Connection of a Distributed Resource to 2-Transformer Spot Network

Transcription

1 1 Connection of Distributed Resource to 2-Trnsformer Spot Network M. Bier, Senior Member IEEE, W. E. Feero, Fellow IEEE, nd D. R. Smith, Fellow IEEE Abstrct--Recent energy crisis brought bout renewed interest in connecting distributed resources (DR) in spot networks of downtown metropolitn res. Severl importnt sfety nd technicl constrints exist tht need to be ddressed by the developer nd the utility. While interconnecting DR to the utility is often complex issue, interconnecting to secondry networks is lwys complex nd not lwys possible. This rticle highlights n pproch tken for successful instlltion of 75kW DR to spot network supplying n office building. In the subject cse n indiscriminte connection of DR would hve resulted in serious consequences to system integrity. The solution consisted of pplying power relys for utomtic supervision of the DR depending on the conditions in the system. The signls were derived directly from the network protectors ensuring full integrity of the system. Tests demonstrted tht the instlltion performed s designed. DR is now in opertion for one full yer. Index Terms--Genertor, distributed genertion, distributed resource, secondry network, spot network, network protector, network trnsformer, overpower, underpower. I. INTRODUCTION The growth of electricity consumption nd our dependence on this form of energy in the United Sttes brought bout some widely publicized concerns of power shortges in the lst decde. These concerns were deepened in some res by the power industry deregultion. Prolifertion of distributed genertion ws nturl rection of the consumers to the vilbility of power reflected by its price. Downtown res re becoming prime cndidtes for the distributed genertion, s these res re growth-constrined. It is in these downtown res where secondry networks re very common type of power distribution system. The types of distributed resources (DR) tht re typiclly found in the municipl setting my rnge from the more conventionl reciprocl engine-powered sources fueled by diesel or nturl gs, to more modern types such s micro turbines, PV nd fuel cells. Smller units typiclly employ induction type genertors while lrger units (from hundreds of kw to megwtt outputs) use synchronous genertors. The genertors re commonly directly coupled to the power system lthough the use of low-hrmonic, solid stte inverters is becoming very common for newer types of DR. Secondry networks were designed to ccommodte the requirement for higher relibility nd better voltge regultion in high lod density res in downtown metropolitn res in North Americ. Since its inception in the twenties, this type of distribution hs seen stedy growth. The networks re multiple feeder systems meshed together through network trnsformers so tht every low voltge secondry node (bus) is supplied from t lest two sources. The most common network voltges re 120/208V nd. Becuse these systems provide service to individul customers, the networks re commonly solidly grounded t the network trnsformers. Individul network brnches re switched by specil circuit breker clled network protector (NWP). The network protector is typiclly housed in wtertight enclosure ttched to the low voltge side of the network trnsformer nd situted in the vult under the street or in the bsement of building. The network protector is designed with very sensitive nd fst reverse power protection with the objective to isolte the network in the event of the outge or fult on the primry feeder. The tripping time of the NWP is s short s few cycles (50ms). Once the fult on the primry of the network trnsformer is clered, the protector utomticlly recloses when the voltge t the network trnsformer is restored to its required level. Norml lods tht re connected to the secondry bus do not produce sustined currents in the tripping direction of the NWP, but genertion connected to the secondry bus my. It is this rpid tripping from bck-fed currents tht poses the concern of system integrity when the distributed resources re connected to the spot network. The consequences of islnding nd the inbility of the networks to self restore their opertion under these conditions re the principl resons for the reluctnce to ccept distributed genertion in secondry networks. As n illustrtion of the sensitivity of the network protector reverse tripping cpbility, severl instnces hve been reported when, during light lod conditions in the spot network-supplied high-rise building, elevtors tripped protectors during their brking regime. Under light lod condition, quite conceivble for lrger DR, n entire spot network cn become n islnd. This is condition tht is totlly uncceptble with one of the following consequences: Ctstrophic filure of the NWP: Most in-service units re not rted for the seprtion of two independent, non-synchronous systems. Islnd gets reconnected by NWP uto reclosing mster nd

