Comparison of Voltage Stability Indices and its Enhancement Using Distributed Generation

Transcription

1 roceedngs of the World Congress on Engneerng 016 ol I WCE 016, June 9 - July 1, 016, ondon, U.K. Comparson of oltage Stablty Indces and ts Enhancement Usng Dstrbuted Generaton I. Kumaraswamy, S.Tarakalyan and B. enkatarasanth Abstract Instablty n a power system may be manfested n many ways dependng on the system confguraton and operatng mode. oltage nstablty n power dstrbuton systems could lead to voltage collapse and thus power blackouts or abnormally low voltages. It s requred to know the strength of the buses to mprove the voltage stablty of the system. Ths paper analyzes the performance of dfferent voltage stablty ndces and ts enhancement by placng optmal locaton of Dstrbuted generaton (DG). Ths work proposes analytcal expressons for fndng optmal sze and power factor of dstrbuted generaton (DG) unts. DG unts are szed to acheve the hghest loss reducton n dstrbuton networks. The large deployement of dstrbuted generaton (DG) sources n dstrbuton network can be an effcent soluton to overcome power system techncal problems and economcal challenges. The method has been tested n 16-bus and 33-bus dstrbuton systems. Index Terms Dstrbuted generaton, oltage stablty, and voltage stablty ndces. I. INTRODUCTION Recently a top prorty s gven to develop a relable, sustanable, envronment frendly as well as low-cost electrcal energy supply. Ths ncludes a sensble energy mx and mprovements n effcency of energy generaton, transmsson and consumpton[1]. As a number of events that have been brought to the vulnerablty of the current centralzed electrcal energy supply nfrastructure, such as terrorst threats, natural dsasters, geopoltcal dsruptons, ageng of a hghly complex nfrastructure, clmate change and regulatory and economc rsks, DG appears to be one of the key answers for dfferent problems[]. In the dstrbuton system, the electrcal power supply wll be transferred from a vertcal one to a horzontal system. In the tradtonal system the electrc power ndustry has been drven by a paradgm where most of the electrcty s generated n large power plants, sent to the consumpton areas through Htransmsson lnes, and delvered to the consumers through a passve dstrbuton nfrastructure that nvolves H, M and networks. In ths paradgm power Manuscrpt receved March 5, 016;. Comparson of voltage stablty ndces and ts enhancement usng Dstrbuted Generaton Kumaraswamy Research scholar JNTUH Hyderabad (e-mal:kumarswamy04@gmal.com ) S.Tarakalyan professor and HoD department of EEE JNTUH, Hyderabad.( e-mal:tarasunder98@yahoo.co.n) B. enkata prasanth professor & HoD, department of EEE IS nsttute of technology ongole.( e-mal:bvenkataprasanth@gmal.com). flows only n one drecton from the power staton to the network and to the consumer. The DG term s used to descrbe small dstrbuton system close to the pont of consumpton. Such generators may be owned by a utlty or more lkely by a customer who may use the entre porton or perhaps all of t to the local utlty combuston turbne generators, nternal combuston engnes and generators, photovoltac panels, and fuel cells. Solar thermal converson, strlng engnes, are consdered as DG. When the penetraton of DG s hgh, the generated power of DG unts not power flow n the dstrbuton network consequence, the connecton of DG to the grd may dfferent techncal ssues, e.g. voltage profles qualty, stablty etc..