comprsed of 147 kw of rooftop solarpv system and two wnd generators (2 1 kw). A 1 kah lthumon battery bank s employed as the man energy storage system for the mcrogrd. The generaton capacty of the SBRC mcrogrd s summarzed n Table II. Table II: Generaton and storage capablty of the SBRC mcrogrd Generaton SolarPV System Wnd Generator (PMSG) Battery storage (Lon) Load 147 kw 2 1 kw 1 kah III. SBRC DYNAMIC SIMULATION MODEL A dynamc smulaton model was developed n DIgSILENT Power Factory consderng the dynamcs assocated wth the varous generators and loads connected to the SBRC mcrogrd. A 6 kvar swtched capactor bank provdes steadystate reactve power requrement for the mcrogrd whch s comprsed of sxsteps and swtched based on mcrogrd common busbar voltage wth a 5 ms delay. The dynamc models of varous generators and loads are descrbed n subsequent sectons. A. Modellng of Generaton Sources 1) SolarPV System The SBRC solarpv system s physcally comprsed of three separate threephase nverter systems. All three nverter systems are connected n parallel. However, ths was modeled as a sngle 147 kw solarpv nverter system n the smulaton model. The current source model of the generc statc generator model n the DIgSILENT Power Factory [19] has been adopted to develop the solarpv model. The solarpv model s comprsed of the PVarray wth embedded maxmum powerpont trackng (MPPT), DClnk and power controller. The controller s capable of controllng the voltage and frequency of the mcrogrd based on the erence values provded by the mode transton control system. 2) Wnd Generaton System A permanent magnet synchronous generator (PMSG) based wnd generator was modeled wth a fullyrated converter. As the PMSG s comprsed of a fully rated nverter, the mechancalsde dynamcs are decoupled from electrcal dynamcs; theore the wnd turbne mechancal system was gnored n the dynamc smulaton model. 3) Battery Storage System A LIon battery bank was modeled wth a battery model and a converter model rated at 1 kw. The chargng characterstcs are ncorporated to the battery model whle the actve and the reactve power control capabltes are ncorporated to the converter model. It must be noted that battery storage system s capable of controllng the mcrogrd voltage and frequency based on the erences provded by the mode transton control system (see Fg. 7). B. Modellng of Loads 1) VSD Motor Loads There are number of VSD drven motor loads are connected to the SBRC mcrogrd. The man VSD motor loads nclude condenser fans at the heat pump unt (2 4 kw), water pumps (1.5 kw) and ar handlng unt plenum fans (2 4 kw). The VSD was modeled wth a frontend controlled rectfer and an nverter system, whle ther load dynamcs were represented by respectve load characterstcs (.e. fan and pump models see Fg. 5). Controlled Rectfer PWM Inverter Inducton machne n T T Load characterstcs Fg.5: The VSD motor load model developed n DIgSILENT power factory. 2) DOL Motor Loads The drectonlne (DOL) nducton motor load s the most domnant load component n the mcrogrd, as the heat pump unt s comprsed of two scroll compressors each rated at 43 kw. These motor loads were modeled wth an nducton motor model and ncorporated the respectve load characterstcs (.e. fan and pump models) n order to replcate the actual load behavor. In addton, softstarter has also modeled at the front end of the nducton motor. 3) Computer, Lghtng and Lab loads The complex load model gven n DIgSILENT power factory [19] has been employed to represent the other loads n the SBRC mcrogrd. The load parameters shown n Table III have been adopted n order to represent the loads located at the ground floor offce, frst floor offce, hghbay area and lab loads, where a, b and c represent the proporton of the lghtng load, computer load and lab load respectvely. Parameters (.e. voltage dependences) for each load type have been derved from [2]. Table III: Exponental load model a b c Ground floor offce Frst floor offce Ground floor labs Hghbay area n

