Protection of power transformers with a common grounding resistor for a low resistance grounded systems

U ovom radu opisana su pogonska stanja energetskog transformatora koja utječu na njegov rad i na rad numeričkih zaštitnih uređaja. Dan je pregled postavki primarnih i sekundarnih zaštita transformatora koje su ispitane na unutarnje i vanjske kvarove. Prikazan je način modeliranja regulatora napona i njegovo korištenje. Izvedeno je snimanje uklopa transformatora radi analiziranja udarne struje magnetiziranja u samostalnom pogonu i paralelnom pogonu uz pojavu simpatetske struje magnetiziranja. Prikazani su valni oblici koji dokazuju pojavu udarnih struja magnetiziranja. Nadalje, predstavljena je simulacija analize prilika jednofaznog kratkog spoja u 110 kV, 220 kV i 400kV mreži koji za posljedicu rezultira tokom struje kroz neutralne točke 35 kV strane dva paralelno spojena energetska transformatora. Te cirkulacijske struje mogu imati negativan učinak na selektivnost zaštite sustava. U radu su dani prijedlozi rješenja neselektivne prorade zaštite u numeričkim relejima.In this paper different transformer operating states that affect and interfere with his and the work of numerical relays are described. Also review of the settings for primary and secondary transformer protection and its testing for internal and external faults has been done. Modeling of the transformer voltage regulator and its testing is shown. The simulation of transformer switch-on has been done for the sake of analyzing the inrush current during stand-alone and parallel operation states in which sympathetic inrush occurs. The wave forms that prove the presence of inrush current are shown. Further the analysis of single phase to ground faults in 110, 220 and 400 kV network which as a result cause a circulating current through neutral points of 35 kV of two parallel power transformers. Those circulating currents can have a negative effect on the selectivity of system protection. Suggestions for the solution of non-selective tripping of relays are given