Discussion of Rural Alaska Electric Power Quality

7/30/2019 Discussion of Rural Alaska Electric Power Quality

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Discussion of Rural Alaska Electric Power Quality

Excerpts of paper by J.D. Aspnes, B.W. Evans and R.P. MerritPresented at the IEEE PES Summer Meeting, 1984

Discussion by F.D. Martzloff, submitted August 1984

Reproduced, with special permission, from IEEE Transactions PAS-104, March 1985

NOTE: This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any wayimply IEEE endorsement of any of NIST's products or services. Internal or personal use of this material is permitted.However, permission to reprint/republish this material for advertising or promotional purposes or for creating newcollective works for resale or redistribution must be obtained from the IEEE by sending a blank email message to

[email protected]. By choosing to view this document, you agree to all provisions of the copyright lawsprotecting it.Significance:

Part 3 Recorded surge occurrences and surveysPart 5 Monitoring instruments

The paper reports an extensive project of monitoring power quality performed with typical instrumentsavailable in the mid-eighties. Note that during the period of monitoring, the proliferation of low-voltagemetal-oxide varistors had probably not yet involved all the sites covered by the monitoring. Nevertheless,

the results indicated that the maximum surges ever recorded in this project had only a 368 V peak. Thissomewhat surprising result needed an explanation.

The discussion offers such an explanation: the monitors that were used for the survey had a built-invaristor in their power supply input, a reasonable precaution from the designers of the instrument. Withhindsight too late for the researchers it was realized that the power supply of the monitor was plugged

in the same receptacle from which the monitored voltage was acquired: small wonder then, that themaximum surge voltage that could be observed was simply the let-through voltage of the varistor.

For the purposes of this discussion, only the most relevant pages concerned with surge occurrences havebeen reproduced. The full copyrighted paper is a rich source of data on other disturbances.


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608 IEEE Transactions on Power Apparatus and Systems, V ol. PAS-104, No. 3, Marc h 1985RURAL ALASKA ELECTRIC POWER QUALITY

3 . D, Aspnes, S en io r Member, IEEE B. W. Evans, St ud en t Member, IEEE R. P. M e r r i t t , Member IEEEElec t r i ca l Eng ineer i ng Depar tmentU n i v e r s i t y o f A la sk aFa irba nks , A1 aska 99701

A b s t r a c t - P oo r q u a l i t y e l e c t r i c powe r h as t r a d i -t i o n a l l y been blam ed f o r e l e c t r i c a l and e l e c t r o n i ce qu ip me nt m a l f u n c t i o n s an d f a i l u r e s i n r u r a l A l as ka ncommunit ies. Th i s paper r ep or ts r es u l t s o f a r ec en t l ycompleted p ro je c t i n wh i ch d is tu rbance ana l yzersp r o v i d e t h e f i r s t c o mp re he ns iv e p ower q u a l i t y d a t a f ro mA1 askan v,i 11ages.Power , sys tems o f f o ur w ide ly separated coimun i t ieswere s tud ied f o r a to ta l o f 1,010 days . These re su l t sa r e i m p o r t a n t b ecause o f t h e t r e n d i n r u r a l A la sk at o wa rd more s o p h i s t i c a t e d e qu ip me nt t h a t i s s e n s i t i v et o power system dis turbanc es. These data repres ent af i r s t s t e p i n d e ve l o p in g a p p r o p r i a t e c ou nt erm ea su re s t op r o t e c t e l e c t r i c a l system s c on ne cte d t o i s o l a t e d r u r a l60 Hz power genera to r fac i l i t i es .

