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1 MURDOCH RESEARCH REOSIORY hs s the author s fnal erson of the work as accepted for publcaton followng peer reew but wthout the publsher s layout or pagnaton. he defnte erson s aalable at : Chandrasena R..S. Shahna F. Rajakaruna S. and Ghosh A. (3) Control operaton and power sharng among parallel conerter-nterfaced DERs n a mcrogrd n the presence of unbalanced and harmonc loads. In: 3rd Australasan Unerstes ower Engneerng Conference (AUEC) 3 September 9 - October 3 3 Hobart asmana Copyrght: 3 IEEE It s posted here for your personal use. No further dstrbuton s permtted.

2 Copyrght IEEE. ersonal use of ths materal s permtted. ermsson from IEEE must be obtaned for all other uses n any current or future meda ncludng reprntng/republshng ths materal for adertsng or promotonal purposes creatng new collecte works for resale or redstrbuton to serers or lsts or reuse of any copyrghted component of ths work n other works.

3 Australasan Unerstes ower Engneerng Conference AUEC 3 Hobart AS Australa 9 September 3 October 3 Control Operaton and ower Sharng among arallel Conerter Interfaced DERs n a Mcrogrd n the resence of Unbalanced and Harmonc Loads Ruwan.S. Chandrasena Farhad Shahna Sumedha Rajakaruna Department of Electrcal & Computer Engneerng Curtn Unersty erth Australa ruwan.chandrasena@curtn.edu.au farhad.shahna@curtn.edu.au s.rajakaruna@curtn.edu.au Arndam Ghosh Electrcal and Computer Eng. School ueensland Unersty of echnology Brsbane Australa a.ghosh@qut.edu.au Abstract hs paper demonstrates power management and control of DERs n an autonomous MG. he paper focuses on the control and performance of conerter nterfaced DERs n oltage controlled mode. Seeral case studes are consdered for a MG based on the dfferent types of loads suppled by the MG (.e. balanced three phase unbalanced sngle phase and harmonc loads). DERs are controlled by adjustng the oltage magntude and angle n ther conerter output through droop control n a decentralzed concept. Based on ths control method DERs can successfully share the total demand of the MG n the presence of any type of loads. hs ncludes proper total power sharng unbalanced power sharng as well as harmonc power sharng dependng on the load types. he effcacy of the proposed power control sharng and management among DERs n a mcrogrd s aldated through extense smulaton studes usng SCAD/EMDC. Index erms Mcrogrd DER Voltage Control ower Sharng. I. INRODUCION he eer ncreasng energy demand along wth the necessty of cost reducton and hgher relablty requrements are drng the modern power systems towards dstrbuted generaton (DG) as an alternate to the expanson of the current electrcty dstrbuton networks [ ]. Mcrogrds (MG) are systems wth clusters of DGs and loads. o deler hgh qualty and relable power the MG should appear as a sngle controllable unt that responds to changes n the system [3]. In MGs parallel DGs are controlled to deler the desred acte and reacte power to the system whle local sgnals are used as feedback to control the conerters. he power sharng among the DGs can be acheed by controllng two ndependent quanttes frequency and fundamental oltage magntude at the conerter output [ 7]. General ntroducton on MG bascs ncludng the archtecture protecton and power management s gen n [8 9]. A reew of ongong research projects on MG n US Canada Europe and Japan s presented n [9 ]. Dfferent power management strateges and controllng algorthms for a MG s proposed n [ ]. Reference [5] has ealuated the feasblty of MGs operaton durng slandng and synchronsaton perods. One of the man ssues stll to be nestgated n MGs s the effect of sngle phase unbalanced and harmonc loads on the control operaton and power sharng among parallel conerter nterfaced DERs. In the presence of balanced and lnear loads the conerters can be ope n oltage control mode [6 9] or current control mode [ 3]. Majorty of the researches for DER conerter control n MGs utlse current control mode [ 3]. For ths the output current reference for each conerter should be calculated properly based on the load demand and the desred power sharng rato among exstng DERs. Later usng a proper swtchng mechansm n the conerter t can