An Adaptive Over-current Protection Scheme for MV Distribution Networks Including DG

Transcription

1 An Adaptve Over-current Protecton Scheme for MV Dstrbuton Networks Includng DG S.A.M. Javadan Islamc Azad Unversty M.-R. Haghfam Tarbat Modares Unversty P. Barazandeh Ghods Nroo Consultng Engneers Abstract- Conventonal electrc dstrbuton systems are radal n nature, and are suppled through a man source. These networks have a very smple protecton system whch s usually mplemented usng fuses, reclosers, and over-current relays. Recently, great attenton has been pad to applyng Dstrbuted Generaton (DG) throughout electrc dstrbuton systems. Presence of such generaton unts n a network, leads to losng coordnaton of protecton devces. Therefore, t s requred to have an algorthm n hand whch s capable of protectng dstrbuton systems that nclude DG, through dagnoss and solaton of occurred faults. In ths paper, a new approach for protectng of dstrbuton networks n presence of DGs wll be presented. The algorthm s based on dvdng an exstng dstrbuton network nto several zones; each of them s capable of operatng n sland operaton. The proposed scheme has been mplemented on some part of a real dstrbuton network and performance of the proposed scheme s tested on t. For smulatng the sample network and for mplementng the relay algorthm, DIgSILENT Power Factory and MATLAB are used respectvely. Index Terms- Dstrbuted generaton, Dstrbuton system, Protecton, Protectve devce coordnaton, Recloser I. INTRODUCTION Tradtonal electrc dstrbuton systems are radal n nature, and are suppled through a man source, therefore t s smple to desgn protecton scheme for such networks. Recently, great attenton has been pad to applyng Dstrbuted Generaton (DG) throughout electrc dstrbuton systems, and presence of these generaton unts results n not havng a radal dstrbuton network, consequently rases some problems such as losng coordnaton of protecton devces [1] [3]. Generally, problems that arse due to applcaton of DGs are: false trppng n feeders, false trppng n generaton unts, protecton blndng, ncreasng and decreasng short crcut levels, undesrable network slandng and preventng automatc and asynchronous reclosng [4]. Appearance of these problems depends on the characterstcs of network and DGs and n most cases network protecton scheme must be thoroughly changed n order to avod the mentoned dffcultes. Such changes may be complcated, snce t s needed to model whole dstrbuton system ncludng dstrbuton network n addton to DG, consequently obtanng the best protecton scheme s stll dffcult [5]. When DG unts are connected to a dstrbuton network, the system wll be radal no longer and ths means losng the exstng coordnaton among network protecton devces. The extent n whch a DG affects protecton coordnaton depends on DGs' capacty, type and nstallaton locaton of them. Due to generaton capacty and nstallaton locaton of DG, there are ranges n whch protecton coordnaton s mantaned and n some cases no protecton coordnaton can be acheved [5] [9]. Regardng the nfluence of DGs on protecton of dstrbuton systems, so many researches have been performed so far as well as some researches concernng how to tackle the resultant problems of applyng DGs [9]-[16]. In ths paper, a new protecton scheme for dstrbuton systems n presence of DGs s proposed. In the proposed scheme, systems protecton s carred out through a computer-based relay whch s nstalled n sub-transmsson substaton. The relay determnes system's status after t receves the requred network data, and n the case of fault occurrence t dagnoses ts type and ts locaton and fnally ssues the proper commands for protecton devces to clear the fault and to restore the network. II. GENERAL VIEW OF THE PROPOSED SCHEME The man purpose of a protecton scheme n dstrbuton systems s to dagnose the faulty part and solate t from the rest of the system. In tradtonal dstrbuton systems, when a fault occurs n a specfc part of t, whole downstream network s dsconnected from the rest of the system or suppled through network te lnes. Assumng t s mpossble to supply through other parts of network and a DG exsts n the downstream network of the faulty part, accordng to conventonal protecton logc, t wll not be possble to utlze the DG any more. Ths wll result n not to be able to utlze DG sources optmally, and amount of ENS (Energy Not Suppled) n network ncreases so system relablty decreases. Thus, n proposed scheme, general approach s to utlze DGs to the fullest n sland operaton when fault occurs. In the suggested scheme, dstrbuton system s dvded nto several zones n such a way that n each zone there s no DG, or f there s any, balance of generaton and consumpton n that zone s mantaned, regardless of whole generaton network, and only usng the power generated by DGs that exst n that zone. In other words, dstrbuton system s dvded nto two categores that have the followng characterstcs: 1. Frst category ncludes those zones whch have not DG and ther loads are fully suppled through the entre network and other zones of dstrbuton network. 2. Second category ncludes those zones whch have DG; t s obvous that at least one generaton unt n each zone /08/$25.00 '2008 IEEE 2520

