CONSIDERAŢII PRIVIND SELECTIVITATEA PROTECŢIILOR UNUI TRANSFORMATOR DE PUTERE DINTR-UN POST AERIAN

CONSIDERATIONS ON THE PROTECTIONS SELECTIVITY

FOR A POLE-MOUNTED TRANSFORMER

Vasile TABACARU Georgeta HARABAGIU Cezar CRISTIAN S.C. FDFEE ELECTRICA MUNTENIA NORD S.A.- SDFEE Galati, Romania, Galati, str. N. Bãlcescu, nr. 35A

Tel. : + 40–236-305812, fax.: +40-236-461795, e-mail : tehnic@electricitate.galati.ro

Rezumat: Instalatiile de joasa tensiune se alimenteaza din instalatiilede medie tensiune peste un transformator de medie pe joasa tensiune.In cazul Postului de Transformare Aerian (PTA) de 20 / 0,4 kV,restrangand sfera de analiza, selectivitatea aparatelor de protectietrebuie sa fie asigurata de la celula de medie tensiune a statiei detransformare de 110/20 kV din care este alimentata linia aerianala care este racordat postul de transformare, pana la ultimul receptorde joasa tensiune, fara a suferi vre-o discontinuitate in zona transfor-matorului. Lucrarea isi propune sa intre in detaliile unei analizecorecte a selectivitatii protectiilor unui transformator de puteredintr-un post de transformare aerian de 20 / 0,4 kV (din mediulrural), cu plecari radiale in linii aeriene de joasa tensiune, exemplifi-cand criteriile de analiza pentru un transformator de 160 kVA. Cuvinte cheie: transformator, sigurante, intreruptor, selectivitate,protectii.

Abstract: The low voltage systems are supplied from the mediumvoltage systems through a medium/low voltage transformers. Atthe 20 / 0,4 kV pole-mounted transformer, limiting the analysis, theprotection selectivity has to be provided from the mediumvoltage bay of the 110 / 20 kV substation that supplies the over-head line where the transformer is connected, up to the last lowvoltage receiver without any discontinuity in the transformerarea. This paper is intended to detail the correct analysis ofprotection selectivity for a power transformer in a 20 / 0,4 kVpole-mounted transformer (in a village), with radial outputs inlow voltage overhead lines. A practical situation of selection of a 160 kVA transformer is presented. Keywords: transformer, fuses, circuit-breaker, selectivity, protections.

1. Introducere

Instalatiile de joasa tensiune se alimenteaza din cele de me-die tensiune peste un transformator de medie / joasa tensiune.

In cazul Postului de Transformare Aerian (PTA) de 20 /0,4 kV, selectivitatea aparatelor de protectie trebuie sa fieasigurata de la celula de medie tensiune (m.t.) a statiei de transfor-mare de 110 / 20 kV din care este alimentata linia aeriana de 20 kV la care este racordat postul de transformare,pana la ultimul receptor de joasa tensiune (j.t.), fara vre-o discontinuitate in zona transformatorului.

Avand in vedere specificul postului de transformare aerian,pe partea de m.t. a transformatorului nu se poate monta unaparat de comutatie cu capacitate de rupere (intreruptor auto-mat tripolar) si se monteaza doar sigurante fuzibile, care saasigure protectia transformatorului la defecte interne (scurt-circuit intre spire sau intre infasurari, atingeri la masa, etc.)si la defecte externe care pot apare pe legaturile cuprinse intrebornele transformatorului si bornele de intrare ale aparatelorde protectie, atat pe partea de m.t. cat si pe partea de j.t.

Pe partea de j.t. a transformatorului se pot monta sigurantefuzibile sau intreruptor automat, echipat cu declansatoaremagnetotermice sau relee electronice, aparate care sa asigureprotectia la suprasarcina si scurtcircuit impotriva defectele externe care pot apare in aval de bornele lor de iesire.

2. Siguranta - siguranta -siguranta 2.1. Siguranta 20 kV – siguranta pe coloana 0,4 kV

Sigurantele fuzibile de pe partea de m.t. a transforma-torului servesc exclusiv la protectia acestuia impotriva scurt-circuitelor interne si externe. In esenta, sigurantele de pe partea de j.t. trebuie sa asigure protectia transformatorului

1. Introduction The low voltage systems are supplied from the m.v

systems through a medium voltage / low voltage transformer.At the 20 / 0.4 kV pole-mounted transformer the protection

selectivity has to be provided from the medium voltage bayof the 110 / 20 kV substation that supplies the overhead linewhere the transformer is connected, up to the last low voltagereceiver without any discontinuity in the transformer area.

Having in view the characteristics of the pole-mountedtransformer, on the medium voltage side of the transformera switchgear with breaking capacity cannot be mounted(circuit breaker) so that only fuses are mounted for the inner(shortcircuit between windings, earthing faults etc.) and outerfaults transformer protection that can occur on the connectionsbetween the transformer terminals and the input protectionterminals, both on medium voltage and low voltage side.

On the l.v. side of the transformer, fuses or automaticcircuit breaker with magneto-thermal trippers or electronicrelays can be mounted, in order to provide overload andshortcircuit protection to outer faults that can occur before their output terminals. 2. Fuse – fuse – fuse 2.1. 20 kV fuse – 0.4 kV fuse

The fuses on the medium voltage side of the transformeruse only for its protection against inner and outer shortcircuits.Mainly the low voltage fuses have to provide transformer protection against shortcircuits and overloads. Therefore the rated current of the low voltage fuse has to be chosen as close as possible to the rated current of the low voltage winding of the transformer.

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impotriva scurtcircui-telor si a suprasarcinilor. Prin urmare, curentul nominal al sigurantei de j.t. se recomanda sa se aleaga cat mai apropiat de curentul nominal al infasurarii de j.t. a transformatorului.