7 5 IV. CONCEPTUAL FRAMEWORK FOR MODE TRANSITION STRATEGY Mcrogrd mode transton can be categorzed nto two types; grd connected to slanded mode and slanded to grd connected mode. In each type, the transton strategy must be desgned to ensure smooth transton wthout leadng to any abnormal operatng condtons n the mcrogrd. However, the scope of ths study s lmted to the mode transfer from grdconnected to slanded mode as t has been dentfed as the most challengng control ssue for the mcrogrd, snce the mcrogrd s suddenly subjected to large voltage magntude and angle varatons soon after dsconnecton from the grd [16, 18] and creates undesrable operatng condtons for the connected loads. The grd connected to slanded mode can be classfed nto two types: () planned slandng; and () unplanned slandng. Though the transton s a sngle process t s essental to desgn separate transton strateges due to unque characterstcs assocated wth each outage type. A. Planned slandng (Planned Outages) The planned outages are determned based on the grdsde ancllary servce requrements and planned network outage condtons, such as servce outages for dstrbuton feeders. Theore, the mode transton strategy must be prudently planned based on the forecasted generaton surplus/defct n the mcrogrd system. The controller must consder avalable generaton capacty, energy storage system stateofcharge (SOC) and forecasted load demand of the mcrogrd system pror carryng out the mode transton. Theore, based on the avalable nformaton, the controller wll carry out a sequence of decsons to match the generaton wth the load demand pror executng the transton from the grd connected to slanded mode. Fg. 6 llustrates the flowchart of the mode transton strategy developed for planned outages. Planned Outage Operatng Mode (Export/ Import) P P P P gen Export load Yes Storage System SOC Not full Charge Storage System SOC 1; Pc, tch mn Battery SOC 1; Pc P, tch T mn P = Yes V loss f No.5pu &.1Hz Import t = t + 1 ms full No Assgn Voltage and Frequency Control Sources Yes Transton to Islanded mode Loadsheddng n ( start n 1) n + 1 No Power Balance res P Yes Update Power erences P gen P P P gen t = t + 1 ms No V.5pu & f.1hz Fg 6: Flow dagram of the mode transton strategy. P bat P P P w load P bat loss P res pre P, P P P. Pgen res pre Pw P Pw, Pw Pw P. Pgen res pre Pbat Pbat, Pbat Pbat P. Pgen where P gen, P load, P loss, P, P w, P bat, P, P c, V, f, t ch, n denote total power generaton, total load demand, total losses, solarpv generaton, wnd power generaton, power output of the battery storage unt, generaton demand mbalance, battery storage system chargng power, voltage sured at the mcrogrd common busbar, frequency surement at the mcrogrd common busbar, chargng tme and load sheddng sequence respectvely. Superscrpts res,, pre denote reserve, new erence, and erence pror to mode transton, respectvely. Pror to mode transton, the mcrogrd decson management system (DMS) determnes the operatng mode (.e. whether mport or export). When the mcrogrd s operatng at power mport mode t s not able to meet the energy demand n the network, hence n order to match the generaton wth the load demand, a load sheddng scheme must be mplemented (er to Table IV). The load sheddng scheme s mplemented n a sequental manner after detectng a power defct, untl energy surplus s acheved. Table IV: Loadsheddng sequence Sequence (n) Load type Maxmum power ratng 1 Heatpumps (DOL) 86 kw 2 VSD pumps 21.4 kw 3 Nonessental lab equpment 15 kw 4 Lghtng loads 25 kw It should be noted that the computer loads and essental lab equpments are not consdered n the load sheddng scheme as they are requred to be onlne under all crcumstances. In addton, for each load sheddng sequence there s a sub sequence whch s capable to solate the ndvdual loads categorzed under same type. For an example, there are two heat pumps operatng at the SBRC mcrogrd, theore the DMS wll decde one or both should be made offlne pror to mode transton. Followng each load sheddng step the mcrogrd power balance s evaluated, and once the voltage and frequency of the mcrogrd are stablzed below the specfed value (.e. V <.5 pu and f <.1 Hz), storage system SOC s evaluated. Furthermore, as the mcrogrd s desgned to acheve net zero energy target, the majorty of the storage system capacty s also utlzed for delverng power to local loads durng power mport mode n order to mnmze power mport. As llustrated n Fg. 6 f the storage system SOC < 1, then battery can be charged tll tme of dsconnecton and subsequently excess generaton ( P) can be curtaled based on the exstng dspatch level from