A New Framework for Reactive Power Dispatch in Electricity Markets

Transcription

1 Research Journal of Appled Scences, Enneern and Technoloy 5(: , 13 ISSN: ; E-ISSN: Maxwell Scentfc Oranzaton, 13 Submtted: Aprl 18, 1 Accepted: June 1, 1 ublshed: January 11, 13 A New Framework for Reactve ower Dspatch n Electrcty Markets 1 Javad Saeb, Hossen Hakmolla, Hassan Soleman Amr and 1 H. Ghasem 1 School of Electrcal and Computer Enneern (ECE, Unversty of Tehran, Tehran, Iran West of Tehran rovnce ower Dstrbuton Company (WTDC, Tehran, Iran Abstract: Ths study proposes a reactve power dspatch model wthn the context of electrcty markets consdern both techncal and economcal ssues. The model utlzes a new metrc ntroduced here for reactve power reserve manaement n voltae control areas. Besdes mnmzn total reactve power payments, the obectve functon manaes for an optmum reserve n each voltae control area. The proposed reactve power dspatch model s decoupled from actve power dspatch and the enerators' actve power s assumed fxed durn the procedure. The relaton between actve power and reactve power of a synchronous enerator s also ncluded n the model by consdern the enerators capablty curves. The CIGRE 3-bus test system s used to demonstrate the feasblty and aspects of the proposed model. Keywords: Ancllary servces, electrcty market, reactve power markets, reactve power reserve INTRODUCTION Reactve power dspatch s a short-term plannn actvty carred out by system operators n order to ensure secure power system operaton. Reactve power has a snfcant effect on system securty as t s drectly assocated wth power system voltae stablty. For nstance, voltae collapse usually occurs n heavly loaded systems and n areas that do not have suffcent reactve power reserve. As ndcated by the Western Electrc Coordnatn Councl (WECC, reactve power reserve n the system can be used as an ndcator for voltae stablty assessment (Abed, Therefore, n order to avod voltae problems across the system, specal case must be taken n obtann suffcent reactve power reserve n all areas of the power system. Due to the nature of reactve power, reactve power cannot be transmtted over lon dstances. So, a power system s usually dvded nto several Voltae Control Areas (VCA that are ndependent from the vewpont of tradn reactve power wthn them (Zhon and Bhattacharya,. In Don et al. (5, reactve power reserve has been used to mprove voltae stablty for the whole system; however, the local nature of reactve power has not been consdered. In ths study, a new metrc for reactve power reserve s ntroduced that consders voltae control areas n reactve power manaement. Federal Enery Reulatory Commsson (FERC has reconzed reactve power and voltae control servces from synchronous enerators as one of sx ancllary servces and the provders are elble for fnancal compensaton. So, n a dereulated envronment, the cost of provdn reactve power must be consdered n reactve power dspatch. Reactve power ancllary servces can be provded based on two dfferent tme horzons: A lon-term stae that s named as reactve power procurement and a short-term stae that s reactve power dspatch (El- Samahy et al., 8. Reactve power procurement s done on a seasonal bass; contracted enerators and prce components of reactve power are determned. In reactve power dspatch, a rescheduln of reactve power must be done based on real-tme loadn condtons. Dfferent obectve functons s used n prevous studes that ust consder techncal ssues assocated wth reactve power dspatch, such as transmsson losses mnmzaton (Lamont and Fu, 1999; Rahel et al., 1, or mzaton of system loadablty to mnmze the rsk of voltae collapse (Mlano et al., 4. In El-Samahy et al. (7, a cost based reactve power dspatch s proposed that only mnmzes total payments to fnd optmal dspatch of reactve power; however, t does not consder techncal ssues to provde a secure operaton condton. In ths study, a new framework for reactve power dspatch n electrcty markets s proposed. In order to consder both techncal and economcal ssues assocated wth reactve power dspatch, the proposed framework consders reactve power reserve manaement as well as reactve power payments. REACTIVE OWER MARKET In a compettve electrcty market, the Independent System Operator (ISO should provde reactve power Correspondn Author: Javad Saeb, School of Electrcal and Computer Enneern (ECE, Unversty of Tehran, Tehran, Iran 38

