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948 IEEE Transactions on Power Delivery,Vol. 8, No. 3, July 1993

PROTECTI ON AGAI NST LOSS OF UTI LI TY GRI D SUPPLY FOR A D SPERSED STORAGE AND GENERATI ON UNI T.

M A Redf ern, Member I EEE, 0 Ust aSchool of El ectr oni c and El ectr i cal Engi neeri ngUni versi t y of BathBath, BA2 TAY, UK.

Abst ract: - The i nstal l at i on of smal l or medi um si zeddi spersed storage and generati on uni t s operati ng i nparal l el w th the ut i l i ty suppl y present s severalt echni cal compl i cat i ons f or t he protect i on and cont rolof t he system Amongst these i s t he need t o protect thesystemf romi sl andi ng caused by t he l oss of the uti l i tygri d suppl y and the possi bl e subsequentout - of- synchroni smr econnect i on of that suppl y.Thi s paper examnes the requi rements of an i sl andi ng,or ' l os s of gri d' , protecti on and out l i nes thepri nci pal methods used f or t hi s t ype of rel ayi ng. A newprotect i on al gori thm i s i nt roduced whi ch i s based onthe rat e of change of power as measured at thegenerator' s t erm nal s. The responses of the di f f erentmeasurands are examned for a sel ect i on of power syst emoperat i ng condi t i ons to demonst rate t he operati on oft hi s type of protect i on.The new protect i on al gori thm i s shown to t ri p f or l ossof gri d, f or l oad f l uctuat i ons whi l e t he di spersedstorage and generati on uni t i s operati ng i ndependent l yof the uti l i ty suppl y fol l ow ng a l oss of gri d, and foran out- of - synchroni sm reconnecti on of the ut i l i t ysuppl y to the di spersed storage and generati on uni t . I ti s al so shown to remai n st abl e f or maj or l oadf l uctuat i ons whi l e the ut i l i ty suppl y remai nsconnected to the di spersed generator syst emKevwords: - Power systemprotect i on, D spersed st orageand generat i on uni t s, I sl andi ng prot ecti on, Non- ut i l i tygenerati on.

I NTRODUCTI ONOver t he l ast decade there has been a grow ng i nteresti n the i nstal l at i on of smal l and medi um si zedgenerat i on uni ts whi ch operate i n paral l el w t h thel ocal el ectri c uti l i ty' s power suppl y. Def i ned asdi spersed storage and generat i on f aci l i t i es, t hei ni t i al i nterest was domnated by the non- ut i l i t ysector , but more recent l y, ut i l i t i es have al so beeni nvest i ng i n thi s t ype of generat i on.Before a di spersed st orage and generati on uni t can beconnected to t he l ocal ut i l i ty network, severalt echni cal i mpl i cati ons must be consi dered. Amongstthese i s t he need to ensure that t he whol e system i sproperl y protected, i ncl udi ng the ut i l i ty net work, thedi spersed storage and generat i on uni t and any on- si tel oad.

2 SM 376- 4 PWRD A paper recommended and approvedby t he I EEE Power Syst emRel ayi ng Commt tee of t heI EEE Power Engi neer i ng Soci ety f or presentati on atthe I EEE/PES 1992 Summer Meet i ng, Seatt l e, WA,J ul y 12- 16, 1992. Manuscr i pt submt t ed J anuary 28,1992; made avai l abl e for pri nt i ng Apr i l 16, 1992.

and G Fi el di ngGEC Al sthomProt ecti on and Cont rolSt Leonards WorksSt af f ord, ST17 4LX, UK.

A par t i cul arl y demandi ng protect i on requi rement i s theneed to guard agai nst the abnormal condi t i on where asmal l secti on of the ut i l i ty' s l oad network remai nsconnected to the di spersed storage and generat i on uni tf ol l ow ng systemsw tchi ng operat i ons. Thi s produces ani ndependent power i sl and. I deal l y, t he protect i onsystem shoul d automati cal l y di sconnect the di spersedstorage and generat i on uni t f rom the ut i l i t y network;i . e. ' l os s of gri d' protecti on. Apart f rom the saf etyaspect of havi ng a secti on of the network energi sedwhen i t woul d general l y be assumed that the l oss of themai n source of suppl y woul d de- energi se the sys t emthere i s al so the potent i al l y di sastr ous scenar i o wherethe mai n source of suppl y coul d be reconnect ed to t hepower i sl and out-of - synchroni sm w th the di spersedst orage and generat i on uni t .Tradi ti onal l y uti l i ti es have resi sted the i nstal l ati onof a di spersed storage and generati on uni t di r ect l yconnected to t he mai n network because of the t echni calcompl i cat i ons i nvol ved. The use of pri vat e generati oni s however wel l establ i shed where generat i on has beeni nstal l ed to operate i ndependent l y f rom the ut i l i ty' ssuppl y, ei ther as stand al one generat i on . di r ect l ysupport i ng the si te l oad, or as stand-by generati on f orsi t uati ons when the ut i l i ty has been unabl e to mai ntai ncont i nuous suppl y.I n r esponse to the grow ng i nterest i n di spersedst orage and generat i on, several gui de- l i nes1S2 ave beeni ntr oduced f or t he connect i on of smal l and medi um si zedgenerati on to thei r networks. The ut i l i t i es' obj ecti vehas been to ensure t hat the presence of the di spersedstorage and generat i on uni t w l l not det ract f rom thequal i ty of suppl y t o al l customers connected to t hei rsystem The protecti on requi rement s for a di spersedgenerat i on and storage faci l i ty are f ur ther expl ai nedi n report s3s4 associ ated w th t he gui del i nes. A commonf eature of t hese, ei t her i mpl i ci t or by i nf erence, i sthe need to protect t he systemagai nst ' l os s of gr i d' .

REQUI REMENTS FOR LOSS OF GRI D PROTECTI ONThe pri nci pal obj ecti ve of ' l oss of gri d' protecti on i sto detect t he condi t i on where the di spersed st orage andgenerat i on uni t i s l ef t connected to a port i on of theuti l i ty' s l oad network w th no mai n source of ut i l i t ypower f ol l ow ng a system sw tchi ng operati on. The mai nsource of ut i l i ty power coul d be obtai ned f romei ther amai n power st ati on or a t ransmssi on suppl y poi nt ,commonl y ref erred to as a gri d connect i on, and hencethe name ' l oss of gri d' .There are several possi bl e causes of l oss of gri di ncl udi ng; sw tchi ng operati ons to cl ear a f aul t , l oadsheddi ng, mai ntenance outages and equi pment f ai l ure.The l oss of gri d protect i on i s requi red to di sconnectthe l i nk between the di spersed st orage and generati onuni t ar.d the ut i l i t y network enabl i ng an uncompl i cat edrestorat i on of the uti l i ty suppl y. The t ypi calrequi rement f or thi s protect i on1 i s t hat i t shoul doperate w t hi n hal f a second f ol l ow ng the i sol at i on ofthe power i sl and, but f aster r el ayi ng i s at t racti ve.

0885-8977/93$03.000 1992 IEEE

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949I n i t s el f , l oss of gri d i s not a probl emprovi ded thatthe source of generat i on mai ntai ns the systems vol t ageand f requency w t hi n the speci f i ed l i mt s. The mai ncause f or concer n5*6, 7, 8, 9, 10s the probabi l i t y that thetwo syst ems w l l be reconnected when t hei r generatorsare out of synchroni sm Al though the best practi ce i sto avoi d recl osi ng, i t i s not reasonabl e to ask aut i l i t y to di sabl e al l recl osi ng cl ose to the di spersedstorage and generati on uni t . Shoul d out of synchroni smrecl osure occur , t here i s a hi gh probabi l i t y t hat thedi spersed storage and generati on uni t w l l be damaged.Where the generator i s used to suppl ement the ut i l i t ysuppl y to t he l ocal si te and does not export power tothe uti l i ty, l oss of gri d w l l resul t i n power f l ow ngf rom the si te to the l ocal uti l i ty l oad. The l oss ofgri d can be detected by a reverse power rel aymoni tori ng the power f l ow i n the i nter- t i e ci r cui t .I n the case of smal l generators, nomnal l y l ess than200 kVA and whi ch are operated such that they canexport power to the l ocal uti l i t y network, l oss of gri dw l l resul t i n severe overl oadi ng of the di spersedstorage and generati on uni t . Thi s w l l cause i ts outputvol tage and f requency to f al l , and the under vol t ageand under f requency protecti on to operat e.Wt h l arge generators, nomnal l y i n excess of 250 kVAand f i t ted w th hi gh speed automati c vol tager egul ator s, there i s a reasonabl e possi bi l i t y that thegenerati on w l l be abl e5*6, 7, 10o mai ntai n the vol tageand fr equency w t hi n speci f i ed l i mt s f ol l ow ng l os s ofgri d. Speci al i st rel ayi ng is therefore requi red todetect the l os s of gri d and tri p the i nter- ti e breaker .The l os s of gri d protecti on i s requi red i n addi t i on tothe other protecti on requi red f or the generator , i t ssi t e, and the ut i l i t y system cl ose to the generator.Check-synchroni sati on rel ayi ng i s requi red for thei ntert i e breakers to ensure that the generator i ssaf el y connected i n paral l el w t h the uti l i ty suppl y.The protecti on scheme used w t h a part i cul ar di spersedgenerator depends on the machi ne and the syst em towhi ch i t i s ~o nn ec t ed ~~~~8.9, 1*12.

TECHNI OUES FOR LOSS OF GRI D PROTECTI ONThe most di rect method for l os s of gri dprotecti on6, ' ***10 i s to moni tor auxi l i ary contacts onal l ci r cui t breakers on the ut i l i t y systembet ween i tsmai n sources of generat i on and the di spersed st orageand generati on uni t . When a sw t chi ng operat i onproduces a l oss of gri d, a t ransf er tr i p scheme cant hen be used to open the i nter- t i e connecti ng the twosystems. Fol l ow ng successful restorati on of theut i l i t y suppl y, the di spersed storage and generati onuni t can be resynchroni sed to t he ut i l i ty and thenreconnect ed. Unf ort unatel y, several ci r cui t breakersare candi dat es f or creati ng the l oss of gri d and acomprehensi ve moni tori ng system i nvol vi ng al l ci r cui tbreakers whi ch coul d be i nvol ved woul d be unmanageabl ef or most uti l i t i es. The i nstal l ati on of an extensi veSCADA systemand net work aut omati on can f aci l i t ate sucha scheme.Recl osure of the ut i l i ty suppl y onto a di spersedst orage and generati on uni t can be avoi ded by usi ngdead ci rcui t pi ck-up supervi si onl o on uti l i ty ci rcui tbreakers. These woul d i nhi bi t breaker cl osi ng unt i l thel oad- si de ci r cui t was de-energi sed and i ni t i ate Rt ransf er t ri p to open the i ntert i e bet ween thedi spersed generator and the uti l i t y.Several techni ques have been devi sed for detecti ng l ossof gri d usi ng measurements taken on the di spersedst orage and generati on si te. These i ncl ude; - reacti vepower export err or dete~t i on' ~ system f aul t l evel

moni t ori ng1", rate of change of f requency mea~ur ement ' ~phase di spl acement m~ni t or i ngl ~- ~~,nd the al gor i t hmsbased on rat e of change of generator power output whi chare presented i n thi s paper. The f i rst two of these canbe descr i bed as acti ve techni ques, si nce they di rectl yi nteract w t h the on- goi ng operat i on of the powersyst em whereas the other techni ques can be consi deredpassi ve, si nce they detect l oss of gri d sol el y bymoni tori ng the power syst ems behavi our .Act i ve Techni aues.The react i ve power export err or detecti on13 rel ayi nter f aces w t h the di spersed generator' s contr olsystem to f orce it to generate a l evel of reacti vepower f l ow i n the i ntert i e between the l ocal si te andthe uti l i t y whi ch can onl y be mai ntai ned when theut i l i ty' s mai n source of generati on i s connected. Rel ayoperat i on i s t r i ggered when there i s an err or betweenthe sett i ng and the actual raact i ve power bei ngexported f or great er t han a preset t i me peri od. Toavoi d mal - operat i on, the t i me set t i ng i s chosen to hegreater t han the durat i on of possi bl e suppl yf l uctuati ons.Al though thi s rel ayi ng approach i s sl ow i t i srecogni sed as bei ng very ef f ect i ve si nce i t can detectl os s of gri d when there i s no change i n the generator' sl oadi ng due to the sw tchi ng operati on. The rel ay i sf requentl y used to provi de a back-up pr otecti on toother ' f aster' systems. Typi cal operati ng t i mes varyf romt wo to f i ve seconds.The system f aul t l evel moni tor1" provi des a f asteroperat i ng protecti on systemand i ts t r i ppi ng depends onmeasurements of the power syst ems sour ce i mpedance ast aken cl ose to the i ntert i e. Thi s i s perf ormed bymoni tori ng the short ci rcui t curr ent and reduct i on i nsuppl y vol tage when a shunt i nductor i s connect edacross the suppl y usi ng poi nt- on-wave t r i ggered,thyri stor sw tches. The fi ri ng of the thyri stors j ustbef ore a curr ent zero causes a short pul se of cur rentto f l ow i n the i nductor and a vol tage gl i t ch. Thedeci si on to t r i p depends on the compar i son of themeasured systemf aul t l evel w th that corr espondi ng toa network f ed f romt he mai n ut i l i t y generati on.Si nce there i s a dramati c di f f erence between the f aul tl evel s of the uti l i t y' s generati on and that of thedi spersed st orage and generati on uni t , the systemneednot be part i cul arl y accurate. The operati ng t i mes canal so be very shor t , w t h a theoreti cal mni mum of hal fa cycl e. However , al though si ml ar t echni ques areal ready used to i mprove the per f ormance of stati cvol tage compensat ors, rel ays have not been produced.Passi ve Techni aues.I n al most al l ci rcuni stances, l oss of gri d resul ts i nchanges t o the l oadi ng of the di spersed storage andgenerati on uni t and hence produces changes i n syst emvol t age, currents and f requency. The behavi our of thesystem i s moni tored by the rel ay and these changesprovi de the basi s f or passi ve protecti on.The most di rect protecti on techni ques are based onunder / over vol t age, and under / over f requency rel ayi ng.I n many smal l generator appl i cat i ons these provi de anacceptabl e l evel of prot ecti on, but they can onl yoperate i f the l oss of gri d produces a change of l oadgreater t han that whi ch can be compensat ed f or by thegenerator' s cont rol systemSeveral of the more sophi st i cated passi ve techniquesdepend on the generat or sw ng equati on1' whi ch def i nesthe rate of change of f requency: -

__

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DF - DP FDt 2 H G-where: DP i s the change i n power out putF i s the power syst emf requencyH i s the i ner t i a const ant of the generator/ systemG i s the rated capaci t y of t he generator/ systemRate of change of f requency13 rel ays moni t or thevol t age wavef orm and t r i p when t he measured rate ofchange of f requency exceeds a preset l evel f or l ongert han a set t i me peri od. The set t i ngs are chosen suchthat the rel ay w l l operate f or f l uctuat i ons associ at edw t h l oss of gri d and when the di spersed generator i soperati ng i ndependent l y f rom the uti l i ty' s source ofsuppl y, but not f or those f l uct uat i ons governed byut i l i t y ti me constant s.For smal l and medi um si zed di spersed st orage andgenerati on uni t , a t ri p set t i ng of 0. 3 Hz/ sec has beenf ound to be opt i mumw t h an operat i ng t i me f rom0. 3 to0 .7 seconds. Under extreme condi t i ons, accompani ed bydramati c changes i n fr equency, t ri ppi ng i n f our or f i vecycl es i s possi bl e.The phase di spl acement m~ni t o r l ~. ~~s generi cal l yrel ated to the rate of change of f requency rel ay andoperat es when t here are phase di spl acement s i n t hevol t age waveform These are a di rect resul t of changesi n the di spersed generator ' s l oadi ng and can cause therel ay to operate i n 50 m f ol l ow ng a l oss of gri dwhi ch resul t s i n a l oad change of greater t han 5% ofi ts rati ng.An i nherent advant age of both r ate of change off requency rel ayi ng and the phase di spl acement moni t ori s that, shoul d the rel ay f ai l to operate f or t he l os sof gri d, any subsequent l oad changes coul d causet r i ppi ng. Si nce these rel ays requi re a si ngl e i nputsi gnal , the system vol t age wavefor m t hey use t hemni mumof i nput ci r cui t ry, t hus of f eri ng an i mmedi ateadvantage for st and al one r el ayi ng.The i ntr oducti on of di gi tal t echnol ogy and the move t oi ntegrated prot ecti on schemes, has enabl ed othert echni ques to be consi dered whi ch requi re more i nputdat a f or the deci si on maki ng. One such techni que usesan al gor i thm based on the rate of change of powerout put measured at the di spersed generator ' s t erm nal s.Consi deri ng the si mpl i f i ed represent ati on of adi spersed generator, i ts si t e l oad and the ut i l i t ynet work as shown i n Fi gure 1, bot h the di spersedgenerator and the uti l i t y suppl y can be model l ed byi deal i sed generators of capaci t y G and G and w t hi nert i a const ant s Hg and Hm respecf i vel y. A change i nsyst eml oad of DP, produces a change i n the generator' sl oadi ng of DPg def i ned by:

Hg GgHg Gg + HmGmDPg - DP,

However, i f the di spersed st orage and generat i on uni toperates i ndependent l y f rom the ut i l i t y suppl y, anychange of l oad w l l di rectl y af f ect t he di spersedgenerator si nce the equat i on above reduces t o: -DPg - DP,

Si nce l os s of gri d general l y produces a l oad change,moni t ori ng changes i n the power out put of the generat orprovi des a di rect method of detect i ng the phenomena.

The i nstant aneous three phase power out put f rom t hegenerator i s deri ved f r om-Pg = vai a + f i i b + vci c

where va, i a, b. i b, vc, and i c represent the sampl edval ues of the l i ne curr ent s and phase vol t ages measuredat the generator' s t erm nal s. Al l of these si gnal s arerequi red by a comprehensi ve i ntegrated protecti onscheme for a di spersed generat or.The protecti on al gor i thmmoni tors the changes i n powerout put and i ntegrates these changes over a def i nedsampl e per i od. Tri ppi ng occurs when t he i ntegratedsi gnal exceeds the t ri p sett i ng, k,: I n the di gi talsampl i ng rel ay the al gori thms tr i p cri t eri a arerepresented by: -

n = O

n - - t xwhere;and, n i s the sampl i ng i nstant of DPgtx i s t he l engt h of the sampl i ng w ndow

(GRID1

\

_,' /" \ POTENTIAL POWER'. _- ISLAND_..FI GURE 1. NETWORK CONFI GURATI ON CONTAI N NG AD SPERSED STORAGE AND GENERATI ON UNI T.The i ntegrati ng f eat ure provi des i mmuni ty tomal - operati on duri ng condi t i ons of ext reme l oadunbal ance or l oss of phase operati on. An unbal ance i nthe i nput wavef orms woul d i ntr oduce si nusoi dal t erms oftw ce the power syst em f requency and these can bef i l t ered out by t he i ntegrati ng process. The l engt h ofthe sampl i ng w ndow was chosen to gi ve a maxi mumoperat i ng t i me of si x cycl es, and ( DP )n was ampl i t udel i mt ed to gi ve a mni mum tri ppi ng t f me of one cycl e.The rel ay' s sett i ng was chosen such t hat the rel aywoul d t r i p whenever the di st urbance produced a l oadchange of 1% of the di spersed st orage and generat i onuni t' s rati ng.

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951I n addi t i on to provi di ng an ef f ect i ve t echni que fordetecti ng l oss of gri d, t hi s systemw l l al so operatef or any subsequent l oad di sturbance shoul d the l oss ofgri d f ai l to produce the di sturbance requi red to t ri p.I t w l l al so tr i p i f the syst ems are reconnected whi l ethe syst ems are out- of- synchroni sm

POWER SYSTEM STUD ESPower syst emst udi es were undertaken usi ng the networkshown i n Fi gure 1. The resul t s presented were obtai nedf rom the anal ysi s of an 11 kV network cont ai ni ng a3.75 MVA di spersed st orage and generati on uni t w th ani nert i a constant of 0.91 MW sec/ MVA, a 3 MW l ocal l oad,and a ut i l i t y suppl y of 2 5 0 MVA and i nert i a constant of10 MWsec/ MVA.The test per i od was chosen to be one second, commenci ng100 m bef ore the di st urbance. The model di d noti ncl ude the act i on of the automat i c vol t age regul atoror the pr i me mover' s governor, si nce the per i od ofi nterest was short i n compari son to thei r t i meconstant s.The var i ati ons i n the di spersed generator' s termnall i ne vol t age and f requency were der i ved f rommeasurement al gor i thm based on the machi ne anal ysi s.Apart f r omthe operati on of the al gori t hms based on therate of change of power , no at tempt was made t o emul atethe operat i on of part i cul ar prot ect i on al gori thm usedf or under/ over vol t age, under/ over f requency or r ate ofchange of f requency protecti on.The perf ormance of prot ect i on syst ems usi ng under/ overvol t age, under/ over f requency or r ate of change off r equency woul d be determned by the changes i n powersystem operati on, t he si gnal processi ng techni quesused, and any enhancement f eatur es i ncl uded. Severaltechni ques exi st f or t hese types of rel ayi ng and al lhave thei r own compl i cati ons and response t i mes. Themeasurements of f requency and rate of change off r equency f roma moni t ored i nput waveformare f ar f romst rai ght f orward and are t i me consumng. The si mpl ertechni ques rel y on zero crossi ngs of the moni toredwaveform whereas t he more robust t echni ques usecompl ex si gnal processi ngl e.The tri p set t i ngs for t he protect i on al gori thm basedon the rate of change of power were chosen such that aone percent change i n l oad resul t i ng f rom the l oss ofgr i d woul d j ust cause t ri ppi ng. Thi s corr esponded to5.0 (MW on the scal e used i n the response curvesshown. The uni ts of MW are der i ved f rom (MWsec)i ntegrated over t i me.The t r i p set t i ngs for t he under/ over vol tage protect i onwere set t o si x percent of vol tage nomnal , theunder/ over f requency protecti on sett i ngs were onepercent of the nomnal f requency, and the rate ofchange of f requency was set to 0.3 Hz/ sec.Loss of Gr i d Studi es.The response to a l oss of gri d resul ti ng i n a f i f typercent i ncrease on the di spersed generator ' s l oadi ngi s shown in Fi gure 2 . I mmedi atel y f ol l ow ng thedi st urbance, t he vol t age and f requency are depressedcausi ng the under/ over vol t age protect i on to f al l bel owthe tr i p set t i ng af ter 36 ms, and the under/ overf requency protecti on to f al l bel ow i ts tri p sett i ngaf ter 4 8 ms. The systems r ate of change of f requencyf al l s al most i mmedi atel y, and exceeds t he t ri p set t i ngaf ter 4 m. The rate of change of power suf f ers vi ol entchanges i mmedi atel y f ol l ow ng the sw t chi ng, t hensett l es to an al most st eady val ue. The al gori thm t ri psaf t er 2 4 ms.

The response to a l os s of gri d resul ti ng i n a fi vepercent i ncrease i n l oadi ng i s shown i n Fi gure 3 . Thegenerator' s term nal l i ne vol t age i s agai n depressedl ess than i n the case above, and i n the test per i odconsi dered, the vol tage l i mt s were not exceeded. Thef al l i n measured f requency onl y exceeded the l owerl i mt after 5 4 0 m. Agai n there was an i mmedi atechange i n the rate of change of f requency and i ts l i mtwas exceeded af t er 4 m. The al gori thmbased on rate ofchange of power t r i pped after 26 m.The margi nal t ri p condi t i on f or the al gori t hm usi ngrate of change of power i s shown i n Fi gure 4 , andproduces a tr i p af t er 1 2 3 ms. I n thi s case, the l i mtsf or under/ over vol t age, under/ over f requency, and rateof change of f requency were not exceeded.Paral l el Ouerat i on Studi es.Fi gures 5 and 6 showthe responses to a doubl i ng of thel ocal l oad and a l os s of the l ocal l oad whi l e thegenerator i s operati ng i n paral l el w th the ut i l i tysuppl y. I n both cases none of the protect i on l i mt s areexceeded and the protect i ons r emai n st abl e. Al l thecurves contai n osci l l at i ons af ter the sw tchi ngoperat i on as the systemre-establ i shes equi l i bri umI ndeoendent ODerat i on Studi es.An exampl e of l oad sw tchi ng whi l e t he generator i soperati ng i ndependent l y of t he ut i l i ty suppl y i s showni n Fi gure 7. Al though the l oad change i s l i mt ed to ani ncrease of f i ve percent of the generators r at i ng, t herate of change of power al gori thm tri ps af ter 26 ms,the fr equency f al l s bel ow i ts l i mt af ter 5 6 4 ms, andthe rate of change of f requency aft er 4 ms. Under thesecondi t i ons, the l oss of gri d protect i on woul d tr i p thei nter- t i e breaker and i sol ate the generator f rom theut i l i t y system I f however the i nter- t i e breaker wasal ready t r i pped, t he protect i on woul d be i nhi bi t ed.Non- synchroni sed Recl osure.The r esponses to non- synchroni sed recl osure resul t i ngi n no l oad changes, but w th phase di spl acements of 1 8 0and 5 degrees are shown i n Fi gures 8 and 9respecti vel y. I n both cases t here are severeosci l l ati ons i n the power taken f rom and fed to thegenerator, but l i t t l e change in ei ther the systemvol tage or f requency. The rate of change of poweral gori thm tri ps aft er 275 and 272 m respecti vel y,whereas the l i mt s on vol t age, f r equency, and rat e ofchange of f requency are not exceeded.

CONCLUSI ON"Provi di ng protect i on agai nst i sl andi ng probabl y i s t hesi ngl e most chal l engi ng aspect of desi gni ng theel ect ri cal system i nvol ved i n cogenerati on. "8 Loss ofgri d protect i on i s arguabl y the greatest chal l enge i nthe prot ecti on package.Several di f f erent t echni ques are avai l abl e f orprotect i ng a di spersed storage and generati on uni tagai nst l oss of gr i d. These i ncl ude networksupervi si on, act i ve techni ques, whi ch di r ect l y i nteractw t h the operati on of the power syst em and passi vetechni ques, whi ch rel y on moni tori ng the power systemsbehavi our .Network supervi si on methods are the most di rect andof f er the greatest l evel of protect i on. However, w t hmost uti l i ty networks where SCADA systems do not coveral l of the di st r i but i on network, t hey present thel argest expense. Acti ve techni ques are general l y veryef f ecti ve i n detecti ng the l os s of gri d, but have the

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2 .4 -

0 . 0

- 2 . 4 -

' z/sec.0. 0 & (c) Rate of Change of Frequency

(e) Rate of Change of Powerprotecti on Al gori t hm(Summat i on Output).- A

t i me (m).5 0 0 . 0

w l s e c+0.2 4 I (d) Rate of Change of Power(Ampl i tude Li mt ed) .-0.2

(e) Rate of Change of PowerProtecti on Al gori t hm(Summat i on Output) .-20.0-30.0FI GURE 2 . LOSS OF GRI D RESULTI NG I NA 50 % I NCREASE I N GENERATOR LOAD NG.

(a) Termnal Vol t age (Ph/Ph).

10.7

49.2 ",sec

o. o ]A ( c ) Rate of Change of Fr equency.

5 0 0 . 0(d) Rate of Change of Power(Ampl i tude Li mted).

- 0 . 2 1 LL(e) Rate of Change of PowerProtecti on Al gori t hm

-10.0 (Summat i on Output).-20.0 -

FI GURE 3 . LOSS OF GRI D RESULTI NG I NA 5 % I NCREASE I N GENERATOR LOAD NG.

'"fit ( c) Rate of Change of Fr equency.+ , . . , I , I . . I . . I . I I . . . . . " . ~ . . " " " " ~ ~ ' " ' ' ' ' ~

M I S ? . (d) Rate of Change of Power(Ampl i tude Li mted).

- 0 . 2

0 . 0 1 (e) Rate of Change of PowerProtecti on Al gori t hm

FI GURE 4 . LOSS OF GRI D RESULTI NG I NA 1 % I NCREASE I N GENERATOR LOAD NG.

10.99 4 b-1 (b) Syst emFr equency

(c) Rate of Change of Fr equency.

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11.0

Ez/sec (c) Rate of Change of Fr equency.0 .16 1 -1 /

2.4 !0 . 0

of Change of Power-2 .4 3 Protecti on Al gori thm(Summat i on Output).FI GURE 6 . LOSS OF LOCAL LOAD DURI NGPARALLEL OPERATI ON WTH THE UTI LI TY GRID

kV11 .0 1 7 (a) Term nal Vol t age (Ph/Ph).

0.0 (c) Rate of Change of Fr equency.

45 0 0 . 0(d) Rate of Change of Power(Ampl i tude Li mt ed) .' 1 10.24 (e) Rate of Change of PowerProtecti on Al gori thm(Summati on Output).

-2 0 .0

~ H z ~ s e c ( (c) Rate of Change of Fr equency.n o

n6.0 70.0 (e) Rate of Change of Power

- 2 4 . 0 protecti on Al gori t hm(Summat i on Output) .1FI GURE 8. AN OUT- OF- SYNCHRONI SATI ON ECONNECTI ON WTHA PHASE D SPLACEMENT OF 180 DEGREES AND NO LOAD CHANGE.kV

\(a) Termnal Vol tage (

(b) Syst emFr equency.5 0 . 0]q49 . 9999 j (o ' o l lc) Rate of Change of Frequency.- o .0121 I

h .J1 2 . 0 1 -

Protection Algorithm(Sm ati on Output).

FI GURE 7. A 5 % I NCREASE I N GENERATOR LOAD NG WHI LETHE D SPERSED GENERATOR I S OPERATI NG ALONE. FI GURE 9. AN OUT- OF- SYNCHRONI SATI ON ECONNECTI ON WTHA PHASE D SPLACEMENT OF 5 DEGREES AND NO LOAD CHANGE.

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954si gni f i cant di sadvantage of requi ri ng a di recti nf l uence on the power syst em Passi ve techni ques avoi dthi s probl em and are general l y the l east expensi ve toi nstal l , but they cannot be guaranteed to operate underal l l oss of gri d condi t i ons.The resul ts of the power system studi es presenteddemonst rate t he fundamental capabi l i t i es andl i mt at i ons of usi ng suppl y vol t age, f requency and r ateof change of f requency f or l oss of gri d protecti on. Theperf ormance of the new al gori thm usi ng the rate ofchange of power i s shown to operate as r equi red.A si gni f i cant advantage of t hi s new approach i s i t sabi l i ty to qui ckl y detect out - of - synchroni smreconnecti on of the ut i l i ty suppl y to a power i sl andcontai ni ng the di spersed stor age and generati on uni t .Thi s i s consi dered to be a par t i cul ar l y onerousrequi rement si nce i t represents the greatest danger tothe di spersed st orage and generat i on uni t .

ACKNOWLEDGEMENTSThe authors are pl eased to acknow edge the hel p andencouragement provi ded by the Uni vers i t y of Bath, GECAl sthom Protect i on and Cont rol , and the Sci ence andEngi neeri ng Research Counci l .

REFERENCES1. ELECTRI CI TY COUNCI L, ' Recommendat i ons f or theConnect i on of Pri vate Generati ng Pl ant t o t heEl ectri ci ty Board' s D str i but i on System - Engi neeri ngRecommendat i on G59. ' The El ectr i ci ty Counci l ( now theEl ectr i ci ty Associ at i on) , London, J une 1985.2. ANSI / I EEE Standard 1001- 1988, ' I EEE Gui de f orI nterf aci ng D spersed Storage and Generati on Faci l i t i esw t h El ectri c Ut i l i t y System. ' I EEE, New J ork, 1989.3. ELECTRI CI TY COUNCI L, ' Notes of Gui dance f or t heProduct i on of Pri vate Generat i ng Set s up to 5 MW f orOperati on i n Paral l el w th El ectri ci ty Board' sD st ri but i on Networks - Engi neer i ng Techni cal Report113. ' The El ect ri ci t y Counci l ( now the El ectri ci tyAssoci at i on) , London, 1989.4. I EEE Speci al Report , ' I ntert i e Protecti on ofConsumer- Ovned Sources of Generat i on, 3 MVA or l ess. '5. PATTON J B and CURTI CE D, ' Anal ysi s of Ut i l i t yProtecti on Probl em Associ ated w t h Smal l Wnd Turbi neI nterconnecti ons, ' I EEE Trans PAS 101, No 10, Oct 1982,6. DUGAN R C and RI ZY D T, ' El ectr i c D str i buti onProbl ems Associ ated w t h the I nt erconnect i on of Smal l ,D spersed Generat i on Devi ces. ' I EEE Trans PAS 103, No6, J une 1984, pp 1121- 1127.7. ROOK M J , GOFF L E, POTOCHNEY G J and POWELL L J ,' Appl i cat i on of Protect i ve Rel ays on a LargeI ndustr i al - Uti l i ty Ti e w th I ndustri al Generati on. 'I EEE Trans PAS 100, No 6, J une 1981, pp 2804- 2812.8. POWELL L J , ' An I ndustri al Vi ew of Uti l i tyCogenerat i on Prot ecti on Requi rements. ' I EEE Trans I A24, No 1, J an/ Feb 1988, pp 75- 81.9. HARLOW J H, ' A Mul t i f uncti on Protecti ve Rel ay f orthe Cogenerat i on I ndustry. ' I EEE Computer Appl i cat i onsi n Power, Vol 3, No 4, Oct 1990, pp 25-30.10. CLARK H K and FELTES J U, ' I ndustri al andCogenerati on Protect i on Probl em Requi r i ng Si mul at i on. 'I EEE Trans I A 25, No 4, ul y/Aug 1989, pp 766- 775.11. FI ELD NG G and BRADLEY J . ' Local Generati on: theDevol uti on of Power. ' I EE Revi ew Vol 4 , No 2, March12. POPE J U, ' Paral l el Operati on of CustomerGenerat i on. ' I EEE Trans I A 19, No 1, J an/ Feb 1983, pp13. WARI N J U, ' Loss of Mai ns Prot ecti on. ' ERAConf erence Ci rcui t Protecti on f or I ndust ri al andCommerci al I nstal l ati ons, London 1990.