INTRODUCTION* I n c r e a s i n g l y s o p h i s t ic a t e d e l e c t r i c a l and e l ec-t r o n i c e qu ip me nt i s b e i n g u t i l i z e d i n s m al l A la sk anv i l l ag es . Th i s r anges f r om communicat ions s a t e l l i t eear th . s t a t i on s , computers and o f f i c e equ ipment t oco n t ro l l e r s and c i r c u l a t i n g pump motors. The i s o l a t edq le c t r i c power sys tems o f these communit ies a re a lmostu n i v e r s a l l y s u p p l i e d by d i e s e l e n g i n e -d r iv e n g e n e r at o r si n . the 100 t o 1,000 kVA range and are o f t en charac-t e r i z e d b y sm a ll d i s t r i b u t i o n t ra n sf o rm e rs w i t h l o n gsecondar ies.Poor e l ec t r i c power qua l i t y has repeated l y been

b lamed f o r e l ec t r i c a l equ ipment ma l func t i on andf a i l u r e s , e ve n t ho ug h d a ta t o s u b s t a n t i a t e t h i s v i ew -po in t were l ack ing . A p r o j ec t under taken to r emedyt h i s l a c k o f d a t a was r e c e n t l y c om p le te d an d t h er e s u l t s a re re p o r t e d i n t h i s p a pe r. A l th o u gh e x t e n s i v es tud ies mon i to r i ng power l i n e d i s tu rbances have been~o mp le te d more than a decade ago [I],h e a u t h o r sb e l i e v e t h a t t h i s p ap er re p re s en t s t h e f i r s tcomprehens i ve e lec t r i c power qua l i t y da ta f r om i so la tedr u r a l A la sk an v i l l a g e s t o a pp ea r i n t h e openl i te ra tu re . Data were co l l e c te d f r om fou r communit iesf o r a t o t a l o f 1,010 d ays. The se d a t a h e l p i d e n t i f yt h e s co pe o f t h e powe r q u a l i t y p ro b le m i n r u r a l A la sk aand help approach the l a rg e r quest ion o f what equipmenti s n e ce ss ary a nd a p p r op r ia t e t o p r o t e c t i n ac o s t - e f f e c t i v e way t h e e l e c t r i c a l and e l e c t r o n i csystems used i n remote Alaskan lo cat io ns.

SITE IDENTIFICATIONThe process o f choos ing v i l l ag es was determined bysevera l cons idera t ions : ( a ) r ev iew ing power av a i l -a b i l i t y f o r s ma ll e a r t h s t a t i o n s [ Z]; ( b ) P u b l ic H e a l th

84 SM 640-9 A paper recommended and approvedby the,IEEE Surge Protective Devices Committee ofthe JEEE Power Engineering Society for presenta-tion.at the IEEE/PES 1984 Summer Meeting, Seattle,Washington, July 15 - 20, 1984 . Manuscript submit-ted February 1 , 1 9 8 4 ; made available for printingMay 1 0 , 1 9 8 4 .