2 must be equpped wth frequency control system n order to be capable of controllng zone frequency n the case that the zone s n sland operaton state. As t can be vewed n Fg. 1, a number of crcut breakers are placed n network to nterconnect the zones. These breakers have fast and consecutve open and close capablty as well as recevng remote open and close command. Besdes, these breakers must be equpped wth check-synchronzaton functon to be able to mantan zones synchronzaton when t s needed to connect two sland zones. To mplement the proposed algorthm, a computer-based relay wth capablty of performng calculatons and storng data must be nstalled n sub-transmsson substaton. Ths relay s able to receve requred nput data (that are provded through measurng some network parameters), to process them, and n the end, to dagnose locaton and type of fault n order to send proper commands to protecton devces. III. NETWORK ZONING PROCEDURE The appled procedure n zonng a dstrbuton system s consderng one zone for each DG, startng from the begnnng of feeder, and each zone extends to the end of feeder as long as the DG wthn that zone s capable of supplyng average load of the zone. When average load of substatons located n the zone exceeds generaton capacty of zone's DG, zone border fnshes, and two crcut breakers are nstalled n the begnnng and n the end of zone ponts. In the case that there exsts a second DG located wthn the supplyng lmt of frst zone's DG, and as long as zone's average load does not exceed generaton capacty of frst DG's capacty, whle movng towards end of feeder, the second DG s regarded wthn the same zone and zone border extends as far as the zone's average load does not exceed summaton of two DGs' capacty. The reason to consder zonng procedure from the begnnng of feeder to ts end s to allow more loads to be suppled through upstream network. Ths approach ncreases network relablty and decreases total ENS of the system. Of course, when DG's capacty s hgher than the loads of substatons located n ts downstream network, zone extenson border should be consdered upward. S CB 1 Z 1 CB 2 Z 2 CB 3 Z 3 Dvdng Network nto Zones Offlne Calculaton Results Measurements Onlne Calculaton Fg. 1. General vew of proposed algorthm Dagnosng Type and Locaton of the Fault Sendng Requred Sgnals for Restoraton The reason to support the dea of consderng average load n zonng s that n dstrbuton systems, daly consumers' loads are varable to a great extent. In other words, hourly curve of consumers loads, and consequently substatons loads, have so many maxmum and mnmum ponts n such a way that peak load mght be four tmes of base load. Regardng the fact that duraton of peak load s short, f zonng s done based on peak load, vastness of zones contanng DGs wll be very small, and therefore more consumers wll face power cut when a fault occurs n the system. On the contrary, f average load s consdered as the bass of zonng nstead of peak load, vastness of zones contanng DGs wll ncrease whch wll result n gvng dstrbuton system consumers more chance of beng suppled n the case of fault occurrence. It s requred to menton that f ths procedure s chosen for zonng; all dstrbuton system loads (.e. all dstrbuton substatons) must be equpped wth load break swtch. Ths enables central relay to supply system loads regardng ther mportance and to dsconnect some of them to mantan the balance between generaton and consumpton n the case of fault occurrence n the peak-load tme. Obvously these swtches must be capable of recevng remote control commands as well. IV. REQUIRED INPUT DATA The needed nput data for correct operaton of relay s as follows: 1. Techncal characterstcs of all network devces such as dstrbuton substatons, lnes and DGs; 2. Estmated hourly load curve for all loads of network and ther degree of mportance; 3. Data regardng network zonng; 4. All operatonal data of the man relay n dfferent faults occurrence; V. REQUIRED MEASUREMENTS To mplement the proposed protecton scheme, t s requred to carry out the followng measurements and contnuously provde the man relay wth ts results. 