An on-line method using no-load closing impulse current of the line to discriminate CT’s polarity is proposed in this paper. No-load closing model for the single-phase transmission line is established firstly, and the mathematical relationship between electrical quantities and line parameters during closing process is described by differential equations. Line parameters such as the inductance or resistance are regarded as unknown quantities, and the characteristics of their values are analyzed when solving the differential equations under different CT polarities. Then the calculation values are normalized. In order to quantify the distribution characteristics of normalized equivalent values, define the forward and reverse coefficients. According to the ratio of these two coefficients, namely recognition coefficient, CT’s polarity in single-phase system can be discriminated. On this basis, CTs’ polarities in three-phase can be discriminated bydifferent phase-mode transformations with specific matrixes. Finally, the effectiveness of the method is verified by simulation.

line parameters; no-load closing current; phase-mode transformation; recognition coefficient; the polarity of current transformer