each generatng source. The chargng tme T must be decded based on the avalable tme perod tll the tme of planned outage. However, t must be noted that battery chargng s an optonal decson for the mcrogrd system. It should also be noted that as the SBRC mcrogrd s operatng at net zero energy target, avalable local storage capacty n the mcrogrd system s kept at a mnmum value durng power mport mode, hence the mcrogrd s desgned to mantan a mnmum of 1 kah reserve at the battery storage unt. Then new power erences are assgned to the generaton sources (.e. P _, P _bat, P _w ) n order to match the generaton wth the load demand of the mcrogrd system. Once the power balance s establshed after assgnng the new power erences, voltage and frequency control schemes are establshed n order to control the voltage and frequency of the

8 6 mcrogrd for seamless mode transton and slanded operaton. The voltage and frequency control responsbltes are assgned to the generaton sources based on the avalable actve and reactve power reserves of the generaton sources (see Fg. 6). After stablzng the mcrogrd voltage and frequency below a predefned value (.e. V <.5 pu and f <.1 Hz) mode transton s executed. B. Forced/Unplanned Islandng Unplanned slandng condtons manly occur due to undesrable network condtons (.e. durng large voltage or frequency varatons), hence mcrogrd must be desgned to contnuously montor these varatons and subsequently swtch to the slanded mode to mantan unnterruptble supply to the mcrogrd loads. Durng such undesrable network condtons, voltage and frequency varatons create adverse operatng condtons for loads operatng n the mcrogrd system. For a successful mode transton, the mcrogrd generaton must be matched wth the load demand n order to allevate transents n the mcrogrd. The transton strategy outlned n Fg. 6 s adapted for the unplanned slandng wth several control modfcatons. The voltage and frequency stablzng loops n Fg. 5 are dsregarded durng unplanned slandng condtons. C. Control Scheme to Facltate Mode Transton A multdroop based control scheme was desgned n order to assgn and control the frequency of the mcrogrd durng mode transton and slanded operaton (see Fg. 7). f V P P bat Assgn Voltage & Frequency Control Sources P Pbat Q Qbat P P P bat P bat Q > Q bat Q < Q bat f V V g + PI f + PI f + PI + PI V g P Battery Q Q SolarPV V g P V u V l V g V u V l SolarPV Battery P P bat q _ q _ bat P P d _ v d _ + + d _ v d _ bat + + Fg. 7: Control strategy for mode transton. The droop control s adopted as the man control mechansm to generate actve and reactve power erences to control frequency and voltage of the mcrogrd. As outlned n Fg. 6 once new power erences are assgned to the generatng sources voltage and frequency control responsbltes are assgned to the generaton sources based on the hghest avalable actve (kw) and reactve (kvar) power reserve (e.g. f P > P bat then the solarpv unts control the frequency based on PI controller whle the battery storage generate the erence usng a frequency droop controller) n the solarpv and battery storage unt. However, t must be noted that sze of the wnd generator unts s relatvely small compared to the solarpv unt and battery energy storage system; hence t has not been consdered n the mode transton control scheme. However, t wll control termnal voltage locally based on local erence and avalable reactve power reserve n the PMSG. Furthermore, as the X/R rato of the cables n the mcrogrd s low (mostly resstve), t s not vable to control the mcrogrd voltage usng reactve power control, hence dual droop control has been adopted for the actve power control. Theore, when voltage at the mcrogrd pont of common couplng (PCC) devates from the voltage deadband (.e..98 pu < V g < 1.2 pu), actve power s also controlled to mantan the voltage due to low X/R rato. As the generatng sources are controlled n the dqerence frame, the daxs current s controlled n order to control the actve power; hence daxs current erence for the solarpv system s derved as follows: ttn 1 P Pbat; k( ferr ) ferrdt d _ v T (1) d _ n t P Pbat; m( f f ) P d _ v where f err, f, f, T n, k, and m denote frequency error (f f ), erence frequency, sured frequency at the SBRC, ntegraton tme constant, proportonal gan, and frequency droop constant