S e r v ic e (PHS) c i r c u i t a y a i l a b i l i y ; ( c ) Federa lA v i a t i o n A d m i n i s t r a t io n (FAA) f a c i l i t y and s e r v i c eo uta ge r e p o r ts ; ( d ) a c c e s s i b i l i t y t o c an d id a tev i l la g e s; and (e ) s t a t e f a c i l i t i e s a v a i l a b i l i t y f o rp lacement o f equ ipment. The power a v a i l a b i l i t y fo re a r t h s t a t i o n s a nd PHs c i r c u i t a v a i l a b i l i t y w erecatego r ized in t o fo u r breakdown causes: (1) outagescaused by power; (2) e nviro nme nta l ; (3) equipment;and (4) unknown. Two separate l i s t s o f candidatev i l la ge s were made; one f rom the ea r th s ta t i o n dataand t h e o t h e r f r om PHS c i r c u i t a v a i l a b i l i t y .The r e s u l t i n g 1 s t s i n c lu d e d v i l l a g e s t h a trepo r ted the most outages. A l l fo u r breakdowncatego r i es were we igh ted equ a l l y i n the p rocess o fse lec t i on . Hence, a v i l l ag e w i th a h igh number o foutages i n one category and low numbers i n the oth ert h r e e w ou ld n o t l e n d i t s e l f t o ca nd id ac y. F i n a l l y ,a s i n g le l i s t was made by choos ing v i l l a ge s f r omeach l i s t t h a t m atc he d i n d a t a r e p o r t e d fr o m t h e t wos ou rc es , t h er e by r e d u c in g th e p o s s i b i l i t y o ferroneou s data. The FAA re p or ts were coded asf o l l o w s : (1) scheduled maintenance; (2) 1 neoutages; (3) improvements; (4) power fa i l u re ; (5)power fa i l u re s tandby ; (6) propagat i on cond i t i ons ;(7 ) weather e f fec ts ; (8) sof tware; (9 ) unknown; and(10) other .The l i s t o f c a n di da te v i l l a g e s r e s u l t i n g f ro mFAA rep or t s was determined i n a s im i l a r fashion ast h a t u se d f o r t h e e a r t h s t a t i o n and PHS c i r c u i ta v a i l a b i l i t y l i s t . The f i n a l se l e c ti o n u t i l i z e ddata f rom a1 T above sources.This s tudy was pr imar i l y concerned w i thmeasur ing e lec t r i c power qua1i y a t u s er 1oca t i ons1 k e l y t o r e c e i v e s e n s i t i v e o f f i c e a ut om a ti on a ndcomputer equ ipment . Therefo re, i n th re e o f the fou rv i l l a g e s t h e da ta c o l l e c t i n g s i t e was an o f f i c e i n ap u b l i c b u i l d i n g . I n t h e f o u r t h s i t e , A mb le r, d a t awere r e co rd e d a t t h e s e r v i c e e n t ra n ce o f t h e p u b l i cschoo l. I n a l l cases a s i n g le phase 120V l i n e t one ut ra l , 60 Hz source was moni tored, su pp l ied by afo ur w i re grounded wye sys tem. Page l i m i t a t i o n sprec lude a more deta i led power sys tem descr ipt ion.A l l f o u r d at a c o l l e c t i n g s i t e s were i n r e l a t i v e l ymodern bu i l d i ngs fo l l ow ing Nat i ona l E le c t r i c a l Coder e qu ir e me n ts f o r w i r e s i z e a nd d i s t r i b u t i o n .Overvo l tage p r o te c t i on was no t p resen t a t any s i te .The power genera t i ng p lan ts and da ta co l l ec t i ngs i t e s w ere c e n t r a l l y l o c a t e d i n a l l co mm un itie s.T yp e and c h a r a c te r o f e l e c t r i c a l l o a d s a t eachs i t e were s tandard o f f i c e equipment, f l uo resc en t andi nc a nd e s ce n t 1 g h t i n g , s m a ll r e f r i g e r a t o r s a ndf reezers , c i r c u la t i ng pumps. and a i r hand1 ngequipment. The prob able maximum in d iv id u a l motorr a t i n g a t a ny l o c a t i o n was l e s s t h an 10 hp.

DESCRIPTION OF DISTURBANCE ANALYZER MEASUREMENTCAPABILITIESThe power 1 n e d i s tu rbance ana l yzers u t i l i z e di n a l l c as es f o r d a t a c o l l e c t i n g w e re D r a ne t z M odel606-3 un i t s w i t h op t i on 101 (over l unde r f r equencymon i to r i ng ) . They p rov ided the fo l l ow ingi n f o r m a t i o n [MI.