[3] In spte of the benefts of utlzng DG unts wthn of the system effcency and the mprovements n the techncal and operatonal challenge unts nto M dstrbuton networks are needed. Moreover, n more detals wth respect to the generaton types. Optmzaton of the M dstrbuton networks wth a large penetraton of DG s Also needed therefore the utltes can get more benefts[4]. Dstrbuted generaton can be consdered as takng power to the load. Dstrbuted generator promses to generate electrcty wth hgh effcency and low polluton. Unlke large central power plants, Dstrbuted generator can be nstalled at or near the load. Dstrbuted generator ratngs range from 5 kw up to 100 MW. Mantenance cost for embedded generaton such as fuel cells and photovoltac s s qute low because of the lack of movng parts. Several recent developments have optmstc the entry of power generaton and energy storage at the dstrbuton level. Some of the maor ones are lsted below. Wth expanded choce, customers are demandng customzed power supples to sut ther needs. Advent of several technologes wth reduced envronmental mpacts and hgh converson effcences. Advent of effcent and cost-effectve power electronc nterfaces to mprove power qualty and relablty. Ablty to effectvely control a number of components and subsystems usng state-of-the-art computers to manage loads, demands, power flows, and customer requrements. Several Dstrbuted generaton technologes are under varous stages of development. They nclude mcro turbnes, photovoltac systems (), wnd energy converson systems (WECS), gas turbnes, gas-fred IC engnes, desel engnes, and fuel cell systems. At present, wnd energy has become the most compettve among all renewable energy technologes. Integraton of DG nto an exstng utlty can ISBN: ISSN: (rnt); ISSN: (Onlne) WCE 016

2 roceedngs of the World Congress on Engneerng 016 ol I WCE 016, June 9 - July 1, 016, ondon, U.K. result n several benefts. These benefts nclude lne loss reducton, reduced envronmental mpacts, peak shavng, ncreased overall energy effcency, releved transmsson and dstrbuton congeston, voltage support. The Dstrbuted Generaton (DG) has created a challenge and an chance for developng varous novel technologes n power generaton. The work dscusses the prmary factors that have lead to an ncreasng nterest n DG. DG reduces lne losses, ncreases system voltage profle. II. RADIA DISTRIBUTION NETWORK The load flow of a power system provdes the steady state soluton through whch varous parameters lke currents, voltages, losses etc can be calculated. It s a very mportant and fundamental tool for the analyss of any power system and s used n the operatonal as well as plannng stages. Many methods such as Gauss-Sedel, Newton-Raphson are well reported to carry out the load flow of transmsson system. The use of these methods for dstrbuton system may not be advantageous because they are mostly based on the general meshed topology of a typcal transmsson system where as most dstrbuton systems have a radal or tree structure. Further dstrbuton system posses hgh R/X rato, whch cause the dstrbuton systems to be ll condtoned for conventonal load flow methods. Some other nherent characterstcs of electrc dstrbuton systems are Radal or weakly meshed structure. Multphase and unbalanced operaton. Unbalanced dstrbuted load. Extremely large number of branches and nodes. Wde-rangng resstance and reactance values. The effcency of the optmzaton problem of dstrbuton system depends on the load flow algorthm because load flow soluton has to run for many tmes. Therefore, the load flow soluton of dstrbuton system should have robust and tme effcent characterstcs. A method whch can fnd the load flow soluton of radal dstrbuton system drectly by usng topologcal characterstc of dstrbuton network [1, ] s used. In ths method, the formulaton of tme consumng Jacoban matrx or admttance matrx, whch are requred n the conventonal methods, s avoded. Ths method s explaned n bref. Mathematcal Model In fg 1, () () and () () are the voltage magntudes and phase angles of two buses and respectvely and current flowng through the lne s I(). The substaton voltage s assumed to be (1+0) p.u. Fg. 1 Equvalent crcut model of RDS of a typcal branch (a) Branch Currents From the electrc equvalent of a feeder branch shown n Fg. 1.1, we can wrte the load current and chargng current equatons (1.1) and (1.) respectvely for bus [5]. S() I() () * () ( ) () * =, 3, nd (.1) IC() y()() =, 3, nd.