Micro-grid operation of inverter based distributed generation with voltage and frequency dependent loads

Transcription

1 Micro-grid operation o inverter baed ditributed generation with voltage and requency dependent load The MIT Faculty ha made thi article openly available. leae hare how thi acce beneit you. Your tory matter. Citation A ublihed ubliher Zeineldin, H. H., and J.L. Kirtley. Micro-grid Operation o Inverter Baed Ditributed Generation with oltage and Frequency Dependent Load. roceeding o the IEEE ower & Energy Society General Meeting, (ES '09), Copyright 2009 IEEE Intitute o Electrical and Electronic Engineer (IEEE) erion Final publihed verion Acceed Tue Sep 04 2:02:05 EDT 208 Citable Link Term o Ue Detailed Term Article i made available in accordance with the publiher' policy and may be ubject to US copyright law. leae reer to the publiher' ite or term o ue.

2 Micro-grid Operation o Inverter Baed Ditributed Generation with oltage and Frequency Dependent Load H. H. Zeineldin, IEEE Member, IEEE, and J.L. Kirtley, Jr., Fellow IEEE Abtract Ditribution ytem are experiencing increaing penetration o Ditributed Generation (DG). One attractive option i to ue the available DG capacity during utility outage by orming planned micro-grid. Load haring among dierent DG, during micro-grid operation, could be accomplihed by equipping each DG with a - and Q- droop characteritic. In thi paper, we analyze the impact o the load voltage and requency dependence on the micro-grid requency and voltage deviation during a planned ilanding condition. The ytem wa modeled and analyzed uing SCAD/EMTDC. The imulation reult and mathematical analyi how that the load voltage and requency dependence i an important actor when chooing the DG droop characteritic. The reult alo how that ome loading condition could lead to an untable micro-grid operation. Index Term Ditributed Generation, Inverter, Micro-grid, Static Load Model. T I. INTRODUCTION HE Micro-grid could be viewed a a group o DG that could operate either connected to the grid or intentionally ilanded. Although both the IEEE Std. 929 and UL 74 tandard do not allow ilanded operation; the IEEE 547 tandard preent it a one o the topic that need to be addreed in the uture[]-[3]. Micro-grid can oer many advantage which include increae ytem reliability, ability to iolate during utility outage, and reduce need or central dipatch [4]. Ditributed Generation are deigned to operate cloe to unity power actor a per the IEEE Std. 547 [3]. It would not be poible to operate a micro-grid with uch control [4]. The operation o a micro-grid require a robut control trategy that would acilitate load haring between dierent DG unit. Two main trategie have been propoed in literature or controlling DG on a micro-grid which include the Mater/Slave approach and the droop control approach. In the mater/lave approach, one DG (mater) operate a a voltage Thi work wa upported by the Madar Intitute o Science and Technology, Abu Dhabi, UAE. H. H. Zeineldin i with the Madar Intitute o Science and Technology, Abu Dhabi, UAE ( hzainaldin@mit.ac.ae). J. L. Kirtley i with the Maachuett Intitute o Technology, Cambridge, MA ource where it i reponible or controlling the micro-grid voltage while the other DG (lave) operate a a current ource [5]-[6]. The DG deigned to operate a lave are reponible or upplying active power to meet the load demand. The ditributed generator communicate with each other or load haring. In the droop control approach, the DG are equipped with a - and Q- droop characteritic [7]-[9]. The amount o load upplied by each DG depend on the DG droop. The main advantage o thi technique i that it doe not require any communication link. In [6] and [9], the mater lave and droop approach were both implemented where ome DG were deigned to upply a predeined active and reactive power and other were equipped with a droop characteritic. In thi paper, we ocu on the micro-grid load and it impact on the voltage and requency deviation during microgrid operation or inverter baed DG equipped with droop control. The load i modeled a a tatic load with both voltage and requency dependence. The tatic load model parameter are varied and both the requency and voltage waveorm are examined. The paper in organized a ollow: Section II provide a brie decription o the load model and DG droop characteritic. Section III preent the micro-grid ytem and DG interace model under tudy. Section I highlight the imulation reult or the dierent load model parameter. Latly, concluion are drawn in Section. II. MICRO-GRID LOAD AND INERTER CHARACTERISTICS The requency and voltage at which the micro-grid will tabilize depend on both the load and DG characteritic. Both the active and reactive power o the load are expreed a unction o voltage and requency [0]. The load voltage dependence could be modeled through either a polynomial or exponential relationhip. The load requency dependence i repreented by multiplying either the polynomial or exponential load model by a actor. An exponential load model with both voltage and requency dependence i given below in () and (2) N = o ( k p )) () o /09/$ IEEE