2 Res. J. Appl. Sc. En. Technol., 5(: , 13 F. 1: Tme scale of reactve power ancllary servce market support from servce provders at mnmum cost whle ensurn a secure operaton of the power system. If reactve power market s run n real tme, ssues such as exercsn reactve market power and system securty problems may arse (El-Samahy et al., 8. Thus, a reactve power procurement market s run whch works on a seasonal tme horzon; contracted enerators and reactve power prce components are determned. Then, n real tme, accordn to pre-determned reactve power prces, reactve power dspatch s run to determne optmal VAr dspatch and payments. In ths study, VAr dspatch s decoupled from MW dspatch,.e., after runnn actve power dspatch and determnn the real power eneraton of each enerator, reactve power dspatch s run and the actve power eneraton of enerators s fxed durn the procedure. The decoupln of real and reactve power s suested n apalexopoulos et al. (1989, El-Keb and Ma (1997 and aucar and Rder (1. Fure 1 llustrates the tme scale of reactve power market. It s mportant to note that a coupled dspatch, smultaneously dspatchn actve power and reactve power, ets the soluton closer to the optmal; however, computatonal burden becomes an ssue for real power systems. Decoupled reactve and actve OF provdes the requred flexblty for spot market applcatons; and reduce the problem assocated wth model complexty (El-Samahy et al., ROOSED REACTIVE OWER DISATCH In ths study, reactve power reserve manaement s used to meet system securty requrements. For ths purpose, a new metrc s ntroduced that consders VCAs n reactve power reserve manaement. Reactve power cost mnmzaton s also ncluded n the obectve functon of reactve power dspatch problem. Reactve power reserve manaement: The reactve power reserve s the ablty of the enerators to support bus voltaes under hh loadn condtons or contnences (Don et al., 5. Reactve power cannot be transmtted over lon dstances thus defnn and calculatn reactve power reserve s more complcated compared to actve power reserve. Due to the nature of reactve power, reactve power and voltae control servces are requred to be provded locally. Also, to restrct the effects of reactve market power, power systems must be dvded nto VCAs and for each VCA, a unform prce s determned. A VCA s ndependent of tradn reactve power n other VCAs and the reactve power transfers between VCAs are normally low. So, reactve power reserve n each VCA must be manaed separately; thus, ths matter must be seen n reactve reserve manaement problem as well. Varous defntons of reactve power reserve can be founded n Leonard and Aarapu (8. The effectve reactve power reserve s nterestn because t

3 consders the reactve power dspatch at the pont of voltae collapse and hence ves the actual amount of reactve reserve. It s ood to menton that all defntons of reactve power reserve reported n Leonard and Aarapu (8 can be used here. Effectve reactve power reserve of a enerator s defned as the dfference between reactve power output of the enerator at the pont of voltae collapse and current reactve power eneraton (V col -. The V col can be calculated by an OF wth the obectve ben, for nstance, mzn voltae stablty marn assumn unform ncrease n loads (El-Samahy et al., 8; Lamont and Fu, In Don et al. (5, reactve power reserve has been used to mprove voltae stablty for the whole system wthout usn VCAs. Here, a Normalzed Effectve Reactve ower Reserve (NERR s ntroduced, whch can be used to take nto account VCAs n the optmzaton process. For ths purpose, the reactve reserve of each VCA s dvded by ts mum reactve reserve wthout consdern other areas. The mum reactve power reserve * Rk at area k s calculated usn an OF as follows: * Vcol Max R ( k G V S k Res. J. Appl. Sc. En. Technol., 5(: , 13 Normally, α = 1 and β k = ; however, the ISO may chane these coeffcents based on ts 38 (1 V V Y cos(, ( G D V V Y sn(, (3 D mn, (4 mn V V, (5 S, (6, By solvn ths optmzaton problem for each area k, we can defne NERR area k as follows: (1 k Vcol NERRk.