(.) Current through the branch s equal to the sum of the all buses load currents and chargng currents beyond the lne by ncludng the lne recevng end bus load current and chargng current Thus, the lne current followng through the lne can be computed usng eqn. (.3): I() I(k) IC(k) avalable k all the buses after the avalable k all the buses after the branch branch = 1,, 3. br (.3) where br s the number of branches (b) Bus oltages Therefore, the generalzed equaton of recevng end voltage, sendng end voltage, lne current and lne mpedance s () () - I()Z() = 1,, 3. br. (.4) where Z() r() x() Impedance of branch = 1,, 3. br (.5) where Impedance of branch (c) Real and Reactve ower osses The real and reactve power loss of each branch s gven by: loss () I() r() = 1,, 3. br (.6) loss() I() x() = 1,, 3. Br--- (.7) (d) roposed Soluton Technque convergence crteron. Here p.u. s taken as tolerance value. The soluton process can easly be carred out by fndng the branch currents computed for each branch by addng load current and chargng current of buses beyond that partcular branch. These branch currents of the branches are dentfed wth the help of bus dentfcaton algorthm, whch s explaned n the followng secton. ISBN: ISSN: (rnt); ISSN: (Onlne) WCE 016

3 roceedngs of the World Congress on Engneerng 016 ol I WCE 016, June 9 - July 1, 016, ondon, U.K. (e) rocess of buses dentfcaton beyond a partcular branch Consder the sngle lne dagram of 16-bus radal dstrbuton system whch s shown n Fg. Buses 1, and 3 are consdered as substatons, and remanng as load buses. Fg. Sngle lne dagram of 16-node Radal Dstrbuton system Consder the sngle lne dagram of 33-bus radal dstrbuton system whch s shown n Fg.3 Bus 1, are consdered as substaton, and remanng as load buses. transmsson lnes and other elements by ther protectve systems. The man factor causng nstablty s the nablty of the power system to meet the demand for the reactve power. The reactve power can be suppled by generators through transmsson networks or compensated drectly at load buses by compensators such as shunt capactors. There are two sde effects of reactve power transmsson: transmsson losses and voltage drops. In response to a dsturbance, power consumed by the loads tends to be restored by the acton of motor slp adustment, dstrbuton voltage regulators, tap-changng transformers and thermostats. Therefore, restored loads ncrease the stress on the hgh voltage network by ncreasng the reactve power consumpton and causng further voltage reducton [7]. It s udged that a system s voltage unstable f, for at least one bus n the system, the bus voltage magntude decreases as the reactve power necton n the same bus s ncreased [5]. The term voltage collapse refers to the process by whch the sequence of events accompanyng voltage nstablty leads to a blackout or abnormally low voltages n a sgnfcant part of the power system [7]. In complex practcal power systems, many factors many factors contrbute to the process of system collapse because of voltage nstablty: strength of transmsson system, powertransfer levels, load characterstcs, generator reactve power capablty lmts and characterstcs of reactve power compensatng devces [5]. Fg. 3 Sngle lne dagram of 33-node Radal Dstrbuton system Consder the sngle lne dagram of 33-bus radal dstrbuton system whch s shown n Fg.. Bus 1, are consdered as substaton, and remanng as load buses. III. OTAGE STABIITY oltage stablty states that voltage stablty s the ablty of a power system to mantan steady acceptable voltage at all buses n the system under normal operatng condtons and after beng subected to a dsturbance. A system enters a state of voltage nstablty when a dsturbance, ncrease n load demand or change n system condtons, causes a progressve and uncontrollable drop n voltage [5]. oltage stablty depends on the ablty to mantan or restore equlbrum between load demand and load supply from the power system. Accordng