respectvely. d_v denotes the output from the voltagedroop, whch s defned as follows: d _ v V u V V u V V l V V ; l ; P ; P n( V n( V l V ) V where V u, V l, V and n denote upper voltage lmt, lower voltage lmt, voltage surement and voltage droop respectvely. In order to control reactve power qaxs current erence s controlled whch s determned for the solar PV system as follows: q _ Q Q Q Q bat bat ; k( V ; s( V err 1 ) T V m t Tm t u ) V err ) Q where V err, V, T m, and s denote voltage error (V V ), erence voltage, ntegraton tme constant and voltage droop respectvely. Smlar functons can be derved for the battery energy storage system. D. Controller Performance In order to evaluate the controller performance a planned outage was smulated for the SBRC mcrogrd model wth 7% load (all VSD and DOL motor loads are onlne). Pror to the planned outage t s assumed that mcrogrd s operatng at ts full generaton capacty (267 kw) and transton process was ntated at t = 2 s. Fg. 8 llustrates the control decson calculaton process of the mode transton strategy. dt (2) (3)

9 Control Sgnal Voltage (pu) Control Sgnal Frequencey (Hz) Control Sgnal Power (MW) Control Decson Power Reserve (kw) Power Reference (pu) New Power Reference Calculaton Fg. 8: Control decson formulaton of the mode transton strategy. Once the planned outage s ntated new power erences are calculated and assgned for the generaton sources drectly wthout makng any chargng decson, snce battery energy storage SOC s at 1. Subsequently, avalable actve power reserve at each generaton source s calculated to determne the frequency control strategy for the mcrogrd system. In Fg. 8 control decson 1 denotes P > P bat whle P < P bat s denoted by. Theore, as llustrated n Fg. 8 solar PV system has taken the prmary responsblty to control network frequency whle the battery energy storage system wll contrbute n droop mode. Fg. 9 llustrates the control sgnals generated at solarpv and battery energy storage system..4 (pu) Fg. 9: Actve power control sgnals of the generaton sources. SolarPV Battery Storage Wnd Generaton SolarPV Battery Storage Wnd Generaton SolarPV Battery Storage (pu).1 Transton from droop based.5 control to PI based control.5 Battery(fdrp) Battery(f).1 Battery(df) (pu) Transton from droop based control to PI based control SolarPV(fdrp) SolarPV(df) SolarPV(f) Battery (d) SolarPV(d) Accordng to Fg. 9 when transton process s ntated solarpv system started to control the mcrogrd frequency based on PI controller output whle swtchng from the droop control scheme as the nverter assocated wth the solarpv system has much larger reserve. Contrary, based on the control decson of the transton strategy, battery storage system has swtched to the droop control mode. V. DYNAMIC PERFORMANCE EVALUATION The dynamc performance of the mode transton strategy was evaluated under planned and forced outage condtons. The smulatons have been performed assumng the worst possble scenaros for the SBRC mcrogrd; hence a load demand scenaros greater than the expected peak demand for the SBRC mcrogrd was nvestgated n the smulatons. A. Planned Outages The planned outages follow the transton strategy outlned n Fg. 6. Furthermore, power export was mantaned at approxmately 5 kw for each load scenaro. For power export mode several load scenaros were consdered (.e. load proportons of.5,.7 and 1 as a rato of total nstalled load n the mcrogrd). As outlned n Secton III, chargng the battery storage unt s an optonal requrement, hence that was not consdered here. Mode transton was ntated at t=1 s and soon after ntaton new power erences are assgned to the generaton sources n order to acheve power balance n the mcrogrd (see Fg. 1) Mode transton after assgnng new power erences at 1.1s SolarPV Battery Storage Wnd Generaton Fg. 1: Actve power response of generator unts durng mode transton for maxmum load (214 kw). As llustrated n Fg. 1 once the transton strategy s mplemented the generatng sources are assgned wth new power erences based on ther extng generaton and ultmately power generaton s curtaled to match wth the load demand of the mcrogrd system Max Load 1. Max Load.7*(Max Load).5*(Max Load).7*(Max Load).5*(Max Load) Fg. 11: Mcrogrd Frequency and voltage response followng a planned outage durng the power export mode.

