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jezgre transformatora L m , inaËe definirana krivuljommagnetiziranja -i m ( je glavni ulanËenimagnetski tok, i m je struja magnetiziranja transformatora),lako se mogu odrediti mjerenjem ili suveÊ dani od strane proizvoappleaËa. Ovaj nelinearniinduktivitet aproksimiran je s dva pravca u -i mkoordinatnom sustavu, slika 2, πto je za energetsketransformatore u praksi uglavnom prihvatljivo.ToËka (i z , z ) predstavlja kritiËnu toËku pri prelaskuiz nezasiÊenog u zasiÊeno podruËje æeljezne jezgre.Koeficijenti pravaca ustvari predstavljaju induktiviteteu nezasiÊenom (L m1 , const 1 ) i zasiÊenompodruËju (L m2 , const 2 ). Na ovaj naËin dobivamofunkcionalnu ovisnost struje magnetiziranja o magnetskomtoku kao (funkcijom sign osiguravamopozicioniranje u odgovarajuÊem kvadrantu):of the transformer iron core L m , otherwise definedby the magnetizing curve -i m ( is the main linkagemagnetic flux, i m is the transformer magnetizingcurrent), can easily be determined through measurementor are already provided by the manufacturer.This nonlinear inductance is approximated with twostraight lines in the -i m coordinate system, Figure 2,which is generally acceptable in practice for powertransformers. Point (i z , z ) represents the criticalpoint at the transition from the unsaturated regionto the saturated region of the iron core. The straightlines coefficients actually represent inductances inthe unsaturated region L m1 , const 1 and saturated regionL m2 , const 2 . In this manner, we obtain the functionaldependence of the magnetizing current on themagnetic flux (we define the position in the correspondingquadrant with the sign function) as follows:(3)(4)3 ANALITI»KI PRISTUPRadi jednostavnosti zanemarit Êe se sve aktivneelemente sa slike 4b. Dakle, pri prijelaznoj pojaviuklapanja transformatora ako trenutaËna vrijednostmagnetskog toka, po apsolutnoj vrijednostipremaπi kritiËnu vrijednost z , induktivnost transformatorase mijenja sa L m1 na L m2 . To ustvariznaËi da se trenutak kada magnetski tok dostignevrijednost z moæe uzeti kao vrijeme t=T>T uk isklapanjainduktiviteta L m1 odnosno uklapanja induktivitetaL m2 . Ekvivalentna shema bi u tom sluËajuizgledala kao na slici 5. SliËno, pri smanjenjumagnetskog toka ispod vrijednosti z iskljuËuje seinduktivitet L m2 i ukljuËuje induktivitet L m1 .3 ANALYTICAL APPROACHFor the purpose of simplicity we shall ignore allthe active elements from Figure 4b. In the transientphenomena during transformer energization,if the instantaneous value of the magnetic flux interms of absolute value exceeds the critical valueof z , the transformer inductance changes from L m1to L m2 . This actually means that the moment whenthe magnetic flux reaches the value of z can betaken as the time t=T>T uk of switching off the inductanceL m1 , respectively the time of switchingon the inductance L m2 . In this case, an equivalentdiagram would look like the one presented in Figure5. Similarly, when the magnetic flux is decreasedbelow the value of z , inductance L m2 is switchedoff and inductance L m1 is switched on.591TokiÊ,A., UglešiÊ, I., NumeriËki proraËun niskofrekvencijskih..., Energija, god. 56(2007), br. 5., str. 584-607TokiÊ, A., UglešiÊ, I.,The Numerical Calculation of..., Energija, vol. 56(2007), No. 5, pp. 584-607