I EEE 88TH0224-6- PWR,1988.

pp 3957-3964.

1990, pp 117- 120.32- 36.

14. COOPER C B, ' Standby Generat i on - Probl ems andProspecti ve Gai ns f rom Paral l el Runni ng. ' Power Syst emProt ecti on ' 89, Si ngapore 1989.15. J ONES R A, THOMAS R S and I MECE I F,' I nvesti gati on of Pot ent i al I sl andi ng of a Sel f -Commutated Stat i c Power Convert er i n Photovol t ai cSystems. ' I EEE Trans EC 5, No 4 Dec 1990, pp 624- 630.16. SCHALTANLAGEN ELECTRON K GERATE GMBH & CO,' Generator/ Mai ns Moni tor - GW2. ' Publ i cat i on GW2/ E/ 810.17. ANDERSON P M and FOUAD A A, ' Power Syst emCont roland Stabi l i t y. ' I owa State Uni versi t y Press, 1977.18. PHADKE A G, THORP J S and ADAM AK M G, ' A NewMeasurement Techni que for Tracki ng Vol t age Phasors,Local System Frequency, and Rate of Change ofFrequency. ' I EEE Trans, Vol PAS-102, No 5, May 1983.

M l es A Redf ern (M 79) recei ved hi s BScdegree fr om Nott i ngham Uni versi t y andPhD degree f rom Cambri dge Uni versi t y i n1970 and 1976 respecti vel y. I n 1970, hej oi ned Bri t i sh Rai l ways Research, andi n 1975, moved to GEC Measurements wherehe hel d var i ous post s i ncl udi ng Head ofResearch and Long Term Devel opment andOverseas Sal es Manager. I n 1986, hej oi ned Bath Uni ver si t y w th i nterests i n Power SystemsProtect i on and Management . He i s curr ent l y a member ofI EE Prof essi onal Group P11, Power Syst emMeasurement ,Protect i on and Contr ol , and i s a correspondi ng memberof the I EEE Li ne Protect i on Sub- commt tee and worki nggroup D13, Si x Phase Power Syst ems.

Omer Ust a was born i n Trabzon, Turkey i n1959 and recei ved hi s BSc and MSc degreesf romthe Techni cal Uni versi t y of I st anbuli n 1984 and 1988 respect i vel y. He becamea research off i cer at that Uni versi t y i n1986. M Ust a j oi ned the Bath Uni versi t yi n 1989, to study for hi s PhD degree. H sresearch i nterests are i n power systemsi mul ati on and protecti on.

Geoff Fi el di ng was apprent i ced by t heLancashi re El ectr i c Power Company and hasbeen empl oyed as an engi neer i nel ectr i ci ty suppl y system si nce 1948.Duri ng hi s career he has been empl oyed byNort h Western El ectr i ci t y, Associ atedEl ectr i cal I ndust ri es, the Cent ralEl ect r i ci ty Generat i ng Board, Ewbank andPart ner s, ESCOM (South Af r i ca) , ARAMCO(Saudi Arabi a), I NGA- SHABA (Zai re), and GEC Al st homMeasurements. He has been awarded the Power Premumandthe Wl l ans Premum f romthe I nsti t ut i on of El ectri calEngi neers (UK) and i s curr ent l y an I ndust r i alConsul tant w t h GEC Al sthomProt ect i on and Cont rol .