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Over lunder f requency moni to r ing t o 0.25 Hzaccuracy.RMS vol ta ge le ve l based on a 10 second mov ingaverage o f th e measured vol ta ge t o an accuracy o f+1%o f r e a d i n g +I%f nom ina l i n pu t a t 60 Hz.T h i s i s a l s o r e f e r r e d t o a s slo w-a ve ra ge v o l t a g ei n the head ing o f Tab le 11.RMS va lue o f each AC cyc le compared w i th t he 10second moving average o f measured vo l ta ge t o anaccuracy of 21% o f r e a d i n g+I%f n om in al i n p u t a t60 Hz. A s ingle-cyc le measurement lower than thes low average i s a sag. A h igher s i ng le - cy c lemeasurement i s a surge.Tran s ien t impulses w i t h du rat ion between 0.5 and800 microseconds are record ed t o an accuracy o f+5% o f r e a d i n g r l% f nominal i np u t +6 dB over th ee n t i r e r a n g e o f i m p u l s e w i d t h . ,O utages, d e f i n e d as l o s s o f l i n e v o lt a g e f o rper io ds gre at er than 0.5 second, are recorded.The d i s tu rbance an a l yzer r e tu rn s t o norma lopera t i ng mode 10 seconds a f t e r l i n e vo l tage i sres to red .S a gl su rg e d u r a t i o n and t i m i n g o f a l l e v en t sin cl u di n g power outages a re a1 so recorded.The Dranetz 606-3 power l i n e d is turbance analyze rhas i s o l a t e d c i r c u i t s f o r t h r e e i n d i v i d u a l phaseinpu ts . These i npu ts a re i so la te d f r om each o ther , thein te rn a l power supp ly and g round. I n a l l bu t one o fthe da ta s i t e s the o n l y power source av a i l a b le was as i n g l e p ha se b ra n ch c i r c u i t f e e d i n g a s i n g l e d u p le xo u t l e t . F o r u n i f o r m i t y , t h e d i s t u r b a n c e a n a l yz e r s w erei n a l l cases powered by the same s ing le -phase c i r c u i tth a t was be ing moni tored.There has been concern that the d is turbanceana l yzer power supp l y m igh t a f fe c t impu lsemeasurements. The u ni ts which prov ided data f o r t h i spaper have power su pp l ie s which pre sen t a 0.01 VFcapac itance ac ross th e power l i n e regard less o f whethert h e u n i t i s tu rn e d on o r o f f . T h is low -pa ss f i l t e rt y p e o f i n p u t c i r c u i t i s common i n h ou se h ol d a nd o f f i c eappl iances. For microsecond pu lses (1 MHz), t h i scapac i tance represen ts 16 a re ac t iv e impedance. Thisimpedance would have a smal l e f f e c t on t r an s i en tsgene rated i n lo w impedance sources such as a powersystem hav ing 100 kVA or greater capac i ty as determinedby ou r own la b or at o ry measurements.

POWER SYSTEM DISTURBANCE DATA SUMMARYThe fo l l ow ing tab les and f i gures p resen t da tat a ke n d u r i n g a t o t a l o f 1,010 days i n f o u r r e mo teAlaskan communit ies. The dat a ca teg or i es are :frequen cy de vi at io ns fro m 60 Hz, 10 second movingaverage, surgelsag, mpulses and known outages.Table I hows the number and percenta ge o f days i nwhich the maximum power system frequency deviat iono c c u rr e d w i t h i n v a r i o u s r an g es f o r e ac h v i l l a g e a nd f o rt h e o v e r a l l p r o j e c t . A +0.5 Hz t h r e s h o l d f o r fr eq u e nc ymon i to r i ng was used a t a l l s i te s except a t Kotzebuew here t h e t h r e s h o l d was s e t a t t 1 . 0 Hz. I n an e f f o r tt o be cons i s ten t w i t h computer manufac tu re r s ' powersystem per formance sp ec i f i ca t io ns , we have repo r tedon l y wors t- case f r equency dev ia t i on d a ta when l i n evol ta ge was w i t h in a useable range. Prec ise f requencye x cu r si on d u r a t i o n i n f o r m a t i o n i s n o t a v a i l ab l e .Table I 1 g ive s the number and percentage o f daysi n wh ich th e sm a l l es t and l a rg es t 10 second moving

average rms system vo l tage occur red w i th i n sp ec i f i e dra ng es f o r each v i l l a g e and f o r t h e t o t a l p r o j e c t .Reference valu e i s take n t o be 120V rms. Thus, the +6%t o + l o% range corresponds w i t h 127V t o 132V; the -13%t o +6% range corresponds wi t h 104V t o 127V; th e -20% t o