Aceasta diferentiere de "sarcini" intre sigurantele fuzi-bile de m.t. si cele de j.t. de pe un transformator are doua aspecte contradictorii:

- pe de o parte, este o imprejurare care favorizeaza compor-tarea lor selectiva;

- pe de alta parte, corespunzator "sarcinilor" ce le revin in protectia transformatorului, este indicat ca sigurantele de j.t. sa aiba caracteristica timp-curent lenta sau lent-rapida,cat timp cele de m.t. se recomanda sa aiba caracteristicatimp-curent rapida, ceea ce reprezinta o dificultate in asigurarea comportarii lor selective. La alegerea sigurantei fuzibile de m.t. se adauga necesi-

tatea de a lua in considerare varful (socul) de curent care seproduce la conectarea trasnformatorului la retea, varf carepoate determina interventia nedorita a protectiei. Curentulde soc la conectarea la retea a transformatorului depinde dematerialului miezului feromagnetic (varf foarte inalt la miezcu pierderi reduse) si de puterea transformatorului, respectiv se micsoreaza cu puterea acestuia.

Nota: Raportul intre curentul de soc si curentul nominalare valorea, in medie, de 14 pentru transformatoare cu puterisub 220 kVA si usc= 4%, si de 6 pentru puteri peste 250 kVAsi usc= 6% [1] .

Alegerea sigurantelor fuzibile de m.t. se va face in functiede curentul nominal pe partea de medie tensiune al trans-formatorului, in baza relatiei:

These differences between medium and low voltage fusesoutlines two issues:

- on one hand, it favoures their selective behaviour; - on the other hand depending on their “goals” for the

transformer protection it is recommended that the low voltage fuses have the time – current characteristic slowor slow – fast, while the ones for medium voltage shouldhave it fast, this making difficult their selective behaviour.When choosing the medium voltage fuse, the current peak

(shock) that occurs when connecting the transformer to thenetwork, has to be also taken into account as this peak valuecould cause the uncommanded action of the protection. Theshock current when connecting the transformer to the networkdepends on the ferromagnetic core material (the peak valueis very high for a ferromagnetic core made of cold rolled platethat has small losses) and on the transformer power i.e de-creases with its power.

Note: The ratio between the shock and the rated currenthas an average value of 14 for transformers with powers upto 220 kVA and usc = 4% and 6 for powers above 250 kVA and usc = 6 % [1] .

The medium voltage fuses are chosen depending on therated current on medium voltage side of the transformeraccording to the following relation:

Insig ≈ (1,3 … 2 ) Intrafo [A] (1)Constituie un avantaj cunoasterea curentului de soc (si

a materialul miezului magnetic) pentru a decide alegerea unuifactor de multiplicare mai mic sau mai mare.

Valorile recomandate difera functie de sursa de docu-mentare, conform Tabel 1; se observa ca un fabricant de si-gurante recomanda valori mai mari pentru Insig fata de instructi-unea MEE, in baza observatiei ca respectivul curent "asigurasuportarea unei suprasarcini de 30 % a transformatorului si o functionare sigura la scurtcircuitele de pe bornele se-cundare ale transformatorului" .

Si fabricantii de sigurante fuzibile de m.t. atrag atentia asupra faptului ca acestea trebuie sa suporte curentul de socla conectarea transformatorului. Iata cateva recomandari [6]:

- valoarea curentului prezumat de pe caracteristica timp-curent a sigurantei fuzibile la momentul 100 ms, trebuiesa fie de cca. 12 ori mai mare decat curentul nominal altransformatorului;

- curentul de intrerupere al sigurantei trebuie sa fie mai mic,iar capacitatea de rupere sa fie mai mare decat puterea descurtcircuit a retelei in locul de montare al transforma-torului;

- curentul nominal al sigurantei trebuie sa fie mai mare (deregula, de 2 ori) decat curentul maxim de sarcina altransformatorului;

- trebuie analizata si selectivitatea intre sigurantele fuzibile de m.t. si cele de j.t. sau intreruptorul automat de j.t.. Pentru a reduce la minimum posibilitatea aparitiei defec-

telor externe din "campul vizual" al sigurantelor de pe parteade m.t., se recomanda utilizarea legaturilor scurte si cu con-ductoare preizolate intre bornele transformatorului si bornele de intrare ale aparatelor de protectie de pe partea de m.t. si j.t.

The shock current (and the material of the magnetic core)is important to be known in order to decide the selection ofa lower or higher multiplication factor.

The recommended values vary depending on the docu-mentation source Table 1:

It can be ascertained that a fuse manufacturer recommendshigher values for Infuse than MEE instruction based on theremark that this current “ensures the support for atransformeroverload of 30 % and a safe operation for the shortcircuitsat the secondary transformer terminals”.

The medium voltage fuse manufacturers underline alsothe fact that the fuses have to stand to the shock current when connecting the transformer. Below are some recom-mendations [6]:

- the forecasted value of the current on the time – currentcharacteristic at 100 ms has to be about 12 times biggerthan the rated current of the transformer;

- the fuse breaking current has to be lower and the breakingcapacity higher than the network shortcircuit power wherethe transformer is mounted;

- the fuse rated current has to be higher (usually two times)than the maximum load current;

- the selectivity between the medium and low voltage fusesor the automatic low voltage circuit breaker has to bealso analysed. In order to reduce as much as possible the possibility of

outer faults occurrence from the “visual field” of the fuseson the m.v side, short connections and pre-insulated conduc-tors have to be used between the transformer terminals andthe protection equipment input terminals on the medium and low voltage side.

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Caracteristicile sigurantelor de m.t. si de j.t. pot fi com-parate direct, intrucat ambele sunt determinate de aceeasi marime (integrala Joule) si tinand seama de raportul de trans-formare al transformatorului. Astfel, cu neglijarea pierde-rilor de mers in gol, cele doua seturi de sigurante fuzibile vor fi parcurse de acelasi curent "raportat" (este indiferent la ce curenti se raporteaza, de regula la partea de j.t.).