2 2 phsing relys, creting hzrdous condition. The NWP relys re not designed nor tested to function in non-synchronous systems. Sfety Hzrd: Losing control of the supply voltge nd exposing the public to potentil life sfety sitution is violtion of regulted utility chrter. Outge: If DR itself doesn't sustin norml opertion (induction genertors, inverters) outge results. Equipment Dmge: DR on n islnd results in hzrdous condition, such s self excittion, nd customer's equipment cn be severely dmged. This rticle demonstrtes tht in certin system configurtions, islnding cn be prevented so tht these very rel concerns will not mterilize. However, even with the prevention of islnding, there re other issues tht must be ddressed such s NWP cycling nd mintining true network qulity service. One such solution ws implemented by the uthors of this rticle nd is described herewith. Finlly, severl importnt conclusions re drwn nd guidelines re provided to determine whether n instlltion qulifies s the DR site. These principles re illustrted on the exmple of 2-trnsformer spot network for trnsprency. However, most of the conclusions nd guidelines derived herewith re believed to be generlly pplicble lso to spot networks contining three or more trnsformers. II. CHALLENGE OF OPERATING DR ON SPOT NETWORKS Mny utilities re reluctnt to ccept indiscriminte deployment of distributed resources in secondry network systems becuse of the concern of DR jeoprdizing life sfety, system integrity, nd power qulity. This is especilly true for spot networks lcking lod diversity where during the minimum lod condition, one or more network protectors could trip on reverse power flow out of the network. Furthermore, under certin circumstnces or when the distributed resource is of type tht could sustin fult current, it could trip one or more network protectors. This is why dditionl mesures my be necessry to sfegurd the instlltion when distributed resources re operted on the spot networks. This rticle discusses n pproch of sfegurding the spot network by detecting the bove conditions nd disllowing the opertion of the DR before the network protectors open. A minor trde-off is necessry under low current bckfeed conditions, s the network protector trip hs to be delyed by frction of second to llow removl of the locl DR from the system. This minor modifiction of the NWP tripping chrcteristic only tkes effect for the low current mgnitudes so tht the power qulity is not compromised during system fults. The concept relies on supervising the power supplied from ech protector to the spot network with three-phse under power rely. True three-phse power elements with instntneous trip contcts re employed. Even in multitrnsformer spot networks, individul monitoring of ech network protector (s opposed to using totl power summtion scheme) my be justified due to unequl lod shring between the trnsformers. The underpower element (ANSI Device Number 37) senses n instntneous power flow to the spot network through the network protectors nd trips the DR unit when such power flow drops below the minimum permissible threshold. Once tripped, DR is not llowed to reconnect to the system until time dely, typiclly five minutes, hs elpsed, during which time certin minimum number of NWPs need to be closed nd their rel lod remining bove the mximum power threshold. The resetting of the circuit logic following the stbiliztion of the lod bove the mximum power threshold is controlled by seprte power element. The time dely is djustble from 1 to 300 second. In ssessing the fesibility of given instlltion, the lod profile of the fcility hs to be well known. It is often tempting to use power billing informtion vilble from the ccounting deprtment or the utility. This my be n entirely cceptble prctice in the instlltions where the size of the connected DR is only smll frction of the minimum lod of the fcility. In more generl cse, or when the DR size pproches or exceeds the minimum lod of the spot network, more cutious pproch is dvised. Specificlly, distinction should be mde between the type of dt vilble from the energy meters nd the specilized power nlyzers. As the demnd redings re typiclly bsed on fixed intervl (commonly 15 or 30 minute) verges, they tend to msk intermittent power fluctutions nd swings. Such power swings cn be often cused by strting of lrge rotting lods or, more importntly, by the regenertive swings (brking) of the elevtors. Lod study is lso necessry to rrive t the suitble power settings, once the instlltion is deemed fesible. The minimum power threshold is estimted from the mesurement of the minimum system lod nd ny experience tht hs demonstrted minimum lod without protector cycling. This estimte should tke into ccount the fct tht the network protectors my not shre the lod eqully. Strting t the instnce of low mgnitude reverse current in network protector which might be cused by the presence of the DR, the network protector tripping hs to be delyed slightly using specil delyed tripping function vilble with modern network protector microprocessor-bsed relys. Emphsis should be given to delying the protector trip only for the low current mgnitudes while preserving the instntneous trip function for the higher currents (fults). This function is similr to the time-current chrcteristic of the combintion of n instntneous nd time-dely overcurrent element (50/51) frequently used for the coordinted protection of rdil distribution feeders. The ppliction lso needs to be scrutinized for the

3 3 scenrio of the fult on the primry side of network trnsformer. This is illustrted on n exmple of 2- trnsformer spot network by Fig. 1 nd Fig. 2 respectively. For the purpose of illustrtion resistnces nd conductor impednces were neglected. Approximte fult current contributions for bolted three-phse fult t the primry of the network trnsformer re clculted using current superposition method. Bse quntities of 480V nd 1202A (1MVA) were used for the clcultions. The results re presented in per unit nottion nd rounded. Fig. 1 illustrtes the effect of fult current contributions in the system with the primry substtion tie open. It is noted tht, in the protector djcent to the one ssocited with the fulted feeder, the fult current contribution from the utility tends to overcome the genertor contribution nd the protector remins closed. In the protector ssocited with the fulted feeder, on the other hnd, both system nd DR contributions ct in the sme direction. The resulting flow into the trnsformer is much higher thn tht of genertor contribution lone nd the protector trips instntly. The reltive mgnitudes of both fult contributions could be of significnt importnce especilly when desensitizing feture is used in the network protector relys. the point tht both network protectors sense the similr reverse flow. The genertor is effectively the only source of the fult current from the network. Becuse this fult current contribution is much less thn with the system contributing, it is possible to dely the tripping for frction of second without exceeding the feeder fult rting. After the feeder breker hs clered the fult, the low mgnitude fult current from the DR will be overwhelmed by lrge fult current contribution from the utility bck feeding the fult nd the NWP will trip instntneously. Only if the fult current stys low, s in the cse of high impednce fult, will the NWP not trip until the full time dely. Under this circumstnce if the instntneous trip point is set low enough, there will be no compromising of the power qulity in the network. 20 MVA, 10% 5.1 % imp. (ssume infinite bus ) TIE CLOSED 3p or L-L fult 5.1 % imp. (ssume infinite bus) 20 MVA, 10% 20 MVA, 10% TIE OPEN 200 pu TRIP! 0.26 pu 0.26 pu TRIP! 5.1 % imp. 3p or L-L fult 5.1 % imp. 9.2 pu 9.5 pu TRIP! INDUCTION GENERATOR 75 kw, 0.8 PF Xd =0.176 pu DR Fig. 2. Fult currents for primry fult, tie closed Fig. 1. Fult currents for primry fult, tie open DR INDUCTION GENERATOR 75 kw, 0.8 PF Xd =0.176 pu Fig. 2 shows the system with the primry tie closed, the preferred feeder configurtion for supplying secondry networks. When the bolted three-phse fult occurs in the electricl vicinity of the primry distribution bus, the system contribution through the network trnsformers diminishes to Fig. 3 uses semi-logrithmic grph of current decrement of both components of the short circuit current to illustrte the time-current coordintion of the NWP trip under the reverse power flow condition. During light lod condition, one protector my open nd remin open while the other protector is crrying the lod. This condition is commonly referred to s "flot" condition. This property brings nother interesting problem to our discussion. Let us consider configurtion with tie open depicted by Fig. 1. Assuming tht the fult on the primry feeder is ssocited with the closed protector, it is esy to follow tht unless the protector tripping is delyed, islnding

4 4 my results s direct consequence of dding DR to the network. Even without split bus feed tht cn cuse power flows which open one of the protectors, mintennce procedures or circulting currents under very light lod conditions cn cuse one of the protectors to open. When this occurs, which ever is the cuse, the NWP mster nd phsing relys re designed to wit for preset voltge mgnitude nd ngle difference cross the protector before they permit its reclosing. This feture is intended to ensure tht power will continue to flow into the network fter the protector closing, therefore preventing its cycling. Becuse of this feture, the presence of DR could significntly dely or prevent the reclosing of the second protector, s DR is effectively reducing the lod on the in-service network protector(s). Such delys would not only increse the chnces of islnding, but lso directly effect the time tht the network is exposed to feeder fult which could result in network outge. Fig. 3. Time-Current Coordintion of NWP Trip In the following section, protective settings will be discussed with regrd to the minimum coordintion of the DR/Network interfce protection with the network protectors. First, n overpower trip setting, P 0, should be determined. This is the power flow tht must be relized in ech closed protector following genertor tripping from n underpower condition, before the genertor is llowed to reconnect. It is bsed on the minimum number of network protectors supplying multi-trnsformer spot network for which DR is to be llowed to remin on line, s follows: P 0 = 1.2(P G /n + P U + P) 1 (1) where P U P G n P desired under power setting per protector (DR trip power level) while positive nd preferbly below the minimum mesured instntneous network lod; rted genertor output (ggregte from ll sources); number of trnsformers or NWPs supplying the spot network; sfety mrgin reflecting the network lod fluctution. An rbitrry multiplier of 1.2 is used here s design sfety mrgin. This multiplier my reflect unequl lod 1 select next higher vilble setting shring between trnsformers in the spot network if the lod summtion scheme is employed. There should be no intentionl dely introduced in the tripping of the DR when n underpower condition occurs. This llows the closest coordintion of the DR tripping time with the tripping chrcteristic of the network protectors. The minimum tripping dely of the network protectors equipped with microprocessor relys, for exmple, is 0.25s. A totl DR tripping dely of 125ms gives protective mrgin of nother 125ms, which is cceptble for the coordintion between two clibrted solid stte protective devices. Selection of the overpower setting needs to reflect the ctul lod profile in the fcility. If it is too low, excessive cycling of the DR my result. Too high setting, on the other hnd, my unduly restrict the opertion of DR thus reducing the expected revenues. Closer settings my be llowed if the DR hs cpbility to regulte its power output s function of the lod in the fcility. As stted erlier, the reverse tripping of ll network protectors in the spot network should be delyed only by minimum time to llow disconnection of DR during specific system conditions tht could cuse the spot network to become n islnd. Further, the overcurrent setting of the time dely function only needs to be set bove the subtrnsient contribution from ll downstrem sources. This wy the network protector trip is delyed only for the current mgnitudes from the distributed resource nd not for the contribution from primry source tht would previl for fults on supply feeders s shown erlier. Network protectors re designed nd tested to trip on the primry fults of very high mgnitudes when the network voltges re severely depressed. The DR/Network protection needs to function t these low voltges, nd should exhibit relible tripping t low power fctor, typiclly 0.15 lgging. III. CASE STUDY A utility in the Est ws pproched by their customer who plnned to connect 75kW induction genertor to the 480V spot network supplying their fcility. The fourwire building network is supplied by two network protectors rted 1875A equipped with microprocessor relys. The protectors re throt-connected to the two 1000kVA network trnsformers in the vult. The NWP mnufcturer nd n independent consultnt were pproched by the utility to propose solution tht would llow connection of the sid genertor without jeoprdizing the life sfety nd relibility of the power system. This section discusses the proposed solution. The existing system prior to modifiction is shown in Fig. 4. The utility ws concerned tht during the minimum lod condition in the network while the DR were running, one or both network protectors could trip on reverse power flow out of the network. Another concern ws the potentil of the protectors' trip during the fults upstrem of the network protectors. The lod of the building consists of lighting,