to [6], voltage nstablty stems from the attempt of load dynamcs to restore power consumpton beyond the capablty of the combned transmsson and generaton system. Instablty occurs n the form of a progressve fall or rse of voltages of some buses. A possble outcome of voltage nstablty s loss of load n an area, or trppng of ISBN: ISSN: (rnt); ISSN: (Onlne) OTAGE STABIIYY INDICES: In voltage stablty analyss, t s useful to assess voltage stablty of power systems by means of voltage stablty ndces (SI), scalar magntudes that can be montored as system parameters change. Operators can use these ndces to know how close the system s to voltage collapse n an ntutve manner and react accordngly. a) Bus voltage computaton ndces: -INDEX: The ndex was frst descrbed n [8] and t s based on a hybrd representaton of the transmsson system wth the followng set of equatons: l I ZU FG I H I K Y (3.1) G G G GG G Where, I are the voltage and current vectors at the load buses G, I G are the voltage and current vectors at the generator buses Z U,K G,F G,Y GG are the sub-matrces of the hybrd matrx H. Thus, the ndex can also be expressed n power terms as followng: The H matrx can be evaluated usng a partal nverson of the Y bus matrx, where the voltages at the load buses are WCE 016

4 roceedngs of the World Congress on Engneerng 016 ol I WCE 016, June 9 - July 1, 016, ondon, U.K. exchanged aganst ther currents. Ths representaton can then be used to defne a voltage stablty ndcator at each load bus. S 0 1 Y (3.) SJ S S corr (3.3) ZS Scorr (3.4) Z The complex term Scorr represents the contrbutons of the other loads n the system to the ndex evaluated at node. When a load bus approaches a collapse pont, the ndex value s 1. The nodes wth the hgher value are consdered the weaker buses of the system. OTSGE STABIITY INDEX (SI): SI s defned by mathematcal expresson, SI(r)= S r r -r(r+x) Z (+)-(3.5) b) INE STABIITY INDICES: Most of lne stablty ndces are formulated based on the power transmsson concept n a sngle lne. A sngle lne n an nterconnected network s llustrated n Fg 4. Fg. 4 Two bus system Where, s and r are the sendng end and recevng end voltages, respectvely. δs and δr are the phase angle at the sendng and recevng buses. Z s the lne mpedance. R s the lne resstance. X s the lne reactance. θ s the lne mpedance angle. r s the reactve power at the recevng end. r s the actve power at the recevng end. Ths ndex defned n [9] uses the same concept as n the prevous ndex mn. Usng the same notaton, the proposed ndex s calculated as followng: X X 4( )( )...( 3.7) () Fast oltage Stablty Index (FSI) Ths ndex proposed by [10] stands for Fast oltage Stablty Index (FSI) and t s also based on the concept of power flow through a sngle lne. 4Z FSI s r...( 3.8) I: OTIMA DG OCATION AND SIZE FOR OSS REDUCTION Due to the ncrease n power demand, the need for generaton of power s steadly ncreasng. To gve unnterrupted servce to consumers, t s necessary to ncrease the penetraton of dstrbuted generaton nto dstrbuton systems. Due to the problems of poor voltage regulaton, shortage of transmsson capactes and ncreased envronmental concerns, the conventonal methods of supplyng power could not meet the whole demand. To overcome ths, dstrbuted generator (DG) has become the alternatve source for power supply[1]. Apart from meetng the energy demand, the optmal locaton and sze of DG unts can reduce dstrbuton losses; mprove voltage stablty and voltage profle. If DG unts are mproperly allocated and szed, the reverse power flow from larger DG unts can lead to hgher system losses. oss mnmzaton s an mportant factor n plannng and operaton of DG. Many technques have been proposed n lterature to fnd the optmal allocaton and optmal sze of DG. The exhaustve load flow (EF) method and an mproved analytcal (IA) method are used for allocaton of DG. Unlke the prevous methods, DG s capable of nectng both real and reactve powers. As DG unts can supply a porton of total power to loads, the feeder current reduces from the source to the DG locaton. a) ROOSED METHODOOGY: The exact power loss equaton s gven by N N...(4.1 ) 1 1 () mn Index Ths ndex proposed n [8] s based on the concept of power flow through a sngle lne and adoptng the technque of reducng a power system network nto a sngle lne. Where r cos, sn r mn [ () Index : s 4Xr sn( - )] 1...( 3.6) the complex voltage at the bus th; r x Z The th element of [Zbus] mpedance matrx ISBN: ISSN: (rnt); ISSN: (Onlne) WCE 016