3 2 NQ Q = Qo ( kq )) (2) o where o repreent the initial operating voltage (rated voltage) and o and Q o repreent the active and reactive power correponding to the initial operating voltage. The parameter and o repreent the operating and rated requency repectively. The parameter repreent the operating voltage. N, NQ, k p and k q correpond to the parameter o the load model [0]. A mentioned earlier, the DG i equipped with a - and Q- droop characteritic a hown in Fig. and Fig. 2. The - characteritic i deigned uch that the DG deliver it maximum power at rated requency. In a imilar manner, the Q- characteritic i deigned uch that the DG upplie zero reactive power at rated voltage. Since both the voltage and requency are controlled by the utility during parallel operation, the DG would operate at unity power actor. During micro-grid operation, the amount o voltage and requency deviation will depend on the amount o active and reactive power mimatch, the load characteritic and the DG droop model. The - and Q- droop characteritic could be mathematically repreented a ollow DG a ) b DG a2 ( o ) b2 Q = (3) = (4) where DG and Q DG are the DG active and reactive power output. arameter a, a 2, b, and b 2 repreent the droop characteritic parameter. The requency and voltage at which the micro-grid will tabilize could be ound by equating () and (2) to (3) and (4) repectively a ollow N o k p )) = a ( o) o Q NQ ( b o kq )) = a2 ( o ) o ( b It can be een rom (5) and (6) that deviation in voltage are dependent on deviation in requency and vice vera. For contant power load (N=NQ=0) with no requency dependence (kp = kq = 0), the voltage deviation and requency deviation are decoupled. y c n e u q e r 3 2 Fig.. - Droop Characteritic Standard droop control max 2 (5) (6) Maximum ower control Real ower Q min Fig. 2. Q- Droop Characteritic p.u Q max III. SYSTEM UNDER STUDY The ytem under tudy conit o a load repreented by a tatic load model and two inverter baed DG a hown in Fig. 3. The ditribution ytem wa modeled by an equivalent ource behind an impedance. Each DG i equipped with the interace control preented in Fig. 4. The requency and voltage at the oint o Common Coupling (CC) are meaured and through the droop characteritic the active and reactive power reerence value ( re and Q re ) are determined. The power reerence value are then compared with the DG output active and reactive power ( DG and Q DG ) and the error i paed to a roportional Integral (I) controller to generate the current d-axi and q-axi reerence value (I dre and I qre ). Through ark tranormation, the DG a-b-c current are tranormed into the d-q axi rame and are compared with the current d-q axi reerence value. The error i paed through a econd I controller to determine the inverter witching ignal. Table I preent the ytem, DG and droop characteritic parameter. DC Microgrid Iolating AC oint Medium oltage Low oltage DG DG2 Fig.3. Sytem under Study. CC Static Load DC AC

4 3 CC CC Q I DGa I DGb I DGc Q re Q DG re abc DG dq Fig. 4. DG Interace Control Deign. K' I K' p i q i d K' I K' p i qre i d re K K p p K I K I qre dre CC requency and voltage. Since the two DG preented in thi work are identical with the ame droop characteritic, the DG hare the load equally. During grid connected operation both DG upply a 00 kw o active power. Since the CC voltage i le than p.u during grid connected operation, each DG will upply 5 kar o reactive power which i dictated by the droop characteritic. From Fig. 5, it can be een that the requency will tabilize at a value o 60 Hz and the voltage will tabilize at approximately 0.9 per unit. Thi reult coincide with the mathematical reult which are preented in Table II in the Appendix. TABLE I SYSTEM, INERTER AND DROO DATA Grid arameter oltage (line to line) 480 Frequency 60Hz Grid Reitance 0.02Ω Grid Inductance 0.3 mh DG Inverter Controller arameter p k = 5 k = k = 3 k = p I I a = 0. b = 0. a 2 = 0.5 b = 2 0 Fig. 5. Active and Reactive power output o each DG with a contant power load. The load on the micro-grid wa modeled uing () and (2). In order to examine the eect o the load model on the microgrid requency and voltage deviation, the dierent load parameter were varied individually while ixing the remaining load parameter. I. SIMULATION RESULTS WITH DIFFERENT LOAD MODELS The ytem under tudy i implemented on the SCAD/EMTDC imulator. A micro-grid cae i imulated by opening the utility breaker at the oint o Common Coupling (CC) at t = 5 econd. Both the CC voltage and requency are monitored to determine the eect o load parameter on the micro-grid voltage and requency deviation. The next ubection highlight the imulation reult or each cae tudy. A. Load Sharing with Droop Control The DG interace control i teted by opening the witch at the micro-grid iolating point (reer to Fig. 3). The load i modeled a a contant power load with no requency dependence (N=NQ=0 and k p =k q =0) and micro-grid operation i initiated at t = 5 econd. The load active and reactive power are 200 kw and 90 kar repectively. Figure 5 and Fig. 6 preent the DG active power, reactive power, Fig. 6. Frequency and voltage at the CC with a contant power load. B. Eect o oltage Dependence In thi cenario, the eect o the load voltage dependence i analyzed by varying the value o N and NQ. Similarly,

5 4 the load active and reactive power are adjuted to 200 kw and 90 kar repectively. A micro-grid i initiated at t = 5 econd. The load requency dependence (kp=kq=0) i et to zero. Figure 7 preent the requency and voltage waveorm or N=NQ=2 (contant impedance load), N=NQ= (contant current load) and or N=NQ=0 (contant power load). The requency deviation decreae a the value o N and NQ decreae. On the contrary, the voltage deviation increae a the value o N and NQ decreae or the preented cae. The reult coincide with the mathematical reult preented in the Appendix. Fig. 8. Frequency and voltage at the CC or dierent value o kp and kq with N=NQ=2. Fig. 7. Frequency and voltage at the CC or dierent value o N and NQ. A econd cae wa imulated where the load i adjuted to 76 kw and 90 kar at rated condition. The load voltage dependence parameter i et equal to zero (N=NQ=0). Figure 9 preent the voltage and requency deviation or variou value o kp and kq. Similarly the requency deviation decreae while voltage deviation increae with the increae in kp and kq. Frequency and voltage deviation are dependent on the load parameter a well a the amount o active and reactive power mimatch. For contant power load with no requency dependence (C and C7 in Appendix), the voltage deviation i independent on the amount o requency deviation. C. Eect o Frequency Dependence Thi cenario ocue on the eect o the load requency dependence on voltage and requency deviation during micro-grid operation. The micro-grid operation i analyzed or dierent value o kp and kq while ixing N= NQ =2. Figure 8 preent the voltage and requency waveorm or variou value o kp and kq. Similarly, the load wa adjuted to conume 200 kw and 90 kar at rated condition. From Fig. 8, the amount o requency deviation will decreae a the value o kp and kq increae. On the other hand, the amount o voltage deviation will increae with the increae in kp and kq (reer to Appendix). Fig. 9. Frequency and voltage at the CC or dierent value o kp and kq with N=NQ=0.

6 5 D. Untable Operation In thi cae, the load wa adjuted to conume 76 kw and 90 kar at rated condition. Similarly, the micro-grid operation wa initiated at t = 5 econd. The load voltage dependence parameter wa et ixed (N=NQ=3). Figure 0 preent the requency and voltage waveorm or dierent value o k p and k q. For the cae where k p =k q =0 and k p =k q =5, the micro-grid wa table and the requency and voltage tabilize at value that coincide with thoe given in Table II. For the cae where k p and k q are et equal to -5, there i no table micro-grid operating point. The requency drit until it reache the interace controller limit. Although cae C0, C, and C2 (reer to Table II) have the ame loading condition, ome cae might reult in table operation while other might reult in an untable operating condition. Table II how that, mathematically, there i no olution or either the requency or voltage or thi peciic loading condition. mathematical analyi coincide with the imulation reult. I. AENDIX The requency and voltage at which the micro-grid will tabilize could be determined rom (5) and (6). Table II preent the value or the requency and voltage during micro-grid operation or the loading condition under tudy in thi paper. TABLE II FREQUENCY AND OLTAGE FOR DIFFERENT LOADING CONDITIONS Cae Q N NQ kp kq (Hz) (p.u) (kw) (kvar) C C C C C C C C C C C C No ol. No ol. Fig. 0. Frequency and voltage at the CC or dierent value o k p and k q with N=NQ=3.. CONCLUSIONS Thi paper examine the eect o the load voltage and requency dependence on micro-grid operation or inverter baed ditributed generation. An exponential load model with both voltage and requency dependence wa modeled. The DG interace control wa equipped with a - and Q- droop characteritic. The reult and analyi how that the requency deviation and voltage deviation during micro-grid operation are dependent on each other. Although the microgrid load could be within the available DG capacity, untable operating condition could occur a a reult o the load voltage and requency dependence. It i important when deigning the DG droop characteritic to take into account dierent poible loading condition and model. The II. REFERENCES [] IEEE Std , IEEE recommended practice or Utility Interace o hotovoltaic () Sytem, April [2] UL 74, Static inverter and charge controller or ue in photovoltaic ytem, Underwriter Laboratorie Inc., Northbrook, IL. [3] IEEE Std , IEEE Standard or Interconnecting Ditributed Reource with Electric ower Sytem, July [4] R. H. Laeter, and. aigi, Microgrid: A conceptual olution, IEEE 35th ower Electronic Specialit Conerence, vol. 6, pp , June [5] J. Liang, T. C. Green, G. Wei, and Q. Zhong, Hybrid control o multiple inverter in an iland-mode ditribution ytem, 2003 IEEE 34th Annual ower Electronic Specialit Conerence, ESC '03, ol., pp. 6 66, June [6] J. A. Lope, C. Moreira, and A. Madureira, Deining Control Strategie or MicroGrid Ilanded Operation, IEEE Trancation on ower Sytem, vol. 2, No.2, pp , May [7] S. Barali, M. Ceraolo,. elacchi, and D. oli, Control technique o Dipered Generator to improve the continuity o electricity upply, IEEE ower Engineering Society Winter Meeting, ol. 2, pp , [8] F. Katiraei, and M. R. Iravani, ower Managemnet Strategie or a Microgrid With Multiple Ditributed Generation Unit, IEEE TRancation on ower Sytem, vol. 2, no.4, pp , Nov [9] C. Moreira, F. Reende, and J. Lope, Uing Low oltage MicroGrid or Service Retoration, IEEE Tranaction on ower Sytem, vol. 22, no., pp , Feb [0] IEEE Tak Force on Load Repreentation or Dynamic erormance, Load repreentation or dynamic perormance analyi. IEEE Tranaction on ower Sytem, ol.8, No.2, pp , May 993. III. BIOGRAHIES H. H. Zeineldin received the B.Sc. and M.Sc. degree in electrical engineering rom Cairo Univerity, Egypt, in 999 and 2002, repectively. In 2006, He received hi h.d. in electrical and computer engineering rom the Univerity o Waterloo, Ontario, Canada. Dr. Zeineldin worked or Smith and Anderen Electrical Engineering Inc. where he wa involved with project involving ditribution ytem deign, protection and ditributed generation.

7 6 Recently, he joined the Madar Intitute o Science and Technology in Abu Dhabi Hi current interet include power ytem protection, ditributed generation, and deregulation. Jame L. Kirtley Jr. attended the Maachuett Intitute o Technology, where he earned the h. D. degree in Electrical Engineering in 97. He ha been a member o the aculty o the Department o Electrical Engineering and Computer Science ince 97 and currently hold the rank o roeor o Electrical Engineering. He ha alo worked or General Electric, Large Steam Turbine Generator Department, a an Electrical Engineer, or SatCon Technology Corporation a General Manager and a Chie Scientit, and wa Gatdozent at the Swi Federal Intitute o Technology. Dr. Kirtley i a pecialit in electric machinery and electric power ytem. He erved a Editor in Chie o the IEEE Tranaction on Energy Converion rom 998 to 2006 and continue to erve a Editor or that journal and a a member o the Editorial Board o the journal Electric ower Component and Sytem. Dr. Kirtley i a Fellow o IEEE and a member o the United State National Academy o Engineering. He wa awarded the Nikola Tela prize in 2002 and the IEEE Third Millenium medal. Dr. Kirtley i a Regitered roeional Engineer in Maachuett.