5 5 computer lods, ir hndling equipment nd elevtors. Oneweek long monitoring of the lod supplied by ech network protector ws conducted to cpture the lod profile nd the instntneous lod swings not pprent from the demnd nlysis. A dedicted 3-phse power nlyzer ws set up to monitor the power with one cycle resolution. The results of lod survey reveled tht the totl building lod reched 500kW during hot spring dy, while the minimum system lod ws bout 120kW. NORMALLY OPEN 62 timer 120 V, 0-300s; 86 lockout rely with mnul reset; 94 trip rely with electricl reset; CPT control power trnsformer 480/120V; PSS power supply sttus contcts of device 37; DR distributed resource (genertor); TCM trip circuit monitor rely. The protection pnel houses two dul-element three-phse power relys (device 37), the control power trnsformer, nd other uxiliry control devices. Sttus indicting lights nd n emergency trip push button were plced on the outside of the enclosure door. The front view of n open cbinet is in Fig. 7. Primry feeders NWP - Network Protector Type CMD,, 1875 A TERTIARY NETWORK VAULT 37 3-ph CONTROL CIRCUIT 3-ph /5 MPCV MPCV CUSTOMER FEEDERS Fig. 4. Existing Distribution System To mintin sttus quo of the protector system until operting experience with this system could be developed, ech underpower rely hs been conservtively set to trip the DR upon reching the underpower threshold of 59.6 primry kw in ech protector. The reset power level (overpower setting) ws set t primry kw per protector. It is nticipted tht once the operting experience with this system is gined, these settings cn be reduced to s low s 30kW for the under power unit nd 88kW for the over power unit. The delyed tripping function of the type MPCV microprocessor rely is known s the BN function. It introduces the trip dely djustble between 0 nd 300 seconds in 0.25s increments for currents within n overcurrent (O/C) threshold djustble between 1% nd 250% of the NWP current trnsformer rting. Above the O/C setting the protector reverts to instntneous tripping. For this instlltion the O/C ws set t 50%. To ccommodte the dditionl protection, seprte protection pnel ws plced outside of the protectors in the vult. The current nd potentil circuits were brought out from ech protector using conduits nd specil wtertight hrdwre. The schemtic representtion of dditionl circuits is in Fig. 5. The new nd retrofitted equipment is shown in dshed lines. Fig. 6 shows the control circuit used to supervise the DR opertion bsed on the lod conditions in the network nd the sttus of the network protectors where NWP is network protector type CMD,, 1875A (1600A CT rting), retrofitted with MPCV rely; 37 Under/Over power rely BE1-32 O/U; Relys power supply 480 V CPT 120 V Gurd Control Ckt. Fig. 5. Proposed Modifictions L N 120 VAC (from CPT) TCM DR Locl Lod b b 37-U-1 37-U TCM b b EM 37-O-1 PSS-1 PSS-2 37-O R 94 O b (On dely) (Mnul reset) b 94 b 86 Fig. 6. Proposed Control Schemtic of DR Control Circuit The three-phse relys sense the rel power in the norml direction (into the network) in ech protector nd instntneously close their contcts when the power in either network protector decreses below n underpower setting. The trip contcts re wired in the control circuit of the genertor. Once the genertor trips, it cnnot be reconnected for up to five minutes fter the power of the fcility returns nd remins bove the relys overpower settings. This feture is incorported in order to prevent on/off cycling of the DR. This is illustrted in Fig. 8. Severl dditionl fetures were incorported in the design to further sfegurd the system from islnding: Permit DR

6 6 DR Trip (opertion suspended): When one or both NWPs open or control power is lost indicted by n mber light "Trip" (this condition is subject to uto reset). DR llowed to operte: Indicted by white light "Redy". DR emergency trip: Loss of rely power or blown control fuse indicted by n mber light "Locked Out" (this condition is subject to mnul reset). IV. OBSERVATIONS The cse discussed llows certin conclusions nd generl projections. From n ppliction stndpoint, some minimum system requirements should be observed, bsed on the concept of preventing network disruption by disllowing DR opertion during power export out of the network or during primry fult bck feed. Severl importnt recommendtions cn be mde tht could be formulted s minimum system criteri for the preliminry system qulifiction: A. DR Size Limittion Idelly the minimum, but certinly the verge dily lod supplied by the spot network should be bove the mximum ggregte DR cpcity. DR utiliztion nd resulting revenues re