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姜龍澈(Yong-Cheol Kang),李美善(Mi-Sun Lee),李炳銀(Byung-Eun Lee),崔在宣(Jae-Sun Choi),張成一(Sung-Il Jang),金容均(Yong-Gyun Kim),柳永植(Young-Sik Lyu) 대한전기학회 2008 전기학회논문지 Vol.57 No.12
This paper proposes an algorithm to estimate the circulating currents in the transformers in parallel in an ultra high voltage system. For the Y-Y-Δ transformers operated in parallel, there exist two kinds of the circulating currents i.e. one is between the tanks and the other between the banks of the delta side. As the former is 90 deg out of phase of the load current, it is estimated by decomposing the line current into the component 90 deg out of phase of the load current in the frequency domain. The latter is estimated in the time domain from applying the Kirchhoff's voltage law on the delta winding which gives a first-order differential equation in terms of the delta winding currents. To estimate the circulating currents between the tanks, the performance of the proposed algorithm is investigated when the impedances of the two transformer tanks are different or the taps of the on-load tap changer of the transformers are mismatched temporarily. To estimate the circulating currents between the banks, the performance of the proposed algorithm is also examined under magnetic inrush and over-excitation. Test results indicate that the algorithm can estimate the two kinds of the circulating currents successfully.