supply the MG by the desred reference current. hs can be relately complex when the MG s supplyng sngle phase unbalanced or harmonc loads. In ths case the DERs not only should share the acte and reacte power but also should share harmonc negate and zero sequence currents. hs needs a fast and accurate negate and zero sequence and harmonc current extracton mechansm as dscussed n [ 5]. If the extracted sequence and harmonc currents msmatch the actual load requrement the operaton and power sharng of DERs can be sgnfcantly affected. Howeer by usng a oltage control mode there wll be no necessty to measure and analyse the load current and to extract the sequence and harmonc current components. In ths paper the operaton control and power sharng among parallel conerter nterfaced mcro sources operatng n oltage control mode are nestgated for grd connected and autonomous modes. hrough the extense smulaton results carred out by SCAD/EMDC t s demonst that the DERs can successfully supply the MG loads wth the desred power sharng ratos n the presence of sngle phase unbalanced and harmonc loads. II. MICROGRID SRUCURE AND CONROL Let us consder the MG system as shown n Fg.. he consdered MG system conssts of three conerter nterfaced DERs. DERs such as photooltac cells (V) fuel cells and batteres are usually connected to the MG through oltage source conerters (VSC) and a properly tuned second order flter to the MG. he DERs are then controlled to supply the loads wthn the MG. It s to be noted that the consdered DERs n ths paper are workng n oltage control mode and ther output power requred by power sharng are wthn ther capabltes. Dstrbuton Statc Compensator (DSACOM) s nstalled at the secondary sde of the dstrbuton transformer to regulate the oltage at ts ont of Common Couplng (CC). he mcrogrd s mantaned and ope usng two control loops. he outer control loop satsfes proper power sharng among the DERs whle the nner control loop s responsble of proper swtchng of DER conerters n order to generate the desred oltage at ther output. he DER and DSACOM conerter structure n addton to outer and nner control mechansms are dscussed n detal below.

4 Australasan Unerstes ower Engneerng Conference AUEC 3 Hobart AS Australa 9 September 3 October 3. DER Conerter Structure Let us assume that the DERs are connected to the MG through oltage source conerters (VSC). he VSC structure conssts of three sngle phase H brdges usng IGBs as shown n Fg.. Each IGB has proper parallel reerse dode and snubber crcuts. he outputs of each H brdge are connected to a sngle phase transformer wth : a rato and three transformers are star connected. In ths fgure the resstance R f represents the swtchng and transformer losses whle the nductance L f represents the leakage reactance of the transformers and the flter capactor C f s connected to the output of the transformers to bypass the swtchng harmoncs. CB M CB G f CB G MV Grd G G Mcrogrd CB L L L 3 L L S 5 DSA DER DER DER 3 Fg.. Schematc dagram of the network under consderaton. a b CC c L L L L Cfa Cfb Cfc ~. Outer Control Loop Lf Rf Cfa Lf Rf Cfb Lf Rf Cfc Lf f Cf + For a conerter nterfaced DER wth the structure shown n Fg. the nstantaneous acte and reacte power flows from conerter output to ts CC are [6] Vdc :a Rf u.a.vdc + V V p L V q L V sn cos V where V = V and V = V are respectely the RMS alue of CC oltage and AC flter capactor oltage of DER. L s the couplng nductance between CC and nerter output whch controls DER output power flow and s the angular frequency of the system. he swtchng frequency components n the nstantaneous acte power (p) and reacte power (q) can be fltered out to yeld the aerage acte power () and reacte power () by passng them through a low pass flter. In [7] t was proposed that decentralzed power sharng among the DERs n an autonomous MG can be acheed smlar to conentonal droop control by changng the oltage magntude and angle of DERs as V V m n where V and are respectely the oltage magntude and angle of the DER when supplyng the load wth acte and reacte power of and whle m and n are the droop coeffcents. he prncple of decentralzed power sharng n the MG s based on keepng the power output of DERs proportonal to ther ratngs whle the