1. Synchronzed three-phase current vectors flowng through all DGs and through man source; 2. Synchronzed three-phase current vectors flowng through laterals, except those branches that have DG; 3. Synchronzed three-phase current vectors flowng through zone-formng breakers; 4. A sgnal whch s ndcatve of current drecton flowng through the breakers that form zones; VI. PROCEDURE OF OFFLINE CALCULATIONS Offlne calculatons consst of load flow studes and short crcut analyss for all types of faults and n all ponts of network. Then for all faults, currents flowng through all DGs, man source and laterals are determned. Also characterstcs of Mnmum Meltng (MM) and Total Clearance (TC) of all fuses n network must be stored. Usng the above characterstcs and storng them n relay, t s possble to perform software 2521

3 coordnaton of fuses and the reclosng operaton; ths wll be dscussed n secton XIII. The tme before fuses to be melted s calculated out of MM characterstcs and short crcut results. It s also needed to update all calculaton results for any network change. For nstance, when there s a change n network confguraton, lke dsconnecton of a lne, t s requred to update network admttance matrx and redo load flow and short crcut analyss. To have DGs' currents, laterals and man source for all types of faults and n all network buses, a table can be provded and through comparng the above values n ths table, t wll be possble to dagnose the exact locaton and type of fault. After dagnoss of locaton and type of fault, whch leads to dagnoss of faulty zone, relay sends dsconnecton sgnals to approprate breakers to solate the zone from other zones of network and consequently the faulty zone s solated from network. In bref, procedure of offlne calculatons can be stated as follows: 1. Recevng network data; 2. Performng load flow calculatons; 3. Performng short crcut analyss for all types of faults and n all network buses; 4. Extractng all requred fault currents for each type of fault and n each network buses; 5. Extractng the tme whch s needed network fuses not to be blown, out of MM curves; VII. ONLINE DIAGNOSIS PROCEDURE OF FAULT LOCATION As t was ponted out before, synchronzed three-phase current vectors for all DGs and man source are avalable and summaton of these values s always equal to network load. If a fault occurs n some pont of network, ths summaton suddenly ncreases and s greater than whole network load. In ths stuaton, the man relay nstalled n sub-transmsson substaton dagnoses the fault. The stuaton n whch summaton of currents s zero shows that the occurred fault s wthn the protecton zone of one of DGs (between DG and the locaton that ts current s measured). In these crcumstances, the man relay wll not ssue any command to let ts own protecton system solate t from the rest of network. To detect fault locaton, currents of DGs, man source and lateral branches are compared wth the values of the table that was provded through offlne calculatons. Ths dea was ntroduced n [1] for the frst tme, but t has some problems n dagnoss of exact locaton of fault and sometmes encounters error and naccuracy regardng ths ssue. However the naccuracy n detecton of fault locaton decreases when number of DGs ncreases but n ths scheme, currents flowng through lateral branches wth no DG are montored as well, n order to ensure whether the locaton of fault has been determned correctly. Ths montorng has two advantages: frst, provdes man relay wth more quanttatve values for dong requred comparsons and consequently the error n determnaton of fault locaton decreases consderably, second snce measurements are carred out n the laterals contanng no DG, for those faults that do not occur n these laterals the measured current s equal to load current and ths determnes that no fault has occurred n laterals. Thus t s possble to ncrease consderably the accuracy of proposed scheme n [1], through applyng ths capablty. VIII. ISOLATION OF FAULTY ZONE AND NETWORK RESTORATION After relay dagnoses fault locaton and faulty part of network s detected, t s tme to solate the faulty zone from the rest of network and restore the network. Needed data s avalable n database to desgnate those breakers that must ssue trppng sgnals. The general approach s that after detecton of faulty zone, the relay sends dsconnecton sgnal to ts solatng breakers, ts downstream network's CBs and all DGs located n the faulty zone. In ths stuaton, upstream network of faulty zone s entrely suppled through man source and ts DGs, the faulty zone faces