-13% range corresponds wi th 96V t o 104V; the -40% t o-20% range corresponds w it h 72V t o 96V; and th ef i n a l -100% t o -40% ca tegory cor responds w i t h OV t o72V. The s low average th re sho ld vo l tage se t t i n g fo ra l l d i s tu rbance ana l yzers was 3V except fo r the u n i ta t Ambler, fo r which the thre sho ld was se t a t 5V.I n the case o f Kotzebue, the re were days i n whichthe 10 second mov ing average both rose in to the 6.1%t o 10% range above 120V and dropped below t h e -13%thresho ld. Both events were counted independent ly ,g i v i ng percentages t h a t do n o t t o ta l 100% and dayswhich do no t sum t o th e t o t a l number o f measurementdays a t tha t s i te . Th i s was done t o p ro v ide ac l e a r e r p i c t u r e o f s ys te m d i s tu r b a n ce s . D u r a t i o n o fs low average dev ia t i on s a re no t r epor ted here i nd e t a i l b u t t y p i c a l l y r an g e be tw ee n one a nd t e nminutes.Table I1 1 prov ides number and percentage o fdays i n which th e wors t case sag and surge occur redw i th i n spe c i f i ed ranges. These ranges a re : 6% tol o%, 10% t o 20% and >20% above a 120V r ef er en ce and-20% to -13%, -40% t o -20% and -100% to -40% belowthe 120V reference . Sag/surge th res ho ld se t t in gswere as fo l l ow s. Ambler: 10V; F o r t Yukon: 3V f o r40 days, the n 5V fo r th e pro je c t durat io n; Kotzebue:5V; S t . Marys: 3V f o r th e f i r s t 146 days, then 5Vf o r th e remainder. The maximum surge du ra t io nmeasured d u r i n g t h e e n t i r e p r o j e c t f o r a d a i l ywors t-cas e surg e was 231 cy cl e s. Maximum sagdu rat ion measured f o r a d a i l y wors t -case sag was 115cyc les . Typ ical saglsurge du rat ion was observed t obe less than 40 cyc les .Table I V shows th e number and percenta ge o fdays i n which th e maximum impulse occur red w i t h in a50V t o 99V range or had an ampl i t ude g re at er th an99V. The number o f impulse s i n each cate gor y i sgive n as we l l as the average o f th e monthly maximumimpulse magni tudes recorded a t each loc at ion . Themaximum impulse measured a t each s i t e i s a l soi n cl u de d . I mp uls e v o l ta g e t h re s h o l d a t a l l s i t e swas 50V.Table V gi ve s a summary o f outage d ata .I n c lu d e d i s t h e t o t a l num ber o f known d u r a t i o no u ta g es a t ea ch d a t a c o l l e c t i o n s i t e , t o t a l numbero f outages , to ta l known dura t i on ou tage t ime,average outage d ur at io n and average number o f daysbetween outages.F igures 1 t h rough 4 p rov ide one represen ta t i vemonth o f power system dis turbanc e data f o r eachcommunity inc lud ed i n th e s tudy. Inc luded on adai ly basis are maximum and minimum frequency,maximum and minimum ave rage vo lta ge , maximum sag andsurge vol tages, maximum impulse vol tage and totalnumber o f impulses (50V th resh 01 d) , number o f sagsand sag du ra t io n and number o f surges and surgedur a t i o n (3 t o 5V th resho ld ) . Thus these f i g ure sg i v e more d e t a i l e d i n f o r m a t i o n t h an i s p o s s i b l e t oshow i n Tables I hrough V.

ESTABLISHED LIMITS OF ACCEPTABLE POWER QUALITYSeveral references d ef ine acceptable powerq u a l i ty l i m i t s f o r computer sys tems [14, 19, 20, 22f o r example] and a t l e a st one addresses communica-t i o ns systems [22]. General agreement e xi s ts th a t+6% and -13% ra ted v o l tage s teady -s ta te l im i t s a renecessary , a l thoug h a t l e a s t one computer manufac-t u r e r i s r e p o r t e d t o r e q u i r e + 4% t o l e r a n c e [2 0].O p in io n s ab o ut a c ce p ta b le powe r q u a l i t y d i f f e r f o rt r a n s i e n t s l a s t i n g l e s s t h a n 2 se co nd s. TheAmer ican Nat ional Standards Ins t i tu te (ANSI)Standard C84.1 re qu ire s +15% and -20% vo l ta ge

to le ranc e f o r t ra ns ien ts between 0.05s and 0.5sdur a t i o n and +20% and -30% vo l tage to l e ranc e f o rtr an si en ts between 0.008s and 0.05s du ra t io n asr e p o r t e d i n 1 201. A d i f f e r e n t t o l e r a n c e e n ve lo pe i ssuggested i n [ I9 1 r es u l t i n g f r om U.S. Navy te s t s and


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Table IV . Impulse Distul

L ocat ion ( t o t a l days)Ambler (147)

St. Marys (331

P r o j e c t T o t a l (1,010)

ance Data SummaryIsul w

2~3C,>%.9 $Z0 m w vL E w sw ~ r nnc 3EC , a>3.- E OZ 3.r U J

66 days44.9%

Lw a,2 E %u s w+ .9 k,O X >m w mL E w mw 7n r s sEC, a93.7 E rZ 3.r C,8 1 days55.1%

10 6 days34.2% 64.5%1122 days55.0% 44.1%I 98

4,489 5,210 1 12OV I 188V68 per day 64 per dayaverage average755 2442 143V 168V7.1 per day 12.2 per dayaverage average

12,098 10,533 168V99 per day 1 107 per day 1 16" 1verage average5,796 28,278 368V88 per day 1 107 per day 1 262v 1verage average

23,138 46,463 172V 368V64 per day 72 per dayaverage average


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DiscussionG. T. Heydt (Pu rdue U niversity, West Lafayette, IN): This is a fascinatingglimpse into an area which few power engineers in the United States see.The problems of rural electric power systems are frequently multifarious,including voltage support, frequency control, surge surpression, conduc-tor sizing, and economic operation. T he authors have focused on a fewarea s which relate to siting of specialized, high technology installations.The ir remarks non etheless apply to a wide range of rural electrificationsettings.I would like to ask the authors whether problems of reconductoringand upgrading of components were also considered. Perhap s a table ofpeak load and MWh served annually would be helpful for the readerto assess the impact of added loads in the four towns included in thestudy. This question comes to mind since many rural sites have littlegeneration m argin for exp ansion. Sites in Latin America, for exam ple,often h ave distribution conductor limitations which would oreclude ex-pansion without considerab le expenditure for upgrading capacity. Is thisalso the case in rural A laska?A second area w hich I would like to discuss concerns the presence ofharmon ics in rural pow er systems. In many isolated systems of limitedcapacity, load s such as fluorescen t lighting and motors with solid statecontrollers cause a significant harmonic distortion. Were tests also ma dein this area? T he total harm onic distortion should be considered in ruralelectric systems for which wm puter loads are planned. In ou r experience,a total harm onic distortion greater than 10% (bus voltage) has the poten-tial of interfe ring with computer timing signals. This has been observedat sites in the continental United S tates and Latin America.

