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Nisko naponski namoti transformatora u spoju zvijezda s izvedenom nultočkom nekako su se udomaćili u proizvodnji i u upotrebi transformatora, dok namoti u spoju slomljena zvijezda nisu tako česti ni opće poznati. Budući da svaki fazni namot u spoju slomljena zvijezda ili cik-cak (oznaka z) ulančuje namote s dva stupa, složeniji je i račun reaktancija transformatora i uklapanje reaktancija mreže u račun struja kvara. Točan račun struja kvara važan je za ispravno dimenzioniranje sekundarne zaštite transformatora. U radu se obrađuje račun struja jednopolnog i tropolnog kratkog spoja na sekundaru transformatora u spoju Dzn, Yzn i YNzn napajanih iz mreže poznatog unutrašnjeg otpora, kakav obično traže kupci takvih transformatora. Dokazuje se, da u sva tri navedena slučaja struje jednopolnog i struje tropolnog kratkog spoja i prateća naprezanja materijala ne ovise o spoju primarnog namota nego samo o geometriji namota i reaktanciji mreže. Transformers with star-connected LV winding and earthed neutral represent a majority of transformers produced or in use today. Zig-zag connected winding is less frequent and therefore less known. Since each phase winding in zigzag (z) connection couples two core limbs, it makes the calculation of transformer reactance and adding the power system network reactance into calculations of fault currents more difficult. Accurate calculation of fault currents is of great importance for the right dimensioning of transformer secondary winding protection. This paper analyzes the calculation of single-phase and three-phase short-circuit currents on the secondary winding of Dzn, Yzn and YNzn transformer winding connections, powered by power system network of known inner reactance, as usually required by the buyers of such transformers. It is proven in this paper that in all three stated cases single-phase and three-phase short-circuit currents and stresses of material do not depend on primary winding connection, but only on geometry of winding and power system network reactance. |