( * (7 Rk k where, α : Coeffcent to show the mportance of enerator ( α 1 β k : Coeffcent to show the mportance of area k ( β k 1 from the ISO s perspectve understandn from the characterstc of the power system under study. Then, total NERR of system can be calculated: NERR NERR k (8 k Althouh calculatn NERR are decoupled from MW dspatch and are less complex compared to solvn coupled MW and MVA dspatch, but an OF s requred to be solved to fnd for each area and hence computatonal tme maybe an ssue n real-tme. To address ths ssue, we suest that for normalzn reactve power reserve of each VCA, ts reactve power reserve can be dvded by ts total reactve power capacty,.e., mn (, nstead of. k By usn normalzed value NERR n each VCA, we can et a better dstrbuton of reactve reserve n each VCA when total NERR of the system s mzed. Ths s due to the fact that n the obectve functon, reserves n all VCA are treated equally. Note that f nstead of NERR the values of reactve reserve are used, some areas wll et hher whle others not havn enouh reserve. Reactve power payments: In a dereulated envronment, the cost of provdn reactve power servces must be also consdered n reactve power dspatch problem. Fure llustrates capablty curve of a synchronous enerator. If real power and termnal voltae are fxed, the armature and feld wndn heatn lmt curves determne the capablty of the enerator to enerate reactve power (Ftzerald et al., 199. These lmts are determned n F., where, V t s the termnal voltae of the enerator, I a s the armature current, E f s the exctaton voltae and X 5 s the synchronous reactance. The GR s determned by ntercepton of the two curves. Assume that real power output at the operatn pont A s GA ; f GA GR, the enerator's feld heatn lmt determnes the mum reactve power capablty of the enerator ( GA ; whereas, when GA GR, ths lmt s mposed by the enerator's armature wndn heatn lmt. Based on a typcal capablty curve for a enerator (F., three reons of reactve power producton n whch the enerator s elble to receve payments can be defned. Note that the reon ( Gblead G Gbla n F. s dentfed as mandatory reon by the ISO and no payment s ssued for producn/absorbn reactve power n ths reon (Zhon and Bhattacharya,.

4 Res. J. Appl. Sc. En. Technol., 5(: , 13 power output $/. The payment to enerators n ths reon can be defned as: III.5.( 3.( G G Gbla GA (11 The prce components of reactve power are determned n lon-term procurement market. Determnn reactve power prces n a tmeframe dfferent from that of actve power reduces prce volatlty and thus helps reduce prce spkes. To restrct the effects of reactve market power, unform prces are determned for each VCA. The ISO seeks to fnd an optmal reactve dspatch that mnmzes the follown cost functon: F. : Capablty curve of a synchronous enerator Reon I ( Gmn G Gblead : In ths reon, enerators expect avalablty payment ( $ and payment for ncrease of loss n wndns for absorbn reactve power more than the mandatory leadn value $/. The payment to enerators n ths reon can be defned as:.( (9 I 1 G Gblead Reon II ( Gbla G GA : It denotes the reactve power that a enerator provdes more than mandatory lan value ( Gbla wthout rescheduln ts real power output. In ths reon, the enerators expect an avalablty payment as well as a payment for the ncreased losses n wndns $/ The payment to enerators n ths reon can be defned as: ayment G3 G1. G (1 roposed reactve power dspatch: The reactve power dspatch s formulated as an Optmal ower Flow (OF wth prmary obectve of mzn NERR to ensure hher voltae stablty marn. A second obectve s to mnmze total payments subect to varous securty constrants. Therefore, reactve power dspatch s formulated as: z Max w1 NERR w ayment (13 Note that n order to mze NERR and mnmze payments, one should choose w1 and w. The constrants to the above OF problem are: G V V Y cos(, (14 D II.( (1 G Gbla Reon III ( GA G GB : It denotes the reactve power that a enerator supples at the expense of reducn ts real power outputs. As mentoned before, the mum reactve power capablty of the enerator at operatn pont A s GA. At ths pont, f more reactve power s requested by the ISO, the enerator must decrease ts real power output to GB to be able to enerate GB. So, n ths reon, the enerators expect an avalablty payment for provdn reactve power support and an opportunty cost payment for reducn ther real 383 V sn(, G D V Y (15 V mn mn, VtE ( X ( V t I s a f V V, ( ( (16 V X, t s, GR GR (17