5 roceedngs of the World Congress on Engneerng 016 ol I WCE 016, June 9 - July 1, 016, ondon, U.K. and The actve power nectons at the th and th buses, respectvely; and The reactve power nectons at the th and th buses, respectvely; N the number of buses. SIZING AT ARIOUS OCATIONS Assumng a = (sgn) tan(cos-1(fdg)), the reactve power output of DG s expressed by eqn. (4.) n whch DG = adg... (4.) sgn = +1: DG nectng reactve power; sgn = 1: DG consumng reactve power; FDG s the power factor of DG. The actve and reactve power nected at bus, where the DG located, are gven by eqn. (4.3) and eqn. (4.4), respectvely, = DG - D.. (4.3) = DG - D = adg - D.(4.4) The power factor of DG depends on operatng condtons and type of DG. When the power factor of DG s gven, the optmal sze of DG at each bus for mnmzng losses can be found n the followng way. 1) Type 1 DG: For Type 1 DG, power factor s at unty,.e., FDG = 1, a = 0. The optmal sze of DG at each bus for mnmzng losses can be gven by equaton (4.5) DG D 1 D N...(4.5) 1 ) Type DG: Assumng FDG = 0 and a =, The optmal sze of DG at each bus for mnmzng losses s gven by reduced equaton (4.6). DG D 1 D N...(4.6) 1 3) Type 3 DG: Assumng 0 < FDG < 1, sgn = +1 and a s a constant, the optmal sze of DG at each bus for the mnmum loss s gven by (4.7) and (4.4), respectvely. ISBN: ISSN: (rnt); ISSN: (Onlne) DG a a D D F D a D D D D D X ay...(4.7) 4) Type 4 DG: Assumng 0 < FDG < 1, sgn = 1 and a s a constant, the optmal sze of DG at each bus for the mnmum loss s gven by (4.7) and (4.4), respectvely. OTIMA OWER FACTOR: Consder a smple dstrbuton system wth two buses, a source, a load and DG connected through a transmsson lne as shown n Fg. 5. Bus Bus 1 R+X Source DG + DG D + D Fg.5. Smple dstrbuton system wth DG. The power factor of the sngle load (FD ) s gven by equaton (4.8) The power factor of the sngle load (FD ) s gven by...(4.8) It can be proved that at the mnmum loss occur when power factor of DG s equal to the power factor of load as gven by equaton (4.9). F D F DG DG DG DG...(4.9) In practce, a complex dstrbuton system ncludes a few sources, many buses, many lnes and loads[1-13]. The power factors of loads are dfferent. If each load s suppled by each local DG, at whch the power factor of each DG s equal to that of each load, there s no current n the lnes. The total lne power loss s zero. The transmsson lnes are also unnecessary. However, that s unrealstc snce the captal nvestment cost for DG s too hgh. Therefore, the number of nstalled DGs should be lmted. To fnd the optmal power factor of DG for a radal complex dstrbuton system, fast and repeated methods are proposed. It s nterestng to note that n all the three test systems the WCE 016

6 roceedngs of the World Congress on Engneerng 016 ol I WCE 016, June 9 - July 1, 016, ondon, U.K. optmal power factor of DG (Type 3) placed for loss reducton found to be closer to the power factor of combned load of respectve system. Fast Approach: ower factor of combned total load of the system (FD ) can be expressed by equaton (4.8). In ths condton, the total actve and reactve power of the load demand are expressed as n D D 1 n D D 1 (4.10) (4.11) The possble mnmum total loss can be acheved f the power factor of DG (FDG) s quckly selected to be equal to that of the total load (FD). That can be expressed by equaton (4.1). Repeated Approach: In ths method, the optmal power factor s found by calculatng