power outage and downstream zones of faulty zone are suppled through DGs wthn them, f that specfc zone has DG or DGs, otherwse faces power outage. Consderng the fact that 80 percent of faults that occur n dstrbuton networks are transent n nature (In ths paper, faults lastng less than few seconds are classfed as the transent faults.), network must be capable of lettng transent faults to be cleared. In conventonal dstrbuton systems ths s done usng reclosers, and n the proposed scheme reclosng operaton s carred out usng zones solatng swtches, through man relay control. The operaton procedure s that after solaton of faulty secton, reclosng s performed through connectng zone breaker to ts upstream network at the command of man relay. After each reclosng operaton, relay nvestgates network status and f the fault stll exsts, relay ssues dsconnecton command. In the case that fault s transent and s removed durng reclosng operaton, relay ssues reclosng as well as synchronzaton and restoraton commands. To have a successful process, t s requred to perform reclosng operaton before fuses of network are begnnng to melt. Regardng the pont that MM curves of all fuses are avalable n relay database, and the tme to avod fuse blowng has been determned as a result of offlne calculatons, consequently relay can perform reclosng operaton at proper tme. Besdes, reclosng operaton must be coordnated wth fuses' characterstcs. To acheve ths coordnaton, MM and TC curves of all fuses are stored n relay and relay determnes and ssues the best dsconnecton tme of related breaker usng dfferent types of faults, ther respectve currents and curves of fuses. For example, n Fg. 1, f a transent fault occurs n Z 2, relay frst sends dsconnectng sgnal to CB 2, CB 3 and all DGs located n Z 2 and then sends reclosng sgnal to CB 2 to dagnose transent fault. In the end, after clearance of fault, relay sends reclosng sgnal n addton to synchronzaton sgnals to CB 3 and all DGs located n Z 2 to restore the network completely. 2522

4 IX. LOAD SHEDDING There are dfferent methods of load estmaton n dstrbuton networks. For nstance, among whch one can menton nstallng load recorders n dfferent ponts of network that measure load at specfc tmes and estmate network load for other tmes. Due to the vastness of load estmaton topc n dstrbuton systems, ths paper has not dealt wth t, and for network zonng and operaton of load sheddng t s assumed that hourly load curves for all substatons of dstrbuton network have been estmated before, and are avalable for performed studes. When t came to zonng ssue, t was stated that operaton of load sheddng must be performed when network faces two condtons. Frst, a fault occurs n a zone where there exsts at least one zone n ts downstream network, ncludng one or more DGs, and the zone goes nto sland operaton status due to the dsconnecton command ssued by relay and consequent operaton of the crcut breakers. Second, a fault occurs when the load of an sland zone s hgher than the generaton capacty of DG or DGs wthn that zone, n other words the load s hgher than zone's average load. In these condtons f dsconnecton of load does not happen, frequency of the zone operatng n sland status drops consderably and ths leads the zone encounters power cut problem. To perform load sheddng accurately, t s requred to have all loads of substatons at each tme as well as ther degree of mportance, n order to be able to determne prorty of loads dsconnecton and mantan the balance between generaton and consumpton. Ths needs precse loads montorng of each and every sngle of the exstng substatons n a dstrbuton network, whch s very costly and s practcal n no network. Thus, t s requred to apply some approxmate approaches and montorng of currents n the most approprate ponts of network to make the best decson, regardng facng the lowest possble power cut n zones as well as mantanng the balance between generaton and consumpton. In ths paper, ths algorthm has been mplemented usng contnuous montorng of currents flowng through crcut breakers that form zones. Ths s done usng the fact that at each tme the whole load of each zone can be calculated usng (1): P P (1) L = PCB + In ths equaton P CB ndcates the whole power flows nto zone and P DGj ndcates the whole power of all DGs placed n that zone. It s obvous that negatve value of P CB for a specfc zone shows that generaton n that zone s hgher than load of that zone and the zone exports energy to other zones. Consderng the pont that at each tme relay montors powers followng through DGs and solatng breakers of zones, t can determne nstantaneous dfference