University of Zagreb. Faculty of Electrical Engineering and Computing. Department of Energy and Power Systems.
Abstract
U radu je nelinearna karakteristika (struja magnetiziranja – magnetski tok) reaktancije praznog hoda transformatora linearno aproksimirana na konačan broj pravaca, pri čemu je utvrđen dovoljan broj pravaca za što točniju aproksimaciju pri različitim prijelaznim pojavama (uklapanje neopterećenog transformatora pri različitim konfiguracijama električnog kruga, isklapanje naponskog mjernog transformatora). Na osnovi matematičkog modela za električni krug s nelinearnim induktivitetom razvijen je kompjutorski program za praćenje valnih oblika varijabli stanja. Rezultati dobiveni tim programom uspoređeni su s namjenskim programom Power System Blockset, koji predstavlja specijalizirani dio Matlaba 5.3 za elektromagnetske prijelazne pojave. Također su sprovedena eksperimentalna mjerenja prilikom uklapanja malog energetskog transformatora te pokazano je da se rezultati pokusa dobro slažu s rezultatima razvijenog programa. Dodatno je razvijen program koji uvažava nelinearnost aktivne otpornosti izazvane Joulovim gubicima u željezu transformatora. Simulacijama su utvrđene električne prilike prilikom uklapanja neopterećenog transformatora. Pri tome su ustanovljena kritična stanja, odnosno najnepovoljnije prilike s obzirom na trenutke uklopa prekidaèa i snage kratkog spoja elektroenergetskog sustava. Na kraju su ispitana energetska naprezanja metal-oksidnih odvodnika prenapona uzrokovana povišenjima napona kod uklapanja neopterećenog transformatora izazvanog prelaskom u zasićeno područje krivulje magnetiziranja nelinearnog induktiviteta. Pokazano je da približavanjem rezonantnim uvjetima dolazi do znatnih energetskih preopterećenja odvodnika prenapona što znaèi da za pravilno dimenzioniranje odvodnika prenapona treba voditi računa i o ovoj vrsti privremenih prenaponskih stanja.Nonlinear curve of non-load transformer reactance (magnetizing current-magnetic flux) was linearized with a finite number of linear functions determined by approximation accuracy at different transients (non-loaded transformer switching at different circuit configurations, voltage transformer switching). State-variable waveform monitoring software based on mathematical model of nonlinear inductive circuit was developed . Results generated by this software were compared to results obtained using Power System Blockset, electromagnetic transient software, integrated in Matlab 5.3. Measurements obtained in the experiment of small power transformer switching in confirmed software results. Software incorporating a nonlinear resistance caused by Joule's losses in a transformer iron core was developed additionally. Conducted simulations established electrical conditions at non-loaded transformer switching in including critical conditions, that is the worst case in the term of switching moment and energy system short circuit power. Energetic stress of metal-oxide surge-arrester caused by overvoltages at non-load transformer switching, as a consequence of transition to saturated area of magnetizing curve, was examined. It was showed that the surge-arrester overload appears approaching to the resonance. According to the previous, it is necessary to take into account such temporary overvoltage conditions in the surge-arrester design