10 Actve power (MW) Voltage (pu) Frequency (Hz) Phase Angle (Degrees) Frequency (Hz) Voltage (pu) 8 After curtalng the power generaton system parameters are verfed aganst the stpulated lmts (.e. V <.5 pu and f <.1 Hz) to ensure voltage and frequency stablty of the mcrogrd, and subsequently the transton process s executed. The voltage and frequency varatons for varous power export scenaros are llustrated n Fg. 11. Accordng to Fg. 11 the planned outage has ndcated mnmal an mpact on SBRC voltage and frequency. For an example, t has ndcated only a.5 pu voltage drop and.1 Hz frequency varaton followng the mode transton even under worst possble operatng condtons (.e. maxmum nstalled load). Furthermore, smulatons have been carred out consderng maxmum load scenaro and assumng a power mport of 5 kw to verfy the mode transton strategy for power mport scenaro. The voltage and frequency varatons for power mport scenaro are llustrated n Fg load sheddng slandng Fg. 12: Mcrogrd Frequency and voltage response followng a planned outage durng the power mport mode. Once the planned outage s ntated at t = 1 s, the load sheddng scheme s mplemented as the mcrogrd generaton s not suffcent to meet the load demand. Theore, one of the 43 kw heat pump unts was taken offlne, and subsequently wth the ad of addtonal battery storage capacty, the mcrogrd generaton s matched wth the load. Then after 1 ms the mcrogrd voltage and frequences were stablzed below the stpulated lmts, and subsequently the mcrogrd slanded from man grd. B. Forced Outages Performance of the mode transton strategy durng forced outages was nvestgated consderng the power export and mport scenaros for the mcrogrd system. It should be noted that durng power export mode the offce, lab and hghbay area loads are assumed to be operatng at.8 pu whle all the motor loads are assumed to be onlne. Both wnd and solarpv generaton s at ther maxmum capacty and the battery storage system also provdes.9 pu actve power output to the grd. Theore, mcrogrd exports 2 kw actve power to the man grd. Durng power mport mode t s assumed that the mcrogrd s operatng at ts peak load whle the total generaton s 181 kw. Theore, the mcrogrd mports 33.4 kw from the man grd pror to the forced outage. In both scenaros outage was ntated at t =1 s and voltage and frequency varatons are shown n Fg Power Export Power Export Power Import Power Import Power Export Power Import Fg. 13: Mcrogrd response durng forced outages. When forced outage occurs durng power mport mode, a 43 kw DOL heat pump was taken offlne as the total power generaton at the SBRC exceeds the local power demand. As shown n Fg. 13, ±.5 Hz frequency oscllatons can be observed n the mcrogrd system. In addton, when the forced outage occurs durng power mport mode the VSD motor loads n the mcrogrd are subjected to commutaton falure due to the large voltage drop and phase angle varatons at ther termnals. For example, when the forced outage occurs durng power mport mode t has ndcated an nstantaneous phase angle shft of 1º, whch has sgnfcantly contrbuted towards the commutaton falure at the VSD motor loads. Theore, the VSD motor loads are dsconnected from the mcrogrd durng power mport condtons (see Fg. 14)..2.1 Power Export Power Import Fg 14: Response of the VSD motor loads durng forced outages. The DOL motor loads are operatng durng modetranston absorbng reactve power from grd whch has resulted n substantal voltage dp (e.g..4 pu durng power mport). However, the mcrogrd voltage recovers wth the reactve power support provded by the battery storage nverter unt. As shown n Fg. 15, the solarpv unt curtals generaton durng power mport mode n order match the generaton wth the load demand. However, n terms of the battery storage