Prin urmare, criteriile de selectivitate pentru sigurantelefuzibile sunt:

1.Caracteristica sigurantelor fuzibile de m.t. sa se gaseascain permanenta deasupra caracteristicii sigurantelor fuzi-bile de j.t.; cele doua carateristici sa nu se atinga si cu atat mai putin sa se intretaie.

2. Integrala Joule dezvoltata in sigurantele fuzibile de m.t., in cursul procesului de topire a fuzibilului si de stingere a arcului electric in sigurantele fuzibile de j.t., trebuie sa aiba o valoare inferioara integralei Joule necesara pentruintreruperea sigurantelor de j.t.. O observatie foarte importanta: analiza si aprecierea

selectivitatii nu trebuie facuta pe toata plaja curentilor din abscisa caracteristicilor de protectie a sigurantelor fuzibile, ci se va lua in considerare curentul de scurtcircuit maximposibil care ar putea sa apara in aval de sigurantele fuzibilede j.t., curent determinat din relatia (2), unde: Sn , Un , uscn -marimile nominale ale transformatorului. Cele precizate mai sus sunt ilustrate in Fig. 1.

The characteristics of medium and low voltage fuses canbe directly compared because both are determined by the samevalue (Joule integral) and taking into account the transformerratio. Neglecting the no-load losses the two sets of fuses will be passed by the same current (regardless what currents arereported, usually on the low voltage side).

Thus the selectivity criterion for the fuses are the fol-lowing:

1. The medium voltage fuse characteristic has to be alwayshigher than the low voltage fuse characteristic; the twocharacteristics should not touch and cross;

2. The Joule integral occurred in the medium voltage fuseswhen the fusible melts and the electric arc is extinguishedat the low voltage fuses, has to be lower than the Jouleintegral required for the breaking of low voltage fuses.

Very important remark: the selectivity analysis andassessment should not be made for the entire current fieldon the abscissa of the fuse protection characteristics; themaximum shortcircuit current that could occur before the lowvoltage fuses has to be taken into account, this current beingknown according to the following relation (2), where: Sn , Un , uscn - rated transformer values.

These are shown in Figure 1.

Isc = ( Sn / 3 Un x uscn ) x 100 [kA] (2)

Fig. 1 Selectivitatea intre sigurantele fuzibile de medie tensiune si de joasa tensiune ale transformatorului

Fig.1. Selectivity between the transformer medium voltage and low voltage fuses

Pentru studiu de caz, transformatorul ales este de 20 / 0,4 kV-160 kVA, In1 / In2= 4,62/231 A, uscn= 4,01 %; se alegaparatele de protectie, initial, dupa urmatoarele criterii:

- la curentulul nominal primar In1= 4,62 A si, pentru a tine seama si de socul la conectare, se aleg sigurantele fuzibilede m.t. tip FEn 10 A

- la curentul nominal secundar In2=231 A, se aleg siguran-tele fuzibile de j.t. tip MPR (gL-gG, ~ 500 V) de 250 A La analiza comportarii lor selective, prin compararea

caracteristicilor, se constata ca sigurantele alese initial nu secomporta selectiv, luand drept "borna" pe axa curentilor cu-rentul de defect maxim posibil de Imax

sc = 5,75 kA care se poateproduce la bornele de j.t. ale transforma-torului; comporta-rea selectiva se manifesta doar pentru Isc < 1,5 kA.

For the case analysis the chosen transformer is 20 / 0.4kV – 160 kVA, In1 / In2 = 4.62 / 231 A, uscn = 4.01%; theprotection equipment is initially chosen according to followingcriterion:

- at the primary rated current In1 = 4.62 A in order to takealso into account the connection shock, medium voltagetype Fen 10 A are chosen ;

- at the secondary rated current In2 = 231 A low voltage typeMPR (NH, gL-gG – 500 V) fuses of 250 A are chosen. The initial chosen fuses have no selective behaviour

chosing as “terminal” on the current axis the maximum faultcurrent Isc

max = 5.75 kA (calculated with relation 2) that can occur at the low voltage transformer terminals; the selec-tive behaviour appears only for Isc < 1.5 kA.

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Rezulta doua variante posibile de rezolvare: {FEn 10 A + MPR 200 A} sau {FEn 16 + MPR 250 A}.

Vom alege initial prima varianta cu FEn 10 A + MPR200 A; dupa analiza finala, daca rezulta curenti nominali preamici pentru sigurantele de pe plecarile de j.t., cu atat mai multin cazul in care rezulta si necesitatea instalarii de cutii de se-lectivitate pe plecari, se poate relua analiza cu a doua varianta.

Dar, nu aceasta trebuie sa fie decizia definitiva, pentru ca, pe de alta parte, trebuie sa se tina cont si de faptul ca transformatorul poate fi supraincarcat cu sarcini, marimea si durata admisibila a acestora depinzand de sarcina anterioara,temperatura ambianta etc. Comportarea transformatorului este descrisa in acest caz prin "caracteristica de suprasar-cina sau supraincarcare" si care se regaseste doar in normainterna sau in catalogul fabricantului; in standardul STAS 440 / 1-90, este specificata doar supraincarcarea admisibila in caz de avarie pentru transformatoarele cu puteri de 16-1600 kVA si tensiuni de 6–24 kV, indiferent de preincarcarea initiala, de durata ei si de temperatura mediului ambiant.

There are two solving ways : {Fen 10 A + MPR 200A} or {Fen 16 + MPR 250 A}.

Initially we will choose the first way: Fen 10 A + MPR 200A; after the final analysis if the rated currents are toosmall for the low voltage output fuses and moreover there isnecessary to install selectivity boxes on outputs, the analysiscan be repeated with the second solving way Fen + MPR250 A.