I would certainly encourage the authors of this paper and others toreport on other findings relating to rural electric power systems. Thespecialized procedures of design and ope ration of these systems have beenneglected in the literature.Manuscript received August 2, 1984

Francois D. Martzloff (General Electric Co., Schenedtady, NY): Thispaper is indeed a very useful contribution to the data base on the occur--rences of power system disturbances. However, the section dealing withthe results obtained on the occurren ce and levels of "imoulses" mightstill be misleading even though the auth ors make a referencetothe q uestion on the effect of the built-in surge suppressor of the Dran etzAnalyzers used the measurements.The problem arises from a characteristic of the Dranetz equipment,which exists for bo th Mod els 606 and 626, and which was no t recogniz-ed a t the time the measurements were made but is now pointed out inmore recent Dranetz instruction manuals.In order to protect the electronics of the Disturbance Analyzer fromdamag e by overvoltages in the power supply to these internal electronics,a surg e suppressor has been provided in the input to the power supply-no t to the monitoring input of course. However, if the ac power systembeing monitored is the same as the power system in which the instru-ment power supply cord is plugged-a likely possibility in the generalcase, a nd which is precisely situation of th e measure ments reported inthe paper-then the observations of surge occurrences on that powersystem are those of a system whose transients have been suppressed!T o support this claim, Fig. 1 shows an oscillogram recorded at theoutput of a surge generator which provides both 120 V ac power andthe IEE E/AN SI C62.41 (formerly IEEE 587) ringwave, Category B. Theoscillogram shows the surge without the Drane tz analyzer plugged in thetest system output, and, superimposed, the effect of plugging the PO WE RCO RD ONL Y of the D ranetz analyzer in the test system output. W ithoutthe analyzer, the open-circuit voltage is 3 kV; with the a nalyzer pluggedin, the output voltage is reduced to 1.1 kV.Th e Category B c haracteristics are 6 kV open-circuit voltage, 500 Ashort-circuit current, therefore a source impedance of 6000 : 500 = 12ohms. From the circuit values of Fig. 2, the unknown effective impedanceof the analyzer, Z, can be computed to be only 7 ohms. Th e authorscite a 16-ohm impedance at 1 MHz; assimilating the first loop of thetest wave to a half sine wave with a duration of 2.5 microseconds, theequivalent frequenc y in Fig. 1 would be 200 kHz (a period of 5microseconds) for which the impeda nce resulting solely from a cap acitorwould be higher than the 16 ohms at 1 MHz. Thus, it appears tha t thereis ad dition al parallel impedance on the line w rd input, since the simplifiedcomp utation of Fig. 2 yields only 7 ohms. That low impedance, when

Fig. 1. Open -circuit voltage outp ut of KeyTek Surge Generator Model711 with P 1 Plug-in and V oltage with Dranetz 606 power cordconnected at output of generator.Vertical : 1 kV/divSweep : ps/div

connected in parallel with the voltage measurem ent leads, will load thesource of the transient and yield lower voltage recordings than the ac-tual occurrence would have been without the analyzer connected. Thissituation makes my facetious remark in the 1970 paper [l] come true("the best surge suppressor is a surge monitor!").The authors make the statement that, according to their own labo ratorymeasurements, this effect should be negligible on a 100 kVA power.system. That statement seems to imply that there is a correlation betweeflthe 60 Hz impedance of a system (which indeed decreases as the kVArating increases) and the effective impedance at the equivalent frequen-cy of th e impulse; I have some difficulty in accepting tha t implication,It w ould be interesting to know the details of the measurements citedby the authors, a nd com pare them to other references, such as the Bullpaper 121 from which Fig. 3 is excerpted. If we accept the value citedby Bull, 50 ohms at 100 kHz, then the 7 ohms effective impedance ofthe analyzer will reduce the voltage recordings in a 8 : 1 ratio (7/57 =8), regardless of the low 60 Hz impedance of the 100 kVA system.While it is too late for data already recorded, there is a very simplesolution to the problem. Ferroresonant line conditioners no t only pro-vide surge isolation at their outp ut, but also decoupling of the input formthe output [3]. Fig. 4 shows the open-circuit output of the surge generatorat 6 kV (upper trace) and the output with the line conditioner feedingthe analyzer plugged in (lower trace). There is no detectable effect onthe impinging surge. Thus, by merely inserting the line conditioner 9the pow er cord of the analyzer, the issue disappears, an d m easurementscan be obtained without the present ambiquity which can cause a senseof false security in the relatively low levels of impulse cited.