between generaton and load of each zone, through calculatng nstantaneous P CB. Therefore, when network faces a fault, relay can determne whether there s shortage or excess of load n zones ncludng DG, usng these values for the tme exactly before occurrence j DGj of fault. For those zones that must operate n sland status and ther generaton s more than ther consumpton, relay does not shed any loads and lets zones' DGs mantan the balance between generaton and consumpton through frequency control system exstng n each zone. But, relay performs load sheddng operaton when there s overload wthn the zone. Relay sends dsconnecton command to load break swtches of loads through consderng a safety factor (for nstance 1.1), and also consderng mportance of loads as well as the estmated values of loads at the tme that fault occurs. The followng equaton shows the load that must be dsconnected n each zone: P = SF (2) cut P CB It mght be possble to mplement ths dea through modelng only some part of network loads and placng load break swtches for those loads (The loads that have lower mportance and ther summaton s equal to the dfference of peak and average loads of each zone). Obvously, ths s smpler and regardng economc consderatons, t has lower costs. But, t s mportant to note that through placng load break swtches for all loads of those zones that contan DG, flexblty of protecton scheme ncreases consderably, n such a way that f generaton capacty of DGs of a zone decreases (for any reason) or even f DGs go out of operaton (due to forced outage or for mantenance), there s no need to change the protecton algorthm of relay and the desgned protecton system wll be stll effectve. For nstance f there are two DGs located n one zone, and one of them goes out of operaton, t s possble the remanng DG not to be able to supply load, even n mnmum load regme. In ths stuaton, the man relay dsconnects loads tll the balance between generaton and consumpton s acheved. Of course, for ths stuaton, and for the stuaton n whch there s only one DG n a zone, and the DG has gone out of operaton for any reason, t s needed to update network data and redo offlne calculatons for the new network n order the relay avods wrong dagnoss of fault locaton. Apparently, for the stuaton n whch there s only one DG n a zone and that DG has gone out of operaton, the zone can be regarded as second type of zone (zones wthout DG) to be needless of operaton of load sheddng n that zone. Fgure 2 llustrates the load sheddng algorthm of the man relay. X. SIMULATION RESULTS Proposed algorthm n ths paper has been mplemented usng MATLAB and a software applcaton has been provded to mplement desgned protecton scheme and to smulate operaton of man relay nstalled n sub-transmsson substaton. The dstrbuton network that the proposed scheme mplemented on t, s a radal 20kVdstrbuton feeder wth a 4.9 MVA desel generator and s smulated usng DIgSILENT Power Factory. For each load a three-step hourly load curve s consdered, whch s shown n Fg. 3. The peak load for all loads s 2 MW and power factor for all of them and n every 2523

5 tme s Performng zonng procedure on ths network causes to have 3 zones that one of them ncludes DG and has the capablty of sland operaton. Fg. 4, shows sngle lne dagram and zonng approach of smulated feeder. To make sure the man relay operates accurately; ts operaton has gone through detaled nvestgaton wth dfferent faults but n ths paper the operatons of the man relay n the case of followng three scenaros are presented: Sngle-phase fault on the lne connectng buses 7 and 8; Two-phase fault on the lne connectng buses 3 and 4; Three-phase fault on the lne connectng buses 5 and 6; Start Sort all loads located n the zone regardng ther prorty durng slow reclosng operaton, Z 1 s suppled wth power and then closng sgnals are sent to CB 2 (n parallel wth network synchronzaton operaton) and CB 3. However, n the case f F 1 s not able to solate the faulty secton, CB 1, CB 2, and CB 3 reman open. Also, f the fault s transent and s cleared durng reclosng operaton, the network must be restored. To make restoraton, frst, connecton sgnal s sent to CB 1, then, CB 2 s closed wth network synchronzaton operaton, and fnally connecton sgnal s sent to CB 3. CB 1 CB 2 CB 3 F 1 F 3 CB 4 Stop No P = Q = L = 1 P>0 OR Q>0? Yes Select the Load NO. L and Dsconnect t P= P P L Q= Q Q L Fg. 2. Load sheddng algorthm of the man relay P Q CB CB L =L+1 1 st Zone 2 nd Zone 3 rd Zone Fg. 4. Sngle lne dagram of the studed dstrbuton feeder and ts zonng after connecton of DG To evaluate relay's load sheddng algorthm, the above