5 S Res. J. Appl. Sc. En. Technol., 5(: , 13 S, (18, Gblead mn G 1, (19 Gbla G GA, ( GA G1 GB, (1 G1.( G G3 ( ( G GA G3. (3 G1 G G 3 (4 Note that constrants 19 to 4 are added to determne the reon of reactve power eneraton of enerator. IMLEMENTATION AND TEST CASE RESULTS The CIGRE 3-bus test system (F. 3 s used here to test the feasblty of usn the proposed reactve power dspatch model (Walve, The test case has enerators that all are assumed to be elble for fnancal compensaton for provdn/absorbn reactve power n all three reons of operaton as dscussed before; so Gblead and bla are assumed to be zero for all enerators wthout any loss of eneralty. The system s splt nto three zones or voltae control areas as reported n Zhon et al. (4. The optmzaton models, whch are essentally a non-lnear prorammn, are smulated n GAMS and solved usn the MINOS solver. To llustrate the effcency of usn NERR for reactve power reserve manaement nstead of usn absolute reactve reserve value that s used n Don et al. (5, two separate OFs wth mentoned obectve functons are executed for CIGRE test system. Fure 4 shows the normalzed amount of reactve reserve n each VCA. As shown n F. 4, usn NERR n the obectve functon of OF leads to better dspatch of reserve n VCAs. Snce reactve power must be provded locally, each VCA s ndependent of tradn reactve power n other VCAs. So, provdn a desrable marn for reactve reserve n each VCA s essental for ncreasn voltae stablty and avodn voltae collapse. Accordn to F. 4, reactve reserve 384 F. 3: CIGRE 3-bus system NERR Absolute value Normalzed value F. 4: Comparson of usn normalzed value of reserve and absolute value n each VCA wll be reater than 6% of mum possble amount of reserve for that VCA (* Rk by usn the concept of NERR; whereas, f the absolute reserve value s used, ths value for Zone B wll be less than 3% and t can create voltae securty problem n Zone B. The proposed reactve power dspatch model s appled on the CIGRE test system. Table 1 shows the nput parameters for the dspatch model 13 to 4 that are the results of real power dspatch and lon-term reactve power procurement (El-Samahy et al., 8. Assumn the obectve functon descrbed by (13, w 1 and w are selected as 1 and -.1, respectvely there

6 Res. J. Appl. Sc. En. Technol., 5(: , 13 Table 1: Input parameters for the dspatch model Zone Bus o ρ ρ 1 ρ ρ A B C condtons, a real power rescheduln may be needed for provdn more reactve power. The ISO would compensate the enerators that are requred to provde more reactve power than A as shown n F. at the Market Clearn rce (MC of real power market. Table 3 shows the results of comparson of the proposed model wth the model n El-Samahy et al. (7. The model proposed n El-Samahy et al. (7 uses reactve power payments as ts obectve functon. We can see from the results that consdern payments only n the obectve functon of reactve power dspatch cannot uarantee a desrable amount of reactve reserve n each VCA. For example, n ths case, the proposed dspatch model secures NER above 5% for all the zones; whle the Model n El-Samahy et al. (7 does not provde ths for Zone C. However, t provdes hher NERR for Zone B (79% compared to 76%. The total reactve power reserve n the system s ncreased by 8.8% at the cost of a 4.3% ncrease n total payments to reactve power provders. Table : Results of proposed reactve power dspatch model Zone Bus G1 G G3 G NERR A B C Table 3: Comparson of the proposed model wth El-samahy model Model n El-samahy arameter roposed model et al. (7 NERR A NERR B NERR C Reactve reserve (p.u. ayment ($ by than total payments. The lower and upper bounds of bus voltaes are selected as.95 and 1.5 n p.u, respectvely. Table shows the results of the proposed reactve power dspatch model. It can be seen from the results that there are no enerators requred to operate n Reon III; hence, no MW rescheduln s requred n reactve power dspatch. In hh loadn 385 CONCLUSION In ths study, a reactve power dspatch model s proposed that consders both techncal and economcal ssues. To ensure system voltae stablty, reactve power reserve manaement s consdered n the model. For ths