power factor testing of power transformers

8/2/2019 Power Factor Testing of Power Transformers

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POWER FACTOR TESTING OF POWER TRANSFORM ERS

werfactortesting of electrical insulation isone ofthe recognizednd highly-regarded procedures in the preventive maintenanceocess of electrical power aparatus. The powerfactor o f electricalsulation, the ratio of the power loss to the volt-amperes, is aeasure of the quality of the insulation. The lower the powerctor, the better the condition of the insulation. With this in mind,e electrical power industry is monitoring the power factor of theirajor system components to avert unwanted failures and out-his paper will consider the requirements for equipment to besed for power factor testing, comment on available equipment,iscuss problems of interference and outlines procedure andeadings to be taken on two-and-three winding transformers.EST EQU IPMENT REQUIREMENTSo say that the test equipment has to measure the power factorf electrical insulation would be greatly oversimplifying the prob-m. To put the problem in a better prospective one should l ist andscuss the requirements of the test equipmentforthis application.The test equipment must measure capacitance and powerfactorof electrical insulation. A measurement of the dissipation factoror power loss is also acceptable, as the power factor can bereadily calculated from these values.The test equipment must measure insulation that is grounded(GST test) or ungrounded (UST test). This is a real necessity, aspractical applications demand such measurements.The test equipment must be equipped with an effective guardcircuit. As most practical applications involve several intercon-nectedcapaehors, itis necessary to separate these into individualsections for analysis. The only practical method of doing this istobe able to eliminate them from the measurement by suitableguarding.should be pointed out that there are two types of guard circuitsossible. Inthe HOT GUARD configuration the guard is atthe testotential of the test set. In the COLD GUARD configuration theuard is at, or near, ground potential. As the HOT GUARDonfiguration requires more dil igence on the part of the operator,ith respect to safety, the COLD GUARD circuit is usually pre-The test equipment must reject or cancel interference. Mostpractical situations involve measurements near live lines orother power equipment. Such live apparatus invariably interferewith measurements by injecting power frequency interferenceinto the measuring circuit. To allow measurements in the pres-ence of interference, the equipment must have provisions forrejecting or cancelling interference.The test equipment must be capable of conducting measure-ments at the traditional stress levels. As most people in theindustry have been making measurements typically at 10 or2.5 kV, the test equipment must be capable of duplicating thesemeasurements so as not to disturb trend analysis.A ILABLE TEST EQU IPMENT

he equipment available for conducting power factor testing isried. Most of the traditional test equipment were bridges suchs the Shering Bridge and the Transformer Ratio Arm Bridge.hese are shown in Figure 1 and 2, respectively. Both of theseeasure the dissipation factor of the insulation being measured,

By O. W. lwanuslwAVO MUL TIAMP CANADA LIM IT ED

rather then the power factor. The difference between the powerfactor and the dissipation factor, however, isvery small, especiallyfor insulation of good quality, that isfor power factors less than 5%.The Transformer Ratio Arm Bridge circuit is more suitable for thisapplication as it offers guard connection that is inherent to thecircuit and passive in nature.To use the above bridge circuits for the measurement ofgrounded specimen (GST test) such measuring equipment mustuse double shielded power sources, as well as double shieldedhigh-voltage cables. Only in this way can power l ine interferenceand internal residuals be reduced to a value that permits ameaningful measurement.

R x

Figure 1. S ch ema tic o f a She ri ng B r idge

Figure 2. S ch ema tic o f a T ra ns fo rm e r R atio A rm B rid ge


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er the years m any pieces o f test equipm ent w ere develo pedd u se d fo r this a pp lica tio n. P erh ap s th e m ost fa mo us o f th ese isD ob le P ow er F acto r T est S et. A sim plifie d d ia gra m o f th is te stet is sho wn in Figure 3. In reality, th is p iece o f test equipm ento es no t m easure pow er facto r, but it do es allo w the user toalculate the po w er facto r fro m tw o m easurem ents. The firstasurem ent is the to ta l current in the m easuring c ircuit. T hec on d m ea su re me nt is th e re sid ua l c urre nt, a fte r a ll th e q ua dra -e c urre nt is s ub tra ct ed , o r c an ce lle d , fro m t he m e as urin g c irc uit .

+----{ t }---+----,

Figure 3. S ch ema tic o f a D oble P ow er Factor T est S ett is no tew orthy to po int o ut that if such a test set is o perated at ae feren ce vo lta ge , sa y 10 ,00 0 vo lts, th en the to ta l curren t in th es urin g c irc uit is p ro p ort io n al to t he c ap ac ita nc e of t he in s ul a-o n a nd th e re sid ua l c urre nt in th e m e as urin g c irc uit is p ro p ort io n alth e p ow er lo ss. T he ra tio o f th e re sid ual to th e to ta l cu rre nt is t heo wer facto r o f the insulatio n being m easured. B eing a reado utpe instrum ent, th is test set has few co ntro ls and is s im ple tohe Do ble set is no t w itho ut its sho rtco m ings. C onsisting o fe sis ta nc es , in du ct an ce a nd c ap ac ita nc e, t he te st s et is s en sitiv efre qu en cy a nd a cc en tu ate s h arm o nic s in th e m ea su rin g c irc uit.m us t re ly o n filte rin g to p ro vid e th e d es ire d re ad in g.here are a variety o f o ther develo pm ents, each w ith its advan-e s a nd disa dva nta ges. T he re is t he "C am ero n H ot B ox" sh ow nF igu re 4. T his is a n Inve rted S herin g B rid ge , w he re p arts o f th etru men t are a t h ig h vo ltag e a nd th e co ntro ls a re o pe ra te d w itha id o f in su la tin g s ha fts . Its u niq ue ne ss is t he u se o f th e in te rn alapacitance between the internal bridge com po nents and thetside ca se fo r th e refe ren ce cap acito r. Its sh ortc om in gs a re itsability to m easure ungro unded specim en and the absence o f ad connect ion .

Figure 4. The Inverted S hering B ridge - T he C am eron H ot B ox

T he re is the O lm an Instru me nts C B6 0. T his is a tra nsfo rm er ra tioa rm b rid ge c irc uit, o f th e H O T g ua rd c on fig ura tio n, w hic h c ou ld beo pe ra te d in U ST a nd the G ST co nn ectio ns. T o o pe ra te in th e G STconnectio n, the instrum ent is insulated from gro und and theco ntro ls a re o pe ra te d b y m ea ns o f in su la te d sh afts , sim ila r to th ein ve rte d S h erin g b rid ge . T his in st ru m en t h as im p re ss iv e fe atu re ssuch as an effective guard fo r bo th U ST and G ST co nnectio nsa nd in te rfe re nc e s up pre ss io n c irc uitry . Its d ow nfa ll w as th e H O TG UA RD circuit w ho se vo ltage w as 10,000 vo lts w hen the instru-m en t w as o pe ra te d a t 1 0,0 00 vo lts in th e G ST co nn ectio n. F ig ure5 sho ws the C B60 in the G ST co nnectio n. This characteristicm ad e th e in stru me nt ra th er u ns afe .T he re a re o th ertra ns fo rm e rra tio a rm b rid ge c irc uit s in tro d uc ed byO lm an In stru me nts a nd T ettex Ins tru me nts. A ll o f th ese fe atu reth e C O LD G U AR D c irc uit, w he re th e p ote ntia l of the guard is at o rv ery n ea r g ro un d p ote ntia l. T he se in stru me nts a re c on sid ere d tobe sa fe r th en th e H OT G UA RD , a nd th ere fo re re ad ily a cce pte d b yth e ind ustry . T he pro ble m w ith m os t o f th ese te st se ts is th at th eyare fu ll a c b rid ges , a nd a s su ch req uire b ala ncin g o f th e in p ha sea nd q ua dra tu re c om po ne nts . U nd er u su al fie ld c on ditio ns , w hic hm ay re qu ire a n a dju stm en t to s up pre ss p ow er fre qu en cy in te rfe r-en ce , th e b ala ncin g pro ce du re g ets to be rath er in vo lve d. T he seo p era tin g c o mp lic atio n s a re re sp o ns ib le f orth es e in str um e nts n o tb ein g w id ely a cc ep te d b y th e in du stry .O ne very desirable characteristic o f the transfo rm er ratio armbridge is that it can be e asily co nfig ure d to ca nce l o r su pp re ssin te rfere nce . T he in terfe ren ce co uld b e re du ce d, o r e ve n e lim i-n ate d, by in je ctin g cu rre nt fro m a n inte rna l o r e xtern al so urc e too ne o f th e w in din gs o n th e tra ns fo rm er.

G U AR D

CD F gJ C N C x

Figure 5. A HOT GUARD T ra nsform er R atio A rm B rid geA g en era l c ha ra cte ris tic o f a ll b rid ge s is th at th ey a re d es ig ne d tote st in su la tio n o f re la tiv ely lo w p o we rf ac to rs , s ay b elo w 1 0% . T he yare no t designed fo r the m easurem ent of e q uip m e nt e x hib it in gh ig h p o we r f ac to rs . L ig htn in g a rre ste rs , fo r e xa m ple , a re ty pic allyv e ry d if fi cu lt if n o t im p o s si bl e to m ea su re o n s om e b rid ge s, a s t he ym ay fa ll o utsid e o f th e d issipa tio n fa cto r ra ng e. T his is a d efinitedraw ba ck n ot o nly fo r the tra nsfo rm er ra tio a rm b ridg e, bu t fo r a llc ap ac ita nc e b rid ge c irc uit in g en era l.

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UTOMATIC POWER FACTOR TEST SETSately severa l auto m ated bridges o r test se ts have beco m eva ilable o n the m arket. A ll o f these m ake the m easurem ent o fu la tio n p ow er fa cto r e as ie r a s th e o pe ra to r n o lo ng er n ee ds toan expe rt in ba lancing a tw o co mpo nen t ac bridge circuit. T heto matio n is even m ore h elpfu l w hen interfe ren ce is enco un-sh ould b e rem em bere d tha t th e au to matic in stru ment w ill s tillu ffer fro m the lim itatio ns o f the circuitry used. T here fo re, theid ge c irc uit, tra ns fo rm e r ra tio a rm o r o th er c on fig ura tio n , w ill b eby its d is sip atio n fa cto r ra ng e a s w a s e xp la in ed a bo ve .

th is tim e I w ish to intro du ce a new in stru me nt co nfig ura tio n.is c on fig ura tio n is v ery p ow erfu l in th at it is a co mbinatio n o f aa ns fo rm e r ra tio a rm b rid ge c irc uit a nd a re ad ou t in stru m en t. In itsim plest fo rm the in stru ment ca n be prese nted a s a m easuringystem co nS isting o f a vo ltm eter, am m eter, w a ttm eter andrm e te r. If t h es e in stru m en ts w e re v ery p re cis e, th en th e c ap ac i-nce and the po wer facto r o f the m easured insulatio n co uld belc ula te d w ith s uf fic ie n t a cc ur ac y. T h e a cc ura cy o f s uc h a s ys te mvery adequate at h igh po w er facto rs, say abo ve 10% , bu t iteco m es pro gress ively less accura te as the po w er facto r isu ce d. A t t yp ic al p o we r fa cto rs o f g o od in su la tio n , s ay 0 .5 % , th eccuracy w ould be no better than 100% . S uch accuracy is no tccep table fo r insu latio n m easurem ent. The disadvan tage o fuch a circu it is that the w attm eter be co mes p ro gre ss ive ly lesscurate a s it is requ ired to m easure p ow er in a lo w p ow er fa cto rrc uit. B y c om b in in g th is re ad ou t in stru m en t w ith a b rid ge c irc uit,ic h in he re ntly is ve ry a ccu ra te a t lo w p ow er fa cto r, re su lts in aa su rin g sy ste m th at is a cc ura te a t a ny p ow er fa cto r.

he functio n o f the bridge circuit is to e ffective ly increase theo wer facto r a t w hich the w attm eter o pera tes. This is acco m-ish ed b y c an ce llin g th e b ulk o f th e ca pa citiv e c ha rg in g c urre nt.c an ce lla tio n is a cc o m plis he d by u sin g cu rre nt fro m th e lo sse e sta nd ard c ap ac ito r w hich is a p art o f th e b rid ge circu it. In th isy the po wer facto r, and there fo re the accuracy o f the po wers ure m en t, is g re atly im p ro ve d. W h en ta ke n to th e lim it o f z eroe rfa cto r, th e a cc ura cy o f s uc h a c on fig ura tio n is lim ite d o nly byp o we r fa cto r o f th e s ta nd ard c ap ac ito r.he o peratio n o f th is type o f test set can be a lso exp la ined as aid ge c irc uit w ith p art ra tio a rm b ala nc e a nd p art re sid ua l re ad o utp ab ility . T he c ap ac ita nc e is d ete rm in ed fro m th e s ettin gs o f th eio a rm s a nd a k no w le dg e o f th e v alu e o f th e s ta nd ard c ap ac ito r.he readings o f the varm eter are used as co rrectio ns to theap acitan ce d eterm ina tio n. T he re ading o f the w attm eter is theue po w er in the test c ircuit. This po w er read ing is used tolc ula te t he p o w er f ac to r.

s in st ru m e nt c o nf ig ur at io n a llo w s it se lf to b e e a sily a u to m a te d .d o es n o t re qu ire a c om p le te n ull b ala nc in g p ro ce du re in vo lv in ge in phase and quadrature co mpo nents. A go od po rtio n o f thead ing can be o bta ined by sim ple reado ut instrum entatio n. Aim plif ied schem atic o f the te st se t d iscussed a bo ve is sho wn ing ure 6 .

e p ro ble m o f in te rfe re nce h as b ee n w ith u s s in ce th e tim e w he nchn icians first tried to m ake a m easurem ent o n a piece o fu ip ment in a live sw itch yard. D ue to its nature, interference isst severe o n specim en that are gro unded and m ust be m ea-re d u sin g th e G S T c on fig ura tio n . In te rfe re nc e le ve ls in vo lv in gU ST co nfig ura tio n a re u su ally a n o rd er o f m ag nitu de s ma lle rn th o se e nc ou nte re d d urin g G S Tte sts .is in te re stin g to n o te th a t t he in te rf er en ce t ha t h a s b ee n a ff ec tin ge ry bo dy's m ea su re me nts h as n ot b ee n th e s ub je ct o f a s tu dy o fs o w n. There are very few peo ple to day that can te ll yo u theu nt o f in te rfe re nc e th at y ou s ho u ld b e e xp ec tin g in a p artic ula rt ua tio n . A s a m a tt er o f in te re st , e ve ry o ne e xp e cts th a t t he h ig h es te rf ere n ce w ill b e p re se nt in t he h ig h es t v o lt ag e s w itc h y ard . T h isy re t be tru e. Interfe re nce is the pro du ct o f vo ltag e and stra y

CURRENT

Figure 6.T he M ulti-A mfP A LF A-1 0 TI J A uto matic P ow er F acto r T est S et

c ap ac ita nc e. A s th e s tra y c ap ac ita nc e is u su ally lo w er in a h ig he rv olta ge s witc h y ard d ue to th e la rg er s pa cin gs , th e in te rfe re nc e ina 500 kV yard co uld be the sam e as the interfe rence in a 345 kVyard. T he largest in te rference is e xp ecte d to b e enco untere d inlo w pro file s ta tio ns. H ere the spacing has been reduced to am in im um , re su ltin g in h ig h stra y ca pa cita nc e a nd th ere fo re h ig hin te r fe rence leve ls.It is app ro priate to po in t o ut th at the vo lt/am plw attlvarm eterm e a su rin g s ys te m d is cu ss ed e a rlie r is v er y s uit ab le f or m e as ur in gth e in te rfe re nc e le ve ls in a te st s itu atio n . T o m e as ure th e in te rfe r-e nc e, o n e o n ly re qu ire s to s hu to fth e te st s et's p o we r s ou rc e. T his ,fo r th e firs t tim e , m a ke s it p o ss ib le fo r te ch nic ia ns to m e as ure a ndk ee p s om e s ta tis tic s o n th e le ve ls o f in te rfe re nc e th ey e nc ou nte r.Th is feature is typica lly no t present in bridge type m easuringc ircuits. 'INTERFERENCE CANCELLAT IONAND REJECTIONAs in dic ate d e arlie r, m o st m e as ure m en ts d o in vo lv e a t le as t s om ein te rfe re nce . D ep en din g o n th e a mo un t o f in te rfe re nc e e nc ou n-te re d, o ne m ig ht u se d iffe re nt te ch niq ue s to s up pre ss th eir e ffe cto n th e m e a su re m e nt .T w o a va ila ble te ch niq ue s o f o ve rc om in g th e e ffe ct o f th e in te rfe r-en ce are rejectin g the in te rference o r ca ncelling it. T he se tw om e th o ds w ill b e d es crib ed b elo w .Interference Rejection.B y c ho o sin g a te st fre qu en cy d iffe re nt fro m th e p o we r fre qu en cy ,o ne w o uld b e in a p os itio n to s up pre ss o r e ve n e lim in ate its e ffe cto n th e m ea su re me nt. D ep en din g o n th e fre qu en cy s ele cte d, o nem ay find the task easier o r harder to acco mplish. Thus, if o nes ele cts 1 00 H z fo r th e te st fre qu en cy, th en th e ta sk o f e lim in atin g60 H z in te rference S ignal fro m the m easurem ent is no t veryd iff icu lt. The 100 H z test frequency, ho w ever, m ay no t be ana cc ep ta b le f re q ue n cy f or p o w er s ys te m a pp lic at io n s.A bette r se lectio n m ay be 55 H z. This frequency is betw een thetw o m ajo r fre qu en cie s u se d fo r p ow er a pp lic atio ns, 6 0 a nd 5 0 H z,an d lN Ould the refo re be quite accep ta ble. T he ta sk o f re je ctinge ith er th e 5 0 o r th e 6 0 H z in te rfe re nc e s ig na ls w o uld n orm ally b ed iff icu lt, due to their pro xim ity to the test frequency. To day,h o we ve r, w ith th e a dv en t o f d ig ita l s ig na l p ro ce ss in g, O S , th is ta skm a y n o t b e v ery d iff ic ult to a cc om p lis h.

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he suppressio n technique is typically lim ited to a s ignal toe rfe re nc e ra tio o f 1 in th e m ea su rin g s et u p. /> Do ve t his le ve le cis e c o rre la tio n t ec hn iq ue s a nd c irc uits w it h e xt re m e lin ea rit ybe e mp lo ye d. T he ra tio o f sig na l to in te rfe re nc e ra tio th atan be to lerated w ill depend o n the linearity o f the m easuring

e rf er en ce C a nc ela tio n .e re a re a t le as t tw o w ay s o f a pp ly in g ca nce lla tio n te ch niq ue sre du ce th e e ffe ct o f in te rfe re nce o n th e m ea su re me nt. O ne o fe se is to m ake two m ea su re me nts, o ne w ith fo rw ard a nd o neth re ve rs e p ola rity o f th e te st v olta ge , a nd th en to a ve ra ge th eesults to o bta in the fin al read ing. As a result o f th is sim plero cedure th e effect o f the interfere nce signal w ill be greatlyeduced o r even co mplete ly elim inated. This arrangem ent isim ple, co nvenien t, an d is applicable to situatio ns w here thee rfe re nc e is s m all, s ay le ss th an 1 0% o f th e to ta l c urre nt. Goodeadin gs can be o btaine d e ven at higher in terfe rence levels ,o vid ed th at th e in te rfe re nc e is v e ry s te ad y a nd th at th e m e as ur-c ir cu it is v er y lin ea r.

o r situ atio ns w here the in terference is h ig her, sa y m ore th en% , o ther tech niques m ust be used . O ne o f th ese is interfer-nce red uctio n by ca ncellatio n. A s w as m en tio ne d ea rlier, th ea ns fo rm e r ra tio a rm b rid ge c irc uit is v ery c on ve nie ntfo rth is a s itllo ws th e ap plicatio n o f a current to a ny o f th e w ind in gs o n thedge t rans fo rmer .n p ra ctic e, th e in te rfe re nc e c urre nt is m e as ure d a nd a c urre nt o fe sa me m agnitude but o f o ppo site po larity is in jected into thedge t rans fo rmer . If a cc ura te ly d on e, th e in te rfe re nc e w ill b eu bsta ntia lly re du ce d. S om e co m plic atio ns w ith th is p rin cip leis e d ue to th e s ta bility o f th e in te rfe re nc e a nd th e s ta bility o f th el ling curren t.is n o tt he in te nt io n o f t yp ic al c an ce lla tio n c irc uits t o e lim in a te th eerference co mp lete ly . A m ea surem ent w ith fo rw ard and re -erse test vo ltage po larity still m ust be do ne to o bta in a go ode ad in g . I nte rf ere nc e c an ce lla tio n p rin cip le s, t og et he r w it h re ad -g averaging, m ake it po ssib le to m ake m easurem ents intu atio n s w he re th e in te rfe re nc e is th ou sa nd s o f p erc en t h ig he rn th e s ig na l to b e m ea su re d.PECIMEN TO BE MEASUREDs th is presentatio n is fo cused m ain ly o n the m aintenance o fa ns fo rm e rs , w e w ill c on sid er th e m e as ure m en t o f a tw o w in din ga ns fo rm er, th e m ea su re me nt o f a th re e w in din g tra ns fo rm erd th e m e as ure m en t o f b us hin gs .

fa cilita te th e m ea su re me nt o f in su la tio n p ow er fa cto r, m o sta ns fo rm ers h ave a ll o f th eir w in din gs te rm in ate d o n b ush in gsven if o ne o f them m ay be d ire ctly g ro un de d. T o m ea su re th es hin gs , th e se a re t yp ic ally e qu ip pe d w ith " Te st T a ps " th a t a llo wbushings to be m ea sured w itho ut d isco nnecting th em fro mt rans fo rmer.W In dIn g T ra ns fo rm e rs .

e s im p lifie d s ch em a tic d ia gra m o f a tw o w in din g tra ns fo rm e r isho wn in F igu re 7. It c on sis t o f th re e c om p on en ts , n am e ly :th e h ig h-v olta ge w in din g to g ro un d c ap ac ita nc e (C h -g ),th e h ig h-to -Io w v olta ge w in din g c ap ac ita nc e (C h-I),th e lo w -v olta ge w in din g to g ro un d c ap ac ita nc e (C I-g ).

e capacitan ces listed abo ve inclu de the capacitan ce o f th eushings. A s the bu shing s a re co nn ected to gro und, the h igh-lta ge w in din g to g ro u nd m e as ure m en t w ill in clu de th e c ap ac i-ce o f th e h ig h-v olta ge b us hin gs . S im ila rly, th e lo w v olta ge toro und m easurem ent w ill inc lude the capacitance o f the lo w-t ag e b u sh in g s.

Figure 7 . S ch ema tic o f a Two W ind in g T ra ns fo rme rT hre e W In dIn g T ra ns fo rm e rs .T h e s im p lif ie d s ch e m atic d ia g ra m o f a th re e- win din g t ra ns fo rm e ris s ho w n in F ig ure 8 . L ab ellin g th e w in din gs in th is tra ns fo rm e r a sh ig h v olta ge , lo w v olta ge a nd te rtia ry , th e s ch em a tic c on sis ts o fs ix c o m po n en ts , n am e ly : th e h ig h-to -Io w v olta ge w in din g c ap ac ita nc e (C h-I), th e h ig h-v olta ge to te rtia ry w in din g c ap ac ita nc e (C h-t). th e lo w -v olta ge to te rtia ry w in din g c ap ac ita nc e (C I-t). th e h ig h-v olta ge w in din g to g ro un d c ap ac ita nc e (C h -g ). th e lo w -v olta ge w in din g to g ro un d c ap ac ita nc e (C I-g ), a nd th e te rtia ry -w in din g to g ro un d c ap ac ita nc e (C t-g ).S im ila rly to th e tw o-w in din g e xa mp le , a ll th e ca pa cita nce s b e-tw ee n th e w in din gs a nd g ro un d in clu de th e ca pa cita nc es o f th ea p p ro p ri at e b u sh in g s.

IICH-TIIH _ : 1 f I: TC H -L CH-T_ . . . . . _ . . . .lH -G I C L ~ C r - : r

-.; G R O U N DFigure 8. S ch ema tic o f a Th ree -W inding T ransforme r

Bushings.T he m ea su re me nt o f b ush in gs is a p articu la r co nn ectio n o f th eu ng ro u nd ed s pe cim e n te st (U S T) c on ne ctio n. T he s ch em a tic o fa ty pic al h ig h-v olta ge b us hin g is s ho w n in F ig ure 9 . It c o n si st s o f : h ig h-v olta ge te rm in al to T es t T ap c ap ac ita nc e (C h -t), a nd T es t T ap to g ro un d c ap ac ita nc e (C t-g ).

Figure 9. S ch ema tic o f a T ransforme r Bush ing4


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KING THE MEASUREMENTSe ne ve r m akin g th e m ea su re me nts, it i s im po rta nt to m ake A LLh e p ossib le m ea su re me nts . T his a llo ws o ne to d ou ble -ch ec k a llh e re su lts o n th e sp ot a nd a vo id re pe ats .a llo w fo r co nvenient m easurem ent w ith the few est po ssib let-u ps , m a ny c om m erc ia l te st s ets p ro vid e tw o m e as urin g le ad sd a v arie ty o f t es t c o nfig ura tio n s. W it h t wo m e as urin g le ad s, a nda lling the instrum ent co nned io ns as H igh Vo ltage (H V), R eda su rin g le ad (R ), B lu e m ea su rin g le ad (B ) a nd G ro un d (G ), it iss ib le t o m a ke t he f ollo w in g m e as ure m en ts :

. H V-RHV-BH V-(R +B )

. H V - (G +R +B)HV-GH V-(G +R ). H V -(G +B)

e lis te d m ea su re me nts o ffe r d up lic atio n a nd a llo w o ne to c he cke r es u lt s. It is su gg es te d th at a ll o f th ese , p lu s e ve n a dd itio na li ca te r ea d in g s be ta ke n to a sc erta in g oo d re su lts .Winding Transformer.

is reco mm ended that, typ ically, fo urteen m easurem ents beo ne o n a single-phase tw o-w inding transfo rm er. These m ea-re m en ts w o u ld in clu de t he f ollo w in g c ap ac ita nc es :. H -L(1). H - G (5). H - (L+G ) (6). L-H (1)L-G (5)

L - (L+G ) (6). TapH 1 - H 1 ( 1). TapH 1 -G (5). TapH 2 - H 2 (1 )0. TapH 2 - G (5)1. TapL1-L1 (1)2. TapL1 -G (5)3. TapL2 - L2 (1)4. TapL2 - G (5)

e a bo ve se qu en ce a ssu me s th at th e h ig h-v olta ge a nd th e Io w-o lta ge b us hin gs h ave te st ta ps . T he se qu en ce , a lth ou gh ra th erg , is n ot d ifficu lt to co nd ud . T he first th re e m ea su re me nts a rearried o ut w ith o ne co nnedio n, the next three w ith ano thero nnectio n, and the bushing m easurem ents are do ne tw o m ea-u re me nts p er c on ne ctio n. T he re fo re s ix d iffe re nt co nn ectio nse re qu ire d to c om ple te th e re co m m en de d s eq ue nc e o f f o urte enis im po rta nt to p oin t o ut th e in te ntio na l d up lica tio n o f re ad in gst a re t ak en :

The H -L capacitance is m easured tw ice independently , anda no th er tw o tim es to ge th er w ith H -G a nd L -G .T he H -G capacitance is m easured tw ice, o nce alo ne and o ncew ith H -L .T he L-G capacitance is m easured tw ice, o nce alo ne and o ncew ith H -L .

a bo v e d up lic at io n a llo w s th e o p era to r t o q uic kly d o ub le -c he cky re ad in g o n th e sp ot. S om e o f th is c he ck in g w ill s oo n be doneo f p erso na l co m pu te rs a tta ch ed to th e te st s ets .d ou ble c he ck in g o fth e b us hin g c ap ac ita nc es a re p os sib le . T heu lts o f th e b us hin g te sts s ho uld be v ery s im ila r a nd th is s erv esa ch eck in itse lf.

s no ted abo ve, the capacitance m easurem ents o f w inding too un d in clu de th e c ap ac ita nc e o f th e a ss oc ia te d b us hin gs . If th eo wer facto r o f the bushings is substantia lly d ifferent fro m theo w er facto r o f the to ta l m easurem ent, then o ne m ay w ish to

subtract the readings o f the bushings fro m the to ta l in o rder too bta in the reading o f o nly the w inding. O ne m ust no t fo rget thatthere are tw o bushings per w inding o n a single-phase trans-fo rm er, and three o r fo ur o f th em o n a th re e-p ha se tra ns fo rm er.A ga in , th is ca lcu la tio n is n ot d ifficu lt to d o a nd c an b e d on e b y th eo pe ra to r o n s ite .Three-W ind ing T rans fo rme rF or th os e w ho c on sid ere d th e tw o -w in din g tra ns fo rm e r e xa mp leto be ra th er te dio us , th ey w ill be th rille d b y a ll th e re ad in gs th ata re s ug ge ste d fo r a t hre e-w in din g s in gle p ha se t ra ns fo rm e r.P ro vid ed th at th ere a re b us hin gs to b e te ste d o n a ll t h re e w in din gso f th e tra ns fo rm er, th en th ere a re 33 te st s s ug ge st ed . T h es e t es tsm e a su re t he f o ll o w in g c a pa c it an c es :1. H -L(1)2. H - T (2)3. H- (L +T) (3)4. H- (G+L+ T) (4)5. H - G (5)6. H - (G +L) (6)7. H - (G+ T) (7)8. L - H (1)9. L - T (2)10. L-(H +T) (3)11. L-(G +H +T) (4)12. L -G (5)13. L - (G+H) (6)14. L - (G+T) (7)15. T - H (1)16. T - L (2)17. T - (H +L) (3)1 8. T - (G + H+ L)(4 )19. T -G (5)20. T - (G + H) (6 )21. T - (G +L) (7)22. TapH 1-H 1 (1)23. TapH 1 - G (5)24. TapH 2 - H 2 (1)25. TapH 2 - G (5)26. TapL1 - L1 (1)27. TapL 1 - G (5)28. TapL2 - L2 (1)29. T apL2 - G (5)30. TapT1 - T1 (1)31. TapT1 - G (5)32. TapT2 - T 2 (1)33. TapT2 - G (5)A ga in , S im ila rly to th e tw o -w in din g tra ns fo rm er, th ere a re fe we rtest set-ups as there are readings to be taken. It ta ke s o nly n in ese t- u ps to d o a ll th e 3 3 te sts liste d a bo ve .Thus, the first seven readings are taken w ith the H V lead co n-nected to the H bushing, the next seven w ith the H V leadco nnected to the L bushing and the next seven w ith the H V leadco nnected to the T bushing. The bushing readings require ad iffe re nt c on ne ctio n fo r e ac h s et o f re ad in gs fo r e ac h b us hin g.A g ain o n e w ill n o te th e n um e ro u s d up lic at io n s t ha t a re s ug ge ste d.T h es e, to g et he r w ith c alc ula tio n s, w ill a ss ure t ha t y o u h av e a v alidse t o f re ad in gs . E ac h o f th e m ain six c ap acita nc es a re m ea su re dtw ic e a lo ne a nd a no th er s ix tim e s in c om bin atio n w ith o th er c ap a-c it an ce s. T h is a llo w s f or a c o mp le te v erific at io n o f th e te st re su lt s.Id en tic ally to th e tw o -w in din g tra ns fo rm e r s itu atio n, if th e p ow erfa cto rs o f th e b us hin gs a re d iffe re nt fro m th e p ow er fa cto r o f th eto ta l reading, then the readings o f the bushings sho uld be sub-tra cte d fro m th e to ta l to o bta in th e re ad in gs o f o nly th e w in din g.

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he m easurem ent o f the bushings is rather sim ple as there arenly two capacitances invo lved in the m easurem ent. The HVad is a lied to the test tap which is especially provided on allig h-v olta ge bu sh ing s fo r th is re as on . T he im po rta nt re sult he rethe Tap-to -co nducto rcapacitance. W hen in service, it has theystem vo ltage applied to it. The Tap-to -gro und capacitance iso rte d o ut d uring n orm al o pe ra tio n o f th e eq uipm en t.e m ust no t fo rget that if the capacitance o f o nly the w inding iseq uire d, it is to be calculated by s ubtra ctin g th e c ap ac itan ce o fb us hin g fro m th e to ta l c apa cita nc e tha t is m ea sured .

SUMMATIONW ith the number o f readings that need to taken on the twow ind in g tra ns fo rm er a nd es pec ially o n the th re e-w ind in g tra ns -fo rmer, there is ample oppo rtunity fo r the test person to makee rro rs in c on nec tio n, re ad in g o r tra ns crip tio n. In o rd er to re duc eth es e to a m in im um a nd to a sc erta in a g o od s eto fte st res ults , firs ttim e, e very tim e, s om e de gree o f au to matio n is desirable.T he m odern auto mated po wer facto r test sets co me to the rescueo f the test technician. Such test sets are capable o f co nductings eries o f m ea surem en ts , p ro vid e printe d re co rds o f tes t re sults .a nd w he n c om bin ed w ith p ers ona l c om pute rs w ill do uble -c hec kevery reading that w as taken and save it in a database fo r futurereference.

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power factor testing of power transformers

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