But this should not be the final decision because thetransformer can also be overloaded, the time and value ofthese allowed overloads depending on the previous load, theenvironmental temperature, a.s.o. In this situation the trans-former behaviour is described by the “overload characteristic”which is to be found only in the internal instruction or themanufacturer’s catalogue; in STAS 440 / 1-90 only theemergency allowed overload for transformers between 16 –1600 kVA and 6 – 24 kV are specified regardless of the initialpreloading, its duration and the environmental temperature.

Tabel 1. Ghid de alegere a sigurantelor fuzibile de medie tensiune Table 1. Recommendations for the selection of medium voltage fuses

3.1.RE - I 15 - 87 / Anexa 9 Recomandare fabricant Un = 6 (7,2) kV Un = 20 (24) kV Un = 6 (7,2) kV Un = 20 (24) kV Sn trafo

[kVA] In trafo [A] In sig [A] In trafo [A] In sig [A] In sig [A] In sig [A]

160 15,2 25 … 31,5 4,6 6,3 … 10 31,5 … 63 16 … 25

Alegerea finala a sigurantei fuzibile de j.t., care asigurasi protectia la suprasarcina a transformatorului, trebuie sa fie rezultatul compararii caracteristicilor de suprasarcina ale transformatorului cu caracteristica timp-curent a sigu-rantei fuzibile

In lipsa caracteristicilor specifice de la fabricant, am facut comparatia cu caracteristica de supraincarcare deavarie a transformatorului, de unde rezulta ca sigurantafuzibila de 200 A indeplineste criteriile 1, 2 si 3; in plus, pebaza masuratorilor de sarcina anterioare si a prognozei deconsum pe urmatorii 15 ani, precum si functie de specificulconsumului (casnic, rural), rezulta ca transformatorul nuva fi incarcat la sarcina nominala sau la suprasarcini.

Concluzii: a) Protectia la suprasarcina nu va putea fi asigurata cores-

punzator prin sigurante fuzibile de j.t. b) Tinand cont şi de faptul ca, din analiza ultimelor ma-

suratori de sarcina si a balantei energetice pe post pentruinterval de 1-15 ani, transformatorul nu va fi incarcat mai mult de 85% din Sn, dar si de specificul consuma-torilor (exclusiv consumatori casnici), se poate stabili varianta finala cu sigurante fuzibile de 100 A, cu com-portare selectiva fata de sigu-rantele de m.t..

c) Rezolvarea corecta se poate asigura numai prin utilizareaunui intreruptor automat de j.t., prevazut cu declansatortermic reglat la curentul nominal al transformatorului si, evident, declansator electromagnetic (cu sau fara temporizare).

The final selection of the l.v fuse that provides also the overload transformer protection should be the result of comparation between the overload transformer character-istics (set by the manufacturer) and the time – current char-acteristic of the fuse.

While the manufacturer’s characteristics were missing we compared with the emergency overload characteristicand we ascertained that the 200 A fuse observes conditions 1, 2 and 3; moreover based on the previous load measure-ments and the consumption forecast for the next 15 yearsas well as depending on the consumption (household, rural)it can be seen that the transformer is not loaded at its ratedvalue and neither at overloads.

Conclusions: a) The overload protection cannot be adequately provided

with low voltage fuses. b) Having in view that the load measurements and power

balance for 1 – 15 years forecasted that the transformerwill not be loaded more than 85 % of Sn and thecomsumer types (only households) the final version with100 A fuses with selective behaviour can be decidedcompared with the medium voltage fuses.

c) Correct solving can only be made by using an automaticlow voltage circuit breaker with thermal tripper adjusted to the transformer rated current and obviously, electro-magnetic tripper (with / without delay).

2.2. Siguranţa fuzibilă pe coloana de 0,4 kV - sigurantafuzibila pe plecare in linia aeriana de 0,4 kV

La alegerea curentului nominal al sigurantelor de pe ple-carile radiale in linie aeriana de j.t. din cutia de distributie a PTA, trebuie sa se tina seama de urmatoarele [2], [3]:

1. Selectivitatea intre sigurantele fuzibile de pe partea dej.t. a transformatorului si sigurantele fuzibile de pe plecari,se asigura daca se prevede o anume diferenta de curentnominal pe scara standardizata intre sigurante de aceeasifabricatie, cu aceleasi caracteristici de topire (practic,

2.2. Fuse on 0.4 kV – fuse on radial output on the 0.4 kV overhead line

When chosing the rated current of the fuses on the radialoutputs on the low voltage overhead line from the distributionbox, following have to be considered [2], [3]:

1. The selectivity between the fuses on the low voltage sideof the transformer and the output fuses is provided if acertain rated current difference between the same manu-facturer’s fuses is provided with the same melting charac-teristics (practically the selectivity will be provided if

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selectivitatea va fi asigurata daca integrala Joule in cursulduratei de topire si stingere a arcului electric in sigu-ranta din aval are o valoare inferioara integralei Joule dezvoltate pe durata topirii sigurantei din amonte). Con-form literaturii de specialitate, sigurantele MPR se comporta selectiv intre ele daca curentii lor nominali se gasesc in raportul 1,6; pentru a tine cont si de bandade neprecizie a caracteristicilor sigurantelor (o toleranta de + 20% pentru curentii de topire), selectivitatea este asigurata daca diferenta intre curentii nominali este de cel putin doua trepte pe scara standardizata.

2. Curentul nominal al sigurantei de pe plecarea de j.t. sa fie un procent din curentul maxim admisibil al conducto-rului de j.t. (neizolat sau torsadat)

the Joule integral during the melting and the electric arcextinguishing is lower than the one occurred when thefuse located after is melting). According to the litera-ture, the MPR fuses are behaving selectively if their ratedcurrents have a ratio of 1.6 ; in order to take into accountalso the imprecision of the fuse characteristics (toleranceof +/- 20 % for melting currents) the selectivity is pro-vided if the difference between the rated currents of thetwo fuses is at least two steps on the standardized scale.

