REAL TME MONTORNG OF DSTRBUTON NETWORKS USNG NTERNET BASED PMU by Akanksha Eknath Pachpnde A Thess submtted to the Faculty of the Gaduate School of the Unvesty at Buffalo, State Unvesty of New Yok n patal fulfllment of the equements fo the degee of (Maste of Scence) Depatment of Electcal Engneeng
Acknowledgements t s wth mmense gattude that acknowledge the suppot and help of my advso, D. HyungSeon Oh, who contnually and convncngly conveyed a spt of adventue n egad to ths eseach and hs teachng. would le to thank my commttee membes, D. Albet Ttus and D. Jonathan Bd fo the gudance and pesstent help wthout whch ths dssetaton would not have been possble. am gateful to the faculty of Unvesty at Buffalo, SUNY and my fellow Buffalo Enegy Scence and Technology goup membes fo the mmense leanng oppotuntes. wll eman deeply ndebted to my paents D. Swat Pachpnde and D. Eknath Pachpnde, my sblngs and dea fends fo the contnued love and suppot.
Abstact Measuement of synchophasos can open up new avenues fo mpoved contol and stablty of the powe system. Phaso Measuement Unt (PMU) can measue postve sequence voltages and cuents along wth fequency and ate of change of fequency. They can dectly measue the state of the system thus elmnatng the need fo state estmaton altogethe, f the system s completely obsevable. Fo ths to become ealty a lage numbe of PMUs need to be deployed that would call fo lage upfont nvestments. Anothe possblty to ensue lage scale deployment of PMUs, s to fnd ways to make the technology moe affodable. Conventonal PMUs whch use global postonng system (GPS) fo tme synchonzaton have some lmtatons. Ths thess poposes a novel concept of PMU usng netwok tme potocol (NTP) to synchonze the local oscllatos of all the PMU. NTP s an attactve altenatve as t uses exstng LAN lnes fo ntenet communcaton and the exstng nfastuctue and has tme accuacy n mll-seconds (lowe than that of GPS). t does not eque, howeve, any majo nfastuctual changes. t s thus sutable fo montong systems that do not eque vey pecse tme stampng. We clam that a dstbuton netwok s an deal canddate fo such PMUs. Cuently dstbuton netwoks have spase montong by supevsoy contol and data acquston (SCADA) system whch has tme synchonzaton wndow of 2-3 seconds. Ths conceptual PMU s elaboated n the followng text and ts feasblty s evaluated. Compaatve measuement accuacy and cost of ths PMU s dscussed n detal n compason wth the conventonal PMUs. The new PMU offes sgnfcant benefts n tems of montong and contollng the dstbuton netwok. One applcaton consdeed n ths study s faultlocaton detecton.
Lst of Fgues Fgue 1: Conventon fo synchophaso epesentaton. Taken and modfed fom Ref [17]... 9 Fgue 2: A snusod wth a fequency s obseved. The phase angle Φ nceases unfomly n elaton to the fequency dffeence, (f f 0 )... 10 Fgue 3: Genec block dagam of PMU... 11 Fgue 4: Typcal synchophaso nstallaton at substaton... 12 Fgue 5: ntenet based PMU block dagam... 17 Fgue 6. Dagammatc depcton of NTP stata... 18 Fgue 7: Example of Synchonzaton Topologes of NTP clocks... 19 Fgue 8: Measung Delay and Offset... 19 Fgue 9: Netwok Tme Potocol... 20 Fgue 10: Genec block dagam of Gadget Box... 22 Fgue 11: Block dagam of the pototype desgn fo the poposed PMU... 23 Fgue 12: A dstbuton netwok substatons wth amount of montong pesent... 27 Fgue. 13. Fomat fo fles tansmtted fom and to PMUs. The numbes below the boxes ndcate length of the wod n bytes.... 28 Fgue 14. Offsets between the local clock and the PPS GPS... 30 Fgue 15: A sngle-lne-to-gound fault on phase A... 39 Fgue 16: Seach fo possble fault locatons... 40 Fgue 17: One-lne dagam fo a 14-Bus dstbuton feede wth two measuement ponts at Bus-1 and Bus-5, espectvely... 40 v
Lst of Tables Table 1: Cost beakdown of the pototype of ntenet based PMU 23 v
Table of Contents Acknowledgements Abstact Lst of Fgues v Chapte 1. ntoducton 1 Chapte 2. Backgound and Lteatue Suvey 3 2.1 Development n PMU technology ove the yeas 3 2.2 Applcatons of PMU n Powe systems 4 2.2.1 State estmaton 4 2.2.2 Powe system potecton 5 2.2.3 Powe system contol 7 Chapte 3. Theoy of conventonal PMU 8 3.1 Synchophasos 8 3.2 Genec model of PMU 10 3.3 Ovevew of synchophaso standad 13 Chapte 4. New conceptual ntenet based PMU 15 4.1 Concept of the ntenet based PMU 16 4.2 ntefacng NTP wth PMU 17 4.2.1 Obtanng tme-stamps 19 4.2.2 Obtanng PPS fom NTP 20 4.3. Desgn of Pototype 22 Chapte 5. Evaluaton of the poposed PMU vesus conventonal PMU 26 5.1 ntoducton 26 v
5.2 Compang conventonal and poposed PMU pefomance 27 5.2.1. Tme synchonzaton accuacy 27 5.2.2. Measuement accuacy 32 Chapte 6. Applcatons and futue pospects 36 6.1 Applcatons of poposed PMU 36 6.2 Futue pospects of the poposed PMU 41 Refeences 43 v
Chapte 1. ntoducton Powe demand s nceasng enomously and the powe system s not equpped to accommodate ths change. The avalablty of elable electc powe supples s an essental pecondton fo the functonng of moden economes [1]. The U.S. Depatment of Enegy plans to addess ths challenge of gowng enegy demand by pomotng dstbuted enegy esouces and enewable enegy ntegaton. ts Renewable and Dstbuted Systems ntegaton (RDS) pogam ecently selected nne pojects to demonstate 15% peak-load educton on a dstbuton feede by ntegaton of dstbuted geneaton [2]. The amount of vaable enewable geneaton s also expected to gow consdeably as polcy and egulatons on geenhouse gas emssons ae developed and mplemented by fedeal authotes, ndvdual states and povnces thoughout Noth Ameca [3]. Mantanng bulk powe system elablty n such a scenao wll be challengng n the nea futue. Potectonsystem msopeatons, patculaly ncoect settngs and ccut beake falue have been lsted amongst the most mpactful causes fo ntatng system dstubances ceatng majo stablty ssues [3]. Real-tme montong at all levels of the powe gd wll play a sgnfcant ole n addessng the stuaton. Phaso measuement unt (PMU) can povde voltages and cuents along wth fequency and ate of change of fequency n eal-tme wth hgh pecson and accuacy. t has the potental to mpove system contols, event analyss, fault detecton and detemnng the locaton of fault though stuaton awaeness tools and montong capablty. t can also povde tools fo supevsng the potecton equpment to pevent false tps and ts 1