11 Reactve power (MVAr) Actve power (MW) 9 system, t supports mcrogrd voltage by njectng reactve power (a) Power export.1. Power export Power mport Power mport (b) Fg. 15: Response durng forced outages; (a) solarpv, (b) battery storage. VI. OTHER PRACTICAL ASPECTS FOR SBRC MICROGRID A. Connecton Agreements Connecton agreements from Australan utltes generally apply several requrements for connecton of mcrogrds: general protecton; voltage rse; antslandng protecton (where applcable); and general safety (labelng, etc.). The man constrant for connecton to downstream parts network s voltage rse requrements. Constrants on voltage rse are also wrtten nto the electrcty wrng rules. Snce most of the commercal buldng mcrogrds are of small scale, more onerous generator and energy market partcpaton requrements are omtted from the connecton agreements wth the utlty. B. Fnancal Constrants Generally, commercal buldng mcrogrds wll not be large enough to partcpate n the electrcty market, and energy prces for exportng power to the grd are negotated drectly wth the retalers. In Australa, energy export prces are generally a fracton of energy mport prces and netmeterng prcng schemes are used. Mcrogrds whch delay or negate network upgrades and enhance network support may be elgble for compensaton from utltes under the connecton agreement. Due to the low energy export prces, utlzng excess energy producton from the SBRC faclty wthn the Unversty precnct s a prorty. Accordngly, the connecton of the SBRC mcrogrd at the LV level wth several adjacent buldng loads may be able to absorb the local generaton. In ths manner, the fnancal benefts mprove the returns based on energy mport prces as opposed to energy export. C. Protecton Adequacy Although not addressed n ths work, protecton adequacy needs to be consdered for both the utlty and wthn the commercal buldng or precnct network, wth the latter beng mperatve f slanded mode s to be mplemented. Nonstandard LV network protecton gradng may be requred for reverse powerflow scenaros (.e. energy export). Protecton adequacy may nclude an addtonal requrement to facltate ntentonal transton to slanded mode. VII. CONCLUSIONS Ths paper has nvestgated the desgn, operatonal and control requrements and strateges requred for a commercal buldng mcrogrd nstallaton for a seamless mode transton. The desgn phlosophy s based around net zero energy target for the mcrogrd. The frequency and voltage control strateges have been proposed to ensure an slanded operaton of the mcrogrd and multdroop control mechansm has been developed for seamless mode transton. However, such capabltes may not be requred durng the grd connected operaton, wheren relatvely robust network condtons are expected. The outcomes of the study presented n ths paper hghlght the necessty of dfferent operatonal tngs for both slanded and grd connected operaton of the commercal scale SBRC mcrogrd under dfferent system condtons. VIII. REFERENCES [1] B. Kroposk, R. Laser, T. Ise, S. Morozum, S. Papatlanassou, N. Hatzargyrou, Makng Mcrogrds Work IEEE Power Energy Magazne, pp. 4 53, May/June 28. [2] N.W.A. Ldula, A.D. Rajapakse, Mcrogrds research: A revew of expermental mcrogrds and test systems Renewable and Sustanable Energy Revews, vol. 15, pp , 211. [3] E. Joseph, R. Laser, B. Schenkman, J. Stevens, H. Volkommer, D. Klapp, et al. CERTS mcrogrd laboratory testbed, Consortum for Electrc Relablty Technology Solutons (CERTS), CEC5, 28. [4] M. Barnes, A. Ds, A. Engler, C. Ftzer, N. Hatzargyrou, C. Jones, et al. 