2. The fuse rated current on the low voltage output should be a percentage of the maximum allowed current of thelow voltage conductor (not insulated or stranded)

Insig = (0,8 – 0,85) Imaxcond [A] (3)

3. Sa se asigure conditia de deconectare a plecarii de j.t. in cazul scurtcircuitului monofazat la capatul plecarii:

3. The l.v output disconnecting condition should be observedwhen there is a single-phase shortcircuit at the output end:

I(1)sc.min > 3 x Infuse [A] (4)

4. In cazul liniilor lungi, unde nu se poate asigura conditia de deconectare, se admite sectionarea plecarii intr-un punct pana la care se asigura aceasta conditie si, in acestpunct, se pot monta in continuare sigurante fuzibile cu separator (in cutie de selectivitate si cu rol de sectionare),sigurante dimensionate conform criteriilor de mai sus. In studiul de caz analizat, pentru cele doua plecari aeriene

din post ar rezulta - Fig.1: - din criteriul de selectivitate fata de sigurantele fuzibile

alese la pct. 1.1, respectiv tip MPR 200 A pe coloana de0,4 kV, rezulta sigurante fuzibile In=125 A;

- plecarea 1: utilizand conductor TYIR 3x50 mm2 cu Imax=141 A, rezulta ca ar trebui sa se "coboare" la sigu-rante fuzibile tip MPR 100 A, iar din criteriul de deco-nectare la scurtcircuit de capat I(1)

sc =198 A, rezulta ca ar fi necesare sigurante cu In < 63 A ! In acest caz, se va pastra siguranta fuzibila de 100 A pe plecare in tabloul postului, din criteriile de selectivitate si de pro-tectie a conductorului, pentru a tine cont si de sarcina de pe plecare si se va monta o cutie de selectivitate (in punctul rezultat din calcul) echipata cu sigurante fuzibilecu In = 63 A (alese din conditiile 1,2,3);

- plecarea 2: utilizand conductor torsadat TYIR 3x70 mm2

cu Imax = 180 A, si urmand acelasi rationament ca mai sus,rezulta siguranta fuzibila de 125 A pe plecare si se vamonta o cutie de selectivitate (in punctul rezultat dincalcul) echipata cu sigurante fuzibile cu In = 80 A. Analiza devine foarte dificila in cazul utilizarii de transfor-

matoare de putere redusa, de 16 si 25 kVA, deoarece: - nu se fabrica sigurante limitatoare rapide pentru protectia

transformatoarelor de putere cu parametri 20 (24 kV) / I1n = 0,46 – 0,72 A , I2n = 23 – 36 A;

- din conditia de selectivitate, rezulta un curent redus pentrusigurantele fuzibile de pe coloana de 0,4 kV si de peplecarile din post, iar asigurarea selectivitatii pana laconsumator devine practic imposibila in conditiileutilizarii de sigurante fuzibile de 16-25 A;

- devine improprie utilizarea conductoarelor torsadate desectiuni mari (35, 50, 70 mm2), cu curenti admisibili mari(> 120 A) fata de curentii nominali pe j.t. Datorita faptului ca, inainte de 1989, nu se punea problema

fabricarii si utilizarii in retelele de distributie de 20 kV atransformatoarelor cu puteri asa de mici, nici o norma tehnicaenergetica sau indreptar de proiectare nu trateaza aceste cazurisi, mai mult, nici dupa 1989 aceste situatii nu sunt tratate.

4. For long lines where the disconnection condition is not observed, it is allowed to cut-off the output in a point up to which this condition is observed and in thatpoint fuses with separator can be mounted (in selectivity boxes with cut-off role), fuses sized according to the above conditions. In the analysed case study for the two overhead outputs,

following would result – Fig 1: - according to the selectivity condition for the chosen

fuses at 1.1, respectively type MPR 200 A on 0.4kV,fuses with Inn = 125 A should be used;

- output 1: using conductor TYIR 3 x 50mm2 with Imax =141 A lower fuses type MPR 100 A should be used andaccording to the shortcircuit disconnection consitionIsc = 198 A so that fuses with In < 63 A should be used. Inthis situation the 100 A fuse is kept on the outputbecause of selectivity and conductor protection reasonsand to take also into account the output load and aselectivity box (in the point resulted from the calculation)provided with fuses with In = 63 A (chosen from condi-tions 1, 2, 3);

- output 2: using conductor TYIR 3 x 70 mm2 with Imax =180 A it can be ascertained that fuses 125 A can be kept andaccording to the end shortcircuit disconnection conditionIsc = 192 A, lower fuses with In < 63 A should be used. Inthis situation also the 125 A fuse can be used on the outputand a selectivity box will be mounted (in the calculated point) with In = 80 A fuses (conditions 1, 2, 3). The analysis is very difficult if low power transformers

of 16 and 25 kVA are used because : - there are no quick limit fuses for power transformer

protection with 20 (24 kV) / I1n = 0.46 – 0.72 A, I2n = 23 – 36 A;

- from the selectivity condition a lower current for the0.4 kV and output fuses results and the selectivity up tothe the consumer is impossible if 16 – 25 A fuses are used;

- large section stranded conductors (35, 50, 70 mm2) cannomore be used with high allowable currents (> 120 A)compared with the rated low voltage currents. As before 1989 such low power transformers would not

be used in the 20 kV distribution networks, the technicalliterature did not study such situations. Unfortunately noteven after 1989 these situations are not analysed. 3. Fuse – circuit breaker – fuse

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3. Cazul siguranta – intreruptor automat – siguranta 3.1. Siguranţa fuzibilă pe 20 kV – intreruptor automat

pe coloana de 0,4 kV In acesta solutie, impusa fie prin concluzia c) de la pct.

2.1 de mai sus, fie ca urmare a unei strategii a distribuitoruluide energie electrica, sigurantele fuzibile de m.t. se aleg respectand criteriile si pasii descrisi la pct. 2.1.

Intreruptorul automat de j.t., avand in vedere ca va trebuisa asigure si protectia la suprasarcina a transformatorului, se va alege pe baza criteriului ca trebuie sa aiba un curent nominal cat mai apropiat de curentul nominal al infasu-rarii de j.t. a transformatorului si, evident, capacitatea sa de rupere sa fie superioara valorii maxime de curent de scurt-circuit in aval de bornele intreruptorului.

In studiul de caz, consideram ca s-au ales deja siguran-tele fuzibile de m.t. tip FEn 10A, si se va alege un intreruptor automat tripolar cu {Un=500V / 50 Hz, In = 250 A, Icu = 35 kA,cu declansator termic reglabil Ith = 250A - reglat la cu-rentul nominal al transformatorului (In = 231 A) si declan-sator electromagnetic fix Im = 3Ith,, fara temporizare}.

Verificarea selectivitatii intre cele doua aparate se varealiza pe baza criteriilor expuse mai sus la pct. 2.1, compa-rand caracteristicile de protectie « raportate » la curentulprimar sau secundar. Evident, selectivitatea este asiguratadaca caracteristica de protectie a sigurantei fuzibile de m.t.este intotdeauna deasupra caracteristicii de protectie a declansatoarelor magnetotermice ale intreruptorului automat, cu raportare fata de "borna" de pe abscisa curentilor, repre-zentata de curentii de intensitate mai mica decat curentul de scurtcircuit maxim care se poate produce in aval de bornele intreruptorului automat.

Referitor la cazul analizat, conform Fig. 2, rezulta caintreruptorul ales functioneaza selectiv cu sigurantele fuzibileFEn 10 A pana la un curent de defect Isc= 4,5 kA. Dar, privind si in avans, spre sigurantele fizibile de j.t. de pe plecari, se observa ca acestea nu pot avea curentul nominal In > 50 A, ceea ce ar face foarte dificila selecti-vitatea protectiei pana la consumatorul final. Prin urmare,se va alege, in final, intreruptorul cu declansator electro-magnetic cu Im= 5 Ith..

Si in acest caz, ar fi necesara, pentru luarea unei decizii fi-nale optime, compararea caracteristicilor de suprasarcina aletransformatorului cu caracteristicle de protectie ale declansa-toarelor magnetotermice ale intreruptorului automat de j.t.

3.1. 20 kV fuse – 0.4 kV circuit breaker For this solution imposed by conclusion c) at above 2.1

or as a result of a utility strategy, the m.v. fuses are chosen observing the conditions and stepts described at 2.1.

Because the low voltage circuit breaker should alsoprovide the overload protection of the transformer, it shall be chosen having in view the fact that its rated current shouldbe as close to the low voltage winding rated current and itsbreaking capacity shall be bigger than the maximum shortcir-cuit current after the circuit breaker terminals.

In the analysed situation we assume that medium voltagefuses of Fen 10 A type were already chosen and a tripolar circuit breaker with { Un = 500 V, In = 250 A, Icu = 35 kA withthermal adjustable tripper Ith = 250 A – adjusted at thetransformer rated current (In = 231 A) and fixed electromag-netic tripper Im = 3 Ith without delay) is selected.

Checking the selectivity between the two devices will beperformed based on the conditions at 2.1 comparing theprotection characteristics “reported” for the primary andsecondary current. It is obvious that the selectivity is providedif the protection characteristic of the medium voltage fuse isalways above the protection characteristic of the magnetic –thermal trippers of the automatic circuit breaker, referred to the“terminal” on the current axis, represented by the lower inten-sity currents compared to the maximum shortcircuit currentthat can occur after the automatic circuit breaker terminals.

Regarding the analysed situation, according to Fig. 2,the chosen circuit breaker is operating selectively with the Fen 10 A medium voltage fuses up to a fault current of Isc =4.5 kA very close to the limit “terminal” 5.75 kA; we considerthat there is no reason to choose a medium voltage fuse typeFEn16 A. However the low voltage output fuses cannot have a rated current In > 50 A so that the protection selectivity up to the final consumer shall be very difficult. Therefore a circuit breaker with electromagnetic tripper Im = 5 Ith will be finally selected.

It should be outlined that the selectivity between themedium voltage fuses and low voltage automatic circuitbreaker is more difficult to be provided as the transformerrated power is higher because when the shock current de-creases at the transformer connection, medium voltage fuseswith rated current according to Table 1 can be chosen. Thesituation is identical for the protection provided only with fuses.

Fig. 2 Selectivitate intre sigurante fuzibile de medie tensiune, intreruptor automat de joasa tensiune si sigurante fuzibile de joasa tensiune

Fig. 3 Selectivity between the medium voltage fuses, low voltage automatic circuit breaker and low voltage fuses

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3.2. Intreruptor automat pe coloana de 0,4 kV - siguranţă fuzibilă pe plecarea radială în linia aeriană de 0,4 kV

In aceasta situatie, analiza poate incepe de la alegereasigurantelor fuzibile de j.t. pe plecarile din post, in baza criteriilor 2, 3, 4 expuse la pct. 1.2 de mai sus.

In cazul analizat, prin alegerea intreruptorului automatcu Im=5Ith ,rezulta ca sigurantele fuzibile pe plecari nu potavea curentul In >80 A, curent care satisface si conditia deprotectie a conductoarelor pe plecari; pentru indeplinirea conditiei de deconectare la I(1)

sc , se pot utiliza cutii desectionare / selectivitate cu separator si sigurante fuzibile alese ca la pct. 1.2.

Avand in vedere faptul ca un intreruptor automat are carateristica de protectie compusa din caracteristicile celordoua declansatoare, termic si electromagnetic, este necesaracompararea fiecareia cu caracteristica sigurantelor fuzibilede j.t. Pentru rezolvarea situatiei, se pot lua in discutie doua solutii:

- se regleaza declansatorul electromagnetic la un curent Im mai mare decat cel stabilit initial, pentru a se "salta" profilul caracteristicii cu un intervalul Δt, dar este insu-ficient; de regula, aceasta solutie impune alegerea unui intreruptor cu un curent nominal mai mare (cu costuri mai mari);

- alegerea unui intreruptor automat cu declansator electro-magnetic cu temporizare, care permite obtinerea uninterval Δt ~ 0,15 sec. ; este solutia practica care asigura o selectivitate satisfacatoare cu costuri mai reduse. In cazul analizat, din Fig. 2 rezulta un interval de timp

Δt = 0,02 sec. fata de caracteristica sigurantei fuzibile cuIn = 80 A, ceea ce atrage o incertitudine in comportarea selec-tiva si impune si cutie de sectionare / selectivtate pe plecari.

Se mai pot analiza si alte doua variante: - sigurante fuzibile cu Insig= 63 A, care satisfac toate

conditiile; este solutia cea mai ieftina, care nu maipresupune utilizarea cutiei de sectionare, realizarea uneiprize de pamant de max. 4 ohmi la stalp etc.

- pentru a putea utiliza sigurante fuzibile cu In = 100 A, sepoate echipa intreruptorul cu declansator electromagneticcu temporizare (varianta mult mai scumpa decat varianta de mai sus cu sigurante fuzibile de 63 A).

4. Concluzii

1. Este cunoscut faptul ca, protectia unei instalatii electricetrebuie sa indeplineasca, in ansamblu si pe componente, urmatoarele conditii: sa fie selectiva – rapida – sensibila –sigura in functionare - economica. Instalatia de protectie este selectiva atunci cand un defect determina actio-narea elementului de protectie din amonte.

2. Lucrarea si-a propus sa se focalizeze pe un subiect "lazi": alegerea aparatelor de protectie a unui transformatordintr-un post de transformare aerian din mediul rural, cuplecari in linii aeriene radiale si analiza detaliata a cri-teriilor de asigurare a selectivităţii tuturor protectiilor aferente, exemplificand cu date concrete pe un caz real.

3. Chestiunea pe care vrem sa o propunem spre dezbaterespecialistilor si factorilor de decizie in domeniu, cu cea mai mare insistenta, este urmatoarea: cui revine sarcinade a face analiza selectivitatii protectiilor la post detransformare aerian ?!

4. Primul impuls, rezultat din multe discutii pe aceasta tema,a fost ca sarcina sa fie transferata imediat proiectantului postului de transformare. Dupa parerea noastra este o

3.2. Low voltage circuit breaker – fuse on the radial outputon the 0.4 kV overhead line

In this situation the analysis can start by chosing the lowvoltage output fuses based on conditions 2, 3, 4 at above 1.2.

In the analysed situation, by chosing the automatic circuitbreaker sith Im = 5 Ith, the output fuses cannot have the currentIn > 80 A, this current observing also the protection conditionof the output conductors; in order to fulfil the disconnectioncondition at Isc

(l) section / selectivity boxes can be used with switch and fuses selected according to 1.2.

Taking into consideration that an automatic circuit breaker has its protection characteristic composed of the characteristics of the two trippers (thermal and electro-magnetic) each of these has to be compared with the low voltage fuse characteristic.

In order to solve this situation, two solutions can beadopted:

- the electromagnetic tripper is adjusted to a current Im bigger than the one initially set in order to “increase” thecharacteristic profile, but the Δt is not enough; usually this situation involves the selection of a higher rated current circuit breaker (with highes expenses);

- the selection of a delay electromagnetic tripper automaticcircuit breaker so that the characteristic will be located with Δt about 0.15 sec; is the practical solution that provides a satisfactory selectivity with lower costs. For the analysed situation according to fig 3 a time Δt =

0.02 s results compared with the fuse characteristic withIn = 80 A this leading to a certain uncertainty in the selectivebehaviour and requires the use of a selectivity box on outputs.

Two other alternatives can also be used: - fuses with Infuse = 63 A fulfilling all the consitions: this is

the cheapest solution that doesn’t require nomore to usethe section box and an earthing socket of max 4 Ω at therod a.s.o;

- in order to use fuses with In = 100 A the circuit breakercan also have delay electromagnetic tripper (version muchmore expensive than the above solution with 63 A fuses).

4. Conclusion 1. It is known that the protection of an electric system should

fulfill entirely and partially following conditions: selec-tive – quick – sensitive – safe – economical. The protec-tion is selective when a fault causes the tripping of the protection located before it.

2. This paper intends to focus on an actual topic: the selec-tion of protections for a pole-mounted transformer in a rural area, with radial overhead outputs and the detailedanalysis of the conditions that should be fulfilled in orderto provide the protection selectivity with real examples.

3. We urge on the following issue : who has to perform theprotection selectivity analysis for a pole-mountedtransformer ?

4. The first answer in mind after several discussion on thisissue, was that this should be performed by the trans-former designer. In our opinion however this position is false because according to the regulations (MF / MLPTL Decision no. 1013/ 873 and no. 1014 / 874) the designer cannot choose a certain maufacturer and switchgear / protection model; he can only set the technical ratedvalues based on the technical and design regulations.

5. The protection selectivity analysis should only be madeby the network designer if the protection equipment are

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pozitie falsa: in condiile legii (Ordin MF / MLPTL nr.1013 / 873 si nr. 1014 / 874) proiectantul nu poate stabiliun fabricant si un anume model de aparat de comutatie si protectie ci, pe baza principiilor de proiectare cuprinse in normele tehnice si indreptarele de proiectare specifice,trebuie sa stabileasca doar caracteristicle tehnice nominale,pe baza carora intocmeste caietele de sarcini.

