The impact of transformer inrush current on operation of differential protection

Transformator je ključna komponenta u prijenosu energije u elektroenergetskom sustavu. Stabilnost kao i ispravna prorada relejne zaštite koja štiti transformatore su veoma važni za pogon i vođenje elektroenergetskog sustava. Glavna tema ovog rada je istraživanje energetskih transformatora u trenucima uklapanja, s posebnim naglaskom na pojavu fenomena uklopne struje, te metoda stabilizacije diferencijalne zaštite transformatora odnosno prepoznavanje stanja kvara transformatora. U trenucima uključenja transformator može povući iz mreže tranzijentnu struju velike amplitude. Uklop transformatora praćen je velikom nesimetrijom magnetskih tokova u željeznoj jezgri. Posljedica takvih nesimetričnih tokova u jezgri javljaju se izobličenja struje magnetiziranja odnosno pojava velikih struja uklopa s izraženom istosmjernom komponentom. Struja uklopa može uzrokovati pogrešan rad zaštitnih releja u elektroenergetskom sustavu kao i mehanička oštećenja transformatora. Amplituda i vrijeme trajanja tranzijentne struje uklopa transformatora ovisi o kutu uklopa, odnosno položaju vektora napona u trenutnu uključenja transformatora, impedanciji mreže na koju se transformator uklapa, iznosu i predznaku remanentnog magnetizma jezgre transformatora i nelinearnosti magnetskog materijala jezgre transformatora, odnosno zasićenju jezgre transformatora. Unutar ovog rada razmatrani su problemi uzrokovani tranzijentnom strujom uklopa transformatora, te su prikazani rezultati dobiveni mjerenjima struja uklopa odabranih transformatora iz prijenosne mreže. Uzimajući u obzir negativno djelovanje struje uklopa na diferencijalnu zaštitu i primjenu metoda stabilizacije releja, izvršena je analiza struje uključenja.Transformer is a key component in power transmission by the electric power system. The stability and security of the transformer protection operation are vital for the power system operation and control. The main topic of this paper is the investigation of the power transformer behaviour under the switching condition. The inrush current phenomena has been particularly discussed including the stabilization of the transformer differential protection and identification of the transformer internal faults condition. When a transformer is switched on, it may draw a high-magnitude transient current from the power grid. The power transformer energizing is accompanied by a high asymmetry of the flux in the iron core. Due to the flux asymmetry, significant distortion of the magnetizing current waveform, very high inrush currents, and a very high DC component of the inrush currents occur. The inrush currents may result in malfunction of the protective relays in the electric power system and a mechanical damage of the transformer. The magnitude and duration of the transient inrush current of the transformer depends on the switching angle and position of the voltage waveform when the transformer is energized. It also depends on the power grid impendence to which the transformer is switched, the value and the sign of the residual flux linkage in the transformer core, and the non-linear magnetic saturation characteristics of the transformer core. This paper addresses the issues caused by the transient inrush current of the transformer and presents the results obtained by measuring the inrush currents on the characteristic transformers from the power transmission grid. Taking into account the negative effect of the inrush current on differential protection and the relay stabilization, the inrush current waveforms have been analyzed