power factors of DG (change n a small step of 0.01) near to the power factor of combned load. The szes and locatons of DG at varous power factors wth respect to losses are dentfed from equaton (4.3). The losses are compared together. The optmal power factor of DG for whch the total loss s at mnmum s determned. FDG D (4.1) RESUTS AND ANAYSIS OF ARIOUS INE AND BUS STABIITY INDICES: Smulatons were run on the 16-buses,33-buses and,69- Buses Radal dstrbuton Test System. The real and reactve power losses obtaned through load flow. The results shown n table 1 are the assorted types of lne ndces and bus ndces. mn, FSI and are the lne ndces, where as -ndex s the bus ndces. To examne the voltage stablty margn the value of varous lne ndces s 1. If all the lne ndces are 1,then the system s secure regon(stable) otherwse t s unstable.,e lne ndces 1. Fg. shows the sngle lne dagram of 16-bus radal dstrbuton system. Buses 1, and 3 are consdered as substatons, and remanng as load buses. by observng the results the all the load buses values are less than one so we conclude that all the lne and bus ndces are stable. Total real power load : kw Total reactve power load : kar Total real power loss : kw Total reactve power loss : kar Mnmum oltage : Table 1: Comparatve Results of varous ndces for voltage stablty analyss-16 bus system S.No mn FSI -Index Table : Comparatve Results of varous ndces for voltage stablty analyss-33 bus system S.No mn FSI -Index ISBN: ISSN: (rnt); ISSN: (Onlne) WCE 016

7 roceedngs of the World Congress on Engneerng 016 ol I WCE 016, June 9 - July 1, 016, ondon, U.K I. CONCUSION Ths paper analyzes the performance of dfferent voltage stablty ndces and ts enhancement by placng optmal locaton of Dstrbuted generaton (DG). Ths work proposes analytcal expressons for fndng optmal sze and power factor of dstrbuted generaton (DG) unts. DG unts are szed to acheve the hghest loss reducton n dstrbuton networks. The large deployement of dstrbuted generaton (DG) sources n dstrbuton network can be an effcent soluton to overcome power system techncal problems and economcal challenges. References [1] Acharya N,Mahat, thulananthann,(006) An Analytcal Approach for DG Allocaton n prmary Dstrbuton network, nt J. Electrc power Energy syst., ol.8,pp [] Borges c, Falcao D,(006) optmal, Dstrbuted Generaton allocaton for relablty, losses,and voltage mprovement, nt J. Electrc power Energy syst., ol.8,pp [3] Hedayat H, Nabavnak sa,akbarmad a,(008) A method for placement of DG unts n dstrbuton networks,ieee transactons on power delvary,vol.3, p.p [4] Khales N, ReZae N, Haghfam M-R,(011) DG Allocaton wth Applcaton of dynamc programmng for loss reducton and relablty mprovement, nt J. Electrc power Energy syst., ol.33,pp [5]. Kundur, ower System Stablty and Control. New York: McGraw-Hll, [6] T. an Cutsem and C. ournas, oltage Stablty of Electrc ower Systems. Norwell, MA: Kluwer, [7]. Kundur et al., Defnton and classfcaton of power system stablty, IEEE Transactons on ower Systems, ol. 19, No., May 004, pp [8]. Kessel and H. Glavtsch, Estmatng the voltage stablty of a power system, IEEE Transactons on ower Delvery, ol.wrd-1, No.3, July 1986, pp [9] B. Gao, G.K. Morson and. Kundur, Towards the development of a systematc approach for voltage stablty assessment of large-scale power systems, IEEE Transactons on ower Systems, ol. 11, No. 3, August 1996, pp [10] C. W. Taylor, ower System oltage Stablty. New York: McGraw-Hll, [11] F. Karbalae, H. Soleyman and S. Afsharna, A comparson of voltage collapse proxmty ndcators, IEC 010 Conference roceedngs, 7-9 Oct. 010, pp [1] M. E.Baran and F. F.Wu, Network reconfguraton n dstrbuton systems for loss reducton and load balancng, IEEE Trans. ower Del., vol. 4, no., pp , Apr [13] J. Y. Fan,. Zhang, and J. D. McDonald, Dstrbuton network reconfguraton: sngle loop optmzaton, IEEE Trans. ower Syst., vol. 11, no. 3, pp , Aug ISBN: ISSN: (rnt); ISSN: (Onlne) WCE 016