Fig. 2. Computation of effective input impedance of the disturbanceanalyzer power supply.


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J. D. Aspnes, B. W. Evans and R. P. Merritt: We thank the discussersfor their thoughtful and detailed comments.With reference to the qu estions posed by D r. H eydt, we recognize theproblems of reconductoring and upgrading rural systems and the im -pact of such system modifications on electric power quality. This w z,however, outside the scope of the project discussed in this p aper. A studyof economic costs and p otential power quality benefits of variou s systemupgrades would certainly be a useful follow-up to our initial work.Peak load and annual megawatt-hour generation data for the fourisolated electrical systems are included in Table 1 111.Tests to determine presence of harmonics were not done. This wouldalso be a valuable addition to future investigations.

--REWEICY IYHJ TAB LE 1-Utility da ta3. Impedance of "Pow er Mains" as a function of frequency, com-pared to SO-ohm, 50-uH series combination. (From Ref. [2].) Utility installed Net generation Peak deman dLocation nameplate capacity (kW) (MWH ) (MW)

Am bler 420 348 0.1Ft. Yuko n 1,050 2,013 0.5(estimated)Kotzebu e 4,825 11,531 2.2St. Marys 1,500 1,599 0.4

Fig. 4. Ou tput v oltage of KeyTek 71 1/P1 surge generator.Top trace: no connected loadLower trace: Line conditionerGE Ca t 9T91L130G3Plugged in outputVertical: 2 kV/divSweep: 2p/div

REFERENCES1. F. D. Martzloff an d G. J. H ah n, "Surge Voltages in Residential an dIndustrial Power Circuits." ZEEE Trans. on Pow er Apparatus andSystem PAS-89, pp. 1049-1056, July-August 1970.J. H. Bull, "Imped ance of the Mains Supply at R adio Frequencies."Proceedings of the 1st Symposium on EMC , Mo ntreux, 75CH1012-4,

pp. 357-362, May 1975.F. D. Martzloff, "The Propagation and Attenuation of Surge Voltagesand Surge Currents in Low-Voltage AC Circuits" Trans. on PowerApparatus and Systems PAS-102, pp. 1163-1170, May 1983.Manuscript received August 10, 1984

We turn next to the interesting points raised by Mr. Mar tzloff. A powersystem impedan ce function characterized by a parallel 50 ohm , 50 fiH,RL network is provided in Fig. 3 of his closure. If this is a true represen-tation of the power systems tested, and if the Dranetz Model 606 analyzermay be represented by a 7 ohm impedance, then we agree with Mr. M art-zloff's conclusions. However, the resulting 8:l impulse attenuatio n wouldimply m easured averages of monthly maximum impulse magnitudes rang-ing between 906V and 2944V. Intu itively, these numb ers seem excessivelylarge.To gain some idea of actual impulse attenuation caused by the Model606 analyzers used in this project, a Dranetz Disturbance Simulator(Model 604A) was used as an impulse generator. We found that theunerengized Model 606 power supply attenuated these impulses approx-imately the same as a 0.01 microfarad capacitor connected across thedisturbance simulator terminals. A higher energy, lower imped ance im-pulse generator was assembled and a correspon dingly lower attenuationwas noted.It would be very interesting to repeat our impulse measurements atthe original data-gathering sites using a suitably isolated surge monitor.However, we have no t yet had the op portunity to d o so.We again wish to express our appreciation of the discussions whichhave added to the value of our paper.

REFERENCE[ I ] U. S. Dept. of Energy, Alaska Power Administration, Alaska Elec-tric Power Statistics, 1960-1981, Seventh Edition, August 1982.

Manuscript received October 25 , 1984

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Discussion of Rural Alaska Electric Power Quality