reduced in direct proportion with the time during which the minimum lod criterion is not met. B. Lod Chrcteristic Billing demnd nlysis my not suffice when minimum system lod ners the ggregte DR size. If this is the cse, the lod chrcteristic hs to be studied in depth, which my require specil instrumenttion. Fig. 7. Front View of Open Protection Cbinet During strtup, series of tests were performed on site to commission the system. The min purpose of the tests ws to scertin tht the selected protective/control scheme would provide the required protection in the bnorml system conditions. Such conditions include, but re not necessrily limited to, the conditions tht could cuse one or both network protectors to trip on reverse power flow contributed to by the genertor. Fig. 8. System Dedbnd Opertion C. Delying Of Network Protector Tripping Any DR ggregte rting pproching the network minimum lod must rely on delying NWP trip t low current levels using discriminted delyed tripping concept. NWP owner (commonly utility) needs to pprove hed of time. Coordinted delying of the network protector tripping is only possible by retrofitting ll network protectors with modern microprocessor relys, the cost of which must be included in the DR s finncil nlysis. D. Fesibility Study If ny of the bove conditions re not met or re met only mrginlly, fesibility study my be necessry which increses the cost of interconnection. E. Testing Commissioning tests re likely to become mndtory for ny instlltion under the new IEEE stndrd P-1547 [1] expected this yer. Under nd over voltge nd frequency settings t the DR protection will not be sufficient to prevent undesired protector tripping even under fult conditions. Suitble testing provision must be provided so tht the protection cn be tested during opertion of the network. Finlly, mintennce testing procedures nd scheduling should be included in the finncil nlysis. Stndrds: [1] Drft 10 IEEE P1547 Stndrd for Interconnection of distributed Resources with Electric Power Systems, Aug Books: [2] R. C. Dugn, M. F. McGrnghn, S. Sntoso, H. W. Bety, Electric Power Systems Qulity, 2nd Edition, McGrw Hill, 2002, Section 9.6.

7 V. BIOGRAPHIES 7

8 8 Mrtin Bier (M 86) ws born in Prgue, Czechoslovki, on September 26, He received his MSEE degree from the University of Prgue in His employment experience included the Czech Energy Enterprises (CEZ), Westinghouse Electric Corportion since 1985, nd with Eton Cutler- Hmmer since His specil fields of interest included Power Qulity, Predictive Dignostics nd Distributed Genertion. Mr. Bier co-uthored severl ptents in the re of power system grounding nd insultion dignostics. He is currently principl engineer of engineering services of Eton Cutler-Hmmer nd n ctive prticipnt in IEEE P1547 Stndrd development. He is registered professionl engineer in Pennsylvni nd Ontrio, Cnd. Dvid R. Smith (F 86) ws born in Altoon, PA on Jnury 1, He received the BSEE degree from the Pennsylvni Stte University in 1963, nd the MSEE degree from the University of Pittsburgh in He ws employed with Westinghouse Electric Corportion from 1963 through 1988, working in res relted to power distribution system nlysis, equipment ppliction, nd specil studies. Since 1988 he hs continued these ctivities for Power Technologies, Inc., currently being n Executive Consultnt. His min res of interest re the design, opertion, nd protection of low-voltge networks. He is co-holder of two ptents on network protector relys. Mr. Smith is registered professionl engineer in Pennsylvni. Willim E. Feero (F 88) ws born in Old Town, ME in He hs degrees from the University of Mine in 1960, University of Pittsburgh in 1969, nd MIT in From 1960 to 1976 he ws employed with Westinghouse Electric Corportion in the Advnced Systems Technology Division specilizing in trnsient nlysis nd protective device coordintion. In 1976 Mr. Feero ws ppointed the progrm mnger of the Power Supply Integrtion Progrm of the Division of Electric Energy Systems of the Deprtment of Energy. The progrm provided for the timely nd orderly integrtion of new source technologies into electric power systems. After tking erly retirement in 2000 from Electric Reserch, compny he co-founded in 1980, Mr. Feero is currently consulting engineer specilizing in power delivery nlysis nd studies of interconnections with utility systems. A mjor reserch focus hs been the chrcteriztion of the dynmics ssocited with interfces between smll power producers nd existing electric utility systems. His creer hs been enhnced by serving on the PES T&D Committee, the PES PSRC, the SCC28, the SCC23, nd the SCC21. He is registered professionl engineer in Pennsylvni nd Mine.