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12 1 [18] V. Jayawardana, L. Meegahapola, S. Perera, D. Robnson, Dynamc Characterstcs of a Hybrd Mcrogrd wth Inverter and Non Inverter Interfaced Renewable Energy Sources: A Case Study, IEEE POWERCON 212, Auckland, New Zealand. Nov [19] Power Factory Manual, DIgSILENT Power Factory Verson 14.1, GmbH, Germany, 212. [2] I. R. Navarro, Dynamc Power System Load: Estmaton of Parameters from Operatonal Data, PhD thess, Lund unversty, 25. Lasantha Meegahapola (S 6, M 11) receved hs BSc.Eng.(Hons.) degree n Electrcal Engneerng (Frst Class) from the Unversty of Moratuwa, Sr Lanka n 26, and hs PhD degree from Queen's Unversty of Belfast, UK n 21. Hs doctoral study was based on the nvestgaton of power system stablty ssues wth hgh wnd penetraton, and research was conducted n collaboraton wth ErGrd (Republc of IrelandTSO). He was a vstng researcher n the Electrcty Research Centre, Unversty College Dubln, Ireland (29/21). Currently he s employed as a lecturer at the Unversty of Wollongong. He s a member of IEEE and IEEE Power Engneerng Socety (PES). P. Cufo (SM 7) receved the B.E. (Hons.) and M.E. (Hons.) degrees n electrcal engneerng from the Unversty of Wollongong, Wollongong, Australa, n 199 and 1993, respectvely, and the Ph.D. degree n electrcal engneerng n 22. Dr. Cufo has had varous stnts n ndustry as an Electrcal Engneer and returned to academa n 27. Hs research nterests nclude modellng and analyss of power dstrbuton systems, dstrbuton automaton, modellng and analyss of ac machnes, power system harmoncs, and power system relablty. Duane Robnson graduated from the Unversty of Wollongong wth a B.E.(Hons.) n Electrcal Engneerng n 1998 after completng a seven year cadetshp wth the BHP Port Kembla Steelworks. In 22 Duane took up a poston as researcher wth the Centre workng on power qualty related research projects for Integral Energy and other consultng actvtes, and later joned the Unversty s academc staff as a Senor Lecturer. He temporarly departed the unversty to pursue more ndustral experence, workng for a multdscplnary consultng frm prmarly on LV and MV electrcal dstrbuton desgn, control, and protecton projects for heavy ndustry clents. In 211 he returned to the Unversty to take up a new research nterest n the area of energy effcency and demand sde ntegraton wth the Sustanable Buldngs Research Centre, and to rejon hs colleagues at the Australan Power Qualty and Relablty Centre. A. P. Agalgaonkar (M'9SM 13) receved the B.E. (Electrcal Engneerng) and M.E. (Electrcal Power System) degrees from Walchand College of Engneerng, Sangl, Inda, n 1997 and 22, respectvely, and the Ph.D. degree n Energy Systems Engneerng from the Indan Insttute of Technology Bombay, Mumba, Inda, n 26. He was a Scentst at the Energy Technology Centre, NTPC Ltd., Greater Noda, Inda, from 25 to 27. In February 28, he took up a poston wth the Unversty of Wollongong, n Wollongong, Australa, as a Postdoctoral Research Fellow. Currently, he s a Lecturer at the Unversty of Wollongong. Hs research nterests nclude plannng and operatonal aspects of renewable and dstrbuted generaton, mcrogrds, electrcty markets and system stablty S. Perera (M 95, SM 13) receved the B.Sc.(Eng.) degree n electrcal power engneerng from the Unversty of Moratuwa, Sr Lanka, the M.Eng.Sc. degree n electrcal engneerng from the Unversty of New South Wales, Australa, n 1978, and the Ph.D. degree n electrcal engneerng from the Unversty of Wollongong, Wollongong, Australa, n He has been a Lecturer at the Unversty of Moratuwa, Sr Lanka. Currently, he s a Professor at the School of Electrcal, Computer and Telecommuncatons Engneerng, Unversty of Wollongong. He s the Techncal Drector of the Australan Power Qualty and Relablty Centre, Unversty of Wollongong.