5. Sarcina de a realiza analiza selectivitatii protectiilortransformatorului ar trebui sa revina, de drept, proiectan-tului de retele doar in cazul in care aparatele de protectiesunt puse la dispozitie de beneficiarul, nemaifacandobiectul unei licitatii.

6. Altfel, dupa adjudecarea lucrarii prin licitatie sau incre-dintare directa, constructorul de specialitate este cel cecomanda, la diversi furnizari/fabricanti atestati in domeniu,elementele ce compun postul de transformare si care intrain analiza selectivitatii protectiei. Constructorul ar puteasa obtina, de la furnizorul/fabricantul sigurantei fuzibilede m.t. si cel al transformatorului, caracteristicile nece-sare analizei selectivitatii protectiei, dar nu cunoastenici el fabricantul aparatelor de comutatie si protectie de j.t. care vor fi incluse in tabloul electric de post.

7. In final, fabricantul tabloului electric este cel ce cunoastetipul aparatelor de j.t. (indigene sau de export) care se vor monta in tablou, aparate alese in baza conditiilor minimale / maximale impuse prin caietul de sarcini de catre proiectantul de retele. Obtinand si caracteristicile sigurantelor fuzibile de m.t. si ale transformatorului, prin intermediul constructorului ce va executa lucrarea in retea, fabricantul tabloului electric este cel care arc putea detine toate datele tehnice pentru a face o corectaanaliza a functionarii selective a protectiilor transfoma-torului in cauza; evident, pentru eventuale schimbari de aparataj, poate si trebuiue sa aiba un dialog interactiv cuproiectantul de retele care a intocmit caietul de sarcini cuprins in proiect.

8. Intrebarea este: care este cadru legal prin care sa sestabileasca aceste responsabilitati, pentru fiecare parte implicata in lucrare ? In lipsa asumarii unor responsa-bilitati, aparatele de protectie vor fi alese (in continuare) doar pe baza parametrilor nominali si a relatiilor de calcul din indreptarele de proiectare, ceea ce poate avea ca rezultat solutii "compromise" tehnic.

9. Nu este dificil ca, dupa ce constructorul a achizitionattoate elementele componente ale postului de transformare,proiectantul sa solicite fabricantilor de aparataj caracte-risticile de protectie ale acestora si sa realizeze o analizacompleta a selectivitatii protectiilor transformatorului,dar intrebarea care se pune este: daca nu se obtineselectivitatea protectiilor cu aparatele furnizate, care esteprocedura de urmat ?

10. O gestionare corecta a functionarii selectivite a protectiilorare implicatii si in activitatea de exploatare a posturilorde transformare aeriene, drept pentru care ne exprimamopinia ca este obligatorie reanalizarea selectivitatiiprotectiilor in situatii precum defectarea sau simpla« rocadare » a transformatorului existent si racordarea denoi consumatori permanenti care duc la modificarea semnificativa a sarcinii (preincarcarea) transformatorului.

11. Normele tehnice energetice si indreptarele de proiectarespecifice trebuie sa fie revizuite pentru a cuprinde sianaliza posturilor de transformare aeriene realizate cutransformatoare de 16 - 25 kVA

supplied by the beneficiary and not through a bid. 6. Thus after the bid or direct contracting the builder orders

to certain approved suppliers / manufacturers the elementscontained by the pole-mounted transformer. The buildercould get from the medium voltage fuse and transformersupplier / maufacturer the data required for the protectionselectivity analysis but he doesn’t know the manufacturerof the switchgear and low voltage protections to beincluded in the board.

7. Finally the board manufacturer is the one that knows thetype of the low voltage devices (home or imported) to be mounted, these being selected based on the minimum / maximum conditions imposed by the designer. Knowing from the builder the characteristics of medium voltage fuses and transformer, the board manufacturer is the one that holds all the technical data for a correct analysis ofthe protection selective operation; obviously if certain devices are changes he can and should contact the designer.

8. The question is: what is the legal framework to set theseresponsibilities for each involved party ? If these responsi-bilities are missing the protection devices will still bechosen based on design rated values and calculations,this could lead to “compromised” technical solutions.

9. It is not difficult that after the builder purchased all thecomponents of the pole-mounted transformer, the designershall require to the manufacturer the protection character-istics in order to perform a complete selectivity analysisbut the question is: if the protection selectivity is notprovided with the selected devices what has to be done ?Who pays for changing the devices ?

10. The situation is different for the premounted transformers: the manufacturer choses all the protections and he is theone to perform the protection selectivity analysis. We evenconsider that the results of this analysis should be sub-mitted to the beneficiary as part of the technical docu-mentation.

11. The technical standards and specific design specificationshave to be reviewed in order to contain also the analysisof 16 – 25 kVA pole-mounted transformers.

Bibliografie (References)

1. Centea, O.: Protectia instalatiilor electrice de joasa tensiune.Editura tehnica, Bucuresti, 1982

2. *** Indreptar de proiectare a protectiilor prin relee si sigu-rante fuzibile in reteaua de joasa tensiune,1RE-Ip 45-85, ICEMENERG, Bucuresti,1987

3. *** Instructiune privind calibrarea, inlocuirea si evidenta sigurantelor fuzibile, 3.1.RE-I15-87, ICEMENERG, Bucuresti,1989

4. *** Trip curves for distribution. Circuit-breakers with thermo-magnetic releases (ABB SACE) si Time current characteristics of fuse link NV, gL-gG ~ 500V prin ELECTROALFA INTL. Botosani

5. *** Elemente de inlocuire de inalta tensiune, limitatoare de curent, de interior si exterior. Caracteristicile timp-curent. SF 01 / 99. EXIMPROD Buzau

6. *** Consulting / FAQ for protection of power transformers. SIBA fuses.