fault has to be smulated for two condtons, peak load and mnmum load condtons. Smulaton results ndcate that at mnmum load condton, relay's load sheddng unt sends no command, but at peak load condton sends dsconnecton command to load break swtch of substaton 4 n order to mantan the balance between generaton and consumpton n Z 2 wth consderng. Output result of relay smulaton software applcaton s as follows: Load (MW) Tme Fg. 3. Hourly load curve of the smulated feeder's loads A. Sngle-phase fault on the lne connectng buses 7 and 8 In ths stuaton, the relay sends dsconnecton commands to CB 1, CB 2, and CB 3 mmedately after t dagnoses fault type as well as t determnes fault occurrence n the frst zone (Z 1 ). Thus, Z 1 and Z 3 face power cut and Z 2 go on operatng as an electrc sland. Then, to dagnose whether fault s transent reclosng operaton s done by CB 1, ths operaton s also coordnated wth F 1 by the man relay. Obvously, there s no need to synchronze the network, snce Z 1 has faced power outage. If the fault s permanent, F 1 cuts the faulty branch and The fault s sngle-phase fault. The faulted secton s secton 3 whch connects bus 7 to 8. The faulted zone s zone 1. CB1 ==> Opened CB2 ==> Opened CB1 ==> Reclosed For transent fault: CB2 ==> Closed CB3 ==> Closed (wth synchronzng functon) For permanent fault: CB1 ==> Opened CB1 ==> Reclosed If F1 removed the fault then: CB2 ==> Closed CB3 ==> Closed (wth synchronzng functon) If F1 dd not remove the fault then: CB1 ==> Opened Load 4 ==> shed B. Two-phase fault on the lne connectng buses 3 and 4 In ths stuaton, the relay sends dsconnecton commands to CB 2, CB 3, and CB 4, mmedately after t dagnoses fault type as well as t determnes fault occurrence n the second zone (Z 2 ). Therefore, Z 2 s solated thoroughly. Thus, Z 1 s suppled by 2524

6 the man source and Z 2 and Z 3 face power cut. Then, to dagnose whether fault s transent reclosng operaton s done by CB 2. Obvously, there s no need to synchronze the network, snce Z 2 has faced power outage. If the fault s permanent, CB 2, CB 3, and CB 4 reman open. But, f the fault s transent and s cleared durng reclosng operaton, the network must be restored. To make restoraton, connecton sgnals are sent to CB 3 and CB 4, as well as network synchronzaton operaton. Output result of relay smulaton software applcaton for ths scenaro s as follows: The fault s two-phase fault. The faulted secton s secton 5 whch connects bus 3 to 4. The faulted zone s zone 2. CB2 ==> Opened CB4 ==> Opened CB2 ==> Reclosed For Transent Fault: CB3 ==> Closed CB4 ==> Closed (wth synchronzng functon) For Permanent Fault: CB2 ==> Opened Load Sheddng s not requred n ths stuaton. C. Three-phase fault on the lne connectng buses 5 and 6 In ths stuaton, the relay sends dsconnecton command to CB 3, mmedately after t dagnoses fault type as well as t determnes fault occurrence n the thrd zone (Z 3 ). Therefore, only Z 3 faces power cut and Z 1 and Z 2 are stll suppled through man source. Then, to dagnose whether fault s transent reclosng operaton s done lke what was done for prevous stuatons, except that ths tme relay assgns ths task to CB 3. If the fault s transent and s cleared, relay sends reclosng sgnal to CB 3 to supply Z 3 through ts upstream network. If the fault s permanent, CB 3 reman open and Z 3 faces power outage. Output result of relay smulaton software applcaton for ths scenaro s as follows: The fault s three-phase fault. The faulted secton s secton 8 whch connects bus 5 to 6. The faulted zone s zone 3. CB3 ==> Reclosed For Transent Fault: No sgnal s needed. For Permanent Fault: Load Sheddng s not requred n ths stuaton. XI. CONCLUSION In ths paper, an algorthm for protecton of dstrbuton networks n presence of DG was proposed. The algorthm uses network zonng approach, n whch each zone s an ndependent secton, capable of sland operaton whenever needed. In the proposed algorthm, after dvdng dstrbuton system nto several ndependent zones, computer-based relay that has been nstalled n sub-transmsson substaton, dagnoses exact fault locaton through makng comparson between needed measured currents and results of offlne calculatons and sends requred commands and sgnals to protecton devces n order to solate the faulty zone from the rest of network. Besdes, the algorthm has the ablty to perform reclosng operaton as well as coordnaton wth fuses of network usng software procedures. In the end, when fault s cleared, network restoraton n addton to synchronzaton operaton s done through sendng reclosng commands to crcut breakers. 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