consequences. Wth ths nfomaton, contol and potecton of the powe system can be sgnfcantly mpoved. The Depatment of Enegy antcpates that once all of the Recovey Act (2009) synchophaso pojects have been completed, thee wll be at least 1,043 netwoked PMUs n place (compaed to 166 n 2010) [4]. Ths numbe s nsgnfcant compaed to the extensve tansmsson netwok n the US. To utlze all the benefts of ths technology, t s necessay to deploy PMUs at a lage scale that would make the netwok completely obsevable. PMUs ae equed at appoxmately one-thd of the Buses to obtan complete obsevablty [5]. Howeve, the ntegaton costs ae too hgh to deploy them on a lage scale. Ths hgh cost s closely elated to tme-synchonzaton tself and to nstall nfastuctue fo communcaton. The thess poposes a novel dea fo an altenatve desgn of PMU by usng exstng technologes such as the tme dstbuton netwok of the ntenet and phase locked loops (PLLs). t elmnates the need to have a dedcated communcaton netwok fo tme dstbuton but banks on the exstng netwok povded by the ntenet. The poposed PMU would be sutable fo ealtme montong of dstbuton netwoks fo ts low costs and the non-necessty of a new nfastuctue fo communcaton. An ovevew of conventonal PMU and the gudelnes povded by the synchophaso standad ae dscussed n Chapte 2 and 3 followed by the concept of the poposed ntenet based PMU n Chapte 4. The evaluaton of ths poposed PMU n compason wth conventonal PMU and ovevew of ts applcatons n dstbuton netwok ae consdeed n Chaptes 5 and 6. 2
Chapte 2. Backgound and Lteatue Suvey The concept of PMU ognated fom Symmetcal Component Dstance Relay (SCDR) as a pat of dynamc elayng technology. PMUs wee fst ealzed n 1988 by D. Phadke and D. Thope. The pape outlned new measuement technques fo voltage phasos, fequency and ate of change of fequency [7]. They nvestgated the possblty of commecally avalable WWVB (Standad Tme and Fequency Staton, 60KHz, Fot Collns, Coloado) eceve ntegated as the souce of tme synchonzaton [7] but eventually fnalzed on usng a GPS eceve due to ts accuacy and elablty. GPS adds a pecse tme stamp to the quanttes and sampled data fo compute elayng applcatons. Thus PMUs equpped wth GPS could measue state of the powe system wth hgh speed, geat accuacy and pecesson [5]. 2.1 Development n PMU technology ove the yeas Enegy Management System (EMS) based on state estmaton came nto beng only afte the 1965 catastophc falue of the Noth- Easten powe gd n Noth Ameca [8]. Detemnng the state of the system thus became mpotant fo all applcatons of contol. t was not possble at that tme to measue the state of the powe system but only to estmate the state fom othe measuements. t was ecognzed that the state obtaned n ths manne at best descbed a quas-steady state appoxmaton to the actual state of the netwok [8]. When PMUs became commecally avalable fo the fst tme n the ealy 1990s, ts applcatons wee lmted to beng used as dgtal system dstubance ecodes (DSDRs). At ths stage, even though the use n potecton and contol was theoetcally establshed, PMUs wee contnued to be used only as DSDRs. n July 1993 dung the Comanche Peak load ejecton test 3
n Texas, fequency measuements evealed the pesence of an electomechancal wave popagatng though the system. Such data ecodngs led to the development of the Fequency Montong Netwok (FNET) that shed lght on many evelatons about these electomechancal waves [9]. Afte the blackouts of 1996 and 2003, PMU technology was hghly encouaged fo use n all utltes. Ths led to the ceaton of the Easten nteconnecton Phaso Poject (EPP), now known as Noth Amecan Synchophaso Poject (NASP). The EPP pefomed the fst ealtme wde-aea montong n the Unted States. PMUs ae expensve and the costs depend on the utlty, locaton, and avalablty of communcaton channels. The need fo eal-tme montong of powe system has led to nceased deployment of PMUs nceased n the Unted States. n Chna, about 400 PMUs wee nstalled at the substatons and powe plants of 500KV and 330KV voltage levels by 2007 [10]. Accodng to the 11 th fveyea plan fo the Chnese powe gd of Chna, all substatons opeatng above 300kV wll have PMU montong. nda and othe Euopean countes ae also keenly pusung PMU technology. 2.2 Applcatons of PMU n Powe systems PMUs have been n use fo a long tme as DSDR n wde aea montong. Ths secton povdes a geneal ovevew of othe applcatons of PMU technology manly n state estmaton, potecton and contol. 2.2.1 State Estmaton 4
State estmaton of the system s pefomed today fom the measuements of powe njecton, voltage values fom tansfomes, etc. Ths estmaton algothm s based on the assumpton that the state of the powe system emans statc when scannng takes place. The system can look completely dffeent by the tme the state s estmated. Ths estmated state s used as the nput data fo many othe calculatons and applcatons le economc dspatch, automatc geneaton contol, automatc voltage contol etc. PMU has the capablty to dectly measue the state of the system.e. voltage at all the Buses equpped wth PMUs. f the ente system becomes obsevable by PMU measuements, the need fo state estmaton would be elmnated altogethe. Howeve, the hgh costs of PMU and lage netwok of Buses makes t pactcally mpossble to ntoduce PMU technology nto the powe systems, at least n the nea futue. Theefoe, the need ases fo algothms to combne the measuements of PMU and exstng montong systems such as SCADA systems fo combned state estmaton [12, 13]. 2.2.2 Powe system potecton Evey lne has dstance potecton of dffeent levels. Zone 1 elays ae pesent at each end of the lne and potect 80% of the lne. Zone 2 potects 100% of the lne and about 20% of the adjonng lne. Zone 3 coves 100% of the lne and moe than 50% of the adjonng lne. Ths means these elays potect the connected lne also to some extent and thus act as backup potecton fo the lnes. Backup elays used fo povdng Zone 2 and Zone 3 potecton ae pone to false tppng. Ths can be catastophc fo a system opeatng on the edge of ts stablty lmts. PMUs at Buses whee elays ae connected can act as supevsoy systems. f the elays see a fault but none of the PMUs ndcate t, then t can be uled out as a false tgge. 5