sum of the gene power from DERs s equal to the total requred demand n the MG. hs was mathematcally smplfed n [8] as Load j m m j DER number j Load j n n j DER number j where and j show the number of each DER. () () (3) Fg.. Schematc dagram of VSC for DERs Sngle phase equalent crcut of VSC..3 Inner Control Loop In ths paper the man concept of DER conerter control s based on fxng the oltage magntude and angle across C f as defned by the droop control from () (3). hs s acheed by approprate swtchng of IGBs n the conerter. For ths let us consder the sngle phase equalent crcut of VSC as shown n Fg.. In ths fgure uav dc represents the conerter output oltage where u s the swtchng functon that can take on alue dependng on whch par of the IGBs s turned on. he VSC s utlzng a closed loop optmal robust controller based on state feedback to generate u. For ths two dfferent state ectors can be assumed. Frst let us assume the state ector be defned as ~ x [ f ] () where represent the nstantaneous oltage and ĩ f s the hgh frequences of f. Now the reference ector x ref s to be calculated for each state arable for each DER as ref A ref B ref C V V V ~ f ~ f ~ ref A ref B f ref C For DER control n ths paper ref s calculated from () when the MG s n grd connected mode and from () when n autonomous mode. From the crcut of Fg. system state space descrpton can be gen as (6) x Ax B uc B where u c s the contnuous tme erson of swtchng functon u and s assumed as load dsturbance nduced to the conerter and neglected here. Dscrete tme equalent of (6) s (5)

5 Current (A) Voltage (pu) Australasan Unerstes ower Engneerng Conference AUEC 3 Hobart AS Australa 9 September 3 October 3 3 x k ) Fx( k) G u ( k) G ( ) (7) ( c k 8 6 Network Acte ower In (7) u c (k) s computed usng a sutable state feedback control law. For ths swtchng control laws are gen by u ( k) K[ x( k) x ( k)] (8) c ref where K s a gan matrx. he gan matrx s obtaned from Lnear uadratc Regulator (LR) method whch ensures the desred results for the system whle the aratons of system load and source parameters are wthn acceptable lmts of realty. From u c (k) the swtchng functon s then gene based on the error leel as If elsef u u c c h h then then u u where h shows the error leel and has a ery small poste alue. More detal on conerter control s gen n [6]. (9) (c) (d).5.5 DER DER3 DER G Sngle-cte ower of DER DER hase-a Acte ower of all DERs DER Network Voltage rofle DER- Output Current. Voltage Control n Autonomous Mode Based on the DER conerter control descrbed aboe there s no drect oltage control n the MG n autonomous mode. hs can be acheed f one of the DERs n MG regulates the network oltage. Howeer the DERs n resdental networks can be owned by customers and are not responsble for network oltage support. Utlzng the conerter of a DER to generate reacte power to support the network oltage profle wll reduce the acte power generaton capacty of the conerter. hs wll not be desred by ther owners. Alternately oltage regulaton n a MG can be acheed usng a DSACOM. hs was presented and dscussed n [7] and s not descrbed here. III. SUDY CASES AND SIMULAION RESULS For nestgatng the performance of the MG wth seeral conerter nterfaced DERs dfferent smulaton cases are consdered. he smulaton cases are bult to demonstrate the effcacy of the proposed oltage control mode for all DERs n proper power sharng among them n the network. he MG performance and power sharng among DERs are studed for dfferent types of loads n the network such as balanced unbalanced and harmonc. he DER conerter flter network and load parameters are gen n the Appendx B. 3.. MG wth balanced loads Let us consder the smple structure of Fg. to nestgate the MG operaton durng grd connected and autonomous modes. In grd connected mode each DER wll generate ts power and the extra load demand wll be suppled by the grd or the extra generaton wll flow back nto the grd. In autonomous mode total power demand s shared among the DERs proportonal to ther ratng. Frst let us assume n the system of Fg. CB G CB M and CB S are closed. he system s assumed to be n steady state condton at t = s and all the DERs are runnng n ther condton. (e) Fg. 3. Smulaton results of MG n case.: Acte power dspatch of grd and 3 DERs Sngle phase acte power output of DER (c) Acte power output of all DERs n ther phase A (d) Network oltage profle n secondary sde of transformer (e) hree phase nstantaneous current output of a DER. At t =.5 s CB G opens and the grd s dsconnected hence MG wll work n autonomous mode. herefore the DERs hae to ncrease ther output power to satsfy the load demand wthn the MG. At t =.5 s one of the exstng loads n the network s ncreased by 3 kw. It can be seen that n both cases all DERs are sharng the load change proportonal to ther ratngs. Fg. 3 shows the acte power dspatch of grd and 3 exstng DERs n the MG between and.5 seconds n the aboe mentoned network. he sngle phase acte power output of one of the DERs (e.g. DER ) s shown n Fg. 3. hs s the same for all phases of A B and C. From ths fgure t can be seen that the network loads are three phase balanced. he acte power output n phase A of all three DERs are also shown n Fg. 3(c). From ths fgure t can be seen that all DERs are sharng the loads accordng to ther ratos and they hae an equal amount of power gene n each phase. he oltage profle of the network s also shown n Fg. 3(d). As t can be seen n ths fgure the network oltage s successfully regulated to pu at all tmes. he three phase nstantaneous current output of a sample DER (e.g. DER ) shown n Fg. 3(e) confrms the DERs are generatng balanced currents n ther output. 3.. MG wth unbalanced loads In ths case let us assume n the system of Fg. at t = s CB G CB M and CB S are closed and the system s n steady state condton. At t =.5 s CB G s opened to dsconnect the grd hence MG wll work n autonomous mode. It s to be noted that dstrbuton transformer s stll connected to the LV sde.

6 Current (A) Current (A) Australasan Unerstes ower Engneerng Conference AUEC 3 Hobart AS Australa 9 September 3 October 3 (c) (d) (e) G Network Acte ower DER DER Acte ower of DER hase-a Acte ower of all DERs DER DER Acte ower Suppled by ransformer DER- Output Current (c) (e).5.5 Acte ower of DER hase-a Acte ower of all DERs DER DER Acte ower Suppled by ransformer DER- Output Current Fg. 5. Smulaton results of MG n case.3: Acte power output of DER Acte power output of all DERs n ther phase A (c) Acte power suppled by the dstrbuton transformer (d) hree phase nstantaneous current output of a DER Fg.. Smulaton results of MG n case.: Acte power dspatch of grd and 3 DERs Acte power output of DER (c) Acte power output of all DERs n ther phase A (d) Acte power suppled by the dstrbuton transformer (e) hree phase nstantaneous current output of a DER. At t =.5 s a new sngle phase kw load s connected to phase A. Later at t =.5 s another sngle phase kw load s connected to phase C. Fg. shows the acte power dspatch of grd and 3 exstng DERs n the MG between and 3.5 seconds n the aboe mentoned network. From ths fgure t can be seen that the total amount of acte power generaton among DERs are kept based on the desred power sharng rato among them. he acte power output of one of the DERs (e.g. DER ) s shown n Fg. for each phase nddually. From ths fgure t can be seen that for t <.5 s all three phases of DER hae an equalent amount of gene acte power. Howeer at t =.5 s when a sngle phase load s connected to phase A the acte power output of all phases of the DER ncreases. hs ncrease s slghtly hgher for phase A. hs slght dfference s further dscussed n Appendx A. In a smlar way at t =.5 s when another sngle phase load s connected to phase C the output acte power of all phases of the DER ncreases but ths ncrease s more for phase C. Smlar results are montored n all the DERs of the MG. From ths fgure t can be seen that sngle phase load power demand s shared among the three phases of the DERs. Fg. (c) shows the acte power output of all DERs n ther phase A for the studed case. From ths fgure t can be seen that for a load change n the network the contrbuton leel of each phase of the DERs n power generaton s also based on the desred sharng rato among them. It s to be noted that all phases of the DERs contrbute to a sngle phase (or unbalanced) load change n the network snce there s a possblty of power crculaton from one phase of MG to the other two phases through the wndngs of the dstr- buton transformer. Fg. (d) shows the acte power suppled by the dstrbuton transformer n the studed case. From ths fgure t can be seen that the dstrbuton transformer output power was zero when the MG was workng n autonomous mode wth balanced loads (.e.5 < t <.5 s). Howeer t can be seen that for.5 < t <.5 s there s a negate acte power flow nto the dstrbuton transformer n phase B and C whch s crculated and retuned to phase A where the sngle phase load s connected to. In a smlar case for.5 < t < 3.5 s there s a negate acte power flow nto the dstrbuton transformer n phase B whch s crculated and retuned to phase A and C where the sngle phase loads are connected to. he three phase nstantaneous current output of a sample DER (e.g. DER ) s shown n Fg. (e) whch shows the DERs are generatng unbalanced current n ther output MG wth unbalanced loads and solated transformer Now let us consder the network of case. where the dstrbuton transformer s solated from the LV sde when the grd s dsconnected. herefore let us assume that at t =.5 s CB G and CB M open and MG works n autonomous mode. As the dstrbuton transformer s solated from the LV sde no power crculaton can happen among the three phases n the autonomous mode. herefore t s expected that by applyng a sngle phase (unbalanced) load change n the network the DERs wll contrbute only n the phase n whch the sngle phase load s appled. he acte power output of one of the DERs (e.g. DER ) s shown n Fg. 5 for each phase nddually. From ths fgure t can be seen that for t <.5 s all three phases of DER hae an equalent amount of gene acte power. Howeer at t =.5 s when a sngle phase load s connected to phase A only the acte power output of phase A ncreases. In a smlar way at t =.5 s when another sngle phase load s connected to phase C only the output acte power of phase C ncreases. Smlar results are montored n all the DERs of the MG. From ths fgure t can be seen that sngle phase load

7 Current (A) (%) Australasan Unerstes ower Engneerng Conference AUEC 3 Hobart AS Australa 9 September 3 October 3 5 power demand s only suppled by the releant sngle phase of the DERs. Fg. 5 shows the acte power output of all DERs n ther phase A for the studed case. From ths fgure t can be seen that for a load change n the network the contrbuton leel of each phase of the DERs n power generaton s based on the desred sharng rato among them. Fg. 5(c) shows the acte power suppled by the dstrbuton transformer n the studed case. As the transformer s solated at t =.5 s then ts output power s zero. he three phase nstantaneous current output of a sample DER (e.g. DER ) s shown n Fg. 5(d) whch shows the DERs are generatng unbalanced current n ther output. 3.. MG wth harmonc loads In ths secton let us consder the MG of Fg.. Let us assume that the system s n steady state condton at t = s and at t =.5 s the grd s dsconnected (.e. CB G s opened) and MG wll work n autonomous mode. At t =.5 s a new three phase harmonc load of 3 kw s connected to the network. At t =.5 s ts demand s ncreased to 6 kw. he harmonc load has a otal Harmonc Dstorton (HD) of 5.5% wth the Fast Fourer ransform (FF) Spectrum as shown n Fg. 6. Fg. 6 shows the acte power dspatch of grd and 3 exstng DERs n the MG between and 3.5 seconds n the aboe mentoned network. he sngle phase acte power output of one of the DERs (e.g. DER ) s shown n Fg. 6(c). hs s the same for all phases of A B and C. he acte power output n phase A of all three DERs are also shown n Fg. 6(d). From these fgures t can be seen that as the load s suppled from the three phase network ts demand s shared equally among the three phases of each DER. Howeer the DERs share the extra demand based on ther desred power sharng rato. he three phase nstantaneous current output of a sample DER (e.g. DER ) s shown n Fg. 6(e) whch shows the output current of the DERs are dstorted as requred by the network harmonc load. IV. CONCLUSION he power management and control of conerter nterfaced DERs were dscussed n ths paper for autonomous operaton of a MG wth balanced unbalanced and harmonc loads. In many MG related researches the conerter nterfaced mcro sources are current controlled. In such a case f the MG supples unbalanced loads detaled calculatons are requred to extract the requred poste negate and zero sequences of the reference current components for the DERs. Smlarly f the MG supples nonlnear and harmonc loads detaled calculatons are requred to extract the requred dfferent harmonc components for the current reference for the DERs. hese calculatons are later utlzed for proper total power sharng unbalanced power sharng and harmonc power sharng among DERs n the MG. Howeer f the conerters are oltage controlled there s no necessty for these complcated current reference extractons and calculatons. In ths paper through seeral case studes t was erfed that proper total power sharng harmonc power sharng and unbalance power sharng among DERs can be acheed through oltage controlled conerters n the MG wthout any complcated current reference extractons for DERs. (c) (d) (e) Harmonc Load Current FF HD = 5.5% Harmonc Order G Network Acte ower DER DER Sngle-cte ower of DER DER hase-a Acte ower of all DERs DER DER- Output Current Fg. 6. Smulaton results of MG n case.: FF Spectrum of harmonc load current Acte power dspatch of grd and 3 DERs (c) Sngle phase acte power output of DER (d) Acte power output of all DERs n ther phase A (e) hree phase nstantaneous current output of a DER. AENDIX A ower Crculaton through ransformer Let us consder the smple network llust n Fg. 7. A three phase conerter nterfaced DER n oltage control mode s connected to Bus 3 to supply a sngle phase load n Bus connected to phase A. A three phase DSACOM s nstalled n Bus to regulate ts CC to a three phase balanced oltage of pu. Only the downstream of /Y Grounded transformer s connected whle ts upstream sde s open crcuted. he /Y Grounded transformer can crculate the reerse fed current and power nto one of ts Y grounded wndngs nto ts two other Y Grounded wndngs due to ts prmary delta confguraton. Frst let us assume that CB s open and the DSACOM s not connected. In ths case the sngle phase load wll be suppled by a and a as a a ga gb ga gc load gb tr _ eq load gc load ( ) tr _ eq ( ) () where a s the phase A current suppled from the generator sde a s the phase A current suppled from the transformer sde ga gb and gc are the three phase output oltage of the DER ga gb and gc are the three phase current output of the DER load s the oltage at load CC s the feeder mpedance between the buses < < and treq s the equalent mpedance between two termnals of the transformer. From Fg. (7) we hae tr _ eq ( ) ()

8 Australasan Unerstes ower Engneerng Conference AUEC 3 Hobart AS Australa 9 September 3 October 3 6 Y Bus- CB- DSA a (-) Fg. 7. Smplfed network under consderaton. load Bus- Load herefore t s expected that a > a. Hence ga gb gc ga gb gc a Bus-3 gc gb ga ga gb gc ga gb gc DER or () From () t can be seen that the amount of current (or power) suppled from DER n ts phase B and C are hghly dependent on the equalent mpedance of the transformer and the lne. Now let us assume that CB s closed and the DSA- COM s regulatng ts CC to pu balanced oltage. In ths case a wll be same as n () whle a wll be a load (3) ( ) where s the angle of phase A n Bus. herefore from () and (3) t can be seen that there wll be always a small dfference between a and a. he same dfference wll be reflected n ga gb and gc n the output of the DER. Hence t s expected that there wll be a slght dfference between ga compared to gb and gc. hs dfference s dependent on the dstance between load and DER and transformer as well as the load demand. AENDIX B able I. echncal data of the network parameters of Fg.. MV Network kv L L RMS 5 Hz MV Lne Impedance R =. L = mh LV Feeder 5 V L L RMS 5 Hz LV Lne Impedance R =. L = mh ransformer 3 kva kv/ 5 V hree hase 5 Hz /Y Grounded I = 5% Balanced hree hase Loads =.7 kw F =.95 Sngle hase Load = kw F =.95 n Sngle hase Load = kw F =.95 n DER VSCs and Flters R f =. L f =.36 mh C f = 5 F V dc = 5 V a = 3.33 h = 5 DSACOM VSC and Flter R f = m L f = mh C f = 5 F L = mh V dc = kv C dc= µf a = h = 5 able II. echncal data of DERs and droop control coeffcents n Fg.. DER DER Ratng [kw] Couplng Inductance (L ) [mh] m [rad/kw] n [V/kVAr] DER DER DER REFERENCES [] B Kroposk C. nk R. es and.k. 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