purpose, Normalzed Effectve Reactve ower Reserve (NERR s ntroduced as a new metrc for reactve power reserve manaement that consders voltae control areas. The ISO s obectve functon n reactve power dspatch s defned usn NERR and total payment to enerators for provdn reactve power servce. The results show that usn NERR for manaement of reactve reserve s more effcent than absolute value of reserve. The proposed reactve power dspatch model s decoupled from actve power dspatch and has not the complexty of coupled OFs. The proposed model determnes an optmal reactve power dspatch by provdn a desrable reactve reserve marn for each VCA of the system thus makn t useful for systems wth several VCAs and local VAr requrements. Vcol NOMENCLATURE Reactve power output of enerator at the pont of voltae collapse * Rk Maxmum reactve power reserve n area k wthout consdern other areas G G D Reactve power eneraton at Bus n p re-determned actve power eneraton of the enerator that s obtaned from the results of the actve power aucton market Actve power demand at Bus n p.u

7 Res. J. Appl. Sc. En. Technol., 5(: , 13 D V δ Y θ Reactve power demand at Bus n p.u Bus voltae mantude n p.u Bus voltae anle n p.u Element of admttance matrx n p.u Anle of Y n radans mn Mnmum and mum reactve power of enerator n p V mn,v Mnmum and mum allowable voltae levels at Bus n p.u S mn Flow lmt for the connectn Bus to Bus n p.u NERR k Normalzed effectve reactve power reserve of area k GR Real power ratn of synchronous enerator GA GB Maxmum reactve power lmt of a enerator wthout reducton n real power eneraton Maxmum reactve power lmt of a enerator wth reducton n real power eneraton ρ Avalablty prce for enerator n $ ρ 1 ρ ρ 3 G1 G G3 rce of losses n under-exctaton reon for enerator n $/MVAr rce of losses n over-exctaton reon for enerator n $/MVAr Lost opportunty prce for enerator n $/MVAr Reactve power eneraton n reon I n p.u Reactve power eneraton n reon II n p.u Reactve power eneraton n reon III n p.u REFERENCES Abed, A.M., WSCC voltae stablty crtera, under-voltae, load sheddn stratey and reactve power reserve montorn methodoloy. IEEE ower Enneern Socety Summer Meetn, Jul. 18-, pp: Don, F., B.H. Chowdhury, M. L. Crow and L. Acar, 5. Improvn voltae stablty by reactve power reserve manaement. IEEE Trans. ower Syst., (1: El-Keb, A. and X. Ma, Calculatn short-run marnal costs of actve and reactve power producton. IEEE Trans. ower Syst., 1(: El-Samahy, I., C. Canzares, K. Bhattacharya and J. an, 7. An optmal reactve power dspatch model for dereulated electrcty markets. IEEE ES, Tampa, Jul. 4-8, pp: 1-7. El-Samahy, I., K. Bhattacharya, C. Canzares, M.F. Anos and J. an, 8. A procurement market model for reactve power servce consdern system securty. IEEE Trans. ower Syst., 3(1: Ftzerald, E., C. Knsley Jr and S.D. Umans, 199. Electrc Machnery. McGraw-Hll, New York. Lamont, J.W. and J. Fu, Cost analyss of reactve power support. IEEE Trans. ower Syst., 14(3: Leonard, B. and V. Aarapu, 8. Investaton of varous Generator Reactve ower Reserve (GRR defnton for onlne voltae stablty/securty assessment. ower and Enery Socety General Meetn-Converson and Delvery of Electrcal Enery n the 1st Century, Jul. -4, pp: 1-7. Mlano, F., C.A. Canzares and M. Invernzz, 4. Voltae stablty constraned OF market models consdern N-1 contnency crtera. Electr. ower Syst. Res., 74(1: apalexopoulos, A.D., C.F. Imparato and F.F. Wu, Lare-scale optmal power flow: Effects of ntalzaton, decoupln and dscretzaton. IEEE Trans. ower Syst., 4: aucar, V. L. and M.J. Rder, 1. Reactve power prcn n dereulated electrcal markets usn a methodoloy based on the theory of marnal costs. roceedns of the IEEE Lare Enneern Systems Conference on ower Enneern, Jul , pp: Rahel, D., M. Khat, A. Chaker and R. Haïmour, 1. Reactve power optmzaton n transmsson power system usn nteror pont method. Int. Rev. Electr. En., 5(: Walve, K., Nordc 3A- A CIGRE Test System for Smulaton of Transent Stablty and Lon Term Dynamcs. Svenska Kranftnat. Zhon, J. and K. Bhattacharya,. Toward a compettve market for reactve power. IEEE Trans. ower Syst., 17(4: Zhon, J., E. Noble, A. Bose and K. Bhattacharya, 4. Localzed reactve power markets usn the concept of voltae control areas. IEEE Trans. ower Syst., 19(3: