An accurate autotransformer model for load flow studies effect on the steady-state analysis of the Hellenic transmission system

Abstract

This paper gives a closed formula which expresses the variation of the leakage reactance of power autotransformer, with tertiary winding, as a function of the autotransformer tap position. The derivation of the formula is based on the analysis of the distortion of the leakage field as portions of the winding are cut out by means of tappings. The case which is examined is the most common when the tap changers on the parallel winding are cut out symmetrically at the end of the winding. The validity of the derived model is tested by comparison with existing autotransformer impedance correction data. The importance of implementing such a model is shown by tap-changing voltage control of the Hellenic power system. The voltage control at the remote buses of the region of East Macedonia and Thrace, is achieved within the specified range, by the implementation of the derived formula on the tap-changing voltage control on load flow calculations.