Tansents occung n the system may cause the geneato to loose synchonzaton, but t s mpotant to detemne f ths s tempoay o pemanent. Out-of-step elays ae desgned to pefom ths detecton and also to take appopate tppng and blockng decsons. These elays ae set accodng to tansent analyss. Wth new lnes beng added to the system, theses elays tend to malfuncton n hghly nteconnected systems. Adaptve Out-of-step Potecton tes to keep eadjustng the elay settngs dynamcally. A elable pedcton algothm should be developed to povde the stable unstable classfcaton of an evolvng swng n a easonable tme [9, 14]. Ths s a developng feld and much wok needs to be done n ths espect. Thee ae two ways n whch the elay can fal to potect the system. t tps when t should not (a false tp) o t does not tp when t should tp. Two types of elablty have been desgnated as secuty and dependablty espectvely [9]. The system moe secue f some o the othe elay always cleas fault but leads to nceased numbe of false tps. The system s moe dependable f elays opeate consevatvely and only clea faults that ae seen wth cetanty. The potecton system of today s moe secue than dependable. We can mantan the balance between secuty and dependablty wth PMU data whch allows the pocess to be adaptve. Loss of mans s a tem descbng a stuaton when one o moe geneato unt gets sepaated fom the system and s also called as slandng. Loss of mans can be detected moe accuately by usng the phase angle dffeence method. Ant-slandng technques based on the ate of change of phase angle (ROCOPA) ae beng exploed. PMUs can be mplemented at dstbuted geneaton (DG) temnals (o at a stategc locaton wthn a potentally slanded secton of netwok) to povde hgh-pecson measuements of fequency and voltage angle to facltate detecton [15]. 6
2.2.3 Powe system contol Wth eal tme data, powe system contol fnds sgnfcant beneft. Contol can now be based on emote quanttes. Wde-aea measuements allow the system to contol nsecue stuatons wthout employng contnuous feedback [14]. n the case of such stuatons, contol actons le econfgung the netwok to ensue the demand s met o cutaled can be taken emotely. Refeence [16] descbes the ncopoaton of synchophaso measuements nto new system ntegty potecton schemes (SPS). PMU measuements enhance obustness of the system by etanng local contol sgnals whle addtonally povdng supevsoy sgnals. Such a edundancy n the system contol would be ndspensable fo fne contol of the system even emotely and a bg step towads a smate gd. 7
Chapte 3. Theoy of conventonal PMU A thoough study of conventonal PMU technology s essental to lay the foundaton fo the poposed concept of PMU. Undestandng the lmtatons and challenges of conventonal PMUs povde motvaton and oom fo mpovement. The chapte also dscusses the synchophaso standad to undestand the qualty of output fom any PMU. 3.1 Synchophasos A phaso s an analytcal and tme-nvaant epesentaton of a snusodal wave. t s commonly used fo AC powe system analyss. f the snusodal Equaton s epesented n Equaton (3.1): x( t) = X m cos( ω t + φ) (3.1) Ths can be epesented n phaso fom as: X X m jφ m X = e = (cosφ + jsnφ) = X + 2 2 j X (3.2) Hee, X m / 2 s the oot mean squae value of the wavefom and φ s the phase angle. Phasos epesent a snusodal wavefom takng nto consdeaton the phase dffeence of the measued wavefom wth espect to a pedefned efeence so that two wavefoms ae dectly compaed wth espect to the same efeence. Synchophasos use coodnated unvesal tme (UTC) as an absolute tme efeence. f UTC s used as a efeence fo all phaso measuements globally then these measuements can dectly be compaed espectve of the locatons of samplng. 8
The synchophaso epesentaton of the sgnal x(t) n Equaton (3.1) s the value X n Equaton (3.2) whee φ s the nstantaneous phase angle elatve to a cosne functon at the nomnal system fequency synchonzed to UTC [17]. One pulse pe second sgnal (PPS) geneated by some vey pecse clocks whch tace UTC s used as efeence fo samplng the wavefom. Fgue 1: Conventon fo synchophaso epesentaton. Taken and modfed fom Ref [17]. Fgue 1 llustates how PPS sgnal s used as a efeence to measue phasos. f the PPS sgnal concdes wth the peak of the measued wavefom, the phase dffeence s zeo hence the phaso epesentaton becomes: X X = m (3.3) 2 Fo the second case, whee the phase dffeence between the efeence peak and the measued sgnal peak s 60 degees, the phaso epesentaton s gven by: Π j 3 X X = m e (3.4) 2 9
A powe system hadly opeates at ts fundamental o nomnal fequency. f fequency f f 0 and f < 2f 0, whee f 0 s the fundamental fequency of a snusodal wave, the obseved phaso wll have a constant magntude. Howeve, the phase angles of the sequence of phasos {X 0, X 1, X 2, X 3, X n, }} wll change unfomly at a ate of 2π (f f 0 ) T 0, whee T 0 = 1/ f 0 and, as shown n Fgue 2 [5, 17]. Fgue 2: A snusod wth a fequency s obseved. The phase angle Φ nceases unfomly n elaton to the fequency dffeence, (f f 0 ) [17] 3.2 Genec model of PMU Ths secton wll talk about how a PMU measues synchophasos and wll descbe a genec fom of a conventonal PMU. Snce dffeent types of PMUs ae manufactued, t s mpotant to undestand such a genec model. PMU can measue synchophaso along wth the fequency and ate of change of fequency (ROCOF), and the measuements ae also tme stamped vey accuately n a PMU. Thus t s possble to algn measuements comng fom dffeent PMUs, espectve of the tme delay n tansfeng the data, and to ndex the data fo a complete snapshot of the system at any gven tme. 10
Fgue 3: Genec block dagam of PMU [5] As shown n Fgue 3, the measued analog nputs ae stepped down fo electonc components usng nstument tansfomes, typcally wthn the ange of ±10 volts. These ae hadwed to the Bus o the lne fom whch wavefom s collected fo measuement (Fgue 4). All thee phase measuements of cuent and voltages ae used to deve postve, negatve and zeo sequence components. The attenuato s a physcal component that s used to educe the powe of the sgnal wthout dstotng the wavefom. These sgnals ae then condtoned by flteng them usng ant-alasng alasng flte. The cut-off fequency of these fltes s less than half the samplng fequency n ode to satsfy the Nyqust cteon [5]. Hghe cut-off fequency may be used fo a hghe samplng ate, n such case a dgtal decmaton flte may be used to downconvet the hghe samplng ate to lowe, thus povdng dgtal ant-alasng [5]. These dgtal fltes ae less senstve to tempeatue vaatons so that the phase angle dffeences and elatve magntudes of the dffeent sgnals ae pactcally unchanged at vaous weathe condtons [5]. 11
Fgue 4: Typcal synchophaso nstallaton at substaton [18] GPS povdes a 1 PPS wavefom eceved anywhee on eath wthn 1µs of each othe. Tme pecson of nanoseconds may be acheved usng GPS. These pulses ae used to synchonze the local oscllato to sample the analog nput n the Analog-to-Dgtal (ADC) convete. The samplng ate s detemned by the local oscllato. Ovesamplng the data helps detemne the phaso wth bette pecson as the estmaton accuacy nceases. GPS also povdes a tme stamp to the measuements evaluated usng the mcopocesso. These tme stamps make t possble to ndex and algn the data comng fom vaous PMUs. The tme-stamps ae ceated fom two of the sgnals deved fom the GPS eceve. The phaso s evaluated and tme-stamped n the mcopocesso unt. Ths data s then communcated to a common Phaso Data Concentato (PDC) that s the next heachcal unt of PMU achtectue. At the PDC level, the data s tme algned, and system state s detemned and necessay contol actons ntated. 12
The communcaton equements of a netwok of PMUs depend on the ate of samplng to detemne the bandwdth of communcaton equed. The data acqued fom PMU may be made compact nstead of aw data beng epoted. At PDC level the snapshot of the system can be obseved and ctcal contol actons ntated. 3.3 Ovevew of synchophaso standad The EEE Std C37.118.1-2011 standad defnes synchophasos, fequency, and ROCOF measuements unde all opeatng condtons [17]. t also povdes fomat of tme tag and othe synchonzaton equements fo mplementng PMUs. Accodng to ths standad, the pefomance of a PMU can be estmated by the elablty and accuacy of ts tme souce capable of tacng coodnated unvesal tme (UTC). The tme tag povded should clealy tace UTC and ndcate tme as well as qualty of measuement of tme. The standad eques value of total vecto eo (TVE), fequency eo (FE), and ROCOF eo (RFE) to be wthn the equed lmts set by the standad. TVE s gven by: 2 2 ( X ' ( n) X ( n)) + ( X ' ( n) X ( n)) TVE( n) = (3.5) 2 2 ( X ( n)) + ( X ( n)) Hee, X (n) and X (n) ae the estmated value of eal and magnay component of the phaso and X (n) and X (n) ae the theoetcal values of the nput sgnal at the nstants of tme (n) [17]. The allowable lmt fo TVE s 1% whch tanslates to a phase eo of 0.57 degees (0.01 adan) by tself and a tme eo of ±26 µs fo a 60Hz system and ±31 µs fo a 50Hz system [17]. gven by: The maxmum allowable FE s 0.005Hz n steady state. Fequency measuement eo s 13
FE = = f tue f measued (3.6) ROCOF s affected by the nstablty of the tme souce and can be gven by: REF = = df dt tue df dt measued (3.7) The synchophaso, fequency and ate of change of fequency ae measued at the same nstance and epoted at a constant ate. 14
Chapte 4. New conceptual ntenet based PMU GPS s ntegated n conventonal PMUs fo povdng pecse tme synchonzaton. Ths eques all the emote unts to be located n ponts fom whee at least fou GPS satelltes must be seen at evey moment [20, 21]. Addtonally, n tems of capacty fo communcaton, the GPS system tself cannot povde communcaton capablty fo PMU. Theefoe, addtonal nfastuctue s equed fo communcaton between devces [20]. An altenatve method s to use a communcaton netwok to dstbute tme le the Pecson Tme Potocol (PTP) [20-25]. Ths s acheved by usng a netwok of dedcated tme seves whch communcate wth each othe va messages to synchonze tme. PTP was ntoduced n EEE 1588-2002, but eques that the substaton be upgaded n accodance wth the EC 61850 standads. t s not always economcally and physcally vable to do so because of the dedcated hadwae equed solely fo tme dstbuton puposes. Both desgns yeld hgh pecson and hgh esoluton n tme measuements that ae necessay fo montong the status of tansmsson netwoks. A PMU wth a lowe pecson n tme measuements mght be acceptable fo dstbuton netwoks. Ou objectve s to desgn a PMU havng lowe cost but accuate enough to monto dstbuton netwoks. We ealze that t s mpotant to mantan hgh esoluton athe than hgh pecson n the tme measuements. The man blocks o components of the poposed ntenet based PMUs ae dscussed n the followng secton along wth the ovevew of the concept. The subsequent sectons wll talk about the mechansm to synchonze tme usng NTP n detal and the equements of the same n tems of 15
hadwae and softwae. The tme synchonzaton accuaces and the mathematcal analyss justfyng the use of such system n dstbuton netwoks s also dscussed. 4.1 Concept of the ntenet based PMU Whle the poposed PMU eplaces GPS tme wth ntenet tme, the wokng of the system emans essentally dentcal to that of conventonal PMUs. Measued analog nputs ae collected fom the seconday of the nstument tansfomes. These sgnals ae then condtoned usng ant-alasng fltes to satsfy the Nyqust cteon as n a conventonal PMU. NTP ove the ntenet netwok wll now povde one PPS. Ths sgnal wll lock the local oscllato to UTC tme and mpove synchonzaton of the maste o efeence clock wth the local oscllato. A method to obtan PPS sgnal s by the addton of a hadwae component called a gadget box that can be connected to the PLL. The gadget box s capable of ecevng a PPS sgnal fom GPS pesent at the seve level va the ethenet/ntenet netwok. The vaous methods to obtan the PPS sgnal usng NTP ae descbed n detal n the subsequent sectons. Tme stamps ae povded by NTP netwok, and the tme taggng of data takes place n the mcopocesso afte the data s sampled and phasos ae calculated. A local clock sgnal of hgh fequency acts as the efeence fo samplng the analog sgnal n ADCs wth hgh samplng ate. Ovesamplng leads to moe accuate phaso estmaton [5]. Hee ovesamplng s done manly to compensate fo the low (as compaed to GPS) synchonzaton accuacy of NTP system. 16
Fgue 5: ntenet based PMU block dagam The phaso data obtaned fom ADC s pocessed n the mcopocesso unt and magntude, phase, fequency and ate of change of fequency of the sgnal s ganed and tme stamped and may be futhe stoed and communcated as necessay. 4.2 ntefacng NTP wth PMU We eque two featues fom the NTP netwok: Tme-stamps and PPS sgnal. n a conventonal PMU tme-stamps ae obtaned fom the GPS usng two sgnals, the measuement made by the pecse clocks pesent on these satelltes and PPS sgnal geneated by these clocks. NTP has a netwok of seves used to dstbute tme. Thee ae two types of clocks pesent n NTP, the maste clock o efeence clock and the slave clock o end devce clock. The efeence clock s typcally a vey pecse clock, such as atomc o ado clock. Ths efeence clock s called Statum 0 and s connected to othe seves 17
whch synchonze the tme n accodance wth ths efeence clock. These seconday seves ae known as Statum 1 and the clocks o seves connected to them as Statum 2 and so on. A depcton of NTP stata s shown n fgue below. Fgue 6. Dagammatc depcton of NTP stata [26] Fgue 7a shows a typcal connecton of seves of dffeent stata; the heavy lnes epesentng the actve synchonzaton paths and decton of tmng nfomaton flow; and the lght lnes epesent backup synchonzaton paths. Fgue 7b shows the econfgued system, whee the lne maked x denotes lne whch s out of sevce [27-31]. 18
Fgue 7: Example of Synchonzaton Topologes of NTP clocks [27-31] 4.2.1 Obtanng tme-stamps Synchonzaton between dffeent clocks s acheved by sendng and ecevng data packets. As shown n Fgue 8, f tme-stamps ae exchanged between two pees, A and B, and the tme-stamps ae T 1, T 2, T 3 and T 4, then the offset (σ ) and delay (δ ) of tme assumng T 3 > T 2 can be gven by; δ = a + b and σ = a + b 2 (4.1) Whee, a = T 2 T1 and b = T 3 T 4 (4.2) Lewse, the delays fom A to B and fom B to A ae smla. Fgue 8: Measung Delay and Offset [27-31] 19
Each NTP message ncludes the latest thee tmestamps T 1, T 2 and T 3, whle the fouth tmestamp T 4 s detemned upon aval of the message. Thus, both pees A and B can ndependently calculate delay and offset. Numeous clocks ae avalable to be synchonzed to the system clock but a clock-selecton algothm detemnes whch clocks have the best accuacy. These offsets fom a system of pees ae combned usng weghted-aveage and act as nput to the PLL ccut. Fgue 9: Netwok Tme Potocol fom Ref. [27] n the PLL the combned effects of the flteng, selecton and combnng opeatons ae to poduce a phase-coecton tem, whch s pocessed by the loop flte to contol the voltagecontolled oscllato (VCO) fequency. The VCO funshes the phase (tmng) efeence to poduce the tmestamps used n all tmng calculatons [27-31]. The voltage contolled oscllato s the system clock n the end devce. Fgue 9 shows a schematc ovevew of the system. 4.2.2 Obtanng PPS fom NTP A PPS sgnal s essental to mantan the synchonzaton between ou system and absolute tme. Wth the loss of PPS sgnal the system clock may dft away fom UTC and eo n tme wll get accumulated. A PPS sgnal can be obtaned by usng only NTP softwae o a combnaton of hadwae and softwae. One method s to use the ASC tmecode whch eques 20
only softwae and can povde PPS wth accuacy of 1ms. ASC tmecode can be ntefaced wth PLL and a local oscllato usng RS-232 o RS-422 nteface. Fo the PPS sgnal to be ntefaced wth PLL some bdge wll be equed that convets data accodng to the RS232 potocol, to the fom equed by PLL. The accuacy of ASC tmecode mght not be acceptable fo ou PMU. We popose to use a combnaton of hadwae and softwae that can povde bette accuaces. The poposed PMU uses gadget box to povde the PPS synchonzaton sgnal. t can be used to synchonze the NTP tme to one PPS sgnal geneated by any maste clock, ethe GPS o the ado clock pesent at the seve level. Ths PPS sgnal helps to algn the slave tme and mantan synchonzaton wth the efeence clock. The combnaton of PLL, NTP and gadget box can povde pecson contol of the local oscllato wthn 1µs [27]. The NTP host can deve the tme of the day, and day of the month fom pees but the PPS sgnal helps t emove jtte and s thus essental fo bette synchonzaton [27-31]. n veson 4 of NTP, NTPD allows end uses to specfy up to 9 NTP seves to synchonze fom and t classfes the seves accodng to the statum. Ths povdes a way to mantan synchonzaton wth othe PMUs n the netwok even of the absolute tme synchonzaton s lost [20]. So even wthout the tme-stamps we can make synchonzed measuements and use a counte o some othe method to ndex the data. A PPS sgnal may also be deved fom the RG- B sgnal poduced by numeous ado clocks. nte-range nstumentaton Goup (RG) sgnal was developed to synchonze nstumentaton ecodes n the ealy days of the U.S. space pogam. The esult can always be eled on to wthn 128 µs [27-31]. 21
4.2.2.1 Gadget Box The gadget box ncludes a level convete and pulse geneato that tuns the PPS sgnal on-tme tanston nto a vald chaacte. The chaacte geneated fo each PPS pulse s ntecepted by the clock faclty and a tme-stamp nseted n the data steam [27]. The genec block dagam of the gadget box s shown n fgue below. Fgue 10: Genec block dagam of Gadget Box The audo output of a shotwave ado usually tuned to CHU at 3330, 7335 o 14670KHz acts as the synchonzaton efeence nput fo the system. Ths sgnal s then condtoned usng bandpass fltes to avod alasng effects. A TTL (Tanssto Tanssto Logc) pulse of specfc wdth s geneated (commonly the wdth s 26µs) fom ths sgnal. Usng a TTL to EA (Electonc ndustes Allance) convete ccut.e. EA level convete, EA output s obtaned whch can then be dectly used as the PPS sgnal [33]. EA s a standad defnng the electcal chaactestcs of dves and eceves of sgnals. t enables the confguaton of nexpensve local netwoks and communcatons channels. 4.3. Desgn of Pototype A pototype beng desgned at Unvesty at Buffalo, SUNY as a collaboatve effot by D. HyungSeon Oh, D. Albet Ttus and the gaduate students s pesented hee. The desgn s n ts ntal stages and wll help to gve a ough dea of the system beng poposed. 22
Fgue 11: Block dagam of the pototype desgn fo the poposed PMU A Lnux o wndows machne wll be used to obtan NTP tme usng ntp daemon and ASC tmecode softwae to obtan the PPS sgnal. To use ths data as the nput fo PLL, some bdge wll be equed fo ntefacng. Hee LPC1768 s used as t can nteface data fom RS232 pot and gve out clock sgnal. PLL wll lock ths nput clock sgnal (OCLK) and povde a synchonzed by hghe fequency clock sgnal as output (NCLK). Ths output clock sgnal wll be used to sample analog data n the ADC. A feld-pogammable gate aay (FPGA) chp s used to synchonze the output clock and the tme obtaned fom ntpd to geneate tme stamps at mcopocesso level. The cost beakdown of buldng ths pototype s ndcated n table 1: 23
Component Estmated Cost ($) ASC tmecode softwae (fo 1 clent) 50 LPC1768 (100pn, 3.3V) 6 to 10 PLL 3 to 7 ADC 6 to 20 FPGA 20 Mcopocesso unt (16bt, 25MHz) 10 Gadget Box CHU ado eceve Bandpass flte (2000 ± 500Hz) TTL pulse geneato (74LS123) EA level convete (CL232) Modem (DB25 female DCE) Resstos, Capactos, LEDs 130 70 1 3 50 1 to 5 Memoy (8GB) 70 Total Estmated cost 446 Table 1. Cost beakdown of the pototype of ntenet based PMU The costs dscussed above ae the component costs only. Addtonal cost wll be equed fo fabcaton and labo [33-47]. Snce the PLL that wll be used to lock the PPS sgnal s not commecally avalable, the costs quoted hee ae fo a egula PLL. The motvaton of ths thess s to develop a new PMU that s substantally cheape to mplement and does not eque addtonal nfastuctue. Ths PMU would stll be accuate 24
enough to be used n dstbuton netwoks. Chapte 5 wll establsh clea goals and methods fo systematcally evaluatng the pefomance of the ntenet based PMU. The poposed PMU wll be compaed wth the conventonal PMU based on the equements of the synchophaso standad descbed n ths chapte and montong equements of the dstbuton netwoks. 25
Chapte 5. Evaluaton of the poposed PMU vesus conventonal PMU 5.1 ntoducton A low cost PMU soluton was poposed n the pecedng chapte. Ths poposed PMU banks on the exstng netwok of NTP seves fo tme synchonzaton thus lagely bngng down the cost of the equed communcaton nfastuctue. NTP has been n use fo a long tme, and the pefomance s nealy pefect fo the PMU tme synchonzaton. Usng NTP, t s not necessay to add o change any majo constucton and equpment. The common tme synchonzaton equpment can have the nteface fo NTP, and the cost s vey low. The pecson povded by NTP synchonzaton howeve, mght not be suffcent fo montong of tansmsson netwoks. Dstbuton netwok on the othe hand has SCADA montong only at HV/MV substatons.e. pmay substaton and only Boolean data fom sectonalzes etc. at seconday substatons [32]. Fgue 12 shows a typcal dstbutons netwok. n SCADA, data s not synchonzed at all but athe sent ove a wndow of a couple of seconds and assumed to be synchonzed. Theefoe, NTP would be an attactve soluton fo eal-tme montong as t s bette than SCADA, and close to the pefomance n conventonal PMU but wth consdeably lowe costs. 26
Fgue 12: A dstbuton netwok substatons wth amount of montong pesent 5.2 Compang conventonal and poposed PMU pefomance Ths secton concentates on compang the pefomance of the poposed ntenet based PMU wth ts conventonal countepat. The compason s done manly wth egads to tme synchonzaton accuacy and accuacy of measuement of data. The objectve s to justfy the use of such a system fo montong dstbuton netwoks and to establsh the adequacy of synchonzaton accuacy fo the same pupose. 5.2.1. Tme synchonzaton accuacy GPS can synchonze tme wth an accuacy of ±1µs. The best synchonzaton accuaces achevable usng GPS ae well wthn nanoseconds ange, but such an accuacy of synchonzaton s not equed fo eal tme montong of dstbuton netwoks. To date, too 27
much emphass has been put on the ssue of synchonzaton accuacy cuacy wth consdeaton of vaed needs. GPS povdes vey pecse tmestamps that consst of data, confguaton, and heade fles. The Heade fle s a human eadable fle whch caes nfomaton about who has poduced the data and he may wsh to shae t. The confguaton and data fles ae machne eadable fles wth fxed fomats. The data fles cay the actual phaso data. As shown n Fgue 13, the fst wod of 2 bytes s fo synchonzaton of the data tansfe (SYNC). The second wod defnes the sze of the total ecod (FRAME SZE), the thd wod dentfes the data ognato unquely (DCODE), and the next two wods povde the second of centuy (SOC) and the facton of a second (FRACSEC) at whch the data s beng epoted. The length of the Data wods whch follow FRACSEC depends upon the specfcatons povded n the Confguaton fle. The last wod s the check sum (CHK) s to help detemne any eos n data tansmsson [5]. Fgue. 13. Fomat fo fles tansmtted fom and to PMUs. The numbes below the boxes ndcate length of the wod n bytes [5]. GPS eceves GPS tme whch s dffeent fom UTC n the espect that leap seconds ae not pesent. These ae added at the GPS eceve level and the tme output thus taces UTC. The accuacy of PPS sgnal eceved anywhee on eath s wthn 1µs. the equement of placng the 28
GPS eceve at a locaton fom whch at least 4 satelltes ae vsble s not feasble n some cases. Ths s a lmtng facto on the use of GPS eceve. Anothe hndance fo usng GPS on a lage scale s the communcaton equement of GPS as t does not have communcaton capabltes. nstead, t has to be ntegated wth some communcaton netwok and havng a dedcated communcaton system ensues bette accuacy but ntoduces extensve monetay and tme nvestments. The epot on DOE expeence wth Recovey Act Synchophaso Pojects confms that, n tems of othe equpment needed fo synchophaso systems, othe than the PMUs and PDCs, the geatest amounts spent by any one poject thus fa wee $15,000,000 on communcatons systems [4]. NTP also povdes tme stamps n SOC and facton of second nfomaton n the fom of a 64-bt unsgned fxed-pont numbe, wth the ntege pat n the fst 32-bt and the facton pat n the last 32 bts and ntepeted n standad seconds elatve to UTC [28]. When a message aves, the tme fom the local clock of the compute nteface s tansfeed to the tme-stamp vaable. The qualty of the tme sgnal geneated on the compute nteface thus depends on the qualty of ntenal compute clock [51]. t also depends on the type of ntenet connecton, the congeston pesent n netwok, but not necessaly the statum that s used fo synchonzaton. Algothms can help decease the latency n pckng up the tme sgnal that s eceved va NTP [51]. The maxmum esoluton of an NTP tme stamp s about 200 ps (about the tme t takes an electcal pulse to tavel though 2 cm of coppe we), so the ultmate accuacy of NTP s lely to be lmted by hadwae and latency concens, athe than by the NTP potocol [26]. The NTP system conssts of a netwok of pmay and seconday tme seves, clents, and nteconnectng tansmsson paths. A pmay tme seve s dectly synchonzed to a 29
pmay efeence souce whch s vey accuate. The seconday seves connect to othe seves and choose the most accuate and elable system of seves to obtan tme. NTP potocol s contnuously mpovng n tems of tme synchonzaton accuacy. A sees of ncemental mpovements n system hadwae and softwae have esulted n sgnfcantly bette accuacy and stablty. These mpovements nclude novel ntefacng technques and opeatng system featues [27]. Fgue 14. Offsets between the local clock and the PPS GPS [27] Synchonzaton accuacy of NTP has mpoved fom tens of mllseconds to submllseconds. Ths has been expementally poven to be tue fo vaous confguatons of NTP. PPS eceved fom ASC tmecode can povde synchonzaton accuacy o aound 25ms [27]. Fgue 14 shows the tme offset the offsets between the local clock and the PPS sgnal fom a GPS eceve measued ove a typcal day usng ths mplementaton. NTP tme pecson has been poven to be adequate fo many PMU applcatons. NTP s beng n PMU fo tme synchonzaton fo detemnng electomechancal oscllatons and 30
fequency measuements [52-53]. Fo obsevng fequency n wde aea, sx PMUs have been setup and synchonzed va NTP (softwae only veson) and the measued fequency s also sent to a common locaton usng the ntenet. Many expements have been conducted to test and calbate the expected accuacy of NTP ove nomal local aea netwok (LAN) and wde aea netwok (WAN) connectons [27-31]. n these expements the PPS obtaned fom GPS s compaed to one whch s obtaned usng NTP. The esults of few such expements by Davd Mlls ae mentoned below [27]: 1. The offset.e. tme eo between PPS geneated by GPS and obtaned va NTP was measued fo a patcula pmay seve wth ove 400 clents beng suppoted usng NTP veson 3. Some of the clents had ntense computatonal and cyptogaphc equements. The esults showed that synchonzaton could be mantaned wthn 400µs. 2. The offset between two pmay seves was measued. These epesent typcal a system whch s geneally used. One of the seves was a dedcated tme seve whle othe was mult-functonal. Occasonal spes of 1ms wee seen, but these would be taken cae of at the PLL end. 3. The esults of an expement n whch the pmay and seconday seve offset was measued show that the maxmum offset was 2.5ms. Ths data, as well as fom seveal othe expements shows that usng only softwae NTP can povde synchonzaton n tens of mllseconds. Wth the addton of hadwae such as a gadget box fo the PPS much bette accuaces can be acheved. Howeve, NTP veson 4 povdes the opton of choosng the seves fo synchonzaton. 9 o less seves can be chosen. Ths gves an oppotunty to connect all the PMUs to the same set of seves fo lowe mutual offset. Also, n case of loss of absolute tme, the PMUs would stll be synchonzed. 31
Usng the gadget box along wth NTP softwae and PLL can povde accuaces n the µs ange. The accuacy of tme-stamps obtaned n such an aangement s 20-30 µs [27]. Howeve, t was clamed that ths confguaton mght yeld 1 µs accuacy n tme-stamps [27]. Ths accuacy s assumed fo the mathematcal analyss n the followng secton. 5.2.2. Measuement accuacy Tansmsson netwoks have a dense netwok of SCADA pesent fo montong. The data eceved fom SCADA along wth othe nfomaton s un though a state estmaton algothm and the quas state of the system s detemned. Ths nfomaton s now used as an nput fo vaous othe applcatons. On the othe hand, SCADA s pesent vey spasely n dstbuton netwoks. Ths mnmal data along wth Boolean data comng fom fault ndcatos, sectonalzes and emote swtches s uploaded at the contol cente. The contol opeato now takes actons manually aganst an nfomaton log. These actons ae thus taken vey consevatvely to ensue that the system s not oveloaded. The pupose of PMU would be dffeent n tansmsson and dstbuton netwoks. n tansmsson netwok, the need of PMU ases manly to measue the state of the system, o mpove state estmaton and contol. Ths would eque PMUs wth vey hgh synchonzaton pecson le the one povded by conventonal PMU. The pupose of PMU n dstbuton netwoks howeve would be manly fo stuatonal awaeness, fault locaton detecton and so on. Fo these puposes, the PMU need not to be so pecse n tme synchonzaton. The poposed PMU accuaces would be adequate fo dstbuton netwoks. The eal and eactve powe flow fom Bus to Bus k can be gven by: P = V V k ( G cosθ + B snθ ) (1) 32
Q = V V k ( G snθ B cosθ ) wheeθ = θ θ k (2) Whee P and Q s the eal and eactve powe flow between th Bus and k th Bus, G s conductance and B susceptance between the th and k th Bus, V and V k espectve voltage magntudes and θ and θ k espectve phase angles at and k Bus. Suppose that the eo n estmates of the eal and eactve powe flow ove a lne connectng Bus and k ae δp and δq espectvely; the eo n measuement of the powe angle dffeence θ =θ θ k s δθ ; and δt s the eo n measuement of tme. Then, smplfyng and eaangng the Equatons gves us: δp P = whee tan 1+ θ + ( B G ) ( B G ) tanθ B tanα = G δθ δt δt tan ( θ α ) θ t δt 2B G θ t δt (5.3) A Taylo s sees expanson s appled to appoxmate the tangent functon, and we tuncate hghe ode tems,.e., 1 B 2 tan( θ α ) = tanα θ + h.o.t + + 1 θ 2 2 cos θ = 0 2B ( α θ ) G G G B (5.4) Note that θ s vey close to zeo n the powe system opeaton, and typcal values fo G /B ae n the ange of 0.5-1. Smlaly, based on the obsevaton; 33
δq Q = 1 tan ( B G ) tanθ θ + ( B G ) δθ δt δt cot θ t 2G B θ t ( θ + α ) δt δt (5.5) And 1 G 2 cot( θ + α ) = cotα θ + h.o.t + 1 θ 2 2 sn θ = 0 2G ( θ + α ) B B B G (5.6) Assumng an eo of 10 µs n tme measuement, then the elatve eos n estmatng P and Q ae vey small as the nstantaneous changes n the voltage angles s vey small wth tme. Ant- slandng technques based on ate of change of phase angle dffeence (ROCPAD) dffeentate slandng and non- slandng condtons by settng a theshold at 50 degee/s to 100 degee/s value of ROCPAD. Consdeng a maxmum vaaton of would be 200 degee/s.e. 3.5 ad/s afte whch [48]; δp P 2B G θ t δt << 2B G θ t max δt << 2B G θ t δt 0.014% (5.7) and δq Q 2G B θ t δt 2G B θ t max G δt << 2 B max θ δt 0.007% t (5.8) Accodngly, δp P ( δp Pj )( P Pj ) ( P Pj ) δp k k = = = P k k k w P k δp Pj δpj N P j 0.14% (5.9) and 34
δq Q k ( δq Qj )( Q Qj ) ( Q Qj ) δq k k = = = w k Q k Q k δq Qj N δq Q j j 0.07% (5.10) whee j = agmax k P P j, w k P P = P j k P P j ( ), w kq = Q Q j k Q Q j ( ) ; (5.11) Also, N s the numbe of the lnes connected to Bus, fo a dstbuton netwok N 10 unless Bus s a feede. Note that these uppe bounds ae sgnfcantly oveestmated. The most sgnfcant oveestmate comes fom the uppe bound eplacng the ate of voltage angle dffeence between two Buses wth the ate of voltage angle at a Bus. Accodng to the cuent opeaton polcy of dstbuton netwok (adal topology and un-dectonal powe flow fom a feede to snks), the ate of voltage angle dffeence s much smalle than that of voltage angle. Theefoe, these values ae exteme cases but eman so small as to be stll acceptable n dstbuton montong. 35
Chapte 6. Applcatons and futue pospects 6.1 Applcatons of poposed PMU Real tme montong n dstbuton netwoks can help to sgnfcantly educe congeston by montong the lne flows contnuously and makng t ease to use lnes to the maxmum capacty [54]. Wth the nceasng amount of sola photovoltacs (PV) and ooftop wnd tubnes beng ntegated nto the gd, the penetaton level of dstbuted geneaton steadly nceases. Real tme montong n such scenaos can go a long way to stengthen the stablty of the system by helpng to obseve and allocate eal as well as eactve powe n a bette way. Moeove, dstbuton netwok topology s not constant and keeps on gettng econfgued. Ths econfguaton occus automatcally n the case of faults o s assumed to cay out mantenance o epa wok. PMUs can help econfgue the netwok effcently by makng the netwok paametes vsble. One mmedate applcaton of ths PMU s to detect faults and fault locaton. Cuently, fault detecton eles heavly on a phone-call fom end-consumes. Needless to say, ths method nvolves a lot of ambguty and s hghly neffcent and tme consumng. Whle many technques exst fo fault locaton detecton fo tansmsson lnes, these cannot be appled to dstbuton netwok due to the nheent dffeence n topologes between tansmsson and dstbuton netwoks. Anothe eason s that most of fault locaton detecton technques ae knowledge based often ely on extenal nfomaton such as SCADA alams, substaton and feede swtch status, feede measuements, load voltage sensos, etc. n dstbuton systems, measuements ae 36
usually only avalable at the substaton and nfomaton about the opeaton of feede potectve devces s nomally unknown [55]. Havng a PMU at evey node would make the system completely obsevable and make way fo utlzaton of many of the fault detecton technques n the dstbuton netwok. The method dscussed n [55] povdes an accuate means to detemne fault locaton n dstbuton netwoks. The dagnostc scheme nvolves thee steps: sgnal pocessng, fault locaton and fault dagnoss. Sgnal pocessng efes to the step of estmatng the phasos. Ths step can now be elmnated as phasos can be dectly measued usng PMU. Fault locaton efes to the step of pefomng fault dstance calculatons, estmatng the ange of faults and fault dagnoss s the dentfcaton of fault locaton and ankng of fault [55]. Consde a sngle lne-to-gound fault on phase A as shown n Fgue 15. Allowng fo the mutual couplngs among dffeent phases, the followng elatonshp exsts fo the faulty phase, V = D( Z + Z + Z ) + R (6.1) a aa a ab b ac c f f whee, D s the fault dstance, f s the fault cuent, R f s the fault esstance, Z aa s selfmpedance of phase a, Z ab s mutual-mpedance between phase a and b, and Z ac s mutualmpedance between phase a and c. The fault cuent wll be equal to: f = ' (6.2) a a Hee, a s the emote nfeed cuent and s a functon of V a, V b and V c. 37
38 Fom Equaton (6.1) and (6.2); f a a c ac b ab a aa a R Z Z Z D V ) ' ( ) ( + + + = (6.3) Sepaatng the eal and magnay pats of Equaton (6.3), we get; ) ( ) ( ' a a f c ac c ac b ab b ab a aa a aa a R Z Z Z Z Z Z D V + + + = (6.4) and ) ( ) ( ' a a f c ac c ac b ab b ab a aa a aa a R Z Z Z Z Z Z D V + + + + + + = (6.5) Equatons (6.4) and (6.5) can be wtten n matx fom as: + + + + + + + = f a a c ac c ac b ab b ab a aa a aa a a c ac c ac b ab b ab a aa a aa a a R D Z Z Z Z Z Z Z Z Z Z Z Z V V ) ( ) ( ) ( ) ( ' ' (6.6) Hee the supescpt denotes the eal component and denotes the magnay component. Solvng Equatons (6.4) and (6.5) smultaneously, the value of D and R f can be calculated.
Fgue 15: A sngle-lne-to-gound fault on phase A [55] Combnng the algothm fo fault dstance fom [55] and the method fo fault locaton mentoned n [40], the complete pocess of detectng and locatng fault (wth the assumpton that system s obsevable usng PMUs) can be summazed n the steps below: 1. Fom the measuement of cuents a and a obtaned fom the PMU, f s calculated. Wth the known fault cuent, the fault dstance and the fault esstance can be calculated by sepaatng Equaton (6.1) nto eal and magnay pats and then solvng the two esultng eal Equatons as shown n Equaton (6.4) and (6.5). 2. Once the fault dstance s calculated, the seach begns by assumng the lne dectly connected to be faulty. Howeve, f the fault dstance s geate than the length of ths lne, the seach moves on to the lnes connected to the ecevng end of the lne and so on. By tavesng the netwok n a cetan sequence, all possble fault locatons can be dentfed [55]. An example s llustated n Fgue 16. 39
Fgue 16: Seach fo possble fault locatons [55] 3. Consde the example of a 14 Bus system shown n Fgue 16. Two PMUs placed at Bus 1 and Bus 5 make the system obsevable. n ths case, thee canddate locatons, desgnated as F, F1, and F2 on lnes 3 4, 10 11, and 8 9, espectvely, ae found. Fgue 17: One-lne dagam fo a 14-Bus dstbuton feede wth two measuement ponts at Bus-1 and Bus-5, espectvely. 40
4. To detemne f the lne s faulted o not, voltage at evey Bus s calculated usng both the PMUs. Fo locaton F2, voltage phasos at Bus-2, denoted as V 1 2 and V 5 2 ae calculated fom Bus-1 and Bus-5, espectvely, as f thee s not a fault at F2. 1 5. f, V 5 ε, whee ε s a pedefned theshold, canddate locaton F2 s the 2 V2 actual fault locaton. Othewse, t s emoved fom the canddate lst [40]. Ths method eques voltage and cuent values at both ends of the lne. 6. Othe methods fo fault locaton nclude the knowledge of status of potectve devces n combnaton wth the fault dstance detemnaton [55]. A majo step n the fault locaton algothm mentoned n [55] was to calculate the emote end cuent nfeed usng the adal powe-flow algothm. Measuements obtaned fom a PMU can elmnate ths step completely by povdng values of the equed quanttes. Ths fault locaton detecton algothm when used n conjuncton wth PMU technology can povde fault locaton wth an accuacy of 1% [56]. 6.2 Futue pospects of the poposed PMU The thess poposed a new concept of PMU that could be bult n only a couple of hunded dollas and mnmum cost fo communcaton nfastuctue. Evaluaton of ths PMU shows that t s adequate n tems of accuacy fo dstbuton netwoks. Fault locaton detecton usng these PMUs can help n effcent estoaton of sevce, educe loss, help n load balancng and load sheddng, topology analyss, event montong and many othe applcatons can beneft fom montong n dstbuton netwok. Futhe eseach ncludes fabcatng such a system usng pnted ccut boad and late n the fom of a chp